This series is meant to give you an inside look at what I'm going to be working on for the next five semesters during my Master of Science in Applied Exercise Science program. Whether it's updated research, new views on anatomy and biomechanics, or just something interesting, I thought it would be a cool little experience to detail along the way. I will give you some quick hitter points and the full assignment in detail after for those that would like to delve in more to the topic at hand. 

(Side bar: for those looking for a little more information, without too much science, dive down to the Discussion and Analysis portions at the bottom for the application of the research)

Enjoy!

Quick hitters

  • Type of article: research review and summary
  • Article name: Isokinetic Rotator Cuff Functional Ratios and the Development of Shoulder Injury in Collegiate Baseball Pitchers 
  • Basic anatomy of the shoulder: external rotators and internal rotators provide support and rotation to the arm, thus functional ratios were developed to determine the best ratio of strength for internal vs external rotation to keep the shoulder healthy. 
  • Mechanism for injury: tissue overload of the internal rotators (subscapularis, pec major, lat dorsi, teres major, and anterior delt) due to stress created from the torque of pitching motion
  • Why is this significant? Most protocols focus on strengthening the external rotators as a weak link in shoulder mechanics, however, this research suggests the internal rotators could be more involved than previously thought. It would change how we approach preventive and rehabilitative exercise in throwing and non-throwing populations. 

Introduction

During a baseball pitch, an incredible amount of force is placed on the shoulder joint, leaving it at risk for chronic shoulder injury due to rotator cuff fatigue, weakness, or imbalances in the joint itself. To address this issue, functional ratios for shoulder rotation strength have been developed as a predictor of potential future shoulder issues, with major focus on the cocking phase and the acceleration phase of throwing. The cocking phase represents the transitioning of the arm from internal rotation (IR) to external rotation (ER), where the internal rotators are required to contract eccentrically to control the rate of external rotation. Conversely, in the acceleration phase, the opposite holds true, the external rotators fire eccentrically to control the internal rotation of throwing, while also keeping the humeral head in the proper position in the glenoid fossa. 

In recreational athletes for overhead throwing and female volleyball players, the cocking phase ratio of 2.09 at 90°s-1 and 2.23 at 60°s-1 were found and acceleration phase ratios of 1.03 to 1.21 were noted. 

Participants and Methods

15 Division I collegiate baseball pitchers from the same team participated in this study, all of which were free of any upper extremity injury at the time of testing. The study was conducted in two parts. The first involved an isokinetic test using a Biodex System 3 dynamometer to test shoulder strength in the off-season. Peak torque was collected for both the concentric and eccentric internal and external rotation at 60°s-1, 180°s-1, and 300°s-1 for six trials, separated by 90 seconds of rest. After a shoulder stretching warm-up, participants were instructed to produce maximal resistance through the entire range of motion in the modified neutral position (45° GH abduction, 30° elbow flexion).

Functional ratios:

Cocking phase: IRecc:ERcon 

Acceleration phase: ERecc:IRcon

Second, follow-up questionnaires were administered during the season to those who developed shoulder injuries. Participants were asked if they had pain during any training, practicing, or competing. If so, participants were then asked to rate their pain on a scale from 1-4, 1 being no pain and no need for medial intervention, 4 representing an intervention requirement and time away from baseball activities. 

Results

Analysis demonstrated that functional ratios were significantly higher during the acceleration phase ratio at 300°s-1 in the injured group as compared to the noninsured group. Secondary analysis found that IR peak torque was significantly lower during the IRcon of the injured group. 

There were no other significant differences in functional ratios between groups. 

These findings suggest that the higher acceleration phase ratios seen in the injured group might demonstrate a link to shoulder injury. For those that were injured, their lower IRcon at 300°s-1 could also be linked to shoulder injury. 

Discussion

As the results suggest, there was a correlation between the significantly higher acceleration phase strength ratio (ERecc:IRcon at 300°s-1, which most likely resulted from lower IRcon peak torque shown in the injured group. 

In attempting to determine the proper strength balance of ERcon:IRcon, it has been reported that normal external rotator strength was 66% the strength of the IR. A more modern study suggested 78%, which is related to the critical nature of the external rotators in the acceleration phase of pitching (they decelerate/stabilize the arm in this motion). 

Using the modified natural position typically leads to stronger readings of IR strength compared to other methods, which makes the findings even more interesting since IR was found to be weaker in the injured group despite the advantageous IR apparatus used in testing. 

Analysis and General Population Application 

Since injured participants struggled producing a forceful IRcon at 300°s-1, it may point out why injury occurred, considering pitching arm speed has been documented at over 6,000°s-1. 

The most interesting aspect of this article relates to the finding that weak internal rotators could be a signifier of potential risk for shoulder injury in throwing athletes. External rotation has long been a primary consideration in injury prevention/rehab approaches to accommodate for the large role the external rotators play in slowing the arm down from such a high peak force. To have a study find that the opposite could play a role in shoulder instability and injury rates in these athletes is a potentially dramatic change to rotator cuff physical therapy and proactive prevention programs. 

Shoulder issues are hardly limited to the throwing athlete, as many in the general population struggle with shoulder injuries. Poor general posture, overuse, and common faciliated/inhibited discrepancies plague these people and a logical next step would be to test this hypothesis on that population. Testing measures for strength would remain, although a new system of measurement outside of throwing may be required to assess peak torque values. Considering the common imbalances most people struggle with, this functional ratio of internal rotator to external rotator strength could eventually reshape standard programming for physical therapists, movement therapists, trainers, and coaches to implement. 

 

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