Experimental determination of force-length relations of intact human gastrocnemius muscles.

The purpose of this study was to determine force-length relations of intact human gastrocnemius muscles experimentally. Changes in maximal active isometric forces of the gastrocnemius muscle were calculated from resultant ankle joint moments of carefully executed maximal effort contractions at systematically changing knee and ankle joint configurations. Changes in muscle lengths were obtained using second-order polynomial regression equations that relate the length of the gastrocnemius muscle-tendon unit to knee and ankle joint angles. From these changes in muscle lengths, changes in muscle fibre lengths were estimated based on assumptions about fibre structure and series elastic properties. The results indicate that intact human gastrocnemius muscles operate on the ascending limb of the force-length relation, and that passive forces are minimal within a normal range of ankle and knee joint configurations.