Background: Neck pain is a growing problem which is linked to occupational factors that include work above shoulder height or sustained neck flexion. These activities may induce fatigue in the neck muscles impairing the muscles’ ability to provide reflex contractions that protect against tissue injury. The aim of this study was to investigate the effect of neck muscle fatigue on reflex activation of the neck muscles.

Methods: Healthy volunteers were subjected to one of two loading protocols. Isometric contractions of neck extensors at 60% MVC were sustained to the endurance limit (n=30) to induce high level fatigue in these muscles. A similar protocol for neck flexors (n=21) was used to initiate low level contraction of the extensors which are co-activated to stabilise the cervical spine under such circumstances. Before and after each loading protocol, reflex activation of the trapezius muscle was assessed using skin surface electromyography (EMG) to measure the latency and amplitude of muscle activation in response to a sudden perturbation of the head.

Results: Reflex latencies increased from 73±17ms to 93±27ms (p=0.0041), and from 72±12ms to 97±28ms (p< 0.0001) following low and high level extensor fatigue, respectively. Time to peak EMG also increased from 122±32ms to 148±39ms (p=0.0093), and from 113±15ms to 138±25ms (p< 0.0001), respectively, although no change in peak EMG amplitude was observed.

Conclusions: Reflex activation of trapezius was substantially delayed following both loading protocols. These findings suggest that even low level postural loading in the workplace may impair neck muscle reflexes rendering the underlying tissues more vulnerable to strain injury.

Conflicts of Interest: None

Source of Funding: BBSRC (Biotechnology and Biological Sciences Research Council, UK)