Biomechanics of shoulder injury after a car crash
Whiplash patients often complain of shoulder soreness, and scientists are beginning to understand why. While previous research has studied shoulder dysfunction in whiplash patients, a new study examined the mechanisms behind this process.
The serratus anterior (SA) and trapezius muscles stabilize the scapula in the shoulder and assist with arm elevation. Using electromyography, researchers discovered that both neck pain and whiplash patients have altered dysfunction the in the SA muscle.
When neck pain and whiplash patients moved to elevate their arms, the SA muscles were less responsive, took longer to activate, and were less active than muscles in healthy patients. This delayed activity of the SA muscle was only weakly correlated with increased pain and disability in whiplash patients. Altered activity of the SA may be more indirectly involved in exacerbating neck pain. As patients compensate for dysfunction in the shoulder muscles, altered activity of the SA could lead to detrimental loading on the thoracic and cervical spine. Detrimental loading could prolong neck pain symptoms.
Surprisingly, patients did not have altered function of the trapezius muscle. Researchers hypothesized that the trapezius muscle could respond differently to neck pain than the SA muscle. A 2009 study suggests that chronic neck pain is associated with dilation of blood vessels in the trapezius muscles but not increased muscle activity.
While more research is needed to fully understand how the muscles respond to whiplash injury, it's clear that whiplash alters the function of the shoulder muscles which could lead to worsening conditions.
Helgadottir, H et al. Altered activity of the serratus anterior during unilateral arm elevation in patients with cervical disorders. Journal of Electromyography and Kinesiology 2011; 21(2): 947-953.