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This Article Full Text Full Text (PDF) All Versions of this Article: 102/3/1890    most recent 00309.2009v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Minetto, M. A. Articles by Farina, D. PubMed PubMed Citation Articles by Minetto, M. A. Articles by Farina, D.

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

Marco A. Minetto^1,2, Ale Holobar^2,3, Alberto Botter² and Dario Farina⁴

¹Division of Endocrinology, Diabetology and Metabolism, Department of Internal Medicine, University of Turin; ²Laboratory for Engineering of the Neuromuscular System, Department of Electronics, Politecnico di Torino, Turin, Italy; ³Faculty of Electrical Engineering and Computer Science, University of Maribor, Maribor, Slovenia; and ⁴Department 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 (R² = 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.