Purpose: The aim of this study was to evaluate the dependence of
motor unit conduction velocity (MUCV) on the length of the muscle.
Methods: Muscle length was modified considering three different
ankle angles: 90°,110° and 130°. For each angle the maximal voluntary
contraction (MVC) was measured during static ankle
dorsiflexion. High-density surface electromyography (HD-sEMG, 128
channels) was recorded from the tibialis anterior muscle (TA) in six
young individuals. Subjects executed a volitional effort in which the
output tension changed in a trapezoidal ramp fashion (15 s transient
and 40 s steady contraction). Two levels of steady contraction were
investigated: 10% and 20% of MVC. Using a novel decomposition
technique based on HD-sEMG processing (Negro et al. 2016), the
individual MUCV values were estimated during the trapezoid steady
part. The decomposition was performed at each ankle angle independently,
and the MU action potentials were not tracked across
different muscle lengths.
Results: MVC mean values at 90° and 130° were 91.78% and 78.77%
of 110° value respectively. Considering that there was no statistical
difference between the MUCV estimations calculated at 10 and 20%
MVC, the values were grouped. The average CV was 4.00 ± 0.54 m/
s for 90°, 3.88 ± 0.19 m/s for 110° and 3.77 ± 0.37 m/s for 130°.
One-Way Anova analysis showed a weak effect between the three
conditions (P = 0.04).
Conclusion: MUCV changes were weakly related to the different
muscle lengths. Explanation of our results should consider that the
muscle fiber can be approximated as a constant volume system and
that, from the cable theory, the smaller the diameter the lower the CV
is. On these bases, the reduction of muscle fibers transverse diameter
during muscle elongation when the ankle angle increases from 90° to
130° could be a possible explanation for our results
