Role of skeletal muscle excitability in strength loss induced by submaximal isometric contractions in males with Duchenne muscular dystrophy.

The role of muscle excitability in the underlying mechanism of contraction-induced strength loss and the recovery processes in those with Duchenne muscular dystrophy is unknown. Strength loss of wrist extensor muscles was induced by an intermittent, submaximal, isometric contraction exercise protocol in males with (n = 10) and without Duchenne muscular dystrophy (n = 10). Muscle strength was measured by torque from isometric maximum voluntary contractions. Muscle excitability was measured by compound muscle action potential evoked by transcutaneous magnetic stimulation. Central component of electromyogram was measured by integral electromyogram during isometric maximum voluntary contractions. Significant reductions in muscle excitability were observed in both groups along with reduced maximum voluntary contractions torque after exercise. Muscle excitability was positively correlated with maximum voluntary contractions torque and did not show between-group differences before or immediately after exercise. However, this correlation showed significant between-group differences during recovery. Results indicate that in males with Duchenne muscular dystrophy, muscle strength loss was accompanied by reduced muscle excitability. Importantly, the difference in excitability-torque correlation between groups during recovery suggests that DMD muscle function was affected by the ability to recover from strength loss more than during the contraction-induced strength loss.