title: Adjustments differ among low-threshold motor units during intermittent, isometric contractions.
abstract: We investigated the changes in muscle fiber conduction velocity, recruitment and derecruitment thresholds, and discharge rate of low-threshold motor units during a series of ramp contractions. The aim was to compare the adjustments in motor unit activity relative to the duration that each motor unit was active during the task. Multichannel surface electromyographic (EMG) signals were recorded from the abductor pollicis brevis muscle of eight healthy men during 12-s contractions (n = 25) in which the force increased and decreased linearly from 0 to 10% of the maximum. The maximal force exhibited a modest decline (8.5 +/- 9.3%; P < 0.05) at the end of the task. The discharge times of 73 motor units that were active for 16-98% of the time during the first five contractions were identified throughout the task by decomposition of the EMG signals. Action potential conduction velocity decreased during the task by a greater amount for motor units that were initially active for >70% of the time compared with that of less active motor units. Moreover, recruitment and derecruitment thresholds increased for these most active motor units, whereas the thresholds decreased for the less active motor units. Another 18 motor units were recruited at an average of 171 +/- 32 s after the beginning of the task. The recruitment and derecruitment thresholds of these units decreased during the task, but muscle fiber conduction velocity did not change. These results indicate that low-threshold motor units exhibit individual adjustments in muscle fiber conduction velocity and motor neuron activation that depended on the relative duration of activity during intermittent contractions.
authors.affiliation: Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg, Denmark. df@hst.aau.dk
authors.forename: Dario
authors.initials: D
authors.lastname: Farina
authors.forename: Ales
authors.initials: A
authors.lastname: Holobar
authors.forename: Marco
authors.initials: M
authors.lastname: Gazzoni
authors.forename: Damjan
authors.initials: D
authors.lastname: Zazula
authors.forename: Roberto
authors.initials: R
authors.lastname: Merletti
authors.forename: Roger M
authors.initials: RM
authors.lastname: Enoka
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000328:Adult; D004576:Electromyography; D004594:Electrophysiology; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology; D011999:Recruitment, Neurophysiological / Q000502:physiology; D012815:Signal Processing, Computer-Assisted