It is generally assumed that the influence of relative muscle length as observed in isometric conditions is similar in dynamic (shortening) conditions, but there is little information available in the literature. The aim of the present study was to compare the influence of relative muscle length between isometric and dynamic contractions of maximally activated skeletal muscles. Medial gastrocnemius muscleÐtendon complexes of anaesthetized (urethane 1.5 g (kg body mass)^-1; I.P.; n = 18) rats were stimulated via the severed sciatic nerve at 35°C. Stimulation frequencies used (250 Hz for isometric contractions and 400 Hz for shortening contractions) were high enough to maximally activate the muscles at all relative muscle lengths. ForceÐvelocity relations were obtained (de Haan, 1998) at four different muscle lengths: isometric optimum length (Lo_iso), Lo_iso -2 mm, Lo_iso -4 mm and Lo_iso -6 mm. LengthÐforce relations were measured at four different velocities: v = 0, 50, 100 and 150 mm s^-1. After the experiments the rats were humanely killed.

Average maximal isometric forces at relative lengths measured below Lo_iso were 96 % (at Lo_iso -2 mm), 88 % (at Lo_iso -4 mm) and 58 % (at Lo_iso -6 mm) of the maximal isometric force at Lo_iso. Maximal rate of force rise was not different between Lo_iso and Lo_iso -2 mm (~0.40 N s^-1) but lower at the lower lengths. There were no significant differences (ANOVA repeated measures) in maximal shortening velocity (~280 mm s^-1) between the different muscle lengths. The forceÐvelocity curves at the lengths below Lo_iso showed that forces at all velocities were higher at Lo_iso -2 mm than at Lo_iso -4 mm and both were higher than at Lo_iso -6 mm. The relation for Lo_iso crossed the curve of Lo_iso -2 mm between v = 25 and 50 mm s^-1 and the curve for Lo_iso -4 mm around v = 100 mm s^-1, about the velocity at which power output was maximal for all relative muscle lengths. The highest power output (P < 0.05) was found at Lo_iso -2 mm (mean ± S.E.M. 435 ± 19 mW; n = 8). Peak power output measured at Lo_iso (390 ± 10 mW) and Lo_iso -4 mm (395 ± 12 mW) were not significantly different, whereas peak power was lowest (P < 0.05) at Lo_iso -6 mm (223 ± 15 mW).

There was a significant (Student’s paired t test; P < 0.01) shift of ~1.5 mm in optimum muscle length for force generation during shortening contractions compared with isometric contractions. Shortening velocity had only a relatively small influence on optimum muscle length. It is concluded that fully activated muscles produce their maximal power at a length lower than Lo_iso. The difference in optimum length between isometric and dynamic contractions may be a consequence of length-dependent differences in heterogeneity of sarcomere length in series during shortening.

All procedures accord with current National guidelines.