Discharge Properties of Motor Units of the Abductor Hallucis Muscle During Cramp Contractions

• Published in: Journal of neurophysiology Vol. 102; no. 3; pp. 1890 - 1901
• Main Authors: Minetto, Marco A, Holobar, Ales, Botter, Alberto, Farina, Dario
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01.09.2009
• Subjects: Action Potentials - physiology; Adolescent; Adult; Electric Stimulation - adverse effects; Electromyography - methods; Evoked Potentials, Motor; Feedback; Female; Humans; Male; Motor Neurons - physiology; Muscle Contraction - physiology; Muscle Cramp - etiology; Muscle Cramp - pathology; Muscle Cramp - physiopathology; Muscle, Skeletal - physiopathology; Neural Conduction; Reaction Time; Time Factors; Young Adult
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00309.2009

1 Division of Endocrinology, Diabetology and Metabolism, Department of Internal Medicine, University of Turin; 2 Laboratory for Engineering of the Neuromuscular System, Department of Electronics, Politecnico di Torino, Turin, Italy; 3 Faculty of Electrical Engineering and Computer Science, University of Maribor, Maribor, Slovenia; and 4 Department of Health Science and Technology, Center for Sensory–Motor Interaction, Aalborg University, Aalborg, Denmark Submitted 6 April 2009; accepted in final form 28 June 2009 Abstract We analyzed individual motor units during electrically elicited cramp contractions with the aim of characterizing the variability and degree of common oscillations in their discharges. Intramuscular and surface electromyographic (EMG) signals were detected from the abductor hallucis muscle of 11 healthy subjects (age 27.0 ± 3.7 yr) during electrically elicited cramps. In all, 48 motor units were identified from the intramuscular EMG. These motor units were active for 23.6 ± 16.2 s, during which their average discharge rate was 14.5 ± 5.1 pulses/s (pps) and their minimum and maximum rates were, respectively, 6.0 ± 0.8 and 25.0 ± 8.0 pps ( P < 0.001). The coefficient of variation for the interspike interval (ISI) was 44.6 ± 9.7% and doublet discharges constituted 4.1 ± 4.7% of the total number of discharges. In 38 motor units, the SD of the ISI was positively correlated to the mean ISI ( R 2 = 0.37, P < 0.05). The coherence spectrum between smoothed discharge rates of pairs of motor units showed one significant peak at 1.4 ± 0.4 Hz for 29 of the 96 motor unit pairs and two significant peaks at 1.3 ± 0.5 and 1.5 ± 0.5 Hz for 8 motor unit pairs. The cross-correlation function between pairs of discharge rates showed a significant peak (0.52 ± 0.11) in 26 motor unit pairs. In conclusion, motor units active during cramps showed a range of discharge rates similar to that observed during voluntary contractions but larger ISI variability, probably due to large synaptic noise. Moreover, the discharge rates of the active motor units showed common oscillations. Address for reprint requests and other correspondence: M. A. Minetto, University of Turin, Molinette Hospital, Department of Internal Medicine, Division of Endocrinology, Diabetology and Metabolism, C.so Dogliotti 14, 10126 Turin, Italy (E-mail: marcominetto{at}libero.it )