Poor Shoulder External-to-Internal Strength Ratios Found to Increase Injury Risk in Overhead Athletes and Their Applicability to Recreational Weight Training Populations

Justin Gibson, BS, CSCS

Introduction

Shoulder performance and injury is a major topic of interest for professional athletes, recreational fitness enthusiasts, and the general population alike. Whether discussing one’s ability to throw a baseball at a consistently high velocity or their ability to hoist a gallon of milk onto a countertop, the strength and health of the glenohumeral joint should and has been the subject of a large body of scientific literature. However, when seeking funding for a research proposal, it is perhaps easier to justify studying a shoulder when it is attached to a body on a $3.1 million dollar 2-year contract than it would be if it were attached to someone lifting weights as a hobby. And so, a large proportion of our scientific literature regarding the shoulder tends to focus on athletes, specifically overhead athletes, when attempting to identify potential risk factors for injury.

          One such risk factor explored in these studies has been the balance between external rotation strength of the shoulder and internal rotation strength. For overhead athletes in particular, it is believed that the eccentric strength of the external rotators should be as strong as the concentric strength of the internal rotators to maintain glenohumeral stability and optimal shoulder function (Wang & Cochrane, 2001). In this paper, I will discuss the scientific evidence linking internal/external rotator imbalance as a predictor of injury in overhead athletes, the similarities between these athletes and recreational weightlifters, and possible data that may build a bridge between them.

External/Internal Rotation Strength Ratios as a Predictor for Injury in Overhead Athletes

          Several studies have noted a link between shoulder muscle imbalance and injury. Two such studies looked at several factors that may contribute to shoulder injury: range of motion, external rotation weakness, scapular asymmetry, and ER/IR strength ratios. Clarsen, Bahr, Anderson, Munk, & Myklebust (2014) measured all of these factors when following the teams in the Norwegian elite handball series for men for three months of the 2011-2012 season. They found significant associations between ER weakness, obvious scapular dyskinesis, and shoulder range of motion, and inury, and “noteworthy” associations between injury and ER/IR strength ratios. To directly quote the study, “non-significant trends in the data suggest that lower ER to IR ratios and abduction strength may also be noteworthy risk factors.”

 In contrast, Wang & Cochrane (2001), in their study on elite male volleyball players, concluded that shoulder rotator muscle strength imbalance (eccentric external/concentric internal <1) played in statistically significant role in the shoulder injuries of their athletes over the course of their season, whereas shoulder mobility, scapular asymmetry, and muscle strength were not significantly associated with injury.  These two studies each have their merits and limitations: The Clarsen study follows a larger group of athletes, while the Wang study measures a longer timeframe. Both studies, however, at least make note of the ER/IR ratios as a risk factor, and both studies extensively reference other studies done on overhead athletes when discussing possible causes of injury.

          Perhaps stronger evidence for the link between ER/IR imbalance and injury is seen in Bryam, et al. (2010). While the researchers’ hypothesis mainly concerned the weakness of shoulder external rotators (which they found to be very strongly associated with injury), they also found significant links between low ER/IR strength and overall throwing injuries and an even stronger association with shoulder injuries specifically. This study was conducted over a 5 year period at looked at 144 Major and Minor League Baseball pitchers, a much larger scope than the previous two studies (and with, as noted earlier, more expensive subjects).

          All three of these studies discuss, independent of their findings, why a proper ER/IR strength ratio may be important in predicting injury. Handball, volleyball, and baseball all involve motions that require significant internal rotation torque to perform at a high level, and as a result, these athletes tend to develop very strong internal rotators without a concurrent strengthening of their external rotators (Codine, L., Pocholle, Benaim, & Brun, 1997). In a high-velocity activity such as throwing, this imbalance may lead to soft tissue injuries as the weak external rotators are unable to provide enough stability to the glenohumeral joint to counteract the immense forces produced by the internal rotators. With this in mind, are these findings relevant to the average weight lifter, or do they only apply to the specific motions of overhead athletes?

The Role of the Rotator Cuff in Glenohumeral Stability and Movement

          Thus far this paper has not attributed internal or external rotation to any specific muscle or muscle group, but of course these motions are achieved by the contraction of specific muscles. Most anatomy textbooks attribute internal rotation to the subscapularis of the rotator cuff, with the larger pectoralis major, anterior deltoid, and latissimus dorsi muscles contributing significantly. The posterior muscles of the rotator cuff, the infraspinatus and the teres minor, are the external rotators, with only the posterior deltoid providing significant additional force, although some studies also suggest the supraspinatus could play a significant role (Dark, Ginn, & Halaki, 2007). This difference in cross-sectional area of muscles that provide internal rotation vs. external rotation could explain why even in non-athletes, internal rotation strength is significantly higher (Noffal, 2003).

          Rotation, however, is not the only function of the rotator cuff. It could be argued that their isometric role in stabilizing the humeral head in the glenoid fossa is as or more important than their isotonic role in providing rotation. Centering the axis of rotation in the glenohumeral joint could be an important factor in preventing shoulder impingement, and it is a role for which the rotator cuff muscles, particularly the inferior and posterior components, are well suited (Sharkey & Marder, 1995). Indeed, instability of the shoulder joint has been shown to be a major predictor of injury in overhead athletes and weightlifters (Gross, Brenner, Esformes, & Sonzogni, 1993).

Applying ER/IR Strength Ratios to the Weight Training Population

          Now that we have established some evidence that a lack of external rotation strength compared to internal rotation strength has been observed to predict injury in overhead athletes, and that specific muscles are responsible for these actions, we can begin attempting to link this information to the recreationally trained weight lifting population. Some common ground has been found between the ER/IR strength ratios of throwers and recreational weight lifters. When comparing throwers to non-throwers, Codine, L. et al (1997) and Noffal (2003) found that throwers had a lower concentric ER/IR strength ratio (about 0.57 and 0.65 at 300 degrees/second, respectively) than non-throwers (about 0.75 at 300 degrees/second in both studies) in their dominant throwing arms, and that this was due to the throwers’ higher IR strength without a significant increase of ER strength over non-throwers.

A similar study which looked at muscluar strength differences between the recreational weight training population and a control group found that these same ER/IR strength discrepencies also exist in the recreational weight training population (about 0.61), and that this imbalance is also due to a strengthening of the internal rotators without concurrent strengthening of the external rotators (Kolber, Beekhuizen, Cheng, & Hellman, 2009). In EMG studies, the pectoralis major, commonly targeted in traditional weight training programs, has been shown to contribute significantly to internal rotation (achieving similar activation rates as the subscapularis), whereas no such large muscle is activated nearly to the same extent as the infraspinatus during external activation (Dark et al, 2007).

          Shoulder instability, particularly anterior shoulder instability, has been observed in weight lifters with shoulder pain. Gross, et al (1993) observed that every patient in their study (all of whom had recurrent instability in one or both shoulders and reported weight lifting as their primary recreational activity) experienced pain in abducted and externally rotated positions when the humeral head is translated anteriorly in the glenoid fossa, and that most of them could not recall any particular event that caused the pain.  In common weight training exercises such as the bench press and row, the rotator cuff functions in a reciprocal fashion to oppose translation of the humeral head by the larger external muscles acting on the shoulder; the infraspinatus is highly active during the bench press to oppose the pectoralis major’s anterior pull, and the subscapularis is highly active during the row to oppose the latissimus dorsi’s posterior pull (Wattanaprakornkul, Halaki, Cathers, & Ginn, 2011). However, as noted in the Kolber study, there is little evidence to suggest that the infraspinatus is significantly strengthened alongside the larger pectoralis major in most common resistance training programs. This may provide a possible explanation as to why some recreational weight lifters experience progressive shoulder pain: large muscles such as the pectoralis major are strengthened at much faster rates and have a much larger total strength capacity than the muscles of the posterior rotator cuff, and eventually these smaller intrinsic muscles cannot adequately stabilize the glenohumeral joint during exercises like the bench press.

Shoulder Injury Prevention in Weight Training Populations and the Case for More Research

          Although not common in traditional weight training programs, the infraspinatus and other rotator cuff muscles can be specifically targeted and relatively isolated from other muscles with the right exercises (Escamilla, Yamashiro, Paulos, & Andrews, 2009). Properly designed strength training programs that include exercises to specifically target external rotators can limit or eliminate the ER/IR strength discrepancy found to be common in weight lifters (Niederbracht, Shim, Sloniger, Paternostro-Bayles, & Short, 2008).

          It is important, however, to note that the comparison between throwing athletes, recreational weight lifters, and the methods used to measure their respective muscle imbalances, has several caveats and outright flaws that need to be addressed. Testing methods for ER/IR rotation varied across the studies I’ve presented in this paper; Bryam (2010), Clarsen (2014), and Kolber (2009) all used isometric testing protocols to measure ER/IR strength, whereas Wang (2001), Codine (1997), Noffal (2003) used isokinetic testing. This is important to note because the sports played by the athletes in the study involve dynamic, rather than isometric, motions of the shoulder, often under immense load and stress. Baker, Wilson, & Carlyon (1994) found that while isometric and dynamic measurements of strength are significantly related, they were not so closely related to imply generality of muscle strength across all contraction types and speeds. It would not be advisable to assume that isometric strength is always going to predict high-velocity eccentric strength, or that changes in isometric strength over the course of a strength training program always reflect dynamic changes in strength.

          Isometric testing, however, may be appropriate for testing the strength of the infraspinatus in recreational weight lifters. As previously stated by Wattanaprakornkul (2011), the infraspinatus plays a major isometric role in glenohumeral joint centration during the bench press as it opposes anterior translation, which contributes to anterior instability. I would make the case that measuring the isometric ER/IR strength ratios of recreational weight lifters might be an effective estimate their posterior rotator cuff’s ability to maintain glenohumeral stability against the pull of the pectoralis major during popular exercises like the bench press. Studies referred to in this paper have only looked at these ER/IR strength imbalances as they relate to overhead athletes, and further study would be recommended to see if such a correlation exists in the recreational weight training population.

Conclusion

The connection between low external-to-internal rotation strength of the shoulder and risk of injury has been well documented in throwing and overhead athletes, and the need to maintain proper balance between the two has been the objective of many strength and conditioning programs for athletes of all levels. Low ER/IR strength ratios similar to that of overhead athletes have also been observed in recreational weight lifters. Considering the dual role of the rotator cuff as shoulder rotators and shoulder stabilizers, and that traditional strength training tends to neglect the external rotators, more research needs to be done to determine whether this same ER/IR strength discrepancy when present in weight lifters carries the same risk of injury.

         

         

Bibliography

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