Sternocleidomastoid Muscle Overactivity: A Potentially Critical Contributor to Postural Abnormalities in Parkinson's Disease

Objective Overactivity of muscles is thought to be involved in postural abnormalities (PA) in Parkinson's disease (PD). Here, we investigated the relationship between muscle activity and postural parameters to explore the peripheral mechanisms of PA. Methods A total of 90 PD patients and 19 healthy controls were enrolled. Posture features (F1–F8) and the index for PA were collected. Surface electromyography was acquired from cervical and thoracolumbar muscles during lying, sitting, standing, and walking, respectively. Follow‐up was completed for a subset of PD patients. Results Root mean square (RMS) amplitudes of the sternocleidomastoid muscle (SCM) and external oblique muscle, were higher in PD patients with PA (PD_PA) than PD patients without PA (PD_NPA) and healthy controls during lying and standing. Extensor activity in PD_PA patients increased only in the antigravity position compared with PD_NPA patients. Spearman's correlation showed that RMS amplitude of SCM in the lying position was associated with index for PA and forward flexion angles including F3–F8, whereas RMS amplitude of the external oblique muscle in the lying position was correlated with F5 alone. Longitudinal analysis showed that changes in F3, F4, and index for PA were significantly correlated with changes in RMS amplitude of the SCM in the lying position. Additionally, PD_NPA patients with SCM overactivity had a significantly higher risk of progressing to PA than those with normal SCM activity. Interpretation The SCM and external oblique muscle are involved in forward trunk flexion in PD patients, while extensors may play a compensatory role. SCM may be the key muscle in PA for PD patients, participating in the pathogenesis of PA. ANN NEUROL 2025;98:967–976