Objective: The purpose of this study was to assess the correlation, concurrent validity, and agreement between the isometric cervical force measurements obtained with fixed and portable dynamometers in asymptomatic individuals.

Methods: Fifty asymptomatic individuals performed 3 maximal isometric contractions for flexion, extension, and lateral flexion of the cervical spine using fixed and portable dynamometers. The correlation and concurrent validity for the measurements of the portable and fixed dynamometers were analyzed using Spearman's correlation coefficient and the intraclass correlation coefficient (ICC), respectively. The agreement between the force values of the portable and fixed dynamometers was measured using the Bland-Altman method.

Results: Isometric cervical force measurements obtained with the fixed dynamometer and portable dynamometer showed a moderately to highly significant correlation for flexion (r_s = 0.74), extension (r_s = 0.82), right lateral flexion (r_s = 0.74), and left lateral flexion (r_s = 0.68). The concurrent validity was moderate to good for all measurements (ICC_2,3 = 0.67-0.80). The fixed and portable dynamometers did not agree, with a significant mean difference between the methods of 2.8 kgf (95% confidence interval [CI], 2.1-3.4 kgf) for cervical flexion, 5.3 kgf (95% CI, 4.2-6.4 kgf) for extension, and 9.1 kgf (95% CI, 0.4-2.1 kgf) for left lateral flexion. The limits of agreement were broad for all movements, with errors that varied between 61% and 77% of the mean force obtained with the fixed dynamometer.

Conclusion: The neck strength measurements obtained with the fixed and portable dynamometers demonstrated high to moderate correlation and had moderate to good comparability for asymptomatic participants. However, they did not agree in that the 2 methods did not provide equivalent measurements, and, therefore, based on these findings, the same equipment should always be used when reassessing an individual.