The shoulder contains a complex group of muscles with varying and important roles in force generation and motion control. There have been a variety of stances taken on the importance of internal and external rotators on shoulder pain. However, shoulder rotators should never be thought about in isolation. Internal rotation (IR) and external rotation (ER) require additional stabilizers and secondary rotators to complete their desired task.
Swimming requires increased IR load of the shoulder regardless of the stroke being performed. In swimmer's who lack a conditioning program, ER/IR strength ratio can be decreased. Although ER is not a force generating component of swimming, without maintenance can cause movement imbalances and scaplular dyskenesis.
Some research discusses the role of training on this imbalance. Ramsi et al. (2004) followed a high school swim team of 27 members and tracked their IR/ER strength ratios over the season. The authors found that IR continued to increase during the season without matched ER strength gains. However, Bak and Magnusson (1997) had conflicting research in which painful and painless swimmers were evaluated. Their results indicated that a decreased IR strength was significantly associated with pain between groups. We think it's important to identify that imbalances can occur as a cause or result of pain. Unfortunately, it is another area to be more closely researched. What is evident is that muscular imbalances, one way or another, can contribute to resting humeral head alignment, dynamic mechanics and pain.
Internal rotation is performed by the anterior deltoid, latissimus dorsi, pectoralis major, teres major and subscapularis. To understand the muscles involved in the IR action is important. As you can see, there are some large, strong muscles involved in internal rotation which often kick in earlier or endure longer than smaller muscle groups. If you think about the "lat" and the "pec" these muscles are going to fire throughout the entire pulling phase of the stroke, biasing an internal rotation moment. This is necessary for propulsion but good to note during conditioning, as these may be adequately strong.
External rotation is performed by the posterior deltoid, teres minor and infraspinatous. These are some small muscle groups and are easily fatigued on their own. Now think of the arm wrestle that has to go on throughout a swim stroke between ER and IR muscle groups. IR often has the upper hand and if shoulder IR continues to bully shoulder ER, injury is on the horizon.
As coaches make their conditioning programs its important to keep in mind what motions/muscle groups are activated and what motions/muscles might be active secondarily. Evaluate your swimmers strength and posture to ensure that the program is a good fit for their presentation.
We want you to understand the necessity of keeping ER/IR ratios consistent even as the swimmer becomes stronger. Imbalances in either direction can cause instability, pain perception, faulty positioning and decreased force production. Keep tuning into our posts and give us some feedback via email, website or Facebook!
Sam LaRiviere & Dan Fay
Grandeur Movement Science
References
1. Ramsi M, Swanik K, Swanik C, Straub S, Mattacola C. Shoulder-rotator strength of high school swimmers over the course of a competitive season. Journal Of Sport Rehabilitation. February 2004;13(1):9-18.
2. Bak K, Magnusson P. Shoulder strength and range of motion in symptomatic and pain-free elite swimmers. The American Journal of Sports Medicine. July 1997;25(4):454 - 459.