Biomechanical mechanisms of multidirectional dynamic compensatory muscle fatigue induced by abnormal cervical curvature: a cross-sectional case-control study based on surface electromyography and Cobb angle

Rui Yang¹Mou Wang²Yifan Yang²Xingyu Zhang³Yiran Wu²Wenna Zhou¹Tao Ding^2*Jing Xu^2*

• ¹School of Rehabilitation, Kunming Medical University, Kunming, China
• ²Department of Rehabilitation, First Affiliated Hospital of Kunming Medical University, Kunming, China
• ³Department of Scientific and Technology, Kunming Medical University, Kunming, China

Introduction: Cervical lordosis, a core indicator of spinal sagittal balance, is closely associated with cervical dysfunction when abnormal (straightening or kyphosis). The muscular system, including the upper trapezius (UT) and sternocleidomastoid (SCM), contributes to the mechanical stability of the cervical spine. However, how abnormal curvature affects dynamic muscle fatigue mechanisms remains unclear. This study investigated how abnormal curvature influences the performance of the UT and SCM during movement and their correlations. Methods: Employing a cross-sectional case-control design, 49 participants were enrolled. Cervical lateral radiographs were used to measure the C2-C7 Cobb angle, categorizing participants into an abnormal curvature group (25 chronic neck pain patients) and a normal curvature group (24 healthy individuals). Surface electromyography (sEMG) monitored the UT and SCM during six cervical movements at maximal voluntary contraction. Frequency-domain parameters analyzed included median frequency (MF), mean power frequency (MPF), and their slopes (MFs, MPFs). Results: The C2-C7 Cobb angle was significantly lower in the abnormal curvature group compared to the normal group (p < 0.01). During dynamic activities, muscle fatigue was exacerbated in the abnormal curvature group: during extension, the left UT showed significantly decreased MPF (p < 0.01) and MF (p < 0.05); during flexion, the left SCM exhibited significantly increased MPFs (p < 0.05) and MFs (p < 0.01). Correlation analysis revealed that within the abnormal curvature group, worsening kyphosis showed a weak positive correlation with the MPF of the right UT during right lateral flexion (r = 0.492, p < 0.05). Conversely, within the normal group, increasing lordosis correlated with reduced fatigue risk. Conclusions: Abnormal cervical lordosis significantly increases dynamic muscle fatigue, manifesting as deteriorated sEMG spectral characteristics in the UT and SCM. Furthermore, a complex relationship exists between the cervical Cobb angle and muscle fatigue. This confirms the hypothesis that abnormal curvature induces compensatory muscle activation. The findings provide a biological basis for rehabilitation strategies, suggesting that clinical interventions should be tailored based on cervical curvature type. Clinical Trail Registration: Chinese Clinical Trial Registry (ChiCTR25001081490)