Unloaded shortening velocity of voluntarily and electrically activated human dorsiflexor muscles in vivo

We have previously shown that unloaded shortening velocity (V ₀) of human plantar flexors can be determined in vivo, by applying the “slack test” to submaximal voluntary contractions (J Physiol 567:1047–1056, 2005). In the present study, to investigate the effect of motor unit recruitment pattern on V ₀ of human muscle, we modified the slack test and applied this method to both voluntary and electrically elicited contractions of dorsiflexors. A series of quick releases (i.e., rapid ankle joint rotation driven by an electrical dynamometer) was applied to voluntarily activated dorsiflexor muscles at three different contraction intensities (15, 50, and 85% of maximal voluntary contraction; MVC). The quick-release trials were also performed on electrically activated dorsiflexor muscles, in which three stimulus conditions were used: submaximal (equal to 15%MVC) 50-Hz stimulation, supramaximal 50-Hz stimulation, and supramaximal 20-Hz stimulation. Modification of the slack test in vivo resulted in good reproducibility of V ₀, with an intraclass correlation coefficient of 0.87 (95% confidence interval: 0.68–0.95). Regression analysis showed that V ₀ of voluntarily activated dorsiflexor muscles significantly increased with increasing contraction intensity (R ²?=?0.52, P<0.001). By contrast, V ₀ of electrically activated dorsiflexor muscles remained unchanged (R ²<0.001, P?=?0.98) among three different stimulus conditions showing a large variation of tetanic torque. These results suggest that the recruitment pattern of motor units, which is quite different between voluntary and electrically elicited contractions, plays an important role in determining shortening velocity of human skeletal muscle in vivo.