Look at your upper arms as you flex (bend your arm at the elbow). You’ll see the top-of-the-arm muscle push up the skin (or “bulge” for those with python upper arms). What you’re observing is the contraction of your bicep. Contractions are the action that happened when muscles shorten. Lengthen or shortening of muscles are involved in most every movement we make.
In the case of contractions, there are two major types: isotonic and isometric. And you can tell the difference by what happens to the length of the muscle (shorten, lengthen, or no change).