Recruitment of motor units in two fascicles of the semispinalis cervicis muscle

Abstract

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

Fig. 1.

Schematic representation of the fascicles of the semispinalis cervicis muscle and the insertion points at spinal levels C2 (A) and C5 (B).

Fig. 2.

Result of decomposition of EMG signals during a 10% maximal voluntary contraction (MVC) contraction for a representative subject. In this case, 3 motor units were identified from C2 and 7 from C5. The action potentials from each of these motor units are identified from the intramuscular EMG recording (A). B: superimposed motor unit action potentials for 3 of the detected motor units. In many cases, not all the identified motor units were included in the analysis since not all were consistently active throughout the contraction. In this example, motor unit (MU)5 and MU10 from C5 were excluded from further analysis since they were active for short periods.

Fig. 3.

Average discharge rate for all motor units identified at each contraction force for C2 (A) and C5 (B), respectively. Lines connect the discharge rates for those motor units that were identified at more than 1 force level. Bold line represents the mean. *P < 0.05.

Fig. 4.

Representative data showing the cross-histograms and cumulative sum (CuSum) for motor unit pairs in a representative subject during a 10% MVC contraction. The motor unit pairs are detected from the C2 level [A; common input strength (CIS): 0.63], the C5 level (B; CIS: 0.59), and between the 2 levels (C; CIS: 0.04). Vertical lines indicate the boundaries of the synchronous peaks as determined from the CuSum.

Fig. 5.

Mean and SE of the synchronization of motor unit pairs within each spinal level separately and between spinal levels. The level of synchronization was similar within spinal levels but differed from the synchronization between levels. *P < 0.05.

Fig. 6.

Mean and SE of the absolute (A) and normalized (B) EMG amplitude detected from the semispinalis cervicis muscle at the levels of C2 and C5 during a ramped neck extension contraction from 0 to 50% MVC.

Fig. 7.

Schematic representation of the moment system for the upper and lower regions of the semispinalis cervicis (SC) during isometric neck extension. The force moment of the reaction force of the head is greater for C5 than for C2. The required force to stabilize C5 is consequently higher than for C2.