Tonic and kinetic motor units revisited: does motor unit firing behavior differentiate motor units?

• Published in: Clinical neurophysiology Vol. 111; no. 12; pp. 2196 - 2199
• Main Authors: Masakado, Yoshihisa, Akaboshi, Kazuto, Kimura, Akio, Chino, Naoichi
• Format: Journal Article
• Language: English
• Published: Shannon Elsevier Ireland Ltd 01.12.2000; Elsevier Science
• Subjects: Action Potentials - physiology; Adult; Biological and medical sciences; Decomposition technique; Electromyography; Exercise - physiology; Fundamental and applied biological sciences. Psychology; Humans; Kinetic; Male; Mean inter-spike interval; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Neurons - classification; Motor Neurons - physiology; Motor unit; Muscle Contraction - physiology; Muscles - physiology; Tonic; Variability; Vertebrates: nervous system and sense organs; Mean inter-spike interval; Kinetic; Decomposition technique; Variability; Motor unit; Tonic; Human; Discharge pattern; Electrophysiology; Isometric muscular contraction; Action potential; Electromyography; Striated muscle; Motor control
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/S1388-2457(00)00450-8

Objective: We tried to differentiate motor unit into two distinct populations, tonic and kinetic, on the basis of the relationship between the mean inter-spike interval and its variability. Methods: During voluntary isometric contraction myoelectric activity was recorded with a special quadrifilar electrode from first dorsal interosseous, biceps brachii, soleus, and tibialis anterior muscle. Motor unit action potentials (MUAP) were decomposed into individual MUAP trains, by electromyography (EMG) signal decomposition. The variability in the instantaneous firing rate was assessed at two or more levels of contraction in each muscle. Results: We found each muscle tested had a homogeneous population. There were no tonic and kinetic motor units. But there were differences in the variability in the instantaneous firing rate in the 4 muscles tested. Conclusion: Motor unit firing behavior in a muscle may be fitted for its function.