Assessment of Neck Muscle Shear Modulus Normalization in Women with and without Chronic Neck Pain

Identifying the objective stiffness of the neck muscles facilitates the early and specific diagnosis of neck pain and targeted therapy. However, individual variation in the muscle shear modulus obscures differences between healthy and diseased individuals. Normalization may improve the comparability between individuals. The shear modulus at different functional tasks served as a reference for normalizing the neck muscles' shear modulus of 38 women, 20 with chronic neck pain and 18 asymptomatic. Reference tasks were maximal voluntary contraction, relaxed sitting, prone head lift, balancing 1 kg on the head, and neck extension at 48 N. The effects of normalization on within-group variation and between-group differences were compared. Normalization with maximal voluntary contraction was discarded due to imaging problems. Normalization with relaxed sitting, prone head lift, balancing 1 kg, and neck extension at 48 N reduced within-group variation, by 23.2%, 26.8%, 11.6%, and 33.6%, respectively. All four normalization approaches reduced the p-values when testing for between-group differences. For the pain group, normalization with relaxed sitting and head lift indicated less normalized muscle stiffness, while normalization with balancing 1 kg and extension at 48 N indicated higher stiffness. The contradictory results are explainable by non-significant group differences in the reference tasks. Normalization of the muscle shear modulus is effective to reduce within-group variation, but a trustworthy normalization approach for group comparisons has yet to be identified.