The Influence of Idiopathic Chronic Neck Pain on Sternocleidomastoid and Upper Trapezius Muscle Activity and Elasticity

Abstract

Approximately half of all adults will experience neck pain. Without early detection and proper treatment, neck pain will likely become chronic. Chronic neck pain is one of the primary causes of disability in the United States and results in great financial and economic burden. Additionally, idiopathic chronic neck pain is difficult to treat due to the unknown etiology underlying the condition. Idiopathic chronic neck pain is often associated with static working conditions, including seated computer work. The stationary nature of modern computer work contributes to a rising incidence of chronic neck pain, highlighting the need to determine the underlying adaptations associated with idiopathic chronic neck pain to improve the prevention and treatment of neck pain. The purpose of this dissertation was to establish the neuromuscular and mechanical adaptations associated with idiopathic chronic neck pain during assessments of functional reaching and seated computer-work ergonomics. In Chapter 2, we used a combination of ultrasound shear wave elastography and electromyography to examine variations in activation and stiffness of the sternocleidomastoid across 3-dimensional isometric neck torques. In Chapter 3, we determined the influence of idiopathic chronic neck pain on the activation and stiffness of the upper trapezius and sternocleidomastoid during the maintenance of unilateral and bilateral functional reaching. Lastly, in Chapter 4, we determined the effect of chair recline and head and neck position on sternocleidomastoid and upper trapezius stiffness and activation during computer work in people with idiopathic chronic neck pain and healthy controls. Results from this dissertation suggest that idiopathic chronic neck pain is associated with changes to both the neuromuscular control and mechanical properties of neck and shoulder musculature during both static reaching tasks and seated computer work. Our results indicate that shoulder position differences between healthy controls and people with idiopathic chronic neck pain may be driving stiffness adaptations in the sternocleidomastoid and upper trapezius muscle during reaching tasks. Our investigation into seated computer work found that head and neck posture has a greater effect than chair recline on upper trapezius stiffness and activation. The self-selected head and neck posture may indicate that individuals are able to determine which posture results in the lowest effort, and therefore activation, of their neck muscles. Findings from this dissertation have broad and important clinical implications. We have established that shear wave velocity scales with muscle activation in muscles that have lines of action in three dimensions. Our novel findings suggest that the use of EMG alone in investigations into idiopathic chronic neck pain is not adequate to fully represent muscle function. The addition of ultrasound shear wave elastography in our studies provided more nuanced insight that would be missed using EMG alone. The findings of this dissertation could have new clinical utility for evaluating individuals with idiopathic chronic neck pain. We found these patients often displayed distinct changes in muscle stiffness that were not related to changes in muscle activity during similar reaching trials and reclined seated computer work. We also observed for the first time that there are clear sex differences in muscle stiffness and activity amongst key neck-shoulder muscles. These results highlight the importance of evaluating both muscle stiffness and activation changes before designing rehabilitation programs and motivates the need for further research to determine the feasibility of using ultrasound shear wave elastography for clinical examination of idiopathic chronic neck pain.