The length dependency of maximum force development in rat medial gastrocnemius muscle in situ.

During many movements (e.g., running, jumping, and kicking) there is little time for skeletal muscles to build up force, thus rapid force development is important. The length dependency of isometric force development was investigated in maximally activated rat medial gastrocnemius muscles in situ with intact blood flow at 35 degrees C. Depending on time available for muscle activation, the length dependency of force development was expected to differ from that of the maximal isometric force (F(max)) reached much later during the contraction. During isometric force development in intact muscle-tendon preparations, the contractile elements actually shortened. Therefore, similar to previous findings on shortening contractions, it was hypothesized that maximal rate of force development (MRFD) would be obtained at a length below the optimum (L(o)) for maximal isometric force production. To measure the effect of the entire time history of activation, force time integrals (FTIs) for different activation times (10-50 ms) were also calculated. The highest MRFD was obtained 1.94 +/- 0.42 mm below (p < 0.05) L(o). When expressed relative to F(max) obtained at each individual length, the optimum was found at L(o) - 4.4 mm. For FTI 10 ms and FTI 20 ms, optimum length was obtained at approximately 2 and 1 mm above (p < 0.05) L(o), respectively, whereas the optima for FTI 30, 40, and 50 ms were approximately 1 mm below (p < 0.05) L(o). In addition, at short lengths (< L(o) - 4 mm) and for all activation times FTIs were relatively more decreased than F(max). In conclusion, length dependency of force output during rapid force development differed from that of maximal isometric force; specifically, MRFD was obtained 2 mm below L(o).