Pii: 0021-9290(90)90017-w

The interaction between the semitendinosus muscle and both hip and knee joint angles was examined in the frog (Ranu pip&s) 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 ;Ls a function of both hip and knee Rexion angle. Based on published frog muscle fiber length-tension [Gordon. A. M. YI al.. J. Phpiol. I&I. 170-192 (196611 and force-velocity [Edman, K. A. P.. J. Physiol. 291. 143-159 (197911 properties. and published joint angles during hopping [Calow. L. J. and Alexander. R. McN.. J. Zoul. (Land.) 171.293-321 (1973)]. muscle sarcomere length. force and hip and knee torque during a hop were predicted. The semirendinosus 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 calctdaled torque profiles at the hip and kncTjoints, this study suggested that the semitcndinosus mu.sclc probably functions more as a hip extensor than a knee flexor. In addition. based on rhc nature of the shortening-lengthening cycle. the semitendinosus may act 10 mechanically link rhe force of knee extension to hip extension.