Neck flexor muscle strength, efficiency, and relaxation times in normal subjects and subjects with unilateral neck pain and headache

Objective: To determine the test-retest reliability of a new method for measuring muscular strength, efficiency, and relaxation times of the neck flexor musculature of healthy adults, and to compare these neck flexor muscle properties in subjects who have unilateral neck pain and headache with those in controls. Design: Subjects lay supine and isometrically flexed their necks against a force transducer attached to the back of a webbing and velcro helmet. Electromyograms (EMGs) were recorded from surface electrodes on the sternocleidomastoid (SCM) muscles. Two consecutive sessions of live contractions of varying levels of effort from minimal through moderate and maximal effort were analyzed. Setting: Ambulatory referral center. Participants: Volunteer control subjects (n = 10, 3 men and 7 women) were recruited from hospital and university personnel. Volunteer neck pain subjects (n = 10; 3 men and 7 women) were recruited from a physiatric chronic pain practice and a hospital outpatient physical therapy practice. Results: In the controls, the intraclass correlation coefficients (ICCs) or the, first two maximum neck; flexion contractions were: peak force ICC = .81; peak force/body weight ICC = .75; average force ICC = .75; force relaxation time ICC = .73; SCM EMG relaxation times: right ICC = .60 and left ICC = .67, Comparing sessions 1 and 2 the intraclass correlations for SCM efficiencies were right ICC = .58 and left ICC = .97. The peak force in controls ((x) over bar = 45.3 +/- 17.6 N) was reduced by 50% in the neck pain subjects ((x) over bar = 22.4 +/- 13.1 N) (p = .004). Similarly, peak force/body weight in the neck pain subjects (x) over bar = 0.3 +/- 0.2 N/kg) was 46% of controls ((x) over bar = 0.7 +/- 0.2 N/kg) (p = .001), and average force in the neck pain subjects ((x) over bar = 12.1 +/- 7.5 N) was 43% of controls ((x) over bar = 28.5 +/- 11.0 N) (p = .001). In two neck pain subjects, SCM EMG and force relaxation times were abnormally long in both the affected and the unaffected SCM muscles, exceeding the control values by greater than 3 standard deviations. The difference between rile right SCM efficiency of the control subjects ((x) over bar = 0.3 +/- 0.2 N/mu V) and the affected SCM efficiency of the neck pain subjects ((x) over bar = 0.1 +/- 0.1 N/mu V) approached the p < .05 criterion for significance (p = .055). Conclusion: The technique was found to be highly reliable for the measurement of neck flexor peak force, peak force/body weight, average force, and force relaxation time, and moderately reliable for the quantitation of SCM EMG relaxation times and SCM efficiency. All force values were significantly lower in the neck pain population compared with the controls. Ln the neck pain population, force and SCM EMG relaxation times, as well as efficiencies, suggested abnormalities. Neck pain subjects showed no significant differences in SCM EMG relaxation time or SCM efficiency between affected and unaffected SCM muscles. (C) 1996 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation