Functional characteristics of the calf muscles of the rat.

Length-force relations, both active and passive, and twitch contraction characteristics were quantified for the entire complex of the superficial calf muscles, as well as individually for the Mm. soleus, plantaris, and gastrocnemius, caput mediale and laterale, of eight male Wistar rats. The M. soleus composes approximately 5% of the weight and cross-sectional area of the entire group of superficial calf muscles and is the only muscle of the group containing mainly slow-twitch fibers. The other superficial muscles of the calf are primarily fast-twitch muscles. The mono-articular M. soleus, the bi-articular M. gastrocnemius, caput mediale and laterale, and the poly-articular M. plantaris differ with respect to the number of joints crossed. However, contrary to the findings for cat hind limbs (Goslow et al. [1977] J. Morphol. 153:23-38), the muscles of the complex of superficial calf muscles of the rat did not differ with respect to a) their fiber optimum length, b) their maximum length range of active force generation, c) the relative increase of passive force owing to lengthening of the muscle, d) the angle of the ankle at which they produce maximal active force (the knee angle was fixed at 90 degrees).