Rat medial gastrocnemius muscles produce maximal power at a length lower than the isometric optimum length.

The interaction of relative muscle length and force-velocity characteristics was investigated in the fully activated rat medial gastrocnemius muscle in situ. Average maximal isometric force (as a percentage of the of the maximal isometric force at L(o,iso)) at relative lengths measured below isometric optimum (L(o,iso)) was 96% at L(o,iso)-2 mm, 88% at L(o,iso)-4 mm and 58% at L(o,iso)-6 mm. Force-velocity curves were obtained at the four relative muscle lengths. There were no significant differences in maximal shortening velocity (approximately 280 mm x s(-1)) between the different muscle lengths. The highest power output (P<0.05) was found at L(o,iso)-2 mm (mean+/-SEM 435+/-19 mW). Peak power values at L(o,iso) (390+/-10 mW) and L(o,iso)-4 mm (395+/-12 mW) were not significantly different, whereas peak power was lowest (P<0.05) at L(o,iso)-6 mm. There was a significant (P<0.01) shift of approximately 1.5 mm in optimum muscle length for force generation during shortening contractions compared with isometric contractions. Shortening velocity had only a minor influence on optimum muscle length for force generation. It is concluded that fully activated muscles produce their maximal power at a length lower than L(o,iso). The difference in optimum length between isometric and dynamic contractions may be related to length-dependent variations in sarcomere length in series during shortening.