Altered control of the trapezius muscle in subjects with non-traumatic shoulder instability

Reflexes of shoulder girdle muscles such as trapezius are evoked from muscle afferents supplying the forearm and hand in healthy subjects. These reflexes are thought to aid the stability of the shoulder during use of the arm and hand [Alexander CM, Harrison PJ. Reflex connections from forearm and hand afferents to shoulder girdle muscles in humans. Exp Brain Res 2003;148: 277-282.]. With this in mind, the objective of this investigation was to examine this trapezius reflex in subjects with non-traumatic shoulder instability (NTSI).

The occurrence and alteration to this supraspinal reflex pathway were investigated using electrical stimulation of the ulnar and the spinal accessory nerves as well as magnetic stimulation of the trapezius motor cortex. These results were compared to a healthy group.

The reflexes to lower trapezius were not usually observed in subjects with NTSI. When evoked, the frequency of occurrence was 27% compared to 87% in a healthy population (p<0.002). When present, the latency of this lower trapezius reflex was later than that evoked in the healthy group (62.8 ms+/-28.1 ms and 38.4 ms+/-3.6 ms, respectively; p<0.009). Although both the conduction velocity of the effective afferents and the latency of the M response to lower trapezius did not differ to the healthy group (p<0.24 and 0.54, respectively), the latency and threshold of the corticospinal evoked potential of lower trapezius did differ (16.7 ms+/-4.7 ms vs 11.2 ms+/-1.8 ms; p=0.006; 57%+/-15.8% vs 36%+/-7.6%, p=0.003). Overall, these results contrast to these measures of control of upper trapezius, which did not differ to a healthy group.

The delay or absence of these reflexes and the delay and change in threshold of the corticospinal response in lower trapezius in subjects with NTSI indicate that the feedback mechanisms that aid shoulder girdle stability, and the voluntary control of lower trapezius are not as proficient in these subjects.

The control of lower trapezius should be considered when treating people with NTSI.