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By Dean Armitage MBA,OPA-C,ATC/L | 
Baseball is a great game, but do baseball players really want to know how the shoulder muscles work during the overhead throwing motion? I think they might. The muscle activity of the shoulder in the overhead throwing motion is broken into three phases:
With these phases the muscles in the shoulder are either acting in an eccentric manner, concentric manner, or are relatively quiet. Athletes need to know that if these muscles are not firing off like a finely tuned engine, it could lead to a "breakdown", an injury. So let's look at the muscles of the shoulders in this chart and see what kind of activity is happening during each phase.
| Muscles |
Acceleration |
Deceleration/
Follow-Through |
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| Supraspinatus |
Eccentric |
Eccentric |
| Subscapularis |
Relatively Quiet |
Concentric |
| Infraspinatus/ Teres |
Minor Eccentric |
Eccentric |
| Deltoid |
Relatively Quiet |
Eccentric |
| Biceps |
Relatively Quiet |
Eccentric |
| Triceps |
Concentric |
Concentric |
| Pectoralis Major |
Concentric |
Concentric |
| Latissimus Dorsi |
Concentric |
Concentric |
| Rhomboids |
Relatively Quiet |
Eccentric |
| Trapezius (Mid & Low) |
Relatively Quiet |
Eccentric |
| Serratus Anterior |
Concentric |
Concentric |
The scapular adduction and humeral external rotators (Teres Minor and Infraspinatus) are weaker on the throwing dominant side than on the non-dominant side, because they are continually being stretched. Baseball players demonstrate greater than normal motion of external rotation at the expense of internal rotation.
Strengthening should be done to balance the muscles of the shoulder. Internal rotators exceed the external rotators and scapular adductors in strength. These weaker muscles should be strengthened to maintain balance in the shoulder throwing motion.
This strength can be developed through some basic shoulder exercises using dumbbells (2 to 5 lbs.), or rubber tubing. Knowing that the Infraspinatus and Teres Minor (External Rotators) are major eccentric muscles, you need to strengthen that aspect of them. The internal rotators out number the external rotators and are much more powerful group of muscles, so the external rotators really need some special time in both the training room and weight room.
How can you reproduce the throwing motion? By throwing! The best way to get the muscles firing at the throwing speed is again, by throwing. By playing long toss at three times a week, as a warm up or in an off-season conditioning program, will help strengthen the shoulder. Long Toss at 90 feet/120 feet/150 feet for twenty throws a distance will work the shoulder muscles at that high velocity motion you need. Proper throwing mechanics are needed during this activity to help prevent injuries. By keeping the elbow at 90 degrees angle above your shoulder and not short arming the throw, plus using a short forward "crow hop" during the throw will only help in decreasing injuries. Do not forget to strengthen your core (Abdominals / Obliques / Lower back) and the lower body; these are also a very important part of the throwing mechanics.
Remember, overhead throwing is one of the most unnatural motions the human body can perform, so knowing what the muscles do, and how to strengthen them, will only enhance your throwing career and athletic performance.
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