Maximal shortening velocity is a fundamental muscle property (Hill, 1938). Implemented by Sasaki and Ishii (2005), the application of extremely high shortening while muscle contracts isometrically (i.e. slack test) would be a more promising method than the ballistic test for evaluating this capacity. The slack test procedure assumes that the assessment of maximal angular velocity reflects the muscular contribution to velocity-generating capacity although it does not measure muscle shortening velocity. The present study aimed to directly assess gastrocnemius medialis (GM) fascicle shortening velocity (VF) during the slack-test. In addition, VF obtained during the slack test was compared to unloaded ballistic condition (Hauraix et al., 2015).
Muscle fascicle behaviour was recorded on GM muscle of 25 participants using ultrafast ultrasound (Hauraix et al., 2015) during slack tests and ballistic test performed with a starting position set at -15° in dorsiflexion. Slack test was applied at various amplitudes (30, 35, 40, 45, 50°) and initial torque levels [5, 10, 20, 40 and 60% of maximal voluntary (MVC) isometric torque] on a mechatronic ergometer (Eracles-Technology, Compiègne, France). Peak VF, tendinous tissues shortening velocity (VTT) and mean articular velocity were calculated in each test.
Maximal VF were higher in slack test at 5% torque level (42.4±12.9 cm.s-1) than in ballistic (36.3±8.7 cm.s-1), and minimal at 60% (29.3±13.2 cm.s-1) indicating that VF decreases with the increase in torque (P<0.001). Inversely VTT showed a significant effect of torque level (P<0.001) and amplitude (P<0.001) indicating that VTT increases with torque. No correlation was found between slack test and ballistic test for maximal VF or maximal articular velocity.
The present study demonstrates that using the slack test in vivo appears debatable to reliably appraise maximal muscle shortening velocity. This finding could partly originate from the contribution of tendinous tissues, which is very difficult to isolate at the initial contraction level increases.
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