dynamic biomechanics of the normal foot and ankle during walkin & running

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1988; 68:1822-1830.PHYS THER.

Mary M RodgersDuring Walking and RunningDynamic Biomechanics of the Normal Foot and Ankle

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D y n a m i c B i o m e c h a n i c s o f t h e N o r m al F o o t an d A n k l e

D u r in g W a lk i n g a n d R u n n in g

M A R Y M . R O D G E R S

T h i s a r ti c l e p r es e n t s a n o v e r vi e w o f d y n a m i c b i o m e c h a n i c s o f t h e a s y m p t o m a t i cf o o t a n d a n k l e t h a t o c c u r d u r i n g w a l k in g a n d r u n n i n g . F u n c t i o n a l d e s c r i p ti o n s f o rw a l k i n g a r e p r o v i d e d a l o n g w i t h a r e v i e w o f q u a n t i t a t i v e f i n d i n g s f r o m b i o m e -c h a n i c a l a n a l y s e s . F o o t a n d a n k l e k i n e m a t i c s a n d k i n e t i c s d u r i n g r u n n i n g a r et h e n p r e s e n t e d , s t a r t i n g w i t h a g e n e r a l d e s c r i p t i o n t h a t i s f o l l o w e d b y m o r es p e c i fi c c u r r e n t r e s e a r c h i n f o r m a t i o n . A n u n d e r s t a n d i n g o f t h e d y n a m i c c h a r a c t e r i s t i c s o f t h e s y m p t o m - f r e e f o o t a n d a n k l e d u r i n g t h e m o s t c o m m o n f o r m s o fu p r i g h t l o c o m o t i o n p r o v i d e s t h e n e c e s s a r y b a s i s f o r o b j e c t i v e e v a l u a t i o n o fm o v e m e n t d y s f u n c t io n .

K ey W o r d s : Ankle ; Foot; K ines io logy/b iomechan ics gai t analys is ; K ine t ics; Lowerextremity nkle an d foot.

The foot and ankle, by virtue of theirlocation, form a dynamic link betweenthe body and the ground. The foot andankle are basic to all upright loco motionperformed by the human, constantly adjusting to enable a harmon ious couplingbetween the body and the environmentfor successful movement. The dynamiccharacteristics of the foot and ankle havebeen inferred traditionally from cadaveric exam ination a nd qualitative clinicalassessment. Advancements in biome-chanical techniques for dynamic analysis have enabled m ore quan titative andaccurate documentation of foot and ankle function during movement, especially during the process of walking.

The objective of this article is to provide a selected review of quantitativeinformation relevant to the dynamicfunction of the foot and ankle complex.Although results have often confirmedtraditional anatomical assumptions regarding foot and ankle function, theyhave also contradicted long-acceptedtheories in certain cases.

The most frequently performedmovements of the foot and ankle forhealthy people occur during walking.Much research has been conducted inthe analysis of walking, and the majorityof this article will concentrate on thedynamic biomechanics of the foot andankle during this activity. A classicaldescription of the biomechanics of gait

as found in clinical literature is followedby an overview of quantitative findingsthat document kinematic and kineticcharacteristics during w alking. As interest in physical fitness con tinues to grow,therapists are treating an increasingnumber of runners, both recreationaland competitive. The foot and anklekinematics and kinetics that occur during running will be presented briefly inthe final section. This review includesinformation relevant to symptom-freeindividuals.

F O O T A N D A N K L E

K I N E M A T I C S D U R I N G W A L K I N G

Although the foot has been viewedtraditionally as a static tripod or a semirigid support for body weight (BW), ithas evolved primarily for walking and istherefore a dynamic mechanism. Thebody requires a flexible foot to accommodate the variations in the externalenvironment, a semirigid foot that canact as a spring and lever arm for the

push off during gait, and a rigid foot toenable BW to be carried with adequatestability. The dynamic biomechanics ofthe foot and ankle complex that allowsuccessful performance of all these requirements can only be understoodwhen studied in relation to the biomechanics of the lower limb duringwalking.

The gait cycle (or stride period) provides a standardized frame of referencefor the various events that occur duringwalking (Fig. 1). The gait cycle is the

period of time for two steps and is measured from initial contact of one foot to

the next initial contact of the same foot.The gait cycle consists of two phase s: 1)stance (when the foot is in contact withthe supporting surface) and 2) swing(when the limb is swinging forward, outof contact with the supporting surface).Along with providing forward mom entum of the leg, the swing phase alsoprepares and aligns the foot for heel-strike and ensures tha t the swinging footclears the floor. Stance comprises about60% of the total gait cycle at freely ch osen speeds and functions to allowweight-bearing and provide body stability. Five distinct events occur durin g thestance phase: heel-strike (HS), foot flat(FF), mid-stance (MS), heel rise (HR ),and toe-off (TO).

G e n e r a l D e s c r i p t i o n

An understanding of the various jointaxes of the foot and ankle (see articlesby Riegger and Oatis in this issue) isessential to the discussion that follows.Figure 1 summarizes these joint mo

tions as they relate to different phases ofgait. Numerous authors have contributed to a clinical description of walkingkinematics based primarily on observation.1-7 To understand the movementsof the foot and ankle during walking,other portions of the lower extremitymust be included.1 D uring walking, rotation of the pelvis causes the femur,fibula, and tibia to rotate about the longaxis of the limb.2 The m agnitude of thisrotational motion increases progressively from pelvis to tibia. Fo r exam ple,

during norm al walking on level ground,the pelvis undergoes a maximum rota-

M. Rodgers, PhD, PT, is Research Health Scientist, Laboratory of Applied Physiology, WrightState University, 3171 Research Blvd, Dayton, OH45420 USA), and Veterans Administration Medical Center, 4100 W Third St, Dayton, OH 45428.

1822 PHYSICAL THERAPY

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One Gait Cyc le Limb Component

Stance hase

Swing hase

0

2 0 -

4 0 -

6 0 -

8 0 -

100

Events

heel-

strike

footflat

mid-stance

heelrise

toe-

off

heel-

strike

LowerL imb

medialrotation

lateral

rotation

medialrotation

AnkleJo in t

plantarflexion

dorsiflexion

plantar

flexion

dorsiflexion

Subta larJo in t

pronation

supination

pronation

TransverseTarsa l Jo in t

freemotion

increasinglyrestricted

freemotion

F i g . 1 . S u m m a r y o f phases o f gait cycle a n d accompanying mot ions o f lower l imb jo ints.Ad apted f rom M ann.

6)

tion in each gait cycle of about 6 degrees,and the tibia undergoes a rotation ofabout 18 degrees in the same period.Generally, the limb rotates medially (internally) during the swing phase andearly stance phase and th en laterally (externally) until the stance phase is com

plete and TO has occurred.3

At HS, the tibia is rotated mediallyabout 5 degrees from its neutral position, and the ankle joint is either in itsneutra l position or in slight plantar flexion.4 According to Perry, compressionof the heel pad occurs at HS and isfollowed by traction on both anteriorand posterior calcaneal attachmentsduring terminal stance.5 Immediatelyfollowing HS , the foot fl x s toward thefloor, with the dorsiflexors controllingthis plantar motion to prevent the foot

from slapping down to the FF position.From HS to just before F F, the increasing medial rotation of the tibia an d fibula is transmitted through the anklemortise to the talus.6 The medial rotation of the mortise, combined with theplantar-flexed position of the ankle,tends to shift the forefoot medially fromits neutral, toe-out position. The heelcontact with the ground is lateral to thecenter of the ankle joint where BW istransmitted to the talus, creating a pro-natory moment at the subtalar joint

that, in turn, stresses the structures ofthe medial arch. The talus rotates me

dially on the calcaneus about the subtalar axis forcing the calcaneus into p ronation. According to Wright and associates, the foot quickly pronates, about10 degrees within the first 8% of stanceat an average walking speed.7 In thispronated position, free motion is avail

able at the transverse tarsal joint so thatthe foot remains flexible, distal to thenavicular and cuboid, and can bend intoclose contact with the supporting surface.

At the FF position, the lower limbbegins to rotate laterally. Because theforefoot is now fixed on the g round, theentire lateral rotation of the ankle mortise is transm itted to the talus. As lateralrotation continues, the foot supinates,increasing stability at the transverse tarsal joint and along the longitudinal arch

of the foot. The stability of the transverse tarsal joint is further improved bythe increasing body load being carriedand by the firm fit of the convex headof the talus into the concave face of thenavicular bo ne.1,3,6

When the leg has passed over the foot,the ankle begins dorsiflexion. After H R,the ankle joint moves back into plantarflexion forcing the metatarsophalangealjoints to dorsiflex. Because the plantaraponeurosis wraps around the metatarsal heads, a windlass effect takes place

that increases tension across the longitudinal arch, further elevating the arch

and increasing foot stability. Just beforeTO, the combination of weight-bearing,windlass effect, and supination ensuresthat the foot is in a maximally stableposition for lift off. After TO, some authors report that the leg rotates medially,again pronating the foot and unlockingthe transverse tarsal join t so that the footreturns to its flexible state for the swingphase of gait.1,3,6 It should be noted thatother authors report that the leg continues to be in lateral rotation throughoutmid-swing and that the foot remainssupinated throughout swing.2

Kinematic Studies

Kinematics refers to the descriptionof motion, independent of the forcesthat cause the movement to take place.Linear and angular displacements, velocities, accelerations, center of r otationfor joints, and joint angles are all ex

amples of kinematics.8 Kinema tic information can be collected using directmeasurement techniques (ie, goniometers, accelerometers) and with indirectmeasurement using imaging techniques(ie, cinematography, high-speed video,stroboscopy). Each technique has advantages and disadvantages that havebeen described by several authors andwill not be detailed in this discussion.9,10

Instead, the results of selected studiesrelevant to dynamic biomechanics ofthe foot and ankle during walking and

running will be presented.Walking cadence and velocity Many

factors affect foot and ankle biom echan ics during walking, including the velocity of gait and anthropometric characteristics (ie, limb length). Winter definesnatural cadence or free cadence as thenumber of steps per minute when asubject walks as naturally as possibleand reports an average natural cadencerange of 101 to 122 steps/min.11 In general, the natural cadence for women is6 to 9 steps/min higher than that of

men. Foot and ankle kinematic measurements also are directly related tothe walking velocity. Studies havedocumented the changes that occurwith increasing speed.12,13 For thisreason, walking velocity mus t be considered when comparing biomechanicalfindings.

Displacements—paths of movementMotion of the heel in walking has beenreported by Winter in a study with 14subjects walking at their natural cadences.11 Vertical displacement of the

heel begins well before TO and reachesmaximum upward velocity just before

V o l u m e 6 8 / N u m b e r 1 2 , D e c e m b e r 1 9 8 8 1823








TO. The heel reaches its highest displacement shortly after T O . Horizontalvelocity increases gradually after HR,reaching its maximum late in the swingphase, and then rapidly decreases justbefore HS. Vertical velocity of the heelslows abruptly at about 1 cm aboveground level, after which the heel islowered very gently to the ground.

The path of the forefoot differs from

that of the heel. For the same sample of4 subjects, Winter rep orts an initial rise

in the forefoot during late push-off andearly swing.11 As the leg and foot areswung forward, the forefoot just clearsthe ground and then rises to a secondpeak just before H S. Because the toe isthe last part of the foot to leave theground, and because of the accompanying leg and foot angles, the toe risesto no more than 2.5 cm above theground and then drops to only 0.87 cmof clearance at mid-swing. As the knee

extends an d foot dorsiflexes, the toe risesto a maximum of 3 cm jus t before HS.

Ankle range of motion, foot placement, and arch movement. Ankle-jointangles, foot-placement angles, and archmovement are other kinematic characteristics that have been investigated.Winter reported mean ankle-jointranges of motion during walking for 19subjects as a maximum of 9.6 degreesof dorsiflexion and 19.8 degrees of plantar flexion.11 Murray and associatesfound that foot-placement angle showedhigh variability on successive steps ofthe same foot.14 A mean value of 6.8degrees of foot abduction (out-toing)was reported, with the average difference between successive foot anglesbeing 2.4 degrees.

Dynamic arch movement was studiedby Kayano using an electro archgauge. 15 He found that the m edial longitudinal arch lengthens from the vertical force of BW from early stance to FF.It then shortens with the decrease in BWand activation of the arch supportingmuscles. As the calf muscles activate forpush-off, the arch lengthens again. Itfinally shortens rapidly because of thewindlass action of the plantar aponeurosis as the toes dorsiflex for T O .

FOOT AND ANKLE KINETICSDURING WALKING

G e n e r a l D e s c r i p t io n

Kinetics is the study of the forces th atcause movemen t, both m edially (muscle

activity, ligaments, friction in musclesand joints) and laterally (from theground, active bodies, passive bodies).8

A large num ber of researchers have analyzed muscular activity and ground reaction forces (GRFs) during gait. Jointmom ents, segmental energy, joint reaction, and pressure distribution beneaththe foot during walking have receivedless attention . The findings from electromyograph ic studies of the foot and anklemuscles during walking will be presented in the first subsection, followed

by findings from force-plate and pressure-distribution studies. Calculated kinetic variables, such as ankle-joint moments and joint reaction forces, will beincluded in the final subsection.

Electromyographic studies of foot nd

ankle muscles during walking. Many researchers have investigated the electricalactivity of muscles during walking, andBasmajian and Deluca have presented a

review of their findings.16 In general,studies have shown that many of thechanges in levels of muscular activityoccur at 15% to 20% of the gait cycle(FF), when the foot adapts to the supporting surface.

Winter and Yack have contributedextensively to the literature on EMGduring walking.17 Specific EMG patterns for several of the foot and ankle

muscle groups that are active duringwalking are shown in Figure 2. Thetibialis anterior m uscle (TA) has its m ajor activity at the end of swing to keepthe foot in a dorsiflexed position. Immediately after HS, the TA peaks andgenerates forces to lower the foot to theground in opposition to the plantar-flexing GRFs. The TA is the only inverting muscle active during the period

F i g . 2 . E lec t romyog raph ic ac t i v i ty (no rma l ized to each s ubjec t s m ean EMG) fo r s ix musc les

dur ing wa lk ing . P lo ts s how m ean EMG (so l id l ine) and one s tandard dev iat i on (do t ted l ines) fo rsamples o f va ry ing s ize . Ac t iv i ty o f med ia l and la te ra l gas t rocnemius musc les i s ve ry s imi la ra n d i s c o m b in e d f o r d iscuss ion i n tex t . (Reprin ted w i t h p e rm i s s i o n .

17)

1824 PHYSICAL THERAPY









F i g . 4. Mean and one standard deviation for center-of-pressure paths during normal walkingin different foot conditions : barefoot and we aring rigid-soled, soft-soled, and high-heeled shoes.(Reprinted with permission.

22)

T A B L E

Co m p a r i s o n o f M e a n R e g i o n a l Pe a k P r e s s u r e s (i n K i l o p a s c a l s ) f r o m Pr e s s u r e -D i s t r i b u

t i o n S t u d i e s

Region

Hallux

Medial toes

Lateral toes

First metatarsal

Second metatarsal

Lateral metatarsals

Medial midfoot

Lateral midfoot

Medial heelLateral heel

Rodgers23

219

180

163

245

336

312

60

103

337333

Soamesa

400

300

200

520

510

550

150

780450

Grieve and

Rashdib

178

163

212

151

68

6

208208

Betts et alc

432

353

392

281

363363

Clarke24

378

160

319

319

324

43

95

443391

by several different investigators isshown in the Table. Differences invalues reported result from the varietyof techniques and subject samples

used by investigators.23 These pressure-distribution studies have shown that allmetatarsal heads are loaded during thestance phase of gait. This finding negatesthe concept of tripod stance, whichwould not allow pressure beneath themiddle m etatarsal heads.

Many variables have been identifiedthat directly influence pressure distribution beneath the foot. Clarke foundthat with increasing speed, pressures increase and shift medially.24 The toescontribute more as the walking speed

increases. Walking barefoot alters bothkinetic and kinematic variables when

compared with walking in shoes.20

Structural characteristics of the foot,such as arch type, also affect pressuredistribution.25 As shown in Figure 5, the

more rigid high-arched foot tends toconcentrate pressure beneath the heeland forefoot, with minimal pressure beneath the midfoot. This absence of midfoot pressure is present even in thehigher loading conditions that occurwith increasing speed of locomotion.The flexible flat-arched foot shows morespreading of pressure, including the areabeneath the midfoot.

The classic Morton's foot structure,characterized by a second metatarsalhead that is placed more distally than

the first, has also been shown to influence pressure distribution. Rodgers and

Cavanagh reported that second m etatarsal head pressures were significantlyhigher in subjects with Morton's footwhen compared with control subjectswithout Morton's foot.26 This findingsuggests that individuals with a Morton's foot structure may be more proneto second metatarsal pressure problemsthan individuals with other foot structures. Pressure-distribution studies havealso been useful in identifying areas ofconcentrated pressure that may lead topressure ulcers for individuals with insensitive feet.27

Joint moments and joint reactionforces. Indirect m ethods have been usedto calculate gait kinetics when directmethods are not feasible. These methodsare necessary to calculate forces withinthe joint because force transducers currently cannot be used safely in subjects.Winter9,11 and Winter and Robertson28

have made significant contributions in

the calculation of joint moments offorce and energy patterns during walking. The mean maximum ankle-jointmoment (normalized to body mass)generated during walking was found tobe a plantar moment of 1.6 N.m/kg,occurring between 40% and 6 0% of thegait cycle. Planta r flexors were found toabsorb energy during the early stanceand MS phases of the gait cycle as theleg rotates over the foot. Late in stance,these same muscles plantar flex rapidly(producing the plantar moment) and

generate an explosive burst of energy(push-off).

As mentioned in the section on force-plate studies, the GRFs during gait aretransmitted proximally to the rest of thebody through the foot and ankle, compressing each join t along the way. Thesecompressive forces have been shown tocontribute to the formation of osteoarthrosis.21,29 Joint reaction studies of theankle have been few, probably becausethis joint demonstrates osteoarthriticchanges less often th an th e hip and knee

joints. Stauffer and co-workers haveshown ankle-joint co mpressive forces ofabout 3 times BW from HS to FF.

30 Afurther rise to a peak value of 4.5 to 5.5times BW occurs during heel-off whenthe plantar flexors are undergoing strongcontraction. Seireg and A rvikar have derived maximal ankle-joint reactionforces of 5.2 times BW from mathem atical models.31 Procter and Paul found apeak of 3.9 times BW for ankle-jointreaction force during walking.32

Stauffer and associates also reported

ankle shear forces of 0.6 times BW in aposterior direction.30 After HR, talo-

a Soames RW: Foot pressure patterns during gait. J Biomed Eng 7:120-126, 1985.

b Grieve DW, Rashdi T: Pressures under normal feet in standing and walking as measured

by foil pedobarography. nn Rheum Dis 43:816-818, 1984.c B etts R P, Franks CI, Duckworth T: A nalysis of loads under the foot: Part 2. Quantification

of the dynamic distribution. Clinical Physics and Physiological Measurement 1(2):113-124,

1980.

1 8 2 6 P H Y S I C A L T H E R A P Y








PEAK PRESSURE - HIG H A RCHDURING 4 ACTIVITIES

PEAK PRESSURE - FLAT ARCHDURINC 4 ACTIVITIES

Fig. 5. Pressure-distribution patterns during slow and fast walking, running, and landing from a jump beneath a high-arched a) and a flat-arched b) foot. The flat-arched foot show s more spreading of p ressure beneath the midfoot region. Reprinted w ith permission of Martinus

Nijhoff/Dr W Junk Publishers.25

)

crural shear was anterior and reduced toless than half of the previous posteriorforces. Subtalar-joint reaction forceshave been calculated by Seireg and Ar-vikar.31 The peak resultant force in theanterior facet of the talocalcaneonavicular joint was 2.4 times BW and for theposterior facet, 2.8 times BW . Peaks forboth locations occurred in the latestance phase of the gait cycle.

FOOT AND ANKLEKINEMATICS DURING RUNNING

A considerable amount of researchhas been conducted in the area of running biomechanics and is presented in adetailed review by Williams.33 The position of other body parts and the timingof their movements are basic to an understanding of the motion of the footand ankle. Although other body parts

(primarily the hip and knee) have received most of the attention, several investigators have contributed to a functional description specific to foot andankle motions during running at moderate speeds.34,35

General Descript ion

For the running gait in which HSoccurs, initial contact is at the lateralheel with the foot slightly supinated.34,35

This position results from swinging ofthe leg toward the line of progression.

Slight plantar flexion of the subtalarjoin t occurs along with supination of theforefoot and calcaneus. The subtalarjoint passes from a supinated to a pro-nated position between HS and 20into the support phase. The foot rem ainspronated between 55 and 85 of thesupport phase. Maximum pronation occurs between 35 and 40 of supportphase, approximately the time when

total-body center of gravity passes overthe base of support. Full pronationmarks the end of the absorbing andbraking period of support as the footbegins its propulsive period. Maximumankle dorsiflexion occurs 50 to 55into the support phase when the centerof gravity is forward of the s upport leg.The foot begins to supinate and returnsto the neutral position at 70 to 90of the support phase. The foot then assumes a supinated position for push-off.

34 35

Kinematic Stud ies

Several stride variables that directlyaffect running kinematics and kineticshave been described by Cavanagh.36

These variables include stride length atdifferent speeds, optimal stride length,timing of the phases of running gait, andfoot placement. Timing of the biome-chanical events in running is variablebecause it depends on running speed,type of shoe, and individual anatomic

variations. For example, Kaelin et alreported the interindividual (N = 70)

and intraindividual variabilities (20 repetitions each for 6 of the subjects) forseveral variables during running.37 Themaximum pronation angle during foot-ground contact showed a range of 20degrees among the subjects, but only 7to 12 degrees within the same individual. Vertical touchdown velocity of thefoot during running varied between 0.64and 2.3 m/sec among the subjects.

Scranton and associates reported an average duration of the suppo rt phase forjogging of 0.2 sec and for sprinting of0.1 sec.38

Clinical evaluations have suggested arelationship between pronation of thefoot during running and a variety oflower extremity problems such as shinsplints and knee pain. C urrently, quantitative data do not support the relationship, although this finding may resultfrom inadequate analytical techniques.For examp le, studies of rear-foot m otion

have been conducted in two dimensions, although pronation occurs inmore than one plane. Clarke and associates have reviewed several differentstudies of rear-foot movement in running (Fig. 6).

39 They reported a n averagemaximum pronation angle of 9.4 degrees over all studies. The authors suggest that a maximum pronation angleof 3 degrees and total rear-foot motiongreater than 19 degrees during runningwould be considered excessive. Currently, however, no single variable reli

ably predicts safe rear-foot movementduring running.

Volum e 68 / Number 12 , December 1988 1827








F i g . 6. Curve showing average rear-foot angular displacement during suppo rt phase of runningbased on rear-foot motion studies conducted by various researchers. The foot remains pronatedfor the majority of the suppo rt phase. (Adapted from Clarke TE, Frederick E C, Hamill CL: Thestudy of rearfoot m ovement in running. In Frederick EC (ed): Sport Shoes and laying Surfaces:Biomechanical Properties Champaign, IL, Human Kinetics Publishers Inc, 1984, p 180.)

FOOT A ND A NKLE KINETICSDURING RUNNING

G e n e r a l D e s c r i p t i o n

Direct measurement of running kinetics poses more difficult technicalproblems than during the slower speedsof walking gait. Targeting a force plateis mo re difficult at higher speeds withoutaltering the normal running gait patterns. The faster motion requires moredistance for running, and longer cablesor telemetry systems therefore must beused for EMG data collection. Treadmill running has been used for EMGdata collection, although the pattern ofrunning is different from that seen overnatural terrain or on a track. Because ofthese problem s, few researchers have di

rectly measured foot and ankle muscleactivity.33 More research has been conducted in GRFs and pressure distribution during running. Indirect calculations of foot and ankle muscle forces,segmental moments, and joint reactionforces during running have been performed by a few researchers.

Electromyographic studies of foot andankle muscles during running. Studieshave shown that EMG activity increaseswith running as compared with walking.Miyashita and associates have reported

that integrated EMG (IEMG) activity ofthe TA and GA increases exponentiallywith increasing speed.40 Ito et al reportthat with increasing running speed, theIEMG increased during swing but remained the same during the supportphase.41

Force-plate studies. Several authorshave suggested a link between commonrunning injuries and the impact forcesat foot-strike that can occur thousandsof times du ring running.34,42 Force-plateanalysis has shown that peak loading

force during run ning is more tha n twicethat of walking and occurs at least twice

as fast. Perry extrapolates tha t the forcesimposed on the supporting tissueswould reflect a fourfold increase instrain.5 Because microtrauma is cumulative, running creates symptoms thatdo not arise with ordinary walking.

Force-plate data for jogging and running are much more variable from stepto step when compared with walking.The pattern and magnitude of the vertical GRFs during running also differsignificantly from those that occur during walking. Variables that affect vertical GRF data include touchdown velocity of the heel, position of the foot andlower leg before contact, and m ovementof these structures during impact.43 Thevertical GRF curve for heel-toe running( heel strikers ) usually shows two dis

tinct peaks: 1) the impact force peakand 2) the active force peak.44,45 Typicalpeak vertical GRF values for distancerunn ing speeds are 2.5 to 3.0 times BW .

The pattern of force is dependent onthe orienta tion of the foot at initial contact, which is determined by whetherthe runn er is a forefoot striker, amidfoot striker, or a rear-foot strik

er. 44 Most runners initially contact theground with the outside border of theshoe, some with the rear lateral border(rear-foot strikers), and some with the

middle lateral border (midfoot strikers).Harrison and associates report thatmean foot contact time is reduced inforefoot strikers as compared with rear-foot strikers (0.20 vs 0.19 seconds, respectively).46 Cavanagh and Lafortunealso found slightly shorter contact tim esfor the midfoot strikers compared withthe rear-foot strikers.44

Additional differences in GRF patterns have been described.44 Rear-footstrikers dem onstrate a sh arp initial spikein vertical GRF that is generally absent

from the midfoot-striker patterns. Midfoot strikers produced two positive

peaks in the anteroposterior force during the braking phase. The mean peak-to-peak amplitude for mediolateral(ML) GRF was three times greater inthe midfoot strikers than that for therear-foot strikers (0.35 and 0.12 BW,respectively). These findings indicatethat the loading rates within the muscleand joints are affected by the type ofinitial foot contact during running.

The path of the CO P also depends onthe type of initial foot contact duringrunning (Fig. 7). Cavanagh and Lafortune found that the COP path for rear-foot strikers followed from the rear lateral border to the midline within 15msec of contact.44 The COP path thencontinued along the midline to the center of the forefoot where it remaine d foralmost two thirds of the entire 200 -msecsupport phase. Midfoot strikers runningat the same running speed made initialcontact at 50% of shoe length. The CO P

path then migrated posteriorly as therear part of the shoe made contact w iththe ground. This posterior movementcoincided with a drop in the AP GRF.When the end of posterior migrationwas reached, the CO P rapidly moved tothe forefoot where it remained for mostof the support phase.

Pressure-distribution studies. Verylittle information is available regardingpressure distribution u nder the foot during running. Pressure patterns duringrunn ing vary with foot type (Fig. 5). The

increased loading that occurs with running remains concentrated under theheel and forefoot in the m ore rigid high-arched foot. In the more flexible flat-arched foot, the increased load is spreadbeneath the entire foot, including themidfoot region.25 Cavanagh and Hennigfound that the average peak pressureduring the contact phase of running(868.0 kPa) occurred under the heel fora sample of 10 rear-foot strikers.47 Although pressures were mu ch higher beneath the heel of these rear-foot strikers,

more of the contact time was spent onthe forefoot.

Muscle forces, segmental impulse,and joint reaction forces. Several investigators have developed mathematicalmodels to predict muscle forces duringrunning. Forces generated by the dorsi-flexors and th e GA hav e been calculatedby Harrison and associates.46 They report peak forces in the dorsiflexors of0.5 times BW, which are active onlyduring the first 10% of the stance phase.The GA generated a substantially

greater peak force of 7.5 tim es BW . Calculations by Burdett revealed that the

1828 P H Y S I C A L T H E R A P Y








F i g . 7. Com par iso n o f center-o f-press ure paths dur ing runn ing for rear- foot (A) and midfo ot

(B) s tr ikers. (Repr in ted wi th permiss ion.44

)

GA-SO group had the highest predictedforce (5.3-10.0 times BW) of the anklemuscle groups.48 Predicted forces in thetibialis posterior, flexor digitorum lon-gus, and flexor hallucis longus muscu

lature ranged from 4.0 to 5.3 times BW.The peroneus tertius muscle and EDLdid not show any predicted force duringthe stance phase of running.

mpulse is the effect of a force actingover a period of time and is determinedmathematically as the integral of theforce-time curve.8 Ae an d associates calculated the impulse generated by different body segments during running.49

The researchers found that the foot generated the largest mean impulse compared with other body segments. This

impulse increased with faster running,suggesting that the foot plays an impor

tant role in projecting the body andincreasing running velocity.

Ankle-joint reaction forces duringrunning have also been calculated byseveral investigators. Harrison and as

sociates reported maximum ankle-jointreactions of 8.97 and 4.15 times BW forthe compressive and shear components,respectively.46 Burdett predicted thatcompressive forces on the foot along thelongitudinal axis of the leg reached peakvalues of 3.3 to 5.5 times BW duringrunning.48 In addition, he reported MLshear forces that ranged from a medialforce of 0.8 times BW to a lateral forceof 0.5 times BW. Furthermore, the vertical reaction forces and oth er calculatedforces were determined to be about 2.5

times larger in running (at a 4.47-m/secpace) when compared with walking.

S U M M A R Y

Physical therapists can provide moreeffective programs for prevention andrehabilitation of foot and ankle injuriesif dynamic characteristics are taken intoconsideration. This article has describedcurrent findings related to the dynamicbiomechanics of the asym ptomatic footand ankle during walking and running.

Functional descriptions of walking andrunning biomechanics have been provided along with quantitative findingsfrom current biomechanical studies. Extensive databases are still unavailable formany of the biomechanical variablesthat affect dynamic foot and ankle motion. As advances in biomechanicalmethods continue and more cliniciansinclude quantitative techniques in theirroutine evaluations, however, more insight into dynamic foot and ankle function will be provided.

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Volume 68 / Number 12, December 1988 1830








1988; 68:1822-1830.PHYS THER.

Mary M RodgersDuring Walking and RunningDynamic Biomechanics of the Normal Foot and Ankle

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