Biomechanical assessment of athletic performance
by Gideon Ariel, Ph.D. Chairman, Division of Biomechanics and Computer Sciences, United States Olympic Sports Medicine Committee and Irving Dardik, M.D., F.A.C.S., Chairman, United States Olympic Sports Medicine Committee
(Editor's Note. In the first part of this two-part series the authors pointed out that with increasing international interest in competitive athletics, recreation and fitness, it was inevitable that computers would be introduced for the analysis of sports techniques.
The authors admitted that biomechanics is a science still in its adolescence with many discoveries yet to be made. They pointed out that the biomechanical research, reported in this second part, relied primarily on data obtained from high speed cinematography, force platforms, and specialized transducers for measuring body motion and forces.)
Long Distance Running Although long distance running is generally considered to be a cardiovascular event, the present study revealed that biomechanical factors are extremely important since cardiovascular demands depend on the individual's work output.
Running speed and the runner's work output depends on the stride length and frequency. Studies have suggested that one advantage to running with long strides is the resulting reduction in the number of strides per mile. However, our study indicated that each running stride is associated with a breaking force which stops the forward motion of the athlete. The larger the stride the greater the breaking force. This phenomenon is a function of the relationship of the location of the body's center of gravity with respect to the location of foot contact.
When the runner extends the forward leg, the contact point is
ahead of the body's center of gravity, a greater breaking force is produced resulting in less efficient running. A stride which is too small, of course, will require a faster leg motion and more strides per mile. It was calculated that each athlete has an optimal stride length with the breaking force at a minimum. Calculation of the precise relationship can improve running efficiency by as much as 20 percent as verified from energy measurement studies. Leaning forward slightly at the hip joint also contributed to running efficiency as did landing on the ball of the foot rather than on the heel - a common characteristic of efficient runners.
Angular displacement measurements at the knee and ankle joints revealed that running is associated with large amounts of elastic energy. The electrical potential of the muscles associated with running are activated prior to contact with the running surface. The muscular contraction is eccentric in nature absorbing kinetic energy in the same manner as bouncing a basketball. In other words, the better runner is the one who can absorb more kinetic energy in the elastic component. This concept is analogous to bouncing an overinflated basketball.
The average runner is less efficient in maximizing elastic energy storage which is like bounding a basketball which has lost some of the air. An under-inflated basketball requires a higher level of energy input yet results in a lower bounce..The average runner requires more energy per step while the elite athlete retains the elastic energy. It remains for future research to determine whether it is possible through training to acquire or increase the amount of elastic energy.
In general, the efficiency of the long distance runner depends on optimal stride length and stride frequency as well as on the capacity of the muscle to develop elastic absorbtion of physiological energy. Training should consist of endurance training as well as specific muscular training in order to develop the elastic capacity of the muscle. Calculations of the stride length-stride frequency relationship should be performed for each
runner in order to optimize the individual's style.
Sprint running The sprint is characterized by a ground contact time of short duration and a high velocity swing phase. Unlike long distance running, sprinting requires, in addition to stored elastic energy, dominant knee and hip extensors to product muscular energy which can be translated into mechanical energy.
Sprinting results in high forces on the contact foot which often reach levels as high as 7G. This foot contact produces rotational force at the hip joint,which must be countered by the opposite arm. Since the arm weight is approximately one fourth the weight of the leg, the arm must accelerate four times faster than the leg in order to counteract the leg's rotation. This fact yields interesting results - the sprinter's limiting factor is not leg speed but rather the arm speed. If the arm cannot accelerate with the leg in a properly coordinated synchronization, the runner automatically slows because of the neuromuscular inhibition. These findings suggest that the training routines of sprinters should include resistance training for the arms.
Another commonly accepted hypothesis is that the ratio of the knee extension muscles to the knee flexors should be approximately 60-40. However, such a ratio was found to be invalid under dynamic conditions with a dynamically correct ratio found to be approximately 50-50. This means that sprinters should develop their knee flexors as well as their knee extensors. Bdrzov, the USSR Olympic gold medalist, was found to have a 1.5 to 1 ratio of the knee flexors to extensors.
In general, successful sprinting results from particular genetic traits. However, proper training for the upper and lower body can optimize each runner's potential.
Kayak The American Kayakers have not performed as well as their rivals from other countries. The reason is not a limitation in the physical characteristics of the U.S. athletes. In fact, the American kayakers exhibit superior physical strength.
However, biomechanical analy-
10 THE OLYMPIAN ï¿½ FEBRUARY
sis of top American and foreign kayakers revealed that the pattern of the paddle is crucial in establishing efficient strokes. There are significant differences in the kayak strokes of the American compared with the non-Americans. For example, the Soviet kayakers reach maximum paddle acceleration after the paddle passes the perpendicular position. That means that the greatest paddle force was applied after rather than before reaching the perpendicular position. The American kayakers accelerated the paddle at the beginning of the stroke which means maximum force was applied in the front of the kayak. These differences are crucially important. If the force is applied in .the front of the kayak the resulting forces push the kayak upward causing significantly greater drag. If, on the other hand, the force is applied in the rear of the kayak the tip of the kayak is pushed down allowing a smoother and faster ride. Therefore, with less energy, the kayakers can achieve greater speed.
peans. Why? The reason may reflect the improved techniques which were developed by the winners. Bulgarians, Soviets and Germans have developed coordinated techniques allowing the lifters to get under the weight when the bar was at a lower point than the less successful lifters and they were still able to accelerate upward from the lower position. Based on our studies the U.S. athletes delayed getting under the bar until the bar had begun accelerating downward. This technique prevented the U.S. athlete from lifting greater loads since, once the weight was descending, the lifter had to overcome both the inertial forces and the weight of the bar.
In addition, the path of the weight of the Europeans was found to coincide with the path of the athlete's center of gravity. The American athletes demonstrated deviations from this center of gravity path resulting in inefficient performances.
Diving Diving is judged by esthetic as well as performance capabilities. The American athletes share successes in this event with
Weightlifting Once an event of other countries. However, a need American glory, the U.S. has lost for a defined base line for successits prowess to the Eastern Euro- ful performances exists.
For example, Greg Louganis has a unique technique which allows him to perform better than most divers. Our research findings revealed that Louganis' technique of absorbing kinetic energy in the diving board differs from the other divers who were tested. This technique incorporates a coordinated movement with Greg collapsing his knees before loading the board. At the same time his arms accelerated downward, a motion which caused the direction of the force to be upward and was counteracted by the collapsing knees. When he reached a knee joint angle of approximately 90 degrees he abruptly decelerated the body downward. This motion caused a loading of the diving board without additional body motion. At this point Louganis accelerated his arms upward in a highly coordinated manner. This movement created an additional downward force adding to the decelerating force of the body and increased the loading force on the diving board. When his arms reached approximately a horizontal position,
Louganis began to decelerate them. At the same instance that his arms began decelerating, the diving board started to unload with a high potential energy that was transferred to kinetic energy. At that point, Louganis prepared for the dive while the diving board provided the upward force. In other words, from this point Louganis was able to concentrate on only the diving stunt without being required to generate additional effort. Most other divers provided muscular forces throughout the dive - a phenomenon which is less efficient than Louganis' unique technique.
Performance profile - Back 1-1/2 layout.
Athletic achievement has emerged into the modern world of measurement and diagnostic expertise. With the engineering principles described by Newton and the rapid calculations provided by the computer, man and machines can lead athletic performance out of the dark ages of witchcraft and into the Renaissance of discovery. The information presented in this paper has briefly described the possibilities that exist for biomechanics and athletic performance.
The art of coaching man will certainly be enhanced by effective and timely utilization of modern medical and scientific techniques which through the efforts of the United States Olympic Sports Medicine Committee will ultimately be made available to coaches and athletes at all levels. '0,