title: Motor unit recruitment and bursts of activity in the surface electromyogram during a sustained contraction.
abstract: Bursts of activity in the surface electromyogram (EMG) during a sustained contraction have been interpreted as corresponding to the transient recruitment of motor units, but this association has never been confirmed. The current study compared the timing of trains of action potentials discharged by single motor units during a sustained contraction with the bursts of activity detected in the surface EMG signal. The 20 motor units from 6 subjects [recruitment threshold, 35.3 +/- 11.3% maximal voluntary contraction (MVC) force] that were detected with fine wire electrodes discharged 2-9 trains of action potentials (7.2 +/- 5.6 s in duration) when recruited during a contraction that was sustained at a force below its recruitment threshold (target force, 25.4 +/- 10.6% MVC force). High-pass filtering the bipolar surface EMG signal improved its correlation with the single motor unit signal. An algorithm applied to the surface EMG was able to detect 75% of the trains of motor unit action potentials. The results indicate that bursts of activity in the surface EMG during a constant-force contraction correspond to the transient recruitment of higher-threshold motor units in healthy individuals, and these results could assist in the diagnosis and design of treatment in individuals who demonstrate deficits in motor unit activation.
authors.affiliation: Department of Integrative Physiology, University of Colorado, 354 UCB, Boulder, Colorado 80309, USA. zachary.riley@colorado.edu
authors.forename: Zachary A
authors.initials: ZA
authors.lastname: Riley
authors.forename: Mary E
authors.initials: ME
authors.lastname: Terry
authors.forename: Alberto
authors.initials: A
authors.lastname: Mendez-Villanueva
authors.forename: Jane C
authors.initials: JC
authors.lastname: Litsey
authors.forename: Roger M
authors.initials: RM
authors.lastname: Enoka
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000328:Adult; D000465:Algorithms; D003627:Data Interpretation, Statistical; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*