The repetitive nature of tennis puts your body under severe stresses, which can result in potential injuries, especially in the knees, ankles, lower back and shoulders.
Shoulder injuries are commonly seen in tennis players because the muscles surrounding the shoulder are relatively small and exposed to tremendous repetitive forces.
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Because of the large range of motion in the shoulder, the ligaments alone cannot provide enough stability through all planes of movements.
The four muscles called the rotator cuff should provide the main stability of the shoulder: supraspinatus, infraspinatus, teres minor, and subscapularis. It's important to regularly strength train the rotator cuff, targeting both external and internal rotation.
Make sure to put more emphasis on the external rotation because the bigger muscle groups in your chest and back help the internal rotators. The external rotators work alone and are generally much weaker than the internal rotators.
All the tennis strokes involve some external rotation, but the greatest stress happens during the serve and overhead. Strong external rotator muscles will prevent future overuse injuries, such as chronic inflammation or even rotator tears, which can be excruciatingly painful and take a long time to heal.
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Most shoulder injuries happen because of the excessive forces that tennis creates on the tendons. If the shoulder muscles are weak or tight, or if there are imbalances, the motion in the shoulder does not happen correctly and the constant repetitive forces of tennis strokes will create problems and injuries over time.