Abstract
The interaction between the semitendinosus muscle and both hip and knee joint angles was examined in the frog ( Rana pipiens) hindlimb. Sarcomere length was measured by laser diffraction in passive muscle during hip and knee rotation. A model was then developed to predict semitendinosus sarcomere length as a function of both hip and knee flexion angle. Based on published frog muscle fiber length-tension [Gordon, A. M. et al., J. Physiol. 184, 170–192 (1966)] and force-velocity [Edman, K. A. P., J. Physiol. 291, 143–159 (1979)] properties, and published joint angles during hopping [Calow, L. J. and Alexander, R. McN., J. Zool. (Lond.) 171, 293–321 (1973)], muscle sarcomere length, force and hip and knee torque during a hop were predicted. The semitendinosus muscle generally operated on the descending limb of the length-tension curve at normal joint angle combinations. The model predicted that, during a single coordinated movement, a period of sarcomere shortening (concentric) was followed by a period of sarcomere lengthening (eccentric). Based on calculated torque profiles at the hip and knee joints, this study suggested that the semitendinosus muscle probably functions more as a hip extensor than a knee flexor. In addition, based on the nature of the shortening-lengthening cycle, the semitendinosus may act to mechanically link the force of knee extension to hip extension.
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