The terms movement and motion are interchangeable in a linguistic sense as they both mean "a change in position of an object".
When you use the term motion, it tends to sound like newtonian physics
The laws of planetary motion
Newton's first laws of motion
Movement is therefore a better word to describe a motion made by a person if you are talking about competitive swimming.
Of course, as suggested by tylerharms, writing just
This article analyzes freestyle arm strokes.
is perfectly fine too. But it is a bit ambiguous as "analyzing freestyle arm strokes" may not necessary be the same as analyzing the movements of freestyle arm stroke. It can be the history of freestyle arm strokes, or something else about freestyle arm strokes.