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  • Title: Motor unit synchronization is increased in biceps brachii after exercise-induced damage to elbow flexor muscles.
    Author: Dartnall TJ, Nordstrom MA, Semmler JG.
    Journal: J Neurophysiol; 2008 Feb; 99(2):1008-19. PubMed ID: 18171708.
    Abstract:
    The purpose of this study was to determine the effect of eccentric exercise on correlated motor unit discharge (motor unit synchronization and coherence) during low-force contractions of the human biceps brachii muscle. Eight subjects (age, 25 +/- 7 yr) performed three tasks involving isometric contraction of elbow flexors while EMG (surface and intramuscular) records were obtained from biceps brachii. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; and 3) sustained discharge of pairs of concurrently active motor units for 2-5 min. These tasks were performed before, immediately after, and 24 h after fatiguing eccentric exercise. MVC force declined 46% immediately after eccentric exercise and remained depressed (31%) 24 h later, which is indicative of muscle damage. For the constant-force task, biceps brachii EMG ( approximately 100% greater) and force fluctuations ( approximately 75% greater) increased immediately after exercise, and both recovered by approximately 50% 24 h later. Motor unit synchronization, quantified by cross-correlation of motor unit pairs during low-force (1-26% MVC) contractions, was 30% greater immediately after (n = 105 pairs) and 24 h after exercise (n = 92 pairs) compared with before exercise (n = 99 pairs). Similarly, motor unit coherence at low (0-10 Hz) frequencies was 20% greater immediately after exercise and 34% greater 24 h later. These results indicate that the series of events leading to muscle damage from eccentric exercise alters the correlated behavior of human motor units in biceps brachii muscle for > or =24 h after the exercise.
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