Effect of pain on the modulation in discharge rate of sternocleidomastoid motor units with force direction

ObjectiveTo compare the behavior of sternocleidomastoid motor units of patients with chronic neck pain and healthy controls.MethodsNine women (age, 40.4 ± 3.5 yr) with chronic neck pain and nine age- and gender-matched healthy controls participated. Surface and intramuscular EMG were recorded from the sternocleidomastoid muscle bilaterally as subjects performed isometric contractions of 10-s duration in the horizontal plane at a force of 15 N in eight directions (0–360°; 45° intervals) and isometric contractions at 15 and 30 N force with continuous change in force direction in the range 0–360°. Motor unit behavior was monitored during the 10-s contractions and the subsequent resting periods.ResultsThe mean motor unit discharge rate depended on the direction of force in the control subjects (P < 0.05) but not in the patients. Moreover, in three of the nine patients, but in none of the controls, single motor unit activity continued for 8.1 ± 6.1 s upon completion of the contraction. The surface EMG amplitude during the circular contraction at 15 N was greater for the patients (43.5 ± 54.2 μV) compared to controls (16.9 ± 14.9 μV; P < 0.05).ConclusionsThe modulation in discharge rate of individual motor units with force direction is reduced in the sternocleidomastoid muscle in patients with neck pain, with some patients showing prolonged motor unit activity when they were instructed to rest.SignificanceThese observations suggest that chronic neck pain affects the change in neural drive to muscles with force direction.