Effects of sternocleidomastoid muscle fatigue on cervical spine kinematics and postural responses to trunk perturbation

This thesis investigated the effects of sternocleidomastoid (SCM) fatigue on cervical spine kinematics and postural responses to trunk perturbation in upright standing. We hypothesized that SCM fatigue would result in greater motion of the head, and that postural control would be impaired as a result. Our findings indicate that SCM fatigue catalyzed a change in postural set that increased trunk joint stiffness. We propose that this change reduced the transfer of momentum from the pendulum to the body to facilitate control of the body's centre of mass. Increased trunk joint stiffness, along with attenuated contractile capacity of the fatigued SCM, resulted in greater magnitudes of head acceleration. Despite the increase in head acceleration, the postural control system effectively controlled motion of the body's centre of mass. The neuromuscular system is highly versatile and can modify a control strategy according to different forms of threat to the maintenance of postural equilibrium.