Forces in gastrocnemius, soleus, and plantaris tendons of the freely moving cat.

The purpose of this study was to gain an insight into the mechanisms of force sharing among muscles in a functional group. Tendon force measurements were obtained simultaneously from gastrocnemius, soleus, and plantaris muscles of 10 cats during a variety of different locomotor tasks using strain gauge based force transducers. In particular, tendon forces were measured for conditions where movement speed was altered systematically, and where movement speed was kept constant but external resistance to walking was varied systematically. The results show that forces in the gastrocnemius and plantaris tendons increase with increasing intensities of movement, independent of intensity being altered by varying speed or external resistance. In contrast, peak soleus forces, on an average, remained nearly the same for all conditions; however, substantial modulations in soleus force were observed for consecutive stride cycles. These results suggest that soleus forces are not limited by peripheral (contractile) conditions but by central mechanisms and, further, that these central mechanisms depend on speed of movement and resistance to movement.