The plantarflexor muscles play a critical role in the successful performance of many motor tasks performed by humans. Foot and ankle structure determines the leverage of the plantarflexors and thus affects how the plantarflexor muscles function. Variation in foot and ankle structure across individuals suggests that certain individuals are better adapted to perform certain tasks, since tasks vary with respect to the requirements placed on the muscles. Previous studies have shown that subjects who specialize in performance of certain activities have foot and ankle structure and muscle-tendon force-generating properties that are different in many respects from those of controls. Results have, however, occasionally not been consistent across studies and the mechanisms that explain how variation in structure affects performance are not always clear. The purpose of this dissertation was to investigate how variation in several ankle and foot structural properties allow for optimal performance in various motor tasks.

In our first study a computational model was created to study maximal-height single-joint ankle jumping. Simulations suggested that bouncing was the optimal strategy for this task, and the best human subject jumpers also jumped highest when bouncing. The model showed that a bouncing strategy allowed for increased elastic energy storage in the plantarflexor tendon. The frequencies of bouncing employed by the computational model and by the best human jumpers further suggest that subjects made use of mechanical resonance to improve performance in this maximal-height explosive task. The exploitation of muscle resonance has been identified previously as a successful strategy in endurance activities requiring a large amount of metabolic energy. The use of resonance for maximizing performance in an explosive type of activity, however, has not been previously reported.

The second study was designed to evaluate the effect of variation of foot and ankle structure on performance in maximal-height single-joint jumping. In this study subjects employed a single upward thrust with no countermovement or bouncing allowed. Significant correlations were found between jump height and heel length, and jump height and toe length. Subjects with longer heels and longer toes jumped highest, although these subjects were not necessarily the largest subjects.

The third study aimed to explain the mechanism by which shorter heels could potentially reduce the metabolic cost in running. It has been suggested that distance runners with shorter heels experience increased plantarflexor muscle force, thereby increasing tendon energy storage and return and thus reducing metabolic cost. An inverse relationship was found between heel length and peak force, but neither heel length nor peak force were significantly correlated with rate of oxygen consumption during running. Increased tendon force could be associated with increased muscle activation which would cause an increase in metabolic cost, negating the benefit of increased energy storage and return.

In the fourth study we employed a modified version of the computational model from the first study to investigate how variation in foot and ankle structure influences performance in three different motor tasks:​ maximal vertical-energy pushoff, maximal horizontal acceleration, and maximal static load support. In both explosive movements, short heels and small normalized tendon lengths allowed for larger force production. High stiffness had a positive effect on performance in the maximal vertical-energy pushoff task, whereas a low stiffness improved performance in the maximal horizontal acceleration task. In the isometric task a long heel, small normalized tendon length, and high stiffness caused larger force production. Variation in toe length did not substantially affect performance. To our knowledge there had been no previous work investigating variation in multiple foot and ankle structural parameters and its influence on performance in multiple tasks.

In conclusion, this dissertation aimed to increase our understanding of the effect of foot and ankle structural variation on performance in different tasks. While our findings have expanded our knowledge of these effects, questions remain as to the exact nature of how variation in structure influences performance of different activities in different individuals.