Fig. 13

Soleus vs. medial gastrocnemius forces (Gastroc. Force) obtained by direct measurement in the cat during a variety of postural and movement tasks. Note, that variability of the force-sharing between these two muscles that occupies the entire solution space, and further note the task-specific nature of the force-sharing between these two muscles. Compare these experimentally observed results also to the common assumption that a muscle contributes force to a synergistic group in correspondence to its physiological cross-sectional area. In a cat, the physiological cross-sectional area of the soleus, and thus its maximal isometric force at optimal length, is approximately 20–25% of that of the medial gastrocnemius muscle. Nevertheless, the soleus produces substantially more force than the medial gastrocnemius for many static and dynamic tasks. (St = standing still, ps = paw shake, j = jumping (estimated from the peak forces), 0.4, 0.7 and 1.2 are the speeds of walking in m/s, 2.4 is the speed of running (trotting) at 2.4 m/s