Hallucis Muscle During Cramp Contractions

33 We analyzed individual motor units during electrically-elicited cramp contractions with 34 the aim of characterizing the variability and degree of common oscillations in their discharges. 35 Intramuscular and surface EMG signals were detected from the abductor hallucis muscle of 11 36 healthy subjects (age 27.0 ± 3.7 yrs) during electrically-elicited cramps. In total, 48 motor units 37 were identified from the intramuscular EMG. These motor units were active for 23.6 ± 16.2 s, 38 during which their average discharge rate was 14.5 ± 5.1 pulses per second (pps) and their 39 minimum and maximum rates were 6.0 ± 0.8 pps and 25.0 ± 8.0 pps (P<0.001). The coefficient 40 of variation for the interspike interval (ISI) was 44.6 ± 9.7% and doublet discharges constituted 41 4.1 ± 4.7% of the total number of discharges. In 38 motor units, the standard deviation of the 42 ISI was positively correlated to the mean ISI (R=0.37, P<0.05). The coherence spectrum 43 between smoothed discharge rates of pairs of motor units showed one significant peak at 1.4 ± 44 0.4 Hz for 29 of the 96 motor unit pairs and two significant peaks at 1.3 ± 0.5 Hz and 1.5 ± 0.5 45 Hz for 8 motor unit pairs. The cross-correlation function between pairs of discharge rates 46 showed a significant peak (0.52 ± 0.11) in 26 motor unit pairs. In conclusion, motor units active 47 during cramps showed a range of discharge rates similar to that observed during voluntary 48 contractions but larger ISI variability, probably due to large synaptic noise. Moreover, the 49 discharge rates of the active motor units showed common oscillations. 50 51 52 53 54 55