Muscle activation and cutaneous reflex modulation during rhythmic and discrete arm tasks in orthopaedic shoulder instability.

In orthopaedic shoulder instability, muscle activity (EMG) is altered during unconstrained discrete arm movement tasks (e.g. elevation against a load). These findings have been ascribed to deficits in afferent feedback and neural control with glenohumeral instabilities resulting from orthopaedic injury. However, the integrity of neural control during shoulder movements in those with unstable shoulders is unclear. It is not known if there are altered EMG patterns during rhythmic arm movement or during discrete tasks involving no load, as would be experienced in many arm motions performed in daily living. The primary objective of this study was to evaluate neural control of arm movements between those with unstable shoulders and control participants, within a constrained arm movement paradigm involving both rhythmic arm cycling and discrete reaching. To achieve this objective, we determined if the amplitude and timing of EMG related to the movement pattern (background EMG) was significantly different between groups. Cutaneous reflexes were used to simulate a perturbation to the upper limb that would typically evoke a coordinated response. In the elevation phase of the movement path for anterior and posterior deltoid, upper trapezius, infraspinatus and serratus anterior, background EMG during rhythmic arm cycling was significantly (24%, p < 0.05) larger in unstable shoulders than in controls. No differences were found in background EMG between the groups during the discrete task. Significant differences (p < 0.05) were also noted in cutaneous reflexes between groups for both the rhythmic and discrete tasks with the reflex amplitudes being either increased or reduced in unstable shoulders as compared to controls. The differences in the background EMG and the cutaneous reflexes patterns in those with shoulder instabilities suggest that neural control is altered during rhythmic movement.