別把手臂伸超過背 ! 當上臂與背平面的角度超過180度，這不但使你難施力，也冒著累積肩傷的風險。
Source: USA Swimming: Coach Education
It’s not necessarily repeated strokes that cause shoulder pain. It’s repeated strokes with flawed technique. The one thing you need to be aware of:
Don’t let your arm go behind your back.
The hard thing is that most swimmers don’t even realize this is happening. They just feel pain as they start pulling the water with their arm. In freestyle, backstroke and butterfly, it’s all too easy to start the pull while your arm is in a high-risk position.
If you’re feeling pain at the beginning of your stroke or as you’re pulling through, you need to be aware of where your arm is when you’re feeling the pain. If your upper-arm (from your shoulder to your elbow) is behind your back (from shoulder to shoulder), that is bad for most people. Your upper-arm and back have to line up if you want to protect your shoulder.
In freestyle, you might feel pain when the stroke starts because your shoulder is rotated down when the arm starts pulling. If you’re feeling pain, either decrease the rotation to that side or pull more in front of you (not as wide). This can be a common problem on a swimmer’s non-breathing side because many swimmers lean on that side when they breathe.
A second problem spot in freestyle is the start of the recovery. If you feel pain there, you might be exaggerating the finish motion too much, and the hand and arm are too high when the recovery starts. Keep the arm lower to the water to start the recovery.
In backstroke, many swimmers press down with their hand and arm to start the stroke. This can definitely make the arm go behind the back. If you’re feeling pain, either rotate more to that side or pull shallower. (I’d recommend a shallower pull.)
In butterfly, the potentially painful position can happen if the swimmer presses deep with their chest and the hands stay high at the surface. If you feel pain, press forward with the chest and have a flatter stroke.
This video and picture shows exactly where the high-risk position typically occurs in freestyle and the type of position to avoid in all strokes: