When swimmers have pain, it is most often in the shoulder. This is usually the result of the thousands of overhead repetitions that occur with the hours of practice often required for age group athletes. With overuse comes fatigue and this can lead to lapses in a swimmer's ability to maintain good swimming technique. Additionally, swimmers will have excessive flexibility or even joint laxity in their shoulders which can be normal, but when swimmers repetitively move through an inappropriate range of motion, micro traumas and discomfort can occur.
Rotator cuff 'impingement', biceps tendinitis or shoulder instability are all common diagnoses in swimmers with shoulder pain. Rotator cuff impingement is the result of pressure on the rotator cuff muscle attachments (tendons) as the hand is ‘catching’ water at the beginning of the pull. Biceps tendinitis (inflammation of the bicep tendon) and shoulder instability might occur when the structures that surround the shoulder joint do not work to maintain the humeral head (ball) within its socket, all can result from fatigue and weakness of the rotator cuff and muscles surrounding the shoulder blade.
Due to the nature of the freestyle (or front crawl) stroke, swimmers tend to develop very tight and strong internal rotator shoulder muscles that are in constant competition with the typically weaker external rotator shoulder muscles.
(In the above picture, the swimmer's right arm is in extension and internal rotation finishing the pull and initiating the recovery phase of the freestyle stroke.)
Working on any imbalance that you might have between the tight internal rotators (subscapularis) and the weaker external rotators (supraspinatus, infraspinatus and teres minor) and thus improving your shoulder stability BEFORE you have any pain in your shoulder is a great way to prevent shoulder pain with swimming.
Resistance Band Pull Aparts
2. Prone Ys
3. Plank Variations
You might also try 'Up-down planks' or 'Plank step ups' to change it up. However, there's no getting around planks 😉