Recent studies have linked plantarflexion moment arm (pfMA) with locomotor function in athletes and elderly adults. Simple biomechanical and computer models suggest that shorter pfMA facilitates energy storage in the Achilles tendon and force generation by the plantarflexors, but no experimental data have been published in support of these modeling results. Though previous investigators found that sprinters possess significantly shorter pfMA than nonsprinters, they did not identify a mechanism responsible for this difference. If sprinters have uniquely shaped joints that provide them with a competitive advantage, this may provide insight to helping individuals with movement disorders. The purpose of this dissertation was to investigate potential mechanisms that link pfMA and locomotor performance.

The first study was designed to identify if ankle joint kinematics differ between sprinters and non-sprinters, and if so, how talocrural geometry affects joint kinematics. Magnetic resonance (MR) images of the foot and ankle in sprinters and non-sprinters were acquired. We found that sprinters have longer forefoot bones and shorter pfMA than non-sprinters. Unlike previous studies that used indirect measurements to link pfMA with function, we identified that the center of rotation (CoR) of the talocrural joint was more posterior in sprinters. These findings are the first to document a difference in ankle joint kinematics in functionally different, but otherwise healthy, adults.

The second study analyzed the three-dimensional geometry of the talocrural joints in sprinters and non-sprinters to determine if CoR differences could be explained by joint structure. We found that sprinters have a less conforming talocrural joint than non-sprinters, which may facilitate muscle force for the task at hand.

The third study investigated the relationship between pfMA and ankle strength in untrained healthy young men. We found that pfMA is positively correlated with ankle strength at all rates of plantarflexion (R² = 0.323 – 0.494). Although these findings do not agree with computer simulations that show short pfMA as protective against torque loss at fast speeds, it is possible that musculoskeletal structure in untrained young men is adapted for submaximal activities and not maximal force generation. To our knowledge, this is the first study to link muscle moment arm and joint strength.

The fourth study directly compared several methods for measuring pfMA in vivo that have been reported in the literature but have not yet been validated. These measurement techniques can be categorized into two subtypes:​ geometric methods, which use MR imaging or external measurements;​ and tendon excursion (TE) methods, which use an ultrasound probe to track the Achilles tendon as it slides during joint rotation. These methods were compared to a highly reliable and commonly used pfMA measurement technique. We found that geometric measures of pfMA have better agreement with the CoR method.

In conclusion, this dissertation is the first study to link pfMA and ankle strength. Previous reports found relationships between pfMA and locomotor performance but were unable to identify a mechanism responsible for differences in function. We also found that shorter pfMA in sprinters are explained by differences in ankle CoR. Three-dimensional analysis of the talocrural joint suggests that a more mobile ankle joint in sprinters may place the foot in a more advantageous position to generate force. While the findings of this dissertation fills some gaps in the literature, they raise further questions about how the musculoskeletal system adapts to geometric constraints and functional demands placed on the system.