chapter 48, factors influencing gait

853

Characteristics of Normal Gaitand Factors Influencing It

THE GAIT CYCLE, THE BASIC UNIT OF GAIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .854

KINEMATICS OF LOCOMOTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .856

Temporal and Distance Parameters of a Stride . . . . . . . . . . . . . . . . . . . . . . . . . . . .856

Angular Displacements of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .857

MUSCLE ACTIVITY DURING LOCOMOTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .861

KINETICS OF LOCOMOTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .863

Joint Moments and Reaction Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .863

Energetics of Gait: Power, Work, and Mechanical Energy . . . . . . . . . . . . . . . . . . .869

FACTORS THAT INFLUENCE PARAMETERS OF GAIT . . . . . . . . . . . . . . . . . . . . . . . . . . .872

Gender . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .872

Walking Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .872

Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .872

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .873

Habitual bipedal locomotion is a uniquely human function and influences an individual’s

participation and interaction in society. Impairments in gait are frequent complaints of

persons seeking rehabilitation services and are often the focus of an individual’s goals of

treatment. Rehabilitation experts require a firm understanding of the basic mechanics

of normal locomotion to determine the links between impairments of discrete segments

of the musculoskeletal system and the patient’s abnormal movement patterns in gait.

Therapists and other rehabilitation experts are called upon daily to analyze a patient’s

movement and determine the cause of the abnormal, often painful, motion. A thorough

understanding of normal locomotion and the factors that influence it, as well as an un-

derstanding of the functions of the components of the musculoskeletal system, provides

a framework for evaluation and treatment of locomotor dysfunctions. This chapter de-

scribes the general characteristics of normal locomotion and introduces the clinician to the

basic concepts central to all movement analysis.

Normal human locomotion consists of stereotypical movement patterns that are imme-

diately recognizable. Yet most individuals also are able to distinguish the gait of close

friends and associates by the sound of their footsteps in the hallway. The purpose of this

chapter is to describe the common characteristics of normal human locomotion and

their variability and to provide insight into how impairments within the musculoskeletal

48C H A P T E R

854 Part V | POSTURE AND GAIT

system may be manifested in altered gait patterns. The specific objectives of this chapter

are to

■ Describe the basic components of the gait cycle

■ Present the temporal and distance characteristics of normal gait

■ Detail the angular displacement patterns of the joints of the lower extremity, the

trunk, and the upper extremities

■ Describe the patterns of muscle activity that characterize normal locomotion

■ Briefly discuss the methods for determining muscle and joint loads sustained during

normal locomotion and present the findings from representative literature

■ Briefly consider the energetics of normal locomotion and the implications of gait

abnormalities on the efficiency of gait

Gait has been studied for millennia, and the last 50 years have seen an explosion in the

research examining the characteristics of gait and the factors that control it. The current

chapter is, of necessity, an overview of the characteristics of locomotion that are useful to

a clinician and that demonstrate the effect of the integrity of the musculoskeletal system

on gait. Several textbooks dealing only with locomotion provide details regarding the

movement and methods of its assessment, and insight into the central nervous system’s

role in controlling and modifying the movement of gait [28,119,127,159].

THE GAIT CYCLE, THE BASICUNIT OF GAIT

Gait is a cyclical movement that, once begun, possesses veryrepeatable events that continue repetitively until the individ-ual begins to stop the motion. The steady-state movement ofnormal locomotion is composed of a basic repeating cycle, thegait cycle (Fig. 48.1). The cycle is traditionally defined as

the movement pattern beginning and ending with groundcontact of the same foot. For example, using the right foot asthe reference foot, the gait cycle begins when the right footcontacts the ground (usually with the heel) and ends when itcontacts the ground again. Thus a gait cycle consists of thetime the reference foot is on the ground (stance) and thetime it is off the ground (swing). The movement of both limbsthat occurs during the gait cycle is known as the stride.

Double support Single support Double support Single support

Right step Left step

Stride

Stance Swing

Figure 48.1: The gait cycle of a single lower extremity consists of a stance and swing period and lasts from ground contact of onefoot to the subsequent ground contact of the same foot. It includes two steps that are defined as the period from ground contact ofone foot to the ground contact of the opposite foot. A single gait cycle includes two periods of double limb support and two periodsof single limb support.

855Chapter 48 | CHARACTERISTICS OF NORMAL GAIT AND FACTORS INFLUENCING IT

The stance phase of gait makes up approximately 60% ofthe gait cycle, so that the remaining 40% consists of the swingphase. The gait cycle with respect to the right limb is slightlyout of phase with the gait cycle of the left limb. At contacton the right, the left limb is just ending its stance phase. Atapproximately 10% of the gait cycle on the right, the left limbleaves the ground and begins its swing phase, returning to theground at approximately 50% of the gait cycle of the rightlimb. Thus the gait cycle is characterized by two brief peri-ods, each lasting approximately 10% of the gait cycle, in whichboth limbs are in contact with the ground. These are periodsof double limb support, and the remaining cycle consistsof single limb support.

The stance phase can be divided into smaller periods as-sociated with specific functional demands and identified bydistinct events (Fig. 48.2) [121]. The period immediately fol-lowing ground contact is known as contact response, orweight acceptance, and ends when the whole foot flattenson the ground. During contact response, the limb absorbs theshock of impact and becomes fully loaded. The foot flat eventthat ends contact response occurs at approximately 15% ofthe normal gait cycle. It is important to recognize that load-ing response includes double limb support and continues intosingle limb support. The period following loading response ismidstance, also known as trunk glide, since during this pe-riod the trunk glides over the fixed foot, moving from behindthe stance foot to in front of it. Heel off ends trunk glide atapproximately 40% of the gait cycle and begins terminalstance, which ends at 50% of the gait cycle when contralat-eral ground contact occurs. The final stage of stance, from 50to 60% of the gait cycle, is preswing and is characterized bydouble limb support. It ends with toe off.

The swing phase also is divided into early, middle, and lateperiods, although it lacks distinctive events to delineate thesephases (Fig. 48.3). Early swing continues from 60% to ap-proximately 75% of the gait cycle and is characterized by therapid withdrawal of the limb from the ground. Midswingcontinues until approximately 85% of the gait cycle and con-sists of the period in which the swing limb passes the stance

limb. Late, or terminal, swing finds the swing limb reach-ing toward the ground, preparing for contact.

Although normal gait is often assumed to be symmetrical,substantial evidence exists to refute that assumption [13,61,93,130]. Although the differences are small among ambulatorswithout pathology, the right and left limb movements are notmirror images of one another. Differences exist in timing andmovement patterns, in muscle activity, and in the loads ap-plied to each limb [51,59]. When evaluating the gait patternsof individuals with asymmetrical impairments, clinicians mustremember that small asymmetries in gait are normal.

Consideration of the basic functional tasks of the swing andstance phase of gait provides a framework for characterizingthe movements in each phase of gait. While the overridinggoal of locomotion is forward progression, the stance and swingphases contribute to that goal in different ways. The stancephase has three tasks in locomotion: providing adequate sup-port to avoid a fall, absorbing the shock of impact between thelimb and the ground, and providing adequate forward andbackward force for forward progress [35,158]. The basic tasksof the swing phase are safe limb clearance, appropriate limb

0% 15% 40% 50% 60%

FFGC HO CGC TO

Figure 48.2: The stance phase is divided into smaller phases that are demarcated by specificevents. GC, ground contact; FF, foot flat; HO, heel off; CGC, contralateral ground contact, TO,toe off.

Earlyswing

Midswing

Lateswing

60-75% 75-85% 85-100%

Figure 48.3: The swing phase is divided into early swing, whenthe limb is pulled away from the ground; midswing, as theswing limb passes the stance limb; and late swing, when theswing limb extends toward the ground.


placement for the next contact, and transfer of momentum.By keeping these tasks in mind, the clinician can understandthe importance of discrete movements of limb segments orthe specific sequencing of muscle activity and can begin to ap-preciate the significance of specific joint impairments.

KINEMATICS OF LOCOMOTION

As noted in Chapter 1, kinematics describes a movement interms of displacement, velocity, and acceleration. The vastmajority of kinematic analyses of gait examines displacementcharacteristics, and although velocity and acceleration dataare available and may provide useful information, this chap-ter reviews the more commonly cited displacement data. Pre-sented first is a description of the movement characteristicsof the stride as a whole followed by descriptions of discretemovement patterns of individual joints.

Many factors affect the kinematic characteristics of gait,including walking speed, age, height, weight or body massindex, strength and flexibility, pain, and aerobic conditioning.Walking speed and age have large and important effects ongait and are discussed later in this chapter. Unless notedotherwise, the data reported come from trials in which thesubjects walk at their self-selected, comfortable, or free,speed.

Temporal and Distance Parametersof a StrideA stride consists of the movement of both limbs during agait cycle and contains two steps. A step is operationallydefined as the movement of a single limb from ground con-tact of one limb to ground contact of the opposite limb(Fig. 48.4). The literature demonstrates that there is consid-erable difference in step and stride characteristics amongsubjects and even among trials of the same subject [50]. De-spite this normal variability, these parameters are capable ofdistinguishing between individuals with and without impair-ments [75,152].

DISTANCE CHARACTERISTICS OF THE STRIDE

The typical distance parameters of gait are defined inTable 48.1. A representative range of values also is presentedfrom the literature [57,74,89,91,107,109,116]. Stride and steplengths depend directly upon standing height, so measures ofabsolute step or stride length, although frequently reported,are difficult to interpret. These measures can be normalizedby standing height or lower extremity length to compare valuesfrom different individuals [27,73]. Estimates of normalizedstride length vary from approximately 60 to 110% of standingheight [27]. Judge et al. report a mean step length of 0.74 �0.04 of leg length in young healthy adults [73]. Step width

Stride length

Steplength

Foot angle

Step width

Figure 48.4: Several distance measures help describe a typicalgait cycle.

TABLE 48.1 Distance Parameters of Stride in Young Healthy Adults

Range of Values Reported inParameter Definition the Literature

Stride length The distance between ground contact of one foot and 1.33 � 0.09 to 1.63 � 0.11 mthe subsequent ground contact of the same foot [57,74,89,91,107,109,116]

Step length The distance between ground contact of one foot and 0.70 � 0.01 to 0.81 � 0.05 mthe subsequent ground contact of the opposite foot [57,107,137]

Step width (also known as The perpendicular distance between similar points on both 0.61 � 0.22 to 9.0 � 3.5 cmbase of support)a feet measured during two consecutive steps [25,104] [91,107,109,137,146]

Foot angle Angle between the long axis of the foot and the line 5.1 � 5.7 to 6.8 � 5.6� [91]of forward progression

a Step width is defined variably in the literature. Some measures incorporate the angle of the foot on the ground.


and foot angle are less frequently reported but provide anindication of the size of the base of support.

TEMPORAL CHARACTERISTICS OF THE STRIDE

The temporal characteristics of the stride are defined inTable 48.2 [40,47,53]. Included in this list is walking speed,or gait velocity, although this is typically computed overseveral strides. The normal gait cycle at free speed lasts ap-proximately 1 second, and walking speed is between 3 and 4miles per hour. Walking speed is a function of both cadence(steps/minute) and step length. An increase in either cadenceor step length contributes to increased walking speed [6,54,89,106,137,152].

Walking speed affects swing and stance time differently.Increased walking speed decreases the overall duration of thegait cycle, but the decrease in cycle duration results in agreater decrease in stance time than in swing time [6,106].As stance time decreases with less change in swing time, dou-ble limb support time decreases, and single limb support timeincreases. The difference between running and walking is theabsence of a double limb support phase in running. The ratiobetween swing and stance time increases toward 1 withincreasing walking speed.

Many gait disorders lead to altered time and distanceparameters, typically decreased speed and stride length and,in the case of unilateral disorders, altered swing and stancetimes with abnormal swing–stance ratios. Such measures arerelatively easy to obtain in the clinic and serve as useful out-come measures, sensitive to change. On the other hand, manydifferent disorders produce similar temporal and distancecharacteristics. For example, a patient with unilateral hip painand a patient with hemiparesis secondary to a stroke bothwalk with decreased velocity, and both demonstrate decreasedsingle limb support time on the affected side and increased

double limb support time [106]. These parameters distinguishbetween normal gait and abnormal walking patterns but areunlikely to identify the differences in gait patterns betweenthe two patients, even though such differences often are easilydetected by an observer. Thus temporal and distance param-eters may be helpful in tracking a patient’s progress but areinsufficient to characterize a gait pattern fully and to identifythe mechanisms driving the movement pattern. Patterns ofjoint excursions, however, can help the clinician to identifythe differences in gait patterns between individuals withsimilar temporal and distance characteristics.

Angular Displacements of JointsThe growth of photography in the mid- to late 19th centuryallowed the systematic observation of discrete movements ofeach joint during the complex activity of normal locomotion[5]. Over the last 50 years improved photographic techniquesand the development of the computer have led to ever moreprecise monitoring of the three-dimensional motion of indi-vidual segments. The sagittal plane motions of the joints of thelower extremity are the most thoroughly studied and best un-derstood, at least in part because sagittal plane motions arethe largest and easiest to measure. In contrast, frontal andtransverse plane motions of the joints of the lower extremitiesand the three-dimensional motions of the upper extremitiesand trunk are less frequently studied. Joint displacement datareveal intra- and intersubject variability in all planes, althoughthe variability is greater in the frontal and transverse planesthan in the sagittal plane and across subjects than between cy-cles of a single individual [14,39,55,74]. The smaller excursionsin the frontal and transverse planes are particularly sensitiveto differences in measurement procedure, which accounts forsome of the increased variability of these motions [67]. Despitethe variability in magnitudes of the movements, the patterns

TABLE 48.2 Temporal Parameters of Stride in Young, Healthy Adults

Parameter Definition Values from the Literature

Stride time Time in seconds from ground contact of one foot to 1.00 � 0.23 to 1.12 � 0.07ground contact of the same foot [91,107,109,116]

Speed (also known Distance/time, usually reported in m/sec 0.82–1.60 � 0.16as velocity) [47,57,73,74,89,91,106,109,116,137]

Cadence Steps per minute 100–131[27,40,47,73,74,91,106,109,137]

Stance time Time in seconds that the reference foot is on 0.63 � 0.07 to 0.67 � 0.04the ground during a gait cycle [91,107,109]

Swing time Time in seconds that the reference foot is off the 0.39 � 0.02 to 0.40 � 0.04ground during a gait cycle [91,107,109]

Swing/stance ratio Ratio between the swing time and the stance time 0.63–0.64 [74,109]

Double support time Time in seconds during the gait cycle that two feet are in 0.11 � 0.03 to 0.141 � 0.03contact with the ground [91,106,107]

Single support time Time in seconds during the gait cycle that one foot is in Not reportedcontact with the ground


and sequencing of joint movements in gait are remarkably con-sistent across trials and across subjects [12,31,32,106].

SAGITTAL PLANE MOTIONS

The classic studies by Murray remain the foundation for un-derstanding sagittal plane motion of the lower extremity[106,107,109,110] (Fig. 48.5). More-recent studies confirm

the overall patterns of motion for the hip, knee, and ankle,although there is variation in the reported maximal jointpositions. Because studies demonstrate both intra- and inter-subject variability, the reader is cautioned that the pattern ofmotion is the focus of the following discussion rather than thespecific magnitudes [43,74,159]. Values of peak excursion arementioned to provide an image of the motion rather than todefine an absolute norm.

The hip exhibits a single cycle of motion. Beginning atground contact, the hip is in maximum flexion (approxi-mately 25�) and gradually extends, reaching maximum hiphyperextension (approximately 10�) at close to 50% of thegait cycle, when contralateral ground contact occurs[80,87,106,159]. The magnitude of apparent hip hyperex-tension excursion depends on the point of reference. Asnoted in Chapter 38, a normal hip exhibits little or no hy-perextension range of motion. Consequently, the hyperex-tension reported at the hip during locomotion is the resultof pelvic motions in the transverse and sagittal planes. Inmost studies, the reported hip hyperextension reflects theorientation of the thigh with the trunk or with the room-fixed reference frame as seen in Fig. 48.6. After reachingmaximum extension, the hip begins flexing again, reachingmaximum flexion late in swing, at 80–85% of the gait cycle.The cycle repeats at ground contact.

The knee exhibits a slightly more complex movement pat-tern, landing in extension, albeit usually a few degrees shortof maximum extension, at ground contact. The knee flexes

Rot

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40

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60 80 100

70

90

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130

Ankle Fl1

A

Fl2

Fl2

Ex1Ex2

Figure 48.5: Sagittal plane excursions of the ankle, knee, andhip (Reprinted with permission from Murray MP: Gait as a totalpattern of movement. Am J Phys Med 1967; 46: 290–333.)

Figure 48.6: In most locomotion studies the hip excursion isdescribed as the angle between the length of the thigh anda room-fixed coordinate system.


10 to 20� immediately after contact, reaching maximum flex-ion at about 15% of the gait cycle when the subject achievesfoot flat. At foot flat the knee begins to extend and reachesmaximum extension at about 40% of the gait cycle as theheel rises from the ground. Flexion of the knee begins againand reaches a maximum of approximately 70� in midswing(approximately 75% of the gait cycle). Knee extension re-sumes, and the knee reaches maximum knee extension justbefore ground contact [20,80,88,106,129].

Ankle motion also exhibits several reversals in direction.Ground contact occurs with the ankle close to neutral in eitherslight plantarflexion or slight dorsiflexion [80,87,106]. Fol-lowing contact, the ankle plantarflexes an additional 5 or 10�,reaching a maximum at about 5% of the gait cycle. As thebody glides over the stance foot, the ankle dorsiflexes, reach-ing a maximum just after the knee reaches full extension.Ankle plantarflexion resumes, and the ankle reaches maxi-mum plantarflexion of approximately 20� just following toeoff. In swing, the ankle dorsiflexes slightly but may remain inslight plantarflexion throughout swing.

Pelvic motions in the sagittal plane are small, with no con-sistent definition of neutral. However, studies suggest that thepelvis anteriorly tilts whenever either hip is extending[107,109,145,147,154]. The anterior pelvic tilt contributes tothe apparent hip hyperextension that occurs in late stance.Upper extremity sagittal plane motion also shows a rhythmicoscillation that is related to the movement of the lower ex-tremities. At free walking speed, flexion of the shoulder andelbow parallel flexion of the opposite hip [106,110,151].

CLINICAL RELEVANCE: ASSOCIATED MOVEMENTSIN AN INDIVIDUAL FOLLOWING STROKEClose examination of the sagittal plane motions of the hip,knee, and ankle reveal that only for a very brief instantfollowing toe off are these three joints moving in the samedirection with respect to the ground. Just following toeoff, all three joints are pulling the foot away from theground, the hip and knee are flexing, and the ankle is dor-siflexing. At other points in the gait cycle the joints moveindependently, so that one or two joints move the foottoward the ground as the other(s) pull it away from theground. A common impairment found in patients follow-ing stroke is an inability to disassociate movements, andas a result, a patient is compelled to move all three jointsof the lower extremity together in the same direction. Forexample, to flex the knee, the patient may flex the kneeand hip and dorsiflex the ankle simultaneously in a flex-ion pattern or extend the knee while simultaneouslyextending the hip and plantarflexing the ankle in an ex-tension pattern. Such obligatory movements interferewith the normal timing and sequencing of joint move-ments in gait. For instance, in late swing, as the patientextends the knee toward the ground, the hip tends toextend, and the ankle plantarflex, producing a foreshort-ened step and an abnormal foot position at ground

contact. A flexion pattern produces similar conflicts as thehip begins to flex in terminal stance. At this time, the hipand knee should be flexing while the ankle continues toplantarflex. A flexion pattern stops the ankle plantarflex-ion and interferes with the normal roll off of late stance.

FRONTAL PLANE MOTIONS

Frontal plane excursions are less well studied and more var-ied than sagittal plane movements (Fig. 48.7). Hip position inthe frontal plane is affected by the motion of the pelvis overthe femur and by the orientation of the femur as the subjecttranslates toward the opposite foot to keep the center ofmass over the base of support. The hip lies close to neutralabduction at ground contact and then adducts during weightacceptance as the pelvis drops on the contralateral side

Add

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Figure 48.7: Frontal plane excursions of the hip (A), knee(B), and foot (C) are much smaller than sagittal plane excursionsbut show characteristic patterns of movement.


[8,70,71,74,147] (Fig. 48.8). Adduction is amplified as the sub-ject shifts toward the stance side to keep the center of massover the foot. Adduction continues until late stance, when load-ing begins on the opposite limb. At that instance, the pelvisdrops on the side in late stance, and the hip moves into ab-duction (Fig. 48.9). Reported knee motion in the frontal planeis slight, with estimates ranging from approximately 2 to 10�of adduction, peaking in early swing [8,20,74,88].

Frontal plane motion of the foot recorded during walkingreflects the inversion and eversion component of supinationand pronation of the foot. Although the position of the hind-foot at ground contact is variable and the magnitude of thereported excursions differs among reports, data consistentlydemonstrate a motion pattern characterized by eversion,consistent with pronation, following ground contact and con-tinuing until mid to late stance when the hindfoot begins in-verting or supinating [26,101,123,164]. Forefoot motion issimilar to hindfoot motion, although forefoot pronation duringstance begins after hindfoot pronation has begun [66,164].

TRANSVERSE PLANE MOTIONS

Transverse plane motions of the limbs and trunk also demon-strate more variability and smaller excursions than those seenin the sagittal plane (Fig. 48.10). Transverse plane rotationsof the hip are a function of the transverse plane motion ofthe pelvis as well as the transverse plane motion of the femur(Fig. 48.11). Pelvic rotation in the transverse plane accom-panies hip flexion, so that the pelvis rotates forward on theside of the flexing hip, reaching maximum forward rotation

Figure 48.8: At weight acceptance, the individual shifts laterally tokeep the center of mass close to the stance foot, and the pelvisdrops on the unsupported side. The stance hip is in adduction.

Figure 48.9: During weight acceptance, the hip drops on theunsupported side, which is abducted.

20 40 60 80 100

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CMC (w) = 0.941CMC (b) = 0.861

Knee Rotation

Figure 48.10: Transverse plane motions of the hip and knee.(Reprinted with permission from Kadaba MP, Ramakrishnan HK,Wootten ME, et al.: Repeatability of kinematic, kinetic, andelectromyographic data in normal adult gait. J Orthop Res1989; 7: 849–860.)


at approximately ground contact [74,106]. Forward rotationof the pelvis contributes to lateral rotation of the hip. At thesame time, the opposite hip is in maximum extension, andthe relative backward position of the pelvis on that side al-lows the hip to appear hyperextended. The transverse planealignment of the pelvis on the extended hip tends to medi-ally rotate the extended hip.

Independent femoral movement provides its own contri-bution to hip position. At ground contact, the femur is alignedclose to neutral but rotates medially from contact to mid-stance. Lateral femoral rotation then begins and continuesinto mid swing when medial rotation resumes. Hip joint po-sition is the sum of the pelvic contribution and the femoralcontribution to joint position. Although there is disagreementabout the hip position at ground contact among the reporteddata, there is good consistency regarding the direction of thehip motion, medial rotation from ground contact to mid- orlate stance and then lateral rotation until late swing or groundcontact [70,71,74,114,147].

The knee, too, exhibits transverse plane motion withmedial rotation following ground contact and gradual lateralrotation from midstance through most of swing, although

there is more disagreement about knee motion in swing[8,20,74,88,114,147]. Transverse plane motion of the knee islinked to the motion of the foot and to the sagittal planemotion of the knee, particularly during stance, when the lowerextremity functions in a closed chain. As the foot pronates,the tibia medially rotates and allows the knee to flex. Thiscoupled motion assists in shock absorption during loadingresponse [122]. Later in stance, the foot supinates as the tibiarotates laterally, and the knee extends while the body rolls for-ward onto the opposite limb.

MOTIONS OF THE TRUNK

Studies of the head and trunk reveal that these segments un-dergo systematic translation and rotation in three dimensionsand exhibit both intrasubject and intersubject variability[85,145,150]. The trunk exhibits slight flexion and extensionduring the gait cycle, is more erect or extended during singlelimb support, and is more flexed during double limb support[29,85]. Frontal plane motion of the trunk is consistent withthe need to keep the center of mass over the stance foot. Sothe trunk leans slightly to the stance limb at each step[85,106,145,150]. In the transverse plane, the rotation of thetrunk is opposite the rotation of the pelvis, with the trunk ro-tating forward on the side in which the shoulder is flexing[85,106,145].

CLINICAL RELEVANCE: THE TRUNK’S CONTRIBUTIONTO SMOOTH GAITThe gait pattern of a toddler learning to walk is charac-terized by large lateral leans with little forward rotationof the trunk and shoulders [9]. As the child matures, thepattern becomes smoother and more stable, and trunk ro-tation moves out of phase with the pelvis. The couplingmotion of the trunk and pelvis contributes to the efficiencyand stability of gait. Patients who lack the ability to ro-tate the trunk separately from the pelvis, such as patientswith Parkinson’s syndrome or patients with low back pain,may lose gait efficiency and require more energy to walk.

MUSCLE ACTIVITY DURINGLOCOMOTION

Studies that examine the electrical activity of muscles duringlocomotion have played a central role in defining the role ofmuscles in producing and controlling locomotion. Data fromWinter and Yack [163] demonstrate the normalized elec-tromyographic (EMG) data for 16 muscles recorded in up to19 subjects (Fig. 48.12). These data reveal important princi-ples regarding muscle activity during gait. First, the durationof large bursts of activity for most muscles is quite brief, andmost of these bursts occur at the transitions between swingand stance or between stance and swing. These data also

Figure 48.11: The pelvic position in the transverse plane and thefemoral rotation in the transverse plane both contribute to thetransverse plane hip joint position during the gait cycle. Atground contact the femur is medially rotating, but the forwardalignment of the pelvis contributes to lateral rotation of thehip. At heel off the opposite is true.


demonstrate the considerable variability in muscle activityacross individuals. Studies also demonstrate variability withina single individual, although there is less than across individ-uals [23,68,74,118,163].

Despite the variability of muscle activity, certain consistentfunctions for specific muscle groups emerge from the EMGdata [10,41,68,74,81,103,138,155]. The gluteus maximus andhamstrings are active prior to and following ground contact,exerting a deceleration force on the hip and knee at the endof swing. Their activity also helps to initiate hip extensionduring early stance. The gluteus medius contracts just beforeground contact and continues its activity through most ofstance, until loading begins on the opposite side. The activityof the hip abductors provides essential frontal plane stabilityto the pelvis. The hip flexors contract in late stance and con-tinue their activity into early swing to slow hip extension andinitiate hip flexion.

Muscle activity at the knee is characterized by co-contraction of the hamstrings and quadriceps for approxi-mately the first 25% of the gait cycle, during loading responseand early midstance. During this period, the knee is flexingand then extending, and the quadriceps activity is essential incontrolling this movement. Some individuals exhibit activityof either the quadriceps, especially the rectus femoris, orhamstrings at the transition from stance to swing, but thisactivity is both variable and smaller in magnitude than theactivity at the beginning of stance [7,112]. Most of swing pro-ceeds with no muscle activity at the knee joint.

The ankle also exhibits co-contraction of the dorsiflexorand plantarflexor muscles. Dorsiflexors of the ankle exhibitslight activity throughout swing to hold the foot away fromthe ground. The activity continues at ground contact andthrough the loading response, controlling the descent of thefoot onto the ground. The plantarflexor muscles gradually in-crease their activity from ground contact through most ofstance, with the greatest burst of activity from heel off to toeoff as the body rolls over the plantarflexing foot.

Review of the muscle activity of these large muscle groupsdemonstrates that much of the activity is characterized by aneccentric contraction followed by a concentric contraction.For example, the gluteus maximus contracts eccentrically asthe hip flexes late in swing and then contracts concentricallyas the hip begins to extend. The same pattern is found in thegluteus medius, hip flexors, quadriceps, and dorsiflexors. Theplantarflexors also exhibit lengthening and then shortening,although at least some of the change in length is a passivestretch and shortening in the tendo calcaneus (Achilles ten-don), so the actual change in muscle fiber length may besmall [49]. The hamstrings also begin their activity with aneccentric contraction in late swing, but their subsequentlength is more difficult to discern, since at loading responsethe hip is extending while the knee is flexing. The overalllength change in the hamstrings during loading response maybe negligible. The lengthening contractions that begin many

200 40 60 80 100

100

200

300

0

% of Stride

200 40 60 80 100% of Stride

100

200

300

0

100

200

300

0

100

200

300

0

Adductorlongus

Adductor magnus

N = 11CV = 51%MEAN = 33.9

N = 11CV = 55%MEAN = 42.7

Tibialisanterior

N = 12CV = 28%MEAN = 135.4

Extensor digitorum longus

N = 12CV = 35%MEAN = 98.4

100200300400500

0

200

400

600

0

100200

300400

0

100200300400500

0

Gluteus medius

N = 17CV = 42%MEAN = 30.6

Gluteus maximus

N = 16CV = 58%MEAN = 16.5

Medial hamstrings

N = 11CV = 60%MEAN = 64.9

Lateral hamstrings

N = 17CV = 59%MEAN = 52.6

200 40 60 80 100

100200

300

400

0

% of Stride

200 40 60 80 100% of Stride

100200

300400

0

100200

300400

0

100

200

300

0

N = 11CV = 61%MEAN = 113.0

N = 10CV = 57%MEAN = 79.2

Soleus N = 18CV = 31%MEAN = 113

Peroneus longus

N = 11CV = 57%MEAN = 54.0

100200300400500

0

200300400

0100

100

200

300

0

100

200

300400

0

Erector spinae N = 11CV = 38%MEAN = 37.0

SartoriusN = 15CV = 54%MEAN = 25.5

Rectus femorisN = 16CV = 46%MEAN = 25.5

Vastus lateralis

N = 15CV = 44%MEAN = 61.5

Lateralgastro-cnemius

Medialgastro-cnemius

A

B

EM

G n

orm

aliz

ed to

mea

n X

100

%E

MG

nor

mal

ized

to m

ean

X 1

00%

Figure 48.12: Electrical activity of lower extremity musclesduring gait. (Reprinted with permission from Winter DA, YackHJ: EMG profiles during normal human walking: stride-to-strideand inter-subject variability. Electroencephalogr ClinNeurophysiol 1987; 67: 402–411.)


muscles’ activity in gait decelerate each joint, and then thesubsequent concentric contractions begin the joint’s forwardmovement.

It is worth noting that at most joints, the motion occurringduring the concentric contraction continues after the con-traction ceases. For example, the hip continues to extend longafter the peak activity of the gluteus maximus and hamstrings,and the hip flexes after cessation of hip flexor activity. Simi-larly, the knee continues to extend without significant quadri-ceps activity, and the ankle continues to dorsiflex after theburst of dorsiflexor activity early in stance. Thus the chieffunctions of the muscles of the lower extremity during loco-motion are to slow one motion and to provide an initial burst,or push, in the opposite direction. How motion continues inthe absence of active muscle contraction is related to the ki-netics of the movement.

KINETICS OF LOCOMOTION

Kinetics examines the forces, moments, and power generatedduring a movement and, in the case of locomotion, includesthe moments generated by the muscles, the forces appliedacross joints, and the mechanical power and energy gener-ated. A discussion of the kinetics of gait allows considerationof the efficiency of gait.

Joint Moments and Reaction ForcesAs indicated in the preceding sections, gait consists of com-plex cyclical movements occurring in a coordinated sequencethat is controlled by muscle activity. In addition, gait entailsthe repetitive impact loading of both lower extremities in eachgait cycle. Thus it is easy to recognize that normal locomo-tion produces large forces between the foot and the ground,requires large muscle forces, and generates significant jointreaction forces. Many impairments in gait are related to anindividual’s inability to generate sufficient muscular supportor to sustain the large reaction forces of gait.

DYNAMIC EQUILIBRIUM

Researchers and clinicians have long been interested in theforces sustained by the muscles and joints during normal andabnormal locomotion [15,96,144]. Chapter 1 of this text de-scribes the principles used to determine the loads in musclesand on joints during activity. Newton’s first law defines theconditions of static equilibrium (�F � 0, �M � 0), stat-ing that an object remains at rest (or in uniform motion) un-less acted upon by an unbalanced external force. Throughoutthis text, two-dimensional examples of static equilibriumproblems are provided to analyze the forces in the musclesand on joints during static tasks or in tasks where accelera-tion is negligible. However, during gait, limb segments

undergo large linear accelerations, and joints exhibit large an-gular accelerations. As a result, the assumption used in staticequilibrium analysis, that acceleration is negligible, is not validwhen applied to gait.

Newton’s second law of motion, �F � ma, states that theunbalanced force on a body is directly proportional to the ac-celeration of that body. The specific relationships between theaccelerations and the forces and moments can be determinedby applying the principles of dynamic equilibrium. Theconditions of dynamic equilibrium are very similar to the con-ditions of static equilibrium. To determine the forces on anaccelerating body in a two-dimensional analysis, the follow-ing conditions must be satisfied:

�FX � maX, �FY � maY,�M � I � � (Equation 48.1)

In three-dimensional analysis, the conditions for dynamicequilibrium are

�FX � maX, �FY � maY,�FZ � maZ

(Equation 48.2)

and

�MX � I � �X, �MY � I � �Y,�MZ � I � �Z

(Equation 48.3)

where Fi is the force in the ith direction, ai is the linear ac-celeration in the ith direction, Mi is the moment about theith axis, �i is the angular acceleration in the ith direction, andI is the moment of inertia. The moment of inertia indicatesa body’s resistance to angular acceleration and depends onthe body’s mass and distribution of mass. The larger the massand the farther the mass is from the body’s center of mass,the larger is the body’s moment of inertia. Elite gymnasts tendto possess short and compact bodies (smaller moments of in-ertia) that allow high angular accelerations producing rapidrotations about horizontal bars and in tumbling routines. Theacceleration quantities in each of the equations of dynamicequilibrium, mai and I � �I, are known as inertial forcesand are intuitively explained by the awareness that it takesmore force to push a car to start or stop its rolling than it takesto keep the car rolling.

Solutions to the conditions of dynamic equilibrium, alsoknown as equations of motion, require knowledge ofseveral parameters, including mass and moment of inertia.Mass is usually determined from tables derived from cadavermeasurements, as demonstrated in examples throughout thistextbook [37]. Similarly, these tables provide means to cal-culate moments of inertia of a limb or limb segment fromeasily obtained anthropometric measurements, althoughmethods also exist to compute the moment of inertia of somesegments directly [22,134]. Regardless of the method cho-sen, the properties of mass and moment of inertia can beestimated and entered into the equations of motion to allowsolutions.


Theoretically, the equations of motion in dynamic equi-librium can be used to calculate a body’s acceleration fromall of the forces on the body. This approach is useful todetermine the response of an airplane or rocket to an appliedforce. However, in the case of human movement, whereforces cannot be measured directly, the equations of motionare used more often to determine the forces on the body whenthe accelerations are known. This approach, known as inversedynamics, allows estimation of the forces on the human bodyand requires direct determination of the acceleration. Appli-cation of inverse dynamics in static equilibrium is straight-forward because the accelerations are, by definition, zero, andthe examples of two-dimensional analysis throughout thisbook demonstrate the use of inverse dynamics.

Chapter 1 reminds the reader that acceleration is thechange of velocity over time, and velocity is the change in dis-placement over time. Therefore, if a body’s displacement isknown over time, then velocity and acceleration can be deter-mined. Precise calculations of velocity and accelerations of the

body or of any limb segment requires careful measurement ofthe displacement, which can be accomplished by a number oftechniques including high-speed cinematography, videogra-phy, or electromagnetic tracking devices [87,106, 111,124].Appropriate signal processing of the displacement data andmathematical calculations yield satisfactory estimations of ve-locity and accelerations of the body of interest. A thoroughdiscussion of the methods and challenges in these techniquesis beyond the scope of this book; suffice it to say that the nec-essary acceleration values are available, so that the equationsof motion can finally be solved for the applied forces.

Examining the Forces Box 48.1 provides an example ofthe equations of motion for the leg–foot segment during theswing phase of gait. Using anthropometric data from Demp-ster [37], the mass (m) and moment of inertia (I) are entereddirectly into the calculations. Videographic data are collectedat a rate of 60 Hz (hertz, or cycles per second) and manip-ulated so that the linear and angular accelerations of theleg–foot segment are determined for every 1/60 of a second

EXAMINING THE FORCES BOX 48.1

EQUATIONS OF MOTION IN TWO DIMENSIONS FORTHE LEG–FOOT SEGMENT DURING EARLY SWING

l3 � the moment arm of the inertial force (�maX)

l4 � the moment arm of the inertial force (�maY)

Since the limb segment accelerates during gait, thedynamic equilibrium conditions apply:

�FX � maX, �FY � maY, �M � I � �

where: aX, aY, and � are the x and y components ofthe linear accelerations and angular accelerations,respectively. These equations can be rewritten as

�FX � maX � 0, �FY � maY � 0, �M � I � � � 0

where (�maX), (�maY), and (�I � � ) are knownas inertial forces. The inertial forces contributeto moments about the knee joint so thattaking moments about the knee, the motionequation is

(W � l1) � (FM � l2) � [(�maX) � l3] � [(�maY) � l4]� I � �

Since the accelerations and anthropometricparameters, W and I, can be measured or determinedfrom available data, the equation can be solved forthe muscle force, F. Once the muscle force isdetermined, the joint reaction forces, JX and JY arecalculated from:

�FX � maX

FMX � JX � maX

�FY � maY

FMY � JY � W � maY

J

max

may

l3

l1

l4

l2

Fm

W

m � the mass of the leg and foot combined

W � the weight of the leg and foot combined

FM � the muscle force

J � the joint reaction force

l1 � the moment arm of the weight of the leg–foot

l2 � the moment arm of the muscle


and entered into the equations. The equations of motion aresolved repeatedly for the muscle force (F) at each incrementof time. A similar procedure is applied to the stance phaseof gait, but the external forces on the foot also include theground reaction forces (Fig. 48.13). The direction and mag-nitude of these forces must be known to solve the equationsof motion during stance and can be measured directly byforce plates. The characteristics of the ground reaction forceduring gait are discussed in the following section.

The example presented in Examining the Forces Box 48.1assumes that only one muscle group is active. However, theEMG data described earlier in this chapter provide convinc-ing evidence that there is co-contraction of the hamstringsand quadriceps during late swing and early stance and some-times at the transition from late stance to early swing as well.

Thus there is more than one muscle applying force at theknee joint, producing a dynamically indeterminate system.As noted in Chapter 1 and elsewhere in this book, sophisti-cated mathematical solutions for indeterminate systems exist,and they are applied frequently in locomotion research toapproximate the muscle and joint reaction forces [24,136].

Using inverse dynamics, many studies report the jointreaction forces in the body during the gait cycle [2,15,30,42,60,83,136,141]. Peak joint reaction forces at the hip, knee,and ankle reported in the literature are presented in Table48.3. These data reveal wide variation in the forces reportedat each joint. Several factors influence these calculations, in-cluding the estimates of the body segment parameters of massand moment of inertia, the accuracy of the displacement dataand the procedures to determine accelerations, the use oftwo- or three-dimensional analysis, as well as the analyticalapproach used to complete the calculations [1,3,33,36,84,166]. Values reported here are intended to demonstratethat regardless of the actual magnitude, all of the joints of thelower extremity sustain large and repetitive loads during lo-comotion. Running and jumping produce even larger muscleloads and joint reaction forces [18,98,149].

To avoid the problem of indeterminacy, researchers oftensolve only the moment equations, calculating the externalmoments applied to the limb by external forces such asweight and ground reaction forces and inferring the internalmoments applied by the muscles and soft tissue [77]. Authorsreport either the internal [156] or external moment [80,86],and the reader is urged to read the literature carefully to iden-tify which moment is reported. The limitation of this approachis that it prevents calculations of the forces in specific musclesand at the joints, but joint moments provide insight into theprimary roles of muscle groups during gait and support theroles already suggested by EMG.

Typical internal moments generated at the hip, knee, andankle in the sagittal plane during normal locomotion are re-ported in Fig. 48.14. The internal moment at the hip joint atground contact and contact response is an extension moment,consistent with the EMG activity of the gluteus maximus andhamstrings. The moment changes direction in midstance atabout the time the hip extensors cease their activity and theflexors become active. The moment at the hip in swing is min-imal until late swing when the hip extensors resume activity.

The knee demonstrates a small and brief flexor moment atground contact, consistent with hamstring activity, but then alarger and more prolonged extensor moment that is consistent

GRF

JM

W

−Iα

−ma

Figure 48.13: Free body diagram of the leg–foot segment duringstance includes the forces: weight of the leg-foot (W), jointreaction force (J), muscle force (M), ground reaction force(GRF), inertial forces �ma and �I�, where m � mass, a � linearacceleration, I � moment of inertia, and � � angular acceleration.

TABLE 48.3 Reported Peak Joint Reaction Forces during Normal Gait in Units of Body Weight

Anderson Duda Seireg and Simonsenet al. [2] Komistek [83] et al. [42] Arvikar [136] Hardt [60] et al. [141]

Hip 4 2.0–2.5 3 5.25 6 6

Knee 2.7 1.7–2.3 n.r.a 7 2.75 4.5

Ankle 6 1.25 n.r. 5 3.5 4a Not reported.


with quadriceps activity. In midstance, the knee exhibits a smallflexor moment that is attributable to activity of the gastrocne-mius. A small extension moment helps control knee flexion atthe end of stance and in early swing, just as the flexion mo-ment at the end of swing slows the rapid knee extension.

A small dorsiflexion moment at ground contact and contactresponse reflects the dorsiflexor activity controlling the de-scent of the foot onto the ground. It is followed by a steadilyincreasing and prolonged plantarflexion moment controllingadvancement of the tibia through the rest of stance. Althoughthere has been disagreement about whether the plantarflexors

actually propel the body forward [120], recent studies provideconvincing evidence that these muscles contribute some ofthe propulsion moving the body forward [21,113,125,132]. Avery small dorsiflexion moment following toe off pulls the footand toes away from the ground.

Moments in the transverse and frontal planes also are re-ported and appear to be important in the mechanics and path-omechanics of locomotion [4,44,94]. However, less consensusexists regarding the magnitude and even the pattern of thesemoments. Moments in the frontal and transverse planes aresmaller than those in the sagittal plane, and smaller momentsare more sensitive to measurement errors, including thelocation of the joint axes and the kinematics of the move-ments [17,64].

Winter describes a support moment for the stance phaseof gait that is the sum of the internal sagittal plane momentsin which all of the moments that tend to push the body awayfrom the ground or support the body are positive (Fig. 48.15)[63,156]. The net support moment during stance is positive,indicating the overall role of the muscles to support the bodyand to prevent collapse during weight bearing. Data suggestthat although the net support moment is consistent across

% N

m/(

BW

•LL)

20 40 60 80 100

5

-5

-15

15

CMC (w) = 0.986CMC (b) = 0.975

% N

m/(

BW

•LL)

% N

m/(

BW

•LL)

Gait cycle (%)

Gait cycle (%)

20 40 60 80 100

5

15

25

-5

-15

A

B

Ext

Flx

Hip Flexion/Extension Moment

CMC (w) = 0.960CMC (b) = 0.944

Knee Flexion/Extension Moment

20 40 60 80 100

0

0

0

-10

-20

10

Gait cycle (%)C

Ext

Flx CMC (w) = 0.992CMC (b) = 0.981

Ankle Flexion/Extension Moment

Flx

Ext

Figure 48.14: Internal moments at the hip, knee, and ankle inthe sagittal plane. (Reprinted with permission from Kadaba MP,Ramakrishnan HK, Wootten ME, et al.: Repeatability ofkinematic, kinetic, and EMG data in normal adult gait. J OrthopRes 1989; 7: 849–860.)

MH

MK

MA

Figure 48.15: The support moment is the sum of the moments atthe hip (MH), knee (MK), and ankle (MA) needed to support thebody weight during stance.


center of mass. The center of mass of the body rises and fallsas the individual moves from double support when the centerof mass is low to single support when the center of mass ishigh [69,106,133]. Similarly, the center of mass moves fromside to side as the individual passes from stance on the right tostance on the left [106]. The ground reaction force is measureddirectly by force plates imbedded in the walking surface.

The ground reaction force typically is described by a ver-tical force as well as anterior–posterior and medial–lateralshear forces. The vertical ground reaction force under onefoot is characterized by a double-humped curve (Fig. 48.17).The two peaks are greater than 100% of body weight and oc-cur when the body accelerates upward. The valley betweenthe peaks is less than 100% of body weight and occurs dur-ing single limb support. Examining the Forces Box 48.2 usesdynamic equilibrium to demonstrate how acceleration of thecenter of mass of the body alters the ground reaction force.The vertical ground reaction force also is characterized by abrief but high peak just following ground contact, whichreflects the impact of loading [140].

CLINICAL RELEVANCE: GROUND REACTIONFORCES AND JOINT PAINVertical ground reaction forces contribute significantly tojoint reaction forces, and large joint reaction forces con-tribute to pain in patients with joint pathology such asarthritis. Patients with arthritis walk more slowly [76], andtheir vertical ground reaction forces demonstrate smallerpeaks and valleys as the result of smaller vertical acceler-ations[139,142]. A reduction in walking speed, producinga reduction in accelerations, may be an effective way toreduce joint loads and, consequently, joint pain. Thesechanges may represent appropriate adaptations to protecta painful joint and to maintain overall function.

Soleus

Gluteusmaximus

Figure 48.16: An individual may increase the activity in thesoleus and the gluteus maximus to support the knee inextension by preventing forward movement of the tibia or thefemur, respectively.

For

ce (

% N

/BW

)

20 40 60 80 100

0

40

60

80

100

120

20

-20

VerticalAnt-PostMed-Lat

Gait cycle (%)

Figure 48.17: Ground reaction forces during gait. (Reprintedwith permission from Meglan D, Todd F: Kinetics of humanlocomotion. In: Rose J, Gamble JG, eds. Human Walking.Philadelphia: Williams & Wilkins, 1994; 23–44.)

walking trials, individuals without pathology demonstratevariability in the individual joint moments, indicating that in-dividuals with normal locomotor systems may exhibit flexi-bility in the ways they provide support [157].

CLINICAL RELEVANCE: A PATIENT WITHQUADRICEPS WEAKNESSA patient with quadriceps weakness lacks the ability tosupport the knee actively during the stance phase of gait.To generate adequate support during the stance phase ofgait, this patient may increase activity of the hip extensormuscles and of the soleus to increase the hip and anklecontributions to the net support moment (Fig. 48.16).

GROUND REACTION FORCES

With every stride, each foot applies a load to the ground andthe ground pushes back, applying a ground reaction forceto each foot. The magnitude and direction of this groundreaction force changes throughout the stance phase of eachfoot and is directly related to the acceleration of the body’s


The posterior and anterior shear components of theground reaction force also demonstrate a consistent patternin normal locomotion. The ground exerts a posterior force onthe foot during the initial portion of stance, decelerating thefoot; consequently this period is known as the decelerationphase. In midstance, the ground applies an anterior shearforce on the foot, contributing to the forward propulsion ofthe body. The second half of the stance phase is known as theacceleration phase of the gait cycle. Walking on ice demon-strates the importance of these posterior and anterior shearforces. Because there is little friction between the foot andthe ice, the posterior and anterior shear forces between theground and the foot are small when walking on ice, and for-ward progress is impaired. In the absence of any posteriorand anterior shear forces, forward progress is impossible.

The medial and lateral shear forces during gait are smallerand more variable than the vertical forces or posterior–ante-rior shear forces. They reflect forces associated with the shiftof the body from side to side between the supporting feet.Although plots of the ground reaction forces demonstraterather stereotypical shapes, it is important to recognize thatlike kinematic variables, these forces exhibit normal intra- andintersubject variability [51,61].

The ground reaction force vector is the sum of the in-dividual components of the ground reaction force. Whetherdescribed as a single force vector or as three individual com-ponents, the ground reaction force generates external mo-ments on the joints of the body in all three planes (Fig. 48.18).Realistic computation of joint moments and forces during gait

EXAMINING THE FORCES BOX 48.2

THE CONTRIBUTION OF ACCELERATION TOTHE VERTICAL GROUND REACTION FORCE

Using the dynamic equilibrium condition,�FY � maY, provides a direct demonstration ofthe role of the acceleration of the body’s center ofmass in generating the vertical ground reactionforce (GRF).

�FY � maY

�FY � maY � 0

�W � maY � GRF � 0

GRF � W � maY

When the body is accelerating toward the ground,the acceleration, aY, is negative, and the GRF is lessthan body weight, W. When the body acceleratesupward away from the ground, acceleration, aY,is positive, and the GRF is greater than bodyweight, W.

a

a

GRF

Figure 48.18: The ground reaction force vector (GRF) is the sumof the vertical, anterior–posterior, and medial–lateral groundreaction forces. The force vector applies external moments to thejoints of the lower extremities about all three axes.


must include the three components of the ground reactionforce or the force vector.

The location of the ground reaction force with respect tothe foot indicates the path of the center of pressure throughthe foot. In the normal foot, the center of pressure progressesin a relatively straight line from the posterior aspect of theplantar surface of the heel through the midfoot and onto theforefoot where it deviates medially onto the plantar surfaceof the great toe [56,58] (Fig. 48.19). Inability to roll over apainful toe or the interrupted forward progress of the body’scenter of mass, because the knee suddenly hyperextends, areexamples of gait deviations that produce changes in the pat-tern of the progression of the center of pressure.

Energetics of Gait: Power, Work,and Mechanical EnergyNormal locomotion appears to be a remarkably efficient move-ment. Individuals without impairments, walking at a self-selected cadence, require less oxygen consumption than whenwalking at lower or higher cadences [65,104]. Individuals withlocomotor impairments expend more energy during ambula-tion than individuals without impairments [16,97,143]. The ef-ficiency of locomotion depends on many factors, including themechanics of the muscular control of gait described earlier inthis chapter and the conservation of mechanical energy thatresults from the synergistic movement of the limb segments.

JOINT POWER

Mechanical power is the product of force and linearvelocity or, in rotational motions such as the joint movementsin locomotion, the product of joint moment and angularvelocity:

P � M � � (Equation 48.4)

where P is power in watts, M is a joint moment, and � is theangular velocity of the limb segment. Power is a usefulindication of the muscles’ role in controlling motion; it is neg-ative when the body absorbs energy during eccentric muscleactivity and is positive when the body generates energy duringconcentric muscle activity. Power also can be described aswork (W) per unit time (t) (i.e., W�t), where work is theproduct of force and displacement, or in angular terms, theproduct of moment (M) and angular displacement (�):

W � M � � (Equation 48.5)

Angular velocity, , is equal to angular displacement over time( � �t) and therefore:

P � M � �t (Equation 48.6)

and

P � W�t (Equation 48.7)

Thus concentric muscle activity generates power, or doeswork, and eccentric activity absorbs power, and work is doneon the segment [162]. A pogo stick (PogoTM) provides a usefulexample of positive and negative power, work done on or bythe pogo stick (Fig. 48.20). In landing, the weight of the childdoes work on the pogo stick, and energy is absorbed by itsspring, but in takeoff, the spring releases its energy and per-forms work on the child, pushing the child and pogo stick offthe ground.

Analysis of joint powers provides increasing understand-ing of the role of muscles in propelling and controlling move-ment during locomotion [21,125,130]. The joint powers at thehip, knee, and ankle during gait derived from two-dimensionalanalysis are pictured in Fig. 48.21. These demonstrate thatpositive power generation, when muscles are generatingpower and doing positive work, occurs at the hip at loadingresponse as the hip extends and again at the end of stance asit flexes. Both of these periods are characterized by concen-tric muscle contractions. In contrast, the knee has only a briefperiod of power generation, producing only a small amountof power. Like the hip, the ankle generates considerable pos-itive power at the end of stance when the plantarflexors con-tract concentrically. These data suggest that the hip flexorsand extensors and the plantarflexors contribute important en-ergy to the lower extremity during normal locomotion. A fullunderstanding of the power generation and absorption in two-and three-dimensional analysis is still emerging and holdspromise for providing more-direct insight into the mecha-nisms of gait deviations.

57%

55%50%

45% 40%35% 30%

25%

20%

15%

10%

5%

2%

Figure 48.19: Progression of the center of pressure duringlocomotion. (Reprinted with permission from Sammarco GJ,Hockenbury RT: Biomechanics of the foot and ankle. In: NordinM, Frankel VH, eds. Basic Biomechanics of the MusculoskeletalSystem. 3rd ed. Philadelphia: Lippincott Williams & Wilkins,2001; 222–255.)


CLINICAL RELEVANCE: JOINT POWERS IN INDIVIDUALSWITH GAIT DYSFUNCTIONSJoint powers during free-speed walking are altered inelders and in individuals with weaker lower extremitymuscles [38,100]. The decrease in plantarflexion powerand concomitant increase in hip flexor power generationnoted in elders and in individuals with weakness may helpto explain the decrease in velocity and step length re-ported in these individuals, as well as their mechanismsof compensation [38,99,100]. As an individual is unable togenerate power through plantarflexion for forward pro-gression, active hip flexion appears to provide the forwardpropulsion needed to swing the limb forward. These

Figure 48.20: Energy storage and release. A. Weight bearing onthe Pogo stick™ compresses its spring and work is done on thestick. B. As weight is removed, the spring is released, and thePogo stick™ does work on the body, lifting it into the air.

Pow

er (

% W

/BW

)

20 40 60 80 100

-5

5

10

15

0

-10

Gait cycle (%)

Abs

Gen Hip

Pow

er (

% W

/BW

)200 40 60 80 100

-5

5

10

15

0

-10

-15

-20

Gait cycle (%)

Abs

Gen Knee

Pow

er (

% W

/BW

)

20 40 60 80 100

10

20

30

40

50

0

-10

-20Gait cycle (%)

Abs

Gen Ankle

0

0

Figure 48.21: Joint powers at the hip, knee, and ankle fromtwo-dimensional analysis. (Reprinted with permission fromMeglan D, Todd F: Kinetics of human locomotion. In: Rose J,Gamble JG, eds. Human Walking. Philadelphia: Williams &Wilkins, 1994; 23–44.)


patients may benefit from exercise to improve plantarflex-ion force production.

The use of joint kinetics in conjunction with EMG is alsouseful in evaluating the complex gait deviations in indi-viduals with central nervous system disorders such as cere-bral palsy. These analyses provide more insight into themechanics of the gait abnormalities than can be providedsolely by clinical observation and lead to more- informedtreatment decisions [117,128].

MECHANICAL ENERGY

The cyclic movement inherent in locomotion and the abilityof the muscles to store energy contribute to the inherent ef-ficiency of normal gait. Mechanical energy, namely potentialand kinetic energy, also provides insight into the efficiencyof gait. Potential (PE) and kinetic (KE) energy are related tothe distance of a body’s center of mass from the earth and tothe body’s linear and angular velocity, as indicated by the fol-lowing relationships:

PE � mgh (Equation 48.8)

where m is the mass of the body, g is the acceleration due togravity, and h is the distance from the body’s center of massto the earth; and

KE � �12� mv2 � �

12� I 2 (Equation 48.9)

where m is the body’s mass, v is its linear velocity, I is itsmoment of inertia, and is its angular velocity. In an idealsystem, conservation of energy dictates a complete transfor-mation between potential and kinetic energy, so that an idealroller coaster continues in motion indefinitely (Fig. 48.22).When the cars are at their peak height, potential energy is

maximized and kinetic energy is minimized. At its lowestpoint, the roller coaster’s potential energy is minimum and itskinetic energy is maximum. Since the work done on a bodyequals the change in total energy, an ideal system requires nowork to continue moving, since the change in the body’s to-tal energy is zero. Studies of the mechanical energy of thelimb segments during gait suggest that an exchange of kineticand potential energy can account for most of the energychange in the distal leg at the beginning and end of swing[126,161]. This energy exchange improves when walking atfree speed and is greater at steady-state walking than at theinitiation of gait [95,102]. These studies demonstrate theefficiency of gait and how dependent the efficiency is on walk-ing speed.

The ability of the muscles to absorb and generate energycontributes to the overall efficiency of gait and explains howmany of the movements can proceed without muscle con-traction. Energy flows between adjacent limb segments dur-ing locomotion in much the same way that energy flowsbetween the vaulter and the pole during a pole vault or amongchildren playing “crack the whip.” Examination of the energyflow between limb segments reveals that the energy gener-ated by the plantarflexors at push off is transferred passivelyto the leg and thigh, facilitating the initiation of swing. Simi-larly, the hamstrings absorb energy at the end of swing, andthat energy is transferred to the trunk at ground contact,assisting in the trunk’s forward progression. The transfer ofenergy from segment to segment depends on the normal se-quencing of the angular changes described earlier in thischapter.

CLINICAL RELEVANCE: ENERGY TRANSFER AMONGLIMB SEGMENTS IN ABNORMAL GAITEnergy transfer among limb segments depends on thepower generated and absorbed at joints and requires pre-cise coordination among the moving segments. Sincepower is a function of the velocity of a limb segment, alimb segment that has a low angular velocity also has lowpower generation or absorption and, consequently, hasless ability to transfer energy from one segment to an-other. A patient with arthritis producing a stiff knee is un-able to transfer energy from the plantarflexors to thethigh; a patient with Parkinson’s disease, which is charac-terized by generalized rigidity, has difficulty transferringenergy through the lower extremity and into the trunkbecause the joints lack the freedom of movement to al-low the sequential movement patterns of the joints of thelower extremity. A study of patients with multiple sclero-sis demonstrates an inverse relationship between themetabolic cost of walking and the patients’ ability to rap-idly flex and extend the knee. This finding is consistentwith a diminished capacity to transfer energy through theknee joint [115]. Thus treatments directed toward reduc-ing joint stiffness or rigidity may lead to improved gaitefficiency in these individuals.

Max PEMin KE

Min PEMax KE

Figure 48.22: In an ideal roller coaster, potential and kineticenergy are transformed from one form to the other with noloss of energy. Potential energy (PE � mgh) is maximum whenthe roller coaster is farthest from the ground, at the same timethe kinetic energy (KE � 1/2 mv2) is at its minimum. As theroller coaster descends the track it gains speed, increasing itskinetic energy while it is losing potential energy as it movescloser to the ground.


FACTORS THAT INFLUENCEPARAMETERS OF GAIT

Several factors influence gait performance and must be con-sidered by clinicians evaluating and treating a person with alocomotor dysfunction. Factors considered here are gender,speed, and age.

GenderAlthough most observers would report differences betweenthe gait patterns of males and females, few studies providedirect comparisons. Women walk with higher cadences thanmen and shorter strides [13,80,106]. Yet when the distancecharacteristics of the gait cycle are normalized by height, fe-males demonstrate a similar or slightly larger stride length[45,80].

A study directly comparing 99 males and females of simi-lar ages reports statistically different joint kinematics, al-though these differences are on the order of 2–4�, and theclinical significance of these differences is negligible [80]. Thesame study also reports that females exhibit a statisticallygreater extension moment at the knee at initial contact and agreater flexion moment in preswing with increases in powerabsorption or generation at the hip, knee, and ankle. The au-thors suggest that these differences in kinetic measures mayhelp to explain the higher incidence of knee osteoarthritis inwomen, but additional research is required to confirm thesefindings and demonstrate a clinical association.

Walking SpeedGait speed affects several parameters of gait performance.As noted in the discussion of the temporal and distance char-acteristics of gait, cadence, step length, and stride lengthincrease with increased walking speed and decrease withdecreased speed [6,106]. Increased speed appears to in-crease the variability of some temporal and spatial gait pa-rameters such as step width [137]. Angular excursions alsoappear to increase with increased walking speed, althoughthese changes are small and differ with the speed and jointexamined [31,106,148]. Increases in joint excursions at theproximal joints are related to the increase in stride lengthassociated with increased speeds [31].

Increased walking speeds also lead to increased ground re-action forces [6,25] and changes in the pattern of muscle ac-tivity. Although the relationship between muscle activity andwalking speed is somewhat complex, there appears to be ageneral increase in the duration of muscle activity with in-creased walking speed, particularly in the muscles around theknee [103,105]. Similarly, joint moments and joint reactionforces increase with increased walking speed [11,135,166].However, muscle activity during free-speed walking is morereproducible than that at speeds slower or faster than freespeed [23,82]. Increased mechanical work and power at theknee and hip also accompany increased walking speed [21,72].

CLINICAL RELEVANCE: WALKING SPEED ININDIVIDUALS WITH GAIT IMPAIRMENTSMany abnormal gait patterns found in individuals withimpairments are characterized by decreased walking ve-locities. Patients with dysfunctions associated with lowback pain, stroke, hemiparesis, and anterior cruciate liga-ment tears all frequently exhibit altered gait patterns thatinclude decreased step length, smaller joint excursions,and decreased walking speed. Because decreased walk-ing speed is associated with decreased step length andjoint excursion, are the gait deviations exhibited by thesepatients merely the consequence of their walking speed?If a goal of treatment is to improve the gait pattern, theclinician must attempt to discern what characteristics ofthe gait pattern are attributable to the gait speed alone,and what characteristics are the result of the patient’simpairments.

AgeAge appears to affect gait rather dramatically, as witnessed bythe development of gait in the toddler and the apparent de-terioration of gait in older adults. While the gradual acquisi-tion of stable bipedal ambulation is a normal part of humandevelopment, it is unclear whether the alterations commonlyseen in gait in the elderly are the normal consequence of agingor reflect the functional deficits resulting from impairmentsassociated with neuromusculoskeletal disorders commonlyfound in elders [34,45,52,90,165].

Table 48.4 lists commonly reported changes in gait withaging. The ages of the elders studied range from approxi-mately 60 years to over 100 years, and studies vary in themagnitude of changes reported. Despite the overwhelmingdata demonstrating changes in gait with increasing age, thenature of the relationship between age and locomotor func-tion remains unclear. One of the most consistent findingswith age is a decrease in free-walking speed [48,62,78,90,92,108,153], but many of the other changes reported withaging also are consistent with the changes reported earlierin this chapter for walking speed alone [46]. Specifically, de-creased walking speed produces reductions in step length,joint excursions, and ground reaction forces [48,62,79,160].Consequently, many of the changes that occur with agingappear to be secondary changes associated with walkingspeed.

Even the decrease in walking velocity reported with ageappears to depend on an individual’s level of fitness and otherfactors besides age itself. Coexisting joint impairments;decreased strength of the quadriceps, plantarflexors, and hipflexors; hip and knee passive ranges of motion; and maximaloxygen uptake all help explain the diminished walking veloc-ity seen with age [19,34,45,52,73,99]. Treatment of gaitdysfunctions in elders requires consideration of the contri-butions made to the dysfunction by discrete impairments inthe neuromusculoskeletal and cardiorespiratory systems.


CLINICAL RELEVANCE: EVALUATION AND TREATMENT OFGAIT DYSFUNCTION IN ELDERSData describing the gait of elderly individuals reveal thatmany of the changes thought to be characteristic of ag-ing can be explained by a reduction in walking speed. Con-sequently, a clinician must alter the standards of “normal”used to judge the adequacy of gait. The gait patterns ofelders walking at reduced speeds are not comparable tothe patterns of subjects walking at faster speeds, regard-less of age. Similarly, treatment may be most successfulwhen directed toward those factors that contribute to di-minished speed, including strength of the quadriceps,plantarflexors, and hip flexors.

SUMMARY

This chapter reviews the kinematic and kinetic variables ofnormal gait. The kinematic variables presented in this chap-ter include the more global parameters of time and distanceas well as the discrete displacement patterns of joints.Although all of these variables are subject to intra- andintersubject variability, representative values from the litera-ture are presented to provide the reader with a frame ofreference for normal locomotion.

Joint excursions are largest in the sagittal plane and exhibitstereotypical patterns and sequences. In normal locomotion,the hip, knee, and ankle rarely move together toward or awayfrom the ground. Activity of the major muscle groups of thelower extremity is reviewed. Their activity is typically brief,characterized by initial eccentric activity followed by concen-tric activity. In most cases, joint movement continues aftermuscle activity has ceased.

The kinetic variables described in this chapter includeground and joint reaction forces, muscle forces, and jointmoments, as well as joint power and mechanical energy. Theprinciple of dynamic equilibrium is used to explain the deri-vation of muscle and joint reaction forces, joint moments, andjoint power. Like the kinematic variables, the kinetic variablesexhibit intra- and intersubject variability that reflects thenormal variability of individuals and populations, but kineticparameters also are quite sensitive to differences in meas-

urement procedures. The kinetic variables reveal thatlocomotion generates large muscle and joint forces. Kineticanalysis also demonstrates the remarkable efficiency of nor-mal locomotion in which energy is stored and released,reducing the amount of work the muscles must perform toachieve the movement. Impairments in the neuromuscu-loskeletal system decrease the efficiency of gait.

Finally this chapter discusses factors that influence walkingpatterns, including gender, walking speed, and age. Thediscussion reveals a complex interdependence betweenwalking speed and age effects on gait, and the clinician iscautioned to keep these factors in mind when judging thewalking performance of an individual.

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TABLE 48.4 Commonly Reported Changes in Gait in Older Adults

Change with Increased Age

Speed Decreased [48,62,78,90,92,108,153]

Cadence Increased [48,72]

Step/stride length Decreased [46,48,62,72,73,108,160]

Double support time Increased [46,72,160]

Joint angular Decreased [72,79,108]excursions Unchanged [48]

Muscle activity Increased [48]

Joint powers Decreased generation in hip extension and plantarflexion and increased generation in hip flexion [72,73,79,160]


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chapter 48, factors influencing gait

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