Bilateral Asymmetry of Semispinalis Capitis Muscle Thickness and Neck Motion during Prone Neck Extension in Subjects with Unilateral Posterior Neck Pain

[Purpose] This study compared the bilateral thicknesses of the semispinalis capitis muscle and neck motions between subjects with and without unilateral posterior neck pain (UPNP). [Subjects] The study recruited 20 young subjects with UPNP at the end-range of neck extension and 20 age- and sex-matched subjects without neck pain as a control group. [Methods] The bilateral thicknesses of the semispinalis capitis in a relaxed prone position were measured bilaterally using ultrasonography. A three-dimensional motion-analysis system was used to measure the asymmetry of neck motions at 45 of prone neck extension. [Results] We found that the asymmetry of the semispinalis capitis muscle was greater in the UPNP group than in the controls. Neck rotation to the painful side and lateral bending to the non-painful side were greater in the UPNP group at 45 of prone neck extension than in the controls. [Conclusion] Asymmetric thickness of the semispinalis capitis muscles might be the cause of asymmetric neck motions in subjects with UPNP.