In vivo moment arm lengths for the Achilles tendon and tibialis anterior (TA) were determined in 10 adult male subjects. Moment arms were measured as the perpendicular distance between the joint center of rotation (CR) and the center of the muscle's tendon on a series of sagittal plane magnetic resonance images. The first set of calculations used a fixed CR and the second a moving CR. The position of the CR was determined using a modification of the graphical method of Reuleaux. For both moving and fixed CR conditions, moment arms increased by approximately 20% for the Achilles tendon and decreased by approximately 30% for the TA when the ankle moved from maximum dorsiflexion to maximum plantarflexion. Moment arms averaged 3.1% greater for the Achilles tendon and 2.5% greater for the TA when calculated using a fixed CR. These data suggest that the averaged moment arm lengths for the Achilles tendon and the TA were relatively unaffected by the use of a fixed versus moving CR.

In Part II of this dissertation the potential contributions of moment arm lengths, muscle lengths, velocities, and activation patterns of the soleus (SOL) and gastrocnemius (GA) were analyzed and compared to the net ankle muscle moment during cycling. Four adult male subjects rode a racing bicycle mounted on a Velodyne trainer at 250 W (90 rpm) for five minutes at three seat heights (96%, 102%, and 108% of greater trochanteric height). Normal and tangential pedal reaction forces were measured using pedals instrumented with dual piezoelectric force transducers. Net joint moments were calculated using inverse dynamics methods. Muscle lengths for the GA and SOL were calculated using the regression equations of Grieve et al. (1978). Muscle end point velocities were calculated from muscle lengths using a finite-difference algorithm. Magnetic resonance images were used to measure moment arm lengths. Peak plantarflexor moments were recorded during the second quadrant of the pedalling cycle and ranged from an average 28.2 Nm to 22.0 Nm from the lowest to highest seat position. In two subjects, peak plantarflexor moments were temporally in phase with maximum normal forces, muscle and moment arm lengths, and activation levels for the GA and SOL. In two other subjects, however, temporal uncoupling between peak plantarflexor moments and peak normal force occurred at both the 102 and 108% seat positions. This temporal uncoupling was not readily explained by ankle muscle mechanics and was attributed primarily to the joint and muscle mechanics at the knee. The results of this study suggested that the net muscle moments acting about the ankle during cycling are influenced by the proximal segments and, therefore, cannot be explained entirely by changes in the mechanics of the ankle joint musculature.