Swimmers should know what a shoulder impingement is (i.e., rotator cuff tendinitis or shoulder bursitis). This shoulder injury is a risk for all the overhead sports — tennis, baseball, volleyball, basketball — but swimming is the most overhead of the overhead sports, so swimmers are the most at risk for this injury, especially if they rotate their arms.
Here is how to prevent impingement injuries.
In freestyle, always little finger first into the water
In a repetitive motion sport like swimming, this is important
Avoid internal shoulder rotation in all strokes
In freestyle, the recovering hand enters the water vertical, little finger first — not thumb first — and stays little finger down as your arm extends forward. This keeps your shoulder in a natural position, promotes torso rotation, makes for a cleaner hand entry, and a nicer, longer, more comfortable arm extension. Staying little finger down prevents your torso from balancing off of your palm. Balancing your torso off of your palm turns your arm into a long lever which puts tension into your shoulder. Don't "platform" on your palms, keep your leading hand edge down, vertical, as it moves through the water.
Does a thumb first hand entry into the water at the front of the stroke contribute to shoulder injuries? You bet it does, it is internal shoulder rotation, a wrenching, difficult, injurious motion for your shoulder.
Keep your hands vertical, little finger down, throughout the stroke.
Keep your hands and forearms close to your long-axis throughout the stroke.