Voluntary activation is impaired in subacromial pain syndrome but improves with pain relief and exercise.

Abstract

Shoulder exercise and subacromial injection are the first line of treatment for patients with subacromial pain syndrome (SAPS) and have been shown to be effective in about 70% of patients. Weakness is common in these patients although its true source is uncertain. The purpose of this study is to determine the changes in rotator cuff voluntary activation (VA), that is, central drive and force, immediately after a pain-relieving subacromial injection, and following a 6-week exercise program and examine baseline differences in patients with SAPS and healthy controls. Voluntary activation, peak normalized external rotation (ER) torque, pain, and self-report function were assessed in 43 patients with positive shoulder impingement. Subjects were tested at baseline (T1), immediately after a pain-relieving injection (T2), and after 6 weeks of exercise (T3). Forty-4 controls were tested at baseline (T1). Participants received 2 stimuli on their infraspinatus, one while contracting maximally and one at rest, to determine the VA. Subjects with SAPS demonstrated lower ER normalized torque (27.3% lower compared with controls, P = .005) and lower infraspinatus VA (median 0.99 in controls and 0.71 in patients, P < .001). As expected, the VA increased (P = .004) from T1 to T2 and remained unchanged at T3 as compared to T2. ER normalized torque increased across all time points. Pain (P < .001) decreased with the injection and exercise, and function (P < .001) improved from baseline to discharge. Understanding neural adaptations with exercise is critical to learning how to best modify the system and optimize current rehabilitation strategies, for example, including exercises focused on motor-control training, biofeedback, or neuromuscular electric stimulation.