title: Motor unit synchronization is increased in biceps brachii after exercise-induced damage to elbow flexor muscles.
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.
authors.affiliation: Discipline of Physiology and Research Centre for Human Movement Control, School of Molecular and Biomedical Science, The University of Adelaide, Adelaide, Australia.
authors.forename: Tamara J
authors.initials: TJ
authors.lastname: Dartnall
authors.forename: Michael A
authors.initials: MA
authors.lastname: Nordstrom
authors.forename: John G
authors.initials: JG
authors.lastname: Semmler
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D001917:Brachial Plexus / Q000473:pathology*; D004550:Elbow; D004576:Electromyography / Q000379:methods; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000293:injuries* / Q000503:physiopathology*; D013647:Task Performance and Analysis; D013997:Time Factors