Fatigue changes neck muscle control and deteriorates postural stability during arm movement perturbations in patients with chronic neck pain

BACKGROUND CONTEXT Multisensory afferent inputs to the cervical spine affect the generation of neck muscle control. Chronic neck pain (CNP) and muscle fatigue are factors that disturb somatosensory function. Whether they affect postural control under self-initiated perturbation in daily activities is still unclear. PURPOSE To investigate the effect of CNP and neck flexor muscle fatigue on muscle control strategy and postural control in young patients performing voluntary shoulder flexion movements. STUDY DESIGN Cross-sectional case-control study. PATIENT SAMPLE Twenty-five patients with CNP and 25 age-matched asymptomatic controls. OUTCOME MEASURES The postural sway, muscle onset time, and activation level of the erector spinae, rectus abdominal, semispinalis capitis (SSC), and sternocleidomastoid (SCM) muscles were recorded and analyzed using two-way ANOVA to evaluate the interaction of CNP and muscle fatigue on standing balance and muscle control upon self-initiated perturbations. METHODS All participants were instructed to perform shoulder flexion movements in the standing position before and after a neck flexor muscle fatigue exercise under either the eyes-open or eyes-closed condition. RESULTS The CNP group exhibited significantly larger body sway, greater neck muscle activation (SCM and SSC), and longer onset time of neck flexor muscle (SCM) compared with the control group. The CNP group also demonstrated a trend of greater postural sway and shorter muscle onset under the eyes-closed condition than those under the eyes-open condition. After muscle fatigue, the CNP group further exhibited (1) greater body sway during the eyes-open condition but decreased body sway during the eyes-closed condition, (2) higher activation of the neck flexor (SCM) and lower activation of the trunk extensor (erector spinae), and (3) early onset of the neck muscles (SCM and SSC). CONCLUSIONS CNP causes poor postural control and altered neck muscle control patterns. The addition of neck flexor muscle fatigue further decreases balance stability and provokes a protective neck muscle control strategy during the shoulder flexion movement. Those findings facilitate the understanding of the strategies adopted by patients and suggest that neck endurance training programs may be beneficial to improve whole postural control in patients with CNP.