Recruitment of Motor Units in Two Fascicles of The

39 This study investigated the behaviour of motor units in the semispinalis cervicis 40 muscle. Intramuscular EMG recordings were obtained unilaterally at levels C2 and C5 in 15 41 healthy volunteers (7 women) who performed isometric neck extensions at 5, 10, and 20% of 42 the maximal force (MVC) for 2 min each, and linearly-increasing force contractions from 0 to 43 30% MVC over 3 s. Individual motor unit action potentials were identified. The discharge 44 rate and interspike interval variability of the motor units in the two locations did not differ. 45 However, the recruitment threshold of motor units detected at C2 (n = 16, mean, SD: 10.3 ± 46 6.0 % MVC) was greater than that of motor units detected at C5 (n = 92, 6.9 ± 4.3 % MVC) 47 (P < 0.01). A significant level of short term synchronization was identified in 246 of the 307 48 motor unit pairs when computed within one spinal level but only for 28 of the 110 pairs of 49 motor units between the two levels. The common input strength, which quantifies motor unit 50 synchronization, was greater for pairs within one level (0.47 ± 0.32) compared to pairs 51 between levels (0.09 ± 0.07) (P < 0.05). In a second experiment on 8 healthy subjects, 52 interference EMG was recorded from the same locations during a linearly-increasing force 53 contraction from 0 to 40% MVC, and showed significantly greater EMG amplitude at C5 than 54 at C2. In conclusion, synaptic input is distributed partly independently and non-uniformly to 55 different fascicles of the semispinalis cervicis muscle. 56 57