Abstract
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.
Highlights
Over the past century, researchers have thoroughly investigated the relationship between the velocity of muscle shortening and the load applied to the muscle, where the velocity of shortening decreases with the progressive increase in load
In a whole muscle preparation, V0 is a measure of the shortening velocity of the fastest fibers, whereas Vmax is a function of the force–velocity characteristics of all the fibers [6,7]
The contraction intensity is expressed as a percentage of maximal voluntary contraction torque (MVC) that was comparable between the two experimental sessions (44.269.7 Nm in voluntary contraction session and 44.1610.0 Nm in electrical stimulation session, P = 0.97)
Summary
Researchers have thoroughly investigated the relationship between the velocity of muscle shortening and the load applied to the muscle (force–velocity relation), where the velocity of shortening decreases with the progressive increase in load. In 1979, Edman [3] developed an alternative approach (referred to as the ‘slack test’) involving multiple quick releases (i.e. rapid shortening) applied to an isometrically contracting muscle fiber, and for the first time measured unloaded shortening velocity (V0) of single muscle fibers. In a whole muscle preparation, V0 is a measure of the shortening velocity of the fastest fibers, whereas Vmax is a function of the force–velocity characteristics of all the fibers [6,7]
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