title: Motor unit firing rates during isometric voluntary contractions performed at different muscle lengths.
abstract: Firing rates of motor units and surface EMG were measured from the triceps brachii muscles of able-bodied subjects during brief submaximal and maximal isometric voluntary contractions made at 5 elbow joint angles that covered the entire physiological range of muscle lengths. Muscle activation at the longest, midlength, and shortest muscle lengths, measured by twitch occlusion, averaged 98%, 97%, and 93% respectively, with each subject able to achieve complete activation during some contractions. As expected, the strongest contractions were recorded at 90 degrees of elbow flexion. Mean motor unit firing rates and surface EMG increased with contraction intensity at each muscle length. For any given absolute contraction intensity, motor unit firing rates varied when muscle length was changed. However, mean motor unit firing rates were independent of muscle length when contractions were compared with the intensity of the maximal voluntary contraction (MVC) achieved at each joint angle.
authors.affiliation: Department of Neurological Surgery, The Miami Project to Cure Paralysis, Lois Pope Life Center, 1095 NW 14th Terrace (R-48), PO Box 016960, Miami, FL 33136, USA.
authors.forename: Alejandro
authors.initials: A
authors.lastname: Del Valle
authors.forename: Christine K
authors.initials: CK
authors.lastname: Thomas
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D018709:Statistics, Nonparametric; D019415:Torque