biomechanics: a bridge builder among the sport sciences

Vol. 22, No. 5 Printed in U.S.A.

In 1987, Peter R. Cavanagh, Ph.D. presented the Wolfe Lecture at the Annual Meeting of the American College of Sports Medicine in Las Vegas. The people inattendance were duly impressed by Dr. Cavanagh's multimedia presentation and the possibilities of interaction that exist between the biomechanist and the phys-iologist. In this age of reductionism, I encouraged Dr. Cavanagh to provide our members a documentation of the integration of two major areas of exercise sciencein which one can identify a multitude of functional questions. This manuscript is a result of this request; despite its lateness of presentation, I believe it remains vitally important and correctly pertinent to all our members.

Peter B. Raven Editor-in-Chief

J.B. WOLFFE MEMORIAL LECTURE

Biomechanics: a bridge builder among thesport sciences

PETER R. CAVANAGH

Center for Locomotion Studies, Penn State University, University Park, PA 16802

I N T R O D U C T I O N

Sports medicine has experienced a rapid evolution over the last several decades, and, during its brief his-tory, it has meant many different things to different people. In the medical community, sports medicine wassynonymous for many years with orthopedic surgery, as evidenced by the American Journal of Sports Medicine, which is an exclusively orthopedic journal. In its early years , the Amer ican Col lege o f Spor t s Medic ine (ACSM) was primarily an organization of exercise phys-iologists who found sports act ivi t ies to be a useful paradigm for their experimental efforts, which would more correctly be described as exercise and sport sci -ence rather than sports medicine. More recently, the delineation between sport science and sports medicine has become sharper as many medical specialty societies

have added sports medicine sections and the individualsport sciences themselves have formed single disciplinegroups. For its part, the ACSM has broadened its scope to include both the scientific and medical communities in an attempt to provide a single forum in which those with varying backgrounds can interact.

In this atmosphere of multidisciplinary contributions

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MEDICINE AND SCIENCE IN SPORTS AND EXERCISE Copyright m 1990 by the American College of Sports Medicine

to a single area, there is a need for each of the subdis-ciplines to have an awareness of the content and bound-aries of all the other related areas in order to maximizethe possibilities for cooperative research. After a decadeof moving apart, many sport science researchers are realizing that progress can best be made by emphasizingthe linkages between their areas. The present paper is offered as a contribution to that end 'and presents a brief history of the development of sports biomechanicstogether with comments on some current issues in thearea. Sports biomechanics is an extremely broad field,and I hope to show, by using locomotor biomechanicsas an example, that there is indeed linkage between biomechanics and many other areas of endeavor of sport science and sports medicine.

H I S T O R I C A L C O N T E X T

The Greeks, as we know from their vase paintings, were fascinated by the human body in motion, and they left a record of both distance runners and sprinters in graphic form. The . reason for the apparent artist ic license expressed in the unopposed representation of the arms ("frontalism") is open to debate, but some of

BIOMECHANICS: A BRIDGE BUILDER AMONG THE SPORT SCIENCES 547

the earliest written statements on locomotor mechanicsare easier to interpret. Aristotle wrote a treatise entitled"Animal Motion " which contained sections on both human and animal gait (32,70). Using the sun as his transducer, he pointed out that, if a person were to walk parallel to a wall in sunshine, the line described by the shadow of his or her head would not be straight but "zigzag." No estimates of this vertical oscillation were made, and Aristotle's contributions to the me-chanics of gait could best be described as structural andphilosophical. For the next significant contribution, as in so many areas in science, we have to move to the Renaissance and to the time of Galileo Galilei, who was quoted as saying, "The universe is written in the language of mathematics, without which it is impossibleto understand a single word." It is the writings of one of Galileo 's contemporaries, Giovanni Borelli, that we find the first use of mechanics to study animal structureand animal motion (4,47).

Although Leonardo da Vinci is best known as an artist and inventor, he was also an anatomist of some note, producing illustrations which have yet to be ri -valed (60). His observations on human motion are less well known, and it is surprising to find that he was intuitively aware of Newton's third law. He pointed outthat, if a man were to leap off a plank supported by two logs, the man would stay in the same place and the plank would accelerate (60). He also contemplated grade locomotion, running into the wind, and con-ducted studies of standing and stepping, pointing out where the projection of the center of gravity fell in relation to the base of support (49).

Etienne Jules Marey must be considered as the fatherof kinematic investigations in locomotor biomechanics(57). Above all, he was an innovator who led a school of workers in the construction of devices for the study of human motion. These instruments enabled questionsthat had been beyond the reach of previous investiga-tors, such as the Weber Brothers (78), to be answered. He built the first force platform, a device to allow the visualization of the forces between the foot and the floor, and used accelerometers and air current foot switches that recorded their signals on clockwork chartrecorders carried by the subject. Cinematographers in both the scientific and commercial arenas count Mareyas the wellspring of their very considerable heritage (57). One gets the impression that Marey was consid-ered by his scientific contemporaries rather as we wouldregard the inventor of magnetic resonance imaging. Hisnew techniques found application in many fields, and he was in great demand in areas as diverse as cardiac physiology and lizard locomotion. But he was more than a gadgeteer and demonstrated insight into con-cepts such as work and energy, and storage of elastic energy in muscles and tendons, that have only now begun to be reexamined (3,12).

Eadweard Muybridge was an adventurer and a pho-tographer rather than a scientist, yet he made a lasting contribution to the study of human movement. He is perhaps most famous for his studies of Governor Ley-land Stanford's trotting horses, but his true legacy can be found in three volumes of studies of human and animal motion that he thought would make his fortune (65). In this day of ubiquitous instant replay and stop action, it is hard for us to imagine never having seen the phases of a runner 's gait in slow motion. Muybridgeenjoyed pointing out in his lectures that equestrian statues in some of the world's most famous squares were actually incorrect in their sequencing of support and swing of the four extremities. His work allowed both scientists and artists to gain insight into critical aspects of normal and pathological gait.

It is a rather sad commentary on human nature that war and conflict have been powerful stimuli to progressin biomechanical analysis. A notable early example is provided by Braune and Fischer (6), who, while workingfor the Prussian army, developed techniques to study the center of gravity of the human body under a varietyof conditions. They went on to publish the first true "engineering analysis" of the lower extremity during gait, concluding from an analysis of joint moments thatthe Weber Brothers had been wrong in their hypothesisthat the limb swung passively like a pendulum during walking (7). The limiting factor to progress was the sheer volume of data that had to be analyzed and processed. Braune and Fischer recount how the analysisof data pertaining to a single step of walking took themthousands of hours to complete.

World War I (1914–1918) left the European nations and the Americas with thousands of disabled veterans. A landmark volume published in 1919 by Jules Amar dealt with the redeployment and re-education of dis-abled veterans based on biomechanical evaluation of their gait and task performance. Amar's work marked the end of the purely kinematic phase of experimenta-tion as he demonstrated the far greater insight that could be obta ined by applying both the force and motion measurement techniques that Marey had de-veloped.

In the early decades of the 20th century, scientists that most people would associate with muscle mechan-ics made significant contributions to locomotor bio-mechanics. Wallace Fenn, like his mentor A. V. Hill, was deeply interested in human motion. Using photo-graphic means, Fenn was able to do the first calculationsof kinetic factors in running (33,34). He calculated the internal work done by changing the velocity of the limbs, and he showed that this is the overwhelming component of energy expenditure. He calculated a 1.68HP output for accelerating the limbs compared to 0.1 HP for doing work against gravity. Fenn's work was truly pioneering and was virtually ignored until the last

548 MEDICINE AND SCIENCE IN SPORTS AND EXERCISE

20 yr. Another muscle scientist who made major con-tributions to locomotor biomechanics was Herbert Elft-man, who, in a landmark series of papers (28-30), was able to argue his way from joint kinematics to the muscular level. In a very creative paper, he hypothe-sized that considerable energy savings result when multi joint muscles perform certain tasks compared to the action of one-joint-equivalent muscles (31).

Major developments in biomechanics were again stimulated by armed conflict as the U.S. National Re-search Council committed significant funding to bio-mechanics research after World War II in order to develop prosthetic limbs for the many amputees (50). The University of California, Berkeley was the focal point of this activity, and the work by Inman and his colleagues advanced the science of biomechanics out ofall proportion to the brief period it took for them to produce their results. In particular, the consideration ofmovement in three dimensions led to the first calcula-tion of the three components of joint forces and mo-ments and to the development of the strain gauge forceplatform by Bresler and Frankel (8).

A mention of the contributions of Wilfred Dempster brings us to an endpoint in this brief historical review. In a report called The Space Requirements of the Seated Operator (25), Dempster provided information on the masses, mass distributions, and volumes of body seg-ments which has only recently been superseded. His characterization of the body as a series of linked body segments with finite mechanical properties influenced a generation of workers in biomechanics who found themselves now able to model body segment motion using the techniques of conventional mechanics.

Following this rather brief survey of forces that have shaped the discipline of sports biomechanics as we know it today, the remainder of the article will discuss a number of areas that are fertile topics of study, with

a particular emphasis on linkages between biomechanics and other sport science disciplines.

K I N E M A T I C A P P R O A C H E S

Kinematics is defined as the description of the move-ments of segments of the body in space without regard to the forces and moments that caused the movement to occur. Those outside the discipline may be forgiven for the impression (albeit mistaken) that biomechanics is simply taking high speed film or video, since historically there has been a tendency to emphasize descriptive rather than mechanist ic approaches. Elaborate and expensive systems are now available to obtain the three-dimensional spatial coordinates of body landmarks using techniques that have been borrowed from the field of close-range photogrammetry (1). It is possible to obtain in a few minutes results that took Braune and Fischer many years. Yet the critical question of what to do with kinematic data has sometimes been ignored in the face of fascination with the data collection process.

There are a number of levels on which kinematic data can be useful in their own right. Level one analysis involves the simple reconstruction of the motion of points of the body in space, and even this s imple approach can sometimes be revealing. In the left panel of Figure 1, for example, the path of a marker on the radial styloid processes is shown in a world class 10,000 m runner. It is apparent that there is an asymmetrical arm action with more motion on the right than on the left. The patterns of vertical oscillation in two 10,000 m Olympic runners, shown in the right panel of Figure 1, indicate that, while one runner is fairly symmetrical, the other could be characterized as having one limb that acts as a strut during the support phase while the

Figure 1—Example of a purely kinematic de-scription of movements of individual targets on the body during running. Left panel: Points on the left and right radial styloids of an elite distance runner traced out in the frontal plane over several cycles of running. Note the asymmetry between the mediolateral movements on the two sides. Right panel: The path of markers on the head in the sagittal plane of two Olympic 10,000 m participants during one cycle of treadmill running. Note the asymmet ry and g rea te r ve r t i ca l oscillation of athlete A.


contralateral limb acts as a shock absorber. Such obser-vations are really just indicators of problems that shouldbe further examined, and they cannot really be consid-ered as results per se. They indicate effect but not cause.

Level two analysis involves a consideration of the motion of anatomical joints, and one of the most effective presentations is an angle/angle diagram devel-oped by Grieve (41). The line enclosing the shaded areain Figure 2 shows the coordinated pattern of motion ofthe hip and the knee during distance running in a groupof elite runners. The other loop shows the contrasting pattern of movement of the late Steve Prefontaine, whose marked knee flexion during swing distinguishedhis pattern from that of his colleagues. These displays have proven useful in fields as diverse as veterinary anatomy (76) and physical medicine (53), and they provide the first example of linkage between biome-chanics and other disciplines. In the context of sport there is also a clear linkage here between biomechanicsand coaching, where quantities that the biomechanist can measure may be useful to a coach as feedback to the athlete. But the question must be raised as to whether this is science or perhaps "high tech coaching".If we are simply providing information at a rate greaterthan that at which the coach can assimilate using his or her own eye, then such activity does not justify the label of science.

Figure 3 shows an extension of the kinematic data described above to include kinetics—the calculation offorces and moments. The two distance runners depictedhere from the work of Hinrichs et al. (44) clearly have different arm actions. That of subject A is limited, whilesubject B has a marked and vigorous arm action. Feed-back at the kinematic level would simply describe the

Thigh Angle (deg)

-40 -20 0 20 40 60

Figure 2—Thigh-knee diagrams for a group of elite distance runners (shaded area) and for the late Steve Prefontaine running at 4.96 m s"' on a treadmill. The exaggerated swing phase knee flexion was a characteristic of Prefontaine's gait.

arm action, its extent and speed. However, if the kine-matic data are processed to obtain derivat ives and complemented by information on segmental masses, we can show that the actual contribution to vertical lift by the arms of the athlete with the vigorous action wasalmost four times greater than that of the other athlete.Kinematic data were not the endpoint here, and the analysis has reached outside the domain of description to produce a result which has some mechanical validity.

M U S C L E M E C H A N I C S

Every student of sport science has, at some stage of his or her career, encountered the issues of the force-length and force-velocity relationships in skeletal mus-cle and this is usually presented based on the classical in vitro experiments of Hill and his colleagues (2). The question that biomechanists have begun to pose is "what relevance do these classical curves have for in vivo human movement?" There are a number of ways to extrapolate from kinematic information and EMG data toward a description of in vivo muscle action, but they all rest on the establishment of a relationship between muscle length and joint angle. First proposed by Elftman (29), this notion has been explored by Grieve et al. (42) and Frigo and Pedotti (36) in humans and by Walmsley et al. (77) in an animal model. Based on measurements in articulated cadaveric specimens, a polynomial of appropriate order can be obtained to predict overall muscle length as a function of the anglesof the joints which the muscle crosses. Once joint anglesare measured during a movement of interest, muscle length and rate of change of length can be estimated instantaneously. Quantitative electromyography pro-vides the final link which is needed to establish the details of muscle action. Some investigators have pro-gressed further to the direct measurement of in vivo tendon forces in both animals (77) and humans (40,51).An approach such as that described above allows us to detect some underlying principles of in vivo muscle action. It is apparent from Figure 4 that, during distancerunning, gastrocnemius muscle activity builds up in theregion of stretch and decays before shortening occurs. Although interpretation is complicated by the issue of electromechanical delay (38), these data are confirma-tion of the "stretch-shorten cycle" hypothesis (3,38), which states that muscles typically actively lengthen and then sometimes actively shorten.

The universality of this kind of muscle action in human and animal movement leads me to believe that the concentric actions of muscle have been overempha-sized in both anatomical and biomechanical texts. We refer to the quadriceps as knee extensors—when they are just as frequently controllers of knee flexion. As wehave seen in Figure 4, the gastrocnemius appears to act

MEDICINE AND SCIENCE IN SPORTS AND EXERCISE

Subject

A

Figure 3—Stick diagrams and the net lift due to the arms (in units of body weight) for a complete cycle of over -ground running in two subjects with contrast ing arm act ions. Note the higher l i f t forces resulting from the more exaggerated arm action of subject B.

in running as a controller of dorsiflexion rather than as a plantar flexor. This emphasis on the concentric action of muscles has led us to use the verb "to contract" for all muscle actions even when they involve maintaining constant length or even lengthening (14). The terms "eccentric contraction" and "isometric contraction" are oxymorons, and in my opinion it is time, as far as human motion is concerned, that we did away with the word "contraction" in favor of the word "action". There is no conflict—either linguistic or mechanistic—with the terms "concentric action", "eccentric action", and "isometric action".

The linkage here among biomechanics, muscle phys-iology, and neurophysiology is an important one since the solution of current problems—such as delayed onsetof muscle soreness following eccentric muscle action—can only be well understood by an interdisciplinary approach. Some of our own recent work in the area of downhill running (10) has shown that, curiously, there was no significant difference between the peak extensormoment or the peak power absorption at the knee between level and 8.3% downhill running. Negative work done on the knee extensors was, however, greater in the downhill condition, as were both power and work at the ankle. Thus, at least at the knee, work would appear to be more important than either peak torque or peak power in producing muscle soreness which is known to result from this exercise.

E C O N O M Y O F M O T I O N

The formulation of an adequate model for the economy of motion is one of the fundamental unsolved problems in the sport sciences. Many authors have

CONCENTRIC dL/dt ECCENTRIC

Figure 4—The rate of change of length of the gastrocnemius muscle (in units of muscle length per second) plotted as a function of integrated EMG (in arbitrary units) for the phase of the cycle when significant EMG activity occurred. Data are for a s ingle subject running on a treadmill at 3.8 m • s ', and the numbers on the curve are one-twentieths of the cycle. The plot begins just before footstrike (nineteen-twentieths of the cycle). Note that the majority of the activity occurs during eccentric muscle action.

demonstrated that there can be up to a 30% difference among fit individuals in the energetic cost of locomotion, even when it is expressed per kilogram of body weight per minute (22,79). There are also a number of paradoxical observations in the literature, including those of Dawson and Taylor (24), who showed that kangaroos actually decrease their oxygen cost-of-trans-port as they run faster. Maloiy et al. (56) have shown that certain African women are able to carry large head loads without increasing the metabolic energy cost of the task.

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The term efficiency has been deliberately avoided in the above accounts although it is widely used in both the biomechanical and physiological literature. Effi -ciency is typically defined as power output divided by power input, and, while the power input is well char-acterized by oxygen consumption, biomechanists have been unable to define the power output within an acceptable error margin (16). Table 1 shows estimates of the power output during running at 3.6 m s -' cal-culated by six different authors (19). The estimates spana ten-fold range from 170 W to 1700 W. An examina-tion of the source of such a large variation indicates that at least four major factors contribute to the error. These are the s torage of e las t ic energy, in ter -segmental transfer of energy, the cost of negative vs positive work,and the numerical methods used in the calculations. Many of these issues have been discussed elsewhere (11,15), but we are still not close to understanding the causes of variations in energy cost during the perform-ance of the same task; nor are we able to intervene to change the cost in individuals judged to be "uneconom-ical"—except in the case of stride length where progresshas been made (18). Following rather mixed results from feedback experiments designed to improve econ-omy (62), we have recently performed an experiment to determine the extent to which economy can be degraded (27). We could only degrade economy by 4% with the arms held behind the back and by 4.6% when the subject 's vertical oscil lation was dramatically in -creased. These results indicate that, although running economy can be negatively influenced by running style,the magnitude of changes for such marked disruption of style is surprisingly low. This study would appear to place upper boundaries on improvements that can be expected from alterations in running mechanics.

Human economy is a classic example of a multi-factorial problem, where an individual probably has set points in the biochemical, biomechanical, psycho-motor, physiological, and environmental domains. Economy is not just a biomechanical problem, and, until we exploit the cross-disciplinary linkages that thisproblem offers, a solution will continue to be elusive.

TABLE 1. Estimates of the work done during running between 3.3 and 3.9 m • s-'

from seven different groups of authors.Authors Method Power Estimate (W)

Fukunaga et al. (37) 1 343Cavanagh et al. (17) 2 556Norman et al. (69) 3 172Gregor and Kirkendall (39) 3 163Luhtanen and Komi (54) 3 1650Williams and Cavanagh (80) 1-4 273-1775Martin and Morgan (59) 1, 4 10-17 x (BW in kg)

Methods used: 1. Center of mass (CoM) alone.2. CoM plus movement of limbs relative to CoM.3. Pseudowork.4. Segmental analysis (including energy transfer).

I N J U R Y E T I O L O G Y , T R E A T M E N T , A N D P R E V E N T I O N

Many running injuries are considered to be the result of repetitive microtrauma, which eventually results in the overuse phenomena that have become so familiar to those who treat runners (9) . While the e lbow and shoulder are the nemeses of the tennis player and gymnast, respectively, the patellofemoral joint is the overwhelming site of injury in runners (20). Patellofem-oral pain has been widely hypothesized to occur in runners who pronate (46), based principally on the "mitered hinge" theory of the subtalar joint proposed by Inman (45) and his colleagues. This theory (which has recently been challenged o n t h e b a s i s o f t h r e e -dimensional stereoroentgenography (55)) suggests that subtalar joint pronation results in concurrent internal rotation of the tibia, consequent axial rotation of the tibiofemoral joint, and a resulting tendency for the patella to sublux or dislocate laterally.

There have been few attempts at experimental con-firmation of this hypothesis although it is regarded almost as an "absolute truth" in many circles. The reason for the lack of attention is that conventional kinematic data collection methods employed to deter-mine patellar motion are doomed to failure since skin markers place over the patella will move almost independently of the underlying bony motion. Lafortune (52) attempted a novel solution to this problem by inserting Steinman pins into the femur, t ibia, and patella of volunteer subjects. The three-dimensional co-ordinates of targets attached to these pins were deter-mined, and, with the appropriate x-ray measurements and coord ina te transformations, the relative movement of the bones of the knee joint could be determined continuously during gait. Although the subjects did not pronate excessively, they wore shoes with 10 degree valgus wedges to enforce pronation and, for comparison, shoes with 10 degree varus wedges to mainta in supination during stance (Fig. 5).

Analysis revealed that internal femoral rotation in five subjects as a result of the 20 degree perturbation at the foot resulted in only a 3 degree difference in peak femoral internal rotation. Clearly a great deal of com-pensation was going on between the hip joint and the foot. The critical observation, however, was that the patella did indeed deviate more laterally in relation to the femur during walking in the valgus shoes, but only by an average of 1.6 mm. It may be that this small amount of additional motion is enough to contribute to patellofemoral joint pain, and, if that is the case, then clearly the techniques employed to measure clinically relevant interventions at the knee will need to be more precise than was previously thought necessary. It must be recalled that the perturbation employed in


Figure 5—A schematic view looking distally down the shaft of the

femur, to show changes in the maximum lateral shift of the patella during walking in shoes with a valgus tilt (bottom left) and a varus tilt (bottom right).

Lafortune's experiment was much larger than might typically be used in, for example, an in-shoe orthotic device (73). It is apparently unrealistic to expect that detectable changes in knee joint motion will result fromsmall interventions inside the shoe.

The linkage between these types of biomechanical experiments and orthopedic sports medicine is a very direct one. In the next decade we can anticipate an even greater marriage between biomechanics and or-thopedics as each group recognizes the benefit that it can derive from the other 's expertise. While biome-chanical research has made a significant contribution to orthopedics, biomechanical analyses have been slow to be assimilated into routine evaluation, and the reasons for this have been the subject of lively debate in the literature (5,72). The research community must assume responsibility to show clinical relevance in order to implement a migration of their techniques from the research laboratory to the clinic.

F O O T S T R U C T U R E A N D F U N C T I O N

The foot is one of the most variable anatomical structures in the body, ranging, in individuals of the same stature and weight, from an extreme planus foot which leaves a 180 cm2 footprint to a cavus foot with aprint less than half of that area. Such variations in structure are commonly accommodated out of necessity by a single design of athletic shoe, and it is not entirely clear how current concepts of static or structural fit have evolved . Al though notable s tudies of footmorphology did exist in the early literature (75), most shoe manufacturers in the past were content to store

their data concerning foot and shoe shape in physical rather than numerical form. That physical form was the "last" on which shoes were built, and we have in our own laboratory recently at tempted to quantify the dimensional relationship between the "last" and a foot that feels comfortable in the shoe made from that "last". A weight bearing cast of both the foot and an actual last w e r e d i g i t i z e d u s i n g a t h r e e -dimensional contactdigitizer. When corresponding "slices" of the two shapesare compared (Fig. 6), one finds that there are regions of the shoe where a good subjective fit implies that the foot is smaller than the shoe and vice versa. For ex-ample, in this particular subject there are almost 12 mm of space for the heel in the shoe, but in the forefoot the foot breadth exceeds that of the shoe by 7 mm. The subject reported that the shoe "fitted well" in all regions. Comfort in footwear is always prized above all else, and recent studies in foot morphology (71) have begun to

5 2 . 8 M M 4 0 . 4 M M

L ack of Fit =12.3 MMFigure 6—Digitized section of a shoe last (A) and the foot of a subject (B) who feels "comfortable" in a shoe built on the last. At each corresponding section, a comparison can be made indicating the degree of "lack of fit" between shoe and foot (C). At the particular heel section shown, the shoe was 12.3 mm wider than the foot.

Valgus Shoes Varus Shoes10°

C .


establish a quantitative basis for the definition of fit and comfort.

While variations in foot structure are readily apparentfrom simple observation, sports biomechanics has con-tributed significantly to the realization that foot function can also vary drastically. For example, some indi -viduals—for reasons that are not yet well understood—make first ground contact during running with the forefoot, while others strike with the rearfoot (Fig. 7). The ground reaction force and center of pressure sig-natures are characteristically different in these two groups (64,68). The classification of runners as rearfoot,midfoot, or forefoot strikers has traditionally been madebased on center of pressure measurements from a con-ventional force platform. This technique is somewhat limited because the center of pressure is an average measurement which could be in the center of the foot either because there were equal forces in both the anterior and posterior regions of the foot or as a result of a locally applied force (13). These situations would need to be treated very differently by the shoe designer, and thus new techniques have emerged in recent years to measure the entire plantar pressure distribution (26,67). Summary pressure distributions showing the largest pressures that were applied to the plantar aspect of the foot during midfoot and rearfoot running are shown in Figure 8. As expected, we see that the rearfoot striker makes high demands of the shoe in both the rearfoot and forefoot regions in contrast to the forefoot striker, who makes only minimal demands of the rear-foot region.

To dramatize the need for shoe designs that are basedon function as well as morphology, the somewhat tongue-in-cheek design shown in Figure 9 is presented. If the individual whose pressure distribution pattern is shown in Figure 8B were running on level ground, it would be possible for him or her to wear a shoe which

Figure 7—Center of pressure plots and strike indices during running at the same speed for a rearfoot striker (Panel A) and a forefoot striker

(Panel B). The strike index is calculated as (A/B) x 100, where A is the heel to first contact distance and B is shoe length.

was entirely without a rearfoot midsole component. This would save about 100 g in weight and possibly 1-2% in energy cost (35). While such a shoe has never been seriously considered, the design of other athletic shoes to "fit" function and not just structure represents a major contr ibut ion of biomechanics to sports equipment des ign . This l inkage be tween b iomechanics and equipment design—which the Europeans have called sports ergonomics—is a powerful one, and many recent sports biomechanics graduates have turned toward in-dustry rather than academia.

This area of endeavor has also taken many biomech-anicians back to their disciplinary roots and provided further linkage between sports biomechanics and the classical engineering disciplines. Sports equipment de-forms under the application of forces according to the same rules as any other solid structure. Modern engi-neering techniques such as finite element modeling (81)are useful in solving otherwise intractable problems. This technique has already proven useful in other fields of biomechanics (such as orthopedic biomechanics). Recent efforts in our own laboratory (61) have modeled the deformation of the rear part of a running shoe under the action of a pressure distribution such as that shown in Figure 8A. The resulting model, shown in Figure 10, offers an extremely powerful tool for evaluating design modifications that would otherwise require empirical testing.

L O C O M O T I O N I N S P A C E

From the early days of space exploration, biome-chanics made a significant contribution toward loco-motion in space. Kane and Scher (48) predicted that, by certain maneuvers of the upper extremities, an astronaut suspended in a weightless environment could reorient his entire body. These hypotheses were con-firmed by direct experiments on Skylab. In 1964 Margaria and Cavagna (58) suggested that it would be possible to move on the moon at speeds of 50 kmh-', but they hypothesized that walking or running would not be the best form of locomotion. Their suggestion tha t "bounding would be bes t" was dramat ica l ly confirmed as the images of Neil Armstrong bounding across the surface of the moon in 1969 rather like a kangaroo were to become etched in our memories.

The biological effects of weightlessness have been widely discussed in the literature (21), and cardiovas-cular deconditioning, decreases in blood volume, muscle atrophy, and bone demineralization (66) have all been identified as major concerns for long duration space flight. Until recently biomechanics at the macro level had little to add on these issues. Bone demineralization in particular was thought to be a factor that had

A. B.

11%


Rearfoot Striker

Forefoot StrikerFigure 9—Two shoe designs that would theoretically serve the needs of the styles shown in Figure 8 during level running. The shoe for the forefoot striker would save approximately 100 g in weight and result in a lower energy expenditure during running. This rather tongue-in-cheek design is intended to indicate that shoes can be designed for function and not just fit and comfort.

an endocrine or dietary etiology. Lately, however, more attention has been given to the hypothesis that the absence of locomotor-like stresses applied on the lower limbs may contribute to bone demineralization. Thorn-ton (74) has pointed out that , in space, the upper extremities become the propulsive organs while the lower extremities are simply used for perching. A num-ber of attempts have been made to introduce locomotor-l ike exercise into a weightless environment (63), probably the most successful of which has been a teth-ered treadmill where gravity is replaced by bungee cordswhich accelerate the body back toward the treadmill surface. A treadmill has already flown many missions on the Space Shuttle (Fig. 11), and Thornton became the first man to run around the world as he ran on the device f o r a c o m p l e t e o r b i t . T o date, no biomechanical experiments have been conducted during a mission although there is a pressing need to determine what kind of mechanical stress different forms of exercise

Figure 10—The result of applying a set of nodal loads to a mathematical model of the rear part of a running shoe. The finite element method was used to predict the deformed geometry assuming material properties, geometry, and loading conditions. The gradation of shading shows the amount of deformation experienced by the various regions of the shoe.

Figure 11—A sketch from a frame of film taken on the Shuttle as an astronaut ran on a tethered treadmill. Treadmill exercise may be an important countermeasure to changes in bone mineral during pro-longed space flight.

can generate in weightlessness. It is important to distin-guish the effects of load per se from the effects of different rates of change of load. This may be critical because different forms of exercise such as rowing, cycling, or running probably offer skeletal structures radically different profiles of load and rate of change ofload. It is not at all clear yet which of these factors is the more important in preventing bone mineral loss

Figure 8—Plantar pressure distributions for the same individual running with a rearfoot strike (A) and a forefoot strike (B). The contour interval is 75 kPa.

≤0.46mm

≤0.91mm

≤1.37mm ≤1.82mm

>1.82 mm


Figure 12—A supine treadmill simulation of microgravity being de-veloped at Penn State based on previous work of Grigor'yev et al. (43). Constant force springs are attached to the segmental centers of mass, and the body is accelerated to the treadmill by elastic restraints. Measurements are made of the force in the suspension cables and the ground reaction forces. Suspension is shown only on the left-hand side of the body for simplicity.

and how either factor interacts with frequency of load application (74).

An alternative to the costly process of experimenta-tion during an actual mission is the simulation of zero gravity on earth, and this has been attempted in many different and creative ways in the last 30 yr. (23). Our own current work involves the construction of a supinetreadmill shown schematically in Figure 12 in a mannersimilar to that used in the Soviet space program for cosmonaut training (43). The subject is suspended by constant force springs at the segmental centers of massand is accelerated to a wall mounted treadmill by bungee cords in a manner similar to the harness used on the Shuttle treadmill. By instrumenting the suspen-sion cords, the treadmill, and the subject, various ex-ercise protocols can be examined to determine the nature of the loads transmitted to the body. The bio-mechanical objective of exercise in space must clearlybe to maximize the appropriate loading parameters that

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CONCLUDING REMARKS

Although limited to studies of locomotion, this re-view of applications and linkages in sports biomechanics has been deliberately broad in order to demonstrate that biomechanics has tremendous potential across the spectrum of sport science. Inevitably, breadth can be misinterpreted as superficiality, but it is the author's hope that an appreciation of the many linkages that exist may encourage sport scientists in other subdisciplines to include their biomechanist colleagues in a team approach to the solutions of important questions in the field. Although the roots of biomechanics in history are firm, it is fair to say that sports biomechanics is still a science in its adolescence, and therefore there is much credibility to be lost by claiming that we can do too much too soon. Spor t b iomechan i s t s shou ld be consc ious o f the boundaries of their discipline and resourceful in striving to extend them.

The material presented in this paper is the result of collaboration with many colleagues and students at Penn State University. Their contributions are gratefully acknowledged. The assistance of Irene McClay, Monica Milliron, and David Sims during the presentation of the original lecture is recalled with appreciation and amazement that audiovisual technology can operate in a trouble-free manner at an important time.

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