Retest Reliability of Force-Time Variables of Neck Muscles Under Isometric Conditions

Abstract

Abstract Context: Proper conditioning of the neck muscles may play a role in reducing the risk of neck injury and, possibly, concussions in contact sports. However, the ability to reliably measure the force-time–based variables that might be relevant for this purpose has not been addressed. Objective: To assess the between-days reliability of discrete force-time–based variables of neck muscles during maximal voluntary isometric contractions in 5 directions. Design: Cohort study. Setting: University research center. Patients or Other Participants: Twenty-six highly physically active men (age  =  21.6 ± 2.1 years, height  =  1.85 ± 0.09 m, mass  =  81.6 ± 9.9 kg, head circumference  =  0.58 ± 0.01 m, neck circumference  =  0.39 ± 0.02 m). Intervention(s): We used a custom-built testing apparatus to measure maximal voluntary isometric contractions of the neck muscles in 5 directions (extension, flexion, protraction, left lateral bending, and right lateral bending) on 2 separate occasions separated by 7 to 8 days. Main Outcome Measure(s): Variables measured were peak force (PF), rate of force development (RFD), and time to 50% of PF (T50PF). Reliability indices calculated for each variable comprised the difference in scores between the testing sessions, with corresponding 95% confidence intervals, the coefficient of variation of the typical error of measurement (CVTE), and intraclass correlation coefficients (ICC [3,3]). Results: No evidence of systematic bias was detected for the dependent measures across any movement direction; retest differences in measurements were between 1.8% and 2.7%, with corresponding 95% confidence interval ranges of less than 10% and overlapping zero. The CVTE was lowest for PF (range, 2.4%–6.3%) across all testing directions, followed by RFD (range, 4.8%–9.0%) and T50PF (range, 7.1%–9.3%). The ICC score range for all dependent measures was 0.90 to 0.99. Conclusions: Discrete variables representative of the force-generating capacity of neck muscles under isometric conditions can be measured with an acceptable degree of reliability. This finding has possible applications for investigating the role of neck muscle strength-training programs in reducing the risk of injuries in sport settings.