With movement of the human body, the origin of the muscle is generally fixed. When the origin is fixed and a muscle contracts, it will pull the insertion toward the origin. This is always not the case however, as there are some instances when the insertion can be fixed, which will cause the origin to move toward the insertion. This is often the case with strength training exercises. For instance, when doing a chin-up, the insertion of the bicep (near the elbow) is fixed, and the origin is getting pulled toward the insertion. This is not the case with a traditional bicep curl, when the origin is fixed (near the shoulder), and the insertion gets pulled toward the origin.