Flexibility and Rehab Tips
Research suggests that up to 67% of the adult population will experience shoulder pain at some point in their lifetime (10). Individuals seeking care for their shoulder are often prescribed activity-based interventions ranging from stretching to muscle performance exercises. The prescription is often based on impairments, activity, or sport-specific needs as well as the individuals' diagnosis. Although the precise etiology of shoulder pain or dysfunction may be elusive, it is not uncommon for rehabilitation specialists to prescribe interventions that seek to improve muscle performance of the rotator cuff. Of the musculature comprising the rotator cuff, the external rotators in particular are often addressed in rehabilitation programs because of their predilection for pathology and weakness. Rehabilitation professionals often focus on the concentric action of a muscle contraction and reserve exercises that isolate eccentric muscle actions for the athletic population despite compelling evidence in favor of eccentric training. Specifically, evidence exists to suggest that eccentric training induces a favorable benefit on muscle strength (13), hypertrophy (13), and stimulation of anabolic hormones and growth factors when compared with isometric- and concentric-based training (4). Moreover, the clinical efficacy of eccentric training for reversing degenerative changes in tendons (12,9), improving function (1,2,5,6), and decreasing pain (1,2,5,6) has been reported in clinical-based investigations.
The purpose of this column is to provide an evidence-based discussion of both the clinical efficacy and the biophysiological adaptations that may result from eccentric training of the shoulder external rotator musculature. This is part 1 of a 2-part column, which will be used to substantiate the practical applications presented in part 2.
From a prescriptive perspective, the efficacy of performing eccentric exercises for the shoulder external rotators has been identified among individuals with shoulder pathology (1,2,5,6). Bernhardsson et al. (1) investigated the effect of eccentric supraspinatus and infraspinatus exercises performed twice a day for 12 weeks among individuals diagnosed with shoulder impingement syndrome and reported significantly improved pain and functional outcome measures. Jonsson et al. (6) studied eccentric loading of the supraspinatus in 9 patients with chronic shoulder impingement syndrome on a waiting list for shoulder surgery. Exercises were performed twice a day, every day for 12 weeks. In 5 of the 9 patients, significant improvement in pain and function was noted, with all 5 patients canceling the scheduled surgical procedures. Camargo et al. (2) investigated the effect of twice a week eccentric exercises on 20 subjects with shoulder impingement syndrome and reported significant improvements in pain, function, and strength at a 6-week follow-up. Holmgren et al. (5) in a randomized trial compared the effect of eccentric training combined with traditional exercises to a control group of nonspecific unloaded exercises in 97 subjects with shoulder impingement syndrome. After 12 weeks of daily exercises, the experimental group had a significant improvement in shoulder function, pain levels, and global change compared with the control group.
The physiological processes that describe the effects of eccentric loading on muscle and tendon tissue are mechanotransduction, microcirculation, and growth differentiation factor/hormonal response. Mechanotransduction describes the process by which the bodies' tissues undergo an adaptive structural change in response to a mechanical load (7). During an eccentric exercise with sufficient overload, a tensile stress is placed on a bone-tendon-muscle complex. This overload-induced stress stimulates protein synthesis, which has been found to result in cellular matrix remodeling and reversal of degenerative changes (12).
Neovascularization is a term used to describe increased and abnormal microvessels in pathological tissue. An association between shoulder tendinopathy and neovascularization has been established as a potential reparative process (9). Eccentric loading has been found to decrease the abnormal circulation while concurrently diminishing a patient's symptoms (8). These results are favorable because while normalized tendon microcirculation has been established, no concurrent detrimental change in tissue oxygen saturation has been identified.
The hormonal response to eccentric loading revolves around the levels of myostatin, insulin-like growth factor (IGF), and mechano growth factor (MGF). Myostatin is a protein found in muscle tissue that has an inhibitory effect on muscle cell differentiation and hypertrophy (3). IGF isoforms have the opposite effect on muscle growth and are an essential component for increasing muscle hypertrophy. MGF also has a positive effect on exercise-induced muscle hypertrophy and occurs at the local muscle tissue level through autocrine secretion. Eccentric exercises have been found to have a favorable effect on decreasing myostatin while increasing IGF and MGF expression when compared with concentric and isometric muscular contractions (4), thus offering utility for facilitating muscle hypertrophy and associated muscle performance improvements.
Eccentric interventions may often become overlooked by clinicians who traditionally use concentric shoulder exercises. Given the association between eccentric weakness and shoulder pathology as well as the clinical efficacy of eccentric exercises, it seems reasonable to recommend the inclusion of eccentric exercises. Moreover, eccentric exercise may be efficacious for individuals such as overhead athletes and weight-training participants who may be at risk for shoulder injury (11). The favorable anabolic hormonal response provides a means for improving strength and hypertrophy, thus augmenting traditional routines would seem to be advantageous. Part 2 of this series will provide exercise description and prescription guidelines. Although the health and fitness benefits attributed to eccentric training are well known, participation may not be without risk. Individuals with an active shoulder injury should consult with an appropriate medical professional before beginning the exercises recommended in part 2 of this 2-part series.
1. Bernhardsson S, Klintberg IH, Wendt GK. Evaluation of an exercise concept focusing on eccentric strength training of the rotator cuff for patients with subacromial impingement syndrome. Clin Rehabil 25: 69–78, 2011.
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9. Lewis JS, Raza SA, Pilcher J, Heron C, Poloniecki JD. The prevalence of neovascularity in patients clinically diagnosed with rotator cuff tendinopathy. BMC Musculoskelet Disord 21: 163, 2009.
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13. Roig M, O’Brien K, Kirk G, Murray R, McKinnon P, Shadgan B, Reid WD. The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: A systematic review with meta-analysis. Br J Sports Med 43: 556–568, 2009.