title: Contraction-induced potentiation of human motor unit discharge and surface EMG activity.
abstract: In quadrupeds, an electrically induced, moderate to high intensity brief muscle contraction potentiates autogenetic excitation and leads to enhanced recruitment and/or tonic firing frequency of alpha-motor neurons. To determine if similar adaptations occur in humans, single motor units (SMUs) and surface electromyographic activity (EMG) were recorded from the right biceps brachii before and immediately after a 5-s 25% or 50% maximum voluntary contraction (MVC), while subjects held a handle (0-1% MVC) attached to a force transducer or maintained a 2% MVC for 30-60 s. Of 26 SMUs recorded, 15 increased, 4 decreased, and 7 showed no change in firing frequency (mean increase: 5 imp/s, P less than 0.01). Twelve SMUs had lower recruitment force thresholds after contraction. There was no significant treatment effect for the % MVC intensity. The postcontraction surface EMG power spectrum broadened, increased in amplitude, and contained a higher frequency component than the control contraction power spectrum. Changes in recruitment and/or frequency coding were reflected in the raw EMG records. Findings agree with previous reports in animals of contraction-induced potentiation of subsequent submaximal muscle contractions. Such acute adaptations in spinal neuromuscular pathways would function to optimize force output to a submaximal range of neural input frequencies.
authors.affiliation: Department of Physiology, Kyorin University, School of Medicine, Tokyo, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Suzuki
authors.forename: K
authors.initials: K
authors.lastname: Kaiya
authors.forename: S
authors.initials: S
authors.lastname: Watanabe
authors.forename: R S
authors.initials: RS
authors.lastname: Hutton
mesh_terms: D004576:Electromyography; D005246:Feedback; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D009045:Motor Endplate / Q000502:physiology*; D009119:Muscle Contraction; D009469:Neuromuscular Junction / Q000502:physiology*; D011999:Recruitment, Neurophysiological; D012684:Sensory Thresholds / Q000502:physiology