VOLUNTARY ACTIVATION OF ROTATOR CUFF IN PATIENTS WITH SHOULDER TENDINOPATHY AFTER PAIN RELIEF AND EXERCISE

K. Shah¹, D. Safford², K. Madara³, A. Karduna⁴, B. Sweitzer⁵, P. McClure²

¹Arcadia University, Physical Therapy, Glenside, United States, ²Arcadia University, Glenside, United States, ³Moravian University, Bethlehem, United States, ⁴University of Oregon, Eugene, United States, ⁵Einstein Healthcare Network, East Norritown, United States

Background: Rotator cuff tendinopathy is the most common cause of shoulder pain. Shoulder exercise and subacromial injection are the first line of treatment, and have been shown to be effective in about 70% of patients. Weakness is common in these patients although its true source is uncertain.

Purpose: The purpose of this study is to determine the changes in rotator cuff voluntary activation (VA), i.e. central drive and force, immediately after a pain-relieving subacromial injection, and following a 6-week exercise program, in patients with rotator cuff tendinopathy.

Methods: 43 patients (21F, mean age 39.2 (11.8), Painful shoulder – 23R) with positive shoulder impingement tests were included. Patients were seated on a custom force testing apparatus. Electrodes were placed on the infraspinatus muscle to deliver stimulation and permit collection of VA data. Two isometric maximal voluntary contractions (MVCs) were collected during external rotation. The patient then received the 1^st brief, strong electrical impulse (3-pulse train of 600µs pulses delivered at 50 Hz at an intensity of 150 mV) when they were contracting at 95% of their MVC, and a 2^nd impulse at rest. The peak force from the voluntary portion of the isometric contraction, the force augmentation produced by the superimposed electrical stimulus, and the force produced by the stimulus applied to the relaxed muscle are measured and used to determine the VA (VA = 1, indicates full activation of the muscle). VA data were collected at baseline (T1), after the subacromial injection (T2), and after the 6-week exercise program (T3) (n = 36, data missing due to COVID). Exercise included strengthening, stretching and joint mobilization. Pain rating with activity was collected at all 3 time points, and self-report Penn Shoulder Score (PSS) was collected at T1 & T3.

Results: The VA increased (p<0.05) from T1 (0.74 (0.27)) to T2 (0.90 (0.16)) and remained unchanged at T3 as compared to T2. External rotation peak force increased (p<0.05) across all time points and averaged 107.2N (50.9) at T1 to 127.4N (57.9) at T3. Pain with activity, measured using numeric pain rating scale, was significantly reduced (p<0.01) from baseline (5.3 (2.1)) to after injection (2.0 (1.7)) and continued to decrease at T3 (1.7 (1.3)). The PSS changed from 62.1 (14.3) at baseline to 84.2 (15.1) at discharge.

Conclusions: As expected, VA was reduced at baseline and improved dramatically after the injection, suggesting that improvements in pain improve muscle activation. At 6-weeks post-exercise, pain with activity, and self-reported pain and function measures all improved. The change in the PSS scores, 25 points, exceeded the previously published MCID of 11.4.

Implications: 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.

Funding acknowledgements: RO1AR063713 NIH/ NIAMS - Andrew Karduna (PI), University of Oregon; Philip McClure (subcontract, PI), Arcadia University

Keywords:
Shoulder

Topics:
Musculoskeletal: upper limb

Did this work require ethics approval? Yes

Institution: Arcadia University

Committee: Arcadia University Institutional Review Board

Ethics number: IRB: 14-10-32