The amount of swimmers is estimated greater than both hiking and jogging in the United States. Overall, the amount of shoulder pain ranges from 10 - 60% of swimmers, which I've previously wrote about. 

Overuse is believed to be one of the main factors for shoulder pain, as swimmers perform ~500,000 strokes per arm per year. Shoulder impingement is the most common injury at the shoulder. There are two types of impingement:

Shoulder external impingement

This is the most common injury associated with a swimmer. In fact, the term “swimmer’s shoulder” is synonymous with shoulder impingement. However, in recent years the volume of external impingements in has decreased swimming is decreasing secondary to an increase in types of impingement (see below). External impingement is usually caused by an inflamed rotator cuff tendon (supraspinatus) that gets pinched while raising your arm overhead. The symptoms are noticeable during a “painful arc” of overhead motion of 60-130 degrees (for example, the recovery of freestyle). There are many causes for the rotator cuff to be pinched, most commonly irritation to a tendon is the cause. Often times, the area is irritated because the muscles around the shoulder are too tight, decreasing the area in the front of the shoulder and rotator cuff, thereby causing compression.

Shoulder internal impingement

Internal impingement involves the infraspinatus, a rotator cuff muscle located in the back of the shoulder.

Repeated internal rotation (for example, the freestyle catch) can pull the labrum. The infraspinatus pulls on the labrum either due to tightness of the infraspinatus, weakness of the shoulder upward rotator muscles, or poor timing of the infraspinatus during stressful tasks causing irritation to the infraspinatus, labrum, or both.

Shoulder impingement prevents many swimmers from competing, sometimes even ending a career. The shoulder blade plays a delicate role in overhead activities, acting as the base of movement of the arm. If the shoulder blade is moving excessively or isn't stable, impingement is likely to occur. Unfortunately, most studies have analyzed and demonstrated altered shoulder blade muscle activity, without assessing actual shoulder blade movement. Also, most of these studies have not analyzed the effects during an actual practice. So, what are the differences in shoulder motion and strength after a practice?

1) Impaired, but Similar Strength:

After a swim session, there was a strength reduction for both the shrug and punch. However, both swimmers with and without shoulder symptoms had similar reductions in force.

2) Reduced Scapular Upward Rotation

Like the muscular activity, scapular motion was similar before practice. However, the scapular orientation altered significantly during arm elevation in the group with shoulder impingement after practice. Specifically, there was less scapular upward rotation after practice. These changes likely decrease the suprahumeral space, resulting in rotator cuff muscle impingement and pain. Also, the greatest difference was with the greater upward rotation rotation, possibly explaining why shoulder pain is more likely with the arm overhead.  

Summary

Although strength of the muscles tested by Su (2004) were unaltered, other muscles, like the lower trapezius, play a role in scapular upward rotation. It seems were do not fully understand the reasons for alteration after practice between those with and without shoulder impingement, as it could be muscular, habit, or even due to pain. Nonetheless, improving these areas is key for shoulder impingement prevention and rehabilitation.

Reference

1. Su KP, Johnson MP, Gracely EJ, Karduna AR.Scapular rotation in swimmers with and without impingement syndrome: practice effects. Med Sci Sports Exerc. 2004 Jul;36(7):1117-23.