Shoulder tendonitis, impingement, and mechanical deficits are probably the single most common injuries sidelining people from being able to swim on a consistent basis.
While in Physical Therapy school, I did extensive research on injuries that occur in swimming. Most of these injuries occur due to the timing and recruitment of specific muscles that either hold the humeral head in the glenoid (ball in the socket) or rotate the scapula (shoulder blade) upward to lift the arm.
This means that most deficits occur in the recovery phase of the stroke when the arm is being lifted out of the water and brought forward for the next power phase. During this time, it is crucial that the muscles of the upper back and posterior rotator cuff are working to prevent the humeral head from sliding forward (anterior) and that the latissimus dorsi (LATS), lower trapezius and upper trapezius are working together to properly rotate the shoulder blade upward.
The most common deficits that occur are that people over recruit their upper trapezius muscles, or the timing and strength of the trapezius and smaller rotator cuff muscles is inadequate, leading to the shoulder and upper arm dropping. In layman's terms, people hike their shoulders up towards their ears, or they let their shoulder joint come forward and leave their arm lagging behind as they go through the recovery phase.
Below are 3 essential exercises that one can do to help prevent shoulder problems during swimming. The first exercise of shoulder "cocking" (shoulder retraction) is the foundation from which the other exercises are built.
One way to ensure that you are doing the shoulder "cocking" exercise correctly is that the front of your shoulder should feel fairly soft . If it feels like there is a bony prominence just under the skin, then the humeral head has moved forward, and you should start at the beginning and "recock" the shoulder.
Suggested Exercises
1. 3X10 reps Shoulder Cocking: Lay face down/prone on a bed. Retract shoulder (humeral head glides posterior)
2. 3X10 reps Cocking + Extension: Same as above and add shoulder extension (lift arm back along the side of your body). The palm should be down, thus externally rotating at the shoulder.
3. 3X10 reps Cocking + Shoulder Abduction: Begin with cocking the shoulder. Next, lift the arm straight out to the side with palm down (shoulder abduction) but not past the midline of your trunk. VERY IMPORTANT to keep shoulder cocked back the whole exercise and "recook" after each repetition.
Although they may seem very basic, if done correctly, these exercises can significantly stabilize your shoulder and help prevent a nagging injury that may hinder your training.