Shoulder impingement during swimming is largely due to dynamic muscle imbalance and biomechanical faults.
Dynamic muscle imbalances can be addressed by strengthening exercises for the rotator cuff and scapular stabilisers together with stretches for the muscles at the back of the shoulder. One good exercise is to stand facing a wall with your arm raised to shoulder height and elbow straight. Hold a tennis ball to the wall by leaning through your shoulder and pinch your shoulder blades together. Roll the ball in small circles for 15 seconds first clockwise then counter clockwise until you become fatigued or for two minutes.
Biomechanical faults can be addressed by correcting your swim technique. Achieving good symmetrical body rotation through an efficient bilateral breathing pattern helps to avoid shoulder impingement during the catch and pull through phase of the freestyle stroke.
Instead of entering thumb first change your technique to enter with a flat hand or fingertip first. This is made easier by avoiding reaching across your midline. A thumb first entry works the shoulder internal rotators excessively and when multiplied by approximately 3,200 strokes per hour contributes heavily to the muscle imbalance of strong internal rotators and weak external rotators causing shoulder impingement.
Most swimmers pull through with a straight arm or dropped elbow. This loads the shoulder muscles excessively rather than utilising the larger and stronger muscles of the chest and upper back.
Making these simple changes can help you achieve many hours of pain free swimming.
For the fitness of you.