Motor Unit Conduction Velocity During Sustained Contraction Of The Vastus Medialis Muscle Injured By Eccentric Exercise

Abstract

PURPOSE: Eccentric contractions induce muscle fiber damage, which is associated with a decreased capacity to generate force. The sarcolemma, which conducts the action potential, is subjected to substantial tears during eccentric contractions. Thus, in this study it is hypothesized that the electrophysiological fiber membrane properties change after muscle damage induced by eccentric exercise. The aim of the study was to investigate the propagation velocity of muscle fiber action potentials in individual motor units following eccentric exercise. METHODS: Multi-channel surface and fine-wire intramuscular EMG signals were concurrently recorded from two locations (proximal and distal) of the right vastus medialis muscle of 10 healthy men during 60-s isometric contractions at 10% and 30% of the maximal force. The measures were performed before (baseline), 24h, and 48h after eccentric exercise. RESULTS: The maximal force decreased by 26.1 ± 16.1% (P < 0.0001) at 24h and remained reduced by 23.6 ± 14.5% (P < 0.0001) 48h post exercise with respect to baseline, which was indicative of muscle damage. Muscle fiber conduction velocity (MFCV) was reduced 24h and 48h post exercise with respect to baseline for both contraction forces and both recording locations. The percent reduction in MFCV from day 1 to day 2 and 3 (average for 24h and 48h post exercise) was 7.7 ± 2.7% (proximal) and 7.2 ± 2.8% (distal) (P < 0.05) for 10% MVC force, and 8.6 ± 3.8% (proximal) and 6.2 ± 1.5% (distal) (P < 0.05) for 30% MVC force. Moreover, motor unit conduction velocity decreased over time during the sustained contractions at a faster rate when assessed 24h and 48h post exercise with respect to baseline for both levels of force and muscle locations (P < 0.05). CONCLUSIONS: These results indicate that the electrophysiological membrane properties of muscle fibers are altered by exercise-induced muscle damage.