Increased Rate Of Force Development Is Correlated To Symptom Improvement In Patients With Rotator Cuff Tendinopathy

Abstract

Rotator cuff (RC) tendinopathy is the most common cause of shoulder pain. Exercise is a beneficial treatment, but many patients have recurrence of pain within one year. Disability is associated with RC muscle performance deficits. However, there is poor understanding of the neuromuscular mechanisms associated with these deficits. Exercise-based clinical trials have shown limited effects on isometric strength as the metric of muscle performance. Exploring neuromuscular mechanisms that account for both neural and muscular components of muscle performance, such as rate of force development (RFD) are warranted. PURPOSE: Determine the effects of an exercise intervention on strength and RFD, and the relationship to changes in patient-reported outcomes. METHODS: Participants (n = 7; age = 26.8 ± 5) diagnosed with RC tendinopathy underwent an exercise protocol for 2 weeks, progressing from exercise with the arm below 90°, to exercise above 90° and increased resistance. Measures were taken at baseline and post-intervention. Pain and function outcomes were measured with the Penn Shoulder Score. Maximum isometric strength via peak force, and RFD for scaption and external rotation (ER) were measured using a fixed hand-held dynamometer. RFD was defined as the area under the force-time curve from 0 to 50, 100 and 200 ms. Paired t-tests compared pre to post-intervention measures. Pearson’s correlation defined the relationship between changes in strength and RFD with outcomes. RESULTS: There were significant changes in pain [mean difference (MD) = -3.3; (95%CI: -0.32, -6.24); p = 0.03], and function [MD = 3.3 (95%CI: 0.05, 6.5); p = 0.04] pre- to post-intervention. There was a significant high correlation between the change in pain and ER RFD at 50 ms (r = 0.81; p = 0.02) and 100 ms (r = 0.74; p = 0.05), but not with maximum isometric strength. CONCLUSIONS: The change in pain can be explained by RFD at 50 ms, and to a lesser extent at 100 ms. RFD at 75 ms or above is associated with muscle contractile properties, while below is associated with neural changes. Our results suggest that neural changes in force production may be more relevant to decreased pain with exercise. Selecting exercises that target an increase in RFD may be beneficial, but more research is needed to explore if this potential mechanism also plays a role in long-term improvements.