Muscle fibre conduction velocity varies in opposite directions after short- vs. long-duration muscle contractions.

Abstract

The effects of muscle contractions on muscle fibre conduction velocity have normally been investigated for contractions of a given duration and intensity, with most studies being focused on the decline on conduction velocity during/after prolonged contractions. Herein, we perform a systematic analysis of the changes in conduction velocity after voluntary contractions of different durations and intensities. Conduction velocity was estimated in the vastus lateralis before and after knee extensor isometric maximal voluntary contractions (MVCs) of 1, 3, 6, 10, 30 and 60s, and after brief (3s) contractions at 10, 30, 50, 70, and 90% of MVC force. Measurements were made during the 10-min period following each contraction. (1) Conduction velocity was increased immediately after (1s) the MVCs of brief (≤ 10s) duration (12 ± 2%, P < 0.05), and then returned rapidly (within 15s) to control levels; (2) the extent of the increase in conduction velocity was similar after the 3-s, 6-s, and 10-s MVCs (P > 0.05); (3) the magnitude of the increase in conduction velocity after a brief contraction augmented with the intensity of the contraction (increases of 4.6, 7.7, 11.4, 14.8, and 15.2% for contractions at 10, 30, 50, 70, and 90% of MVC force, respectively); (4) conduction velocity was not decreased immediately after the 30-s MVC (P > 0.05); and (5) conduction velocity did not reach its minimum 1s after the long (≥ 30s) MVCs. Brief (≤ 10s) muscle contractions induce a short-term increase in conduction velocity, lasting 15s, while long (≥ 30s) contractions produce a long-term decrease in conduction velocity, lasting more than 2min.