Skeletal muscles are attached to bones on each end by tendons. The origin is the fixed attachment, while the insertion moves with contraction. The action, or particular movement of a muscle, can be described relative to the joint or the body part moved.
The insertion of the muscle is defined as that end of the muscle that attaches to the freely moving bone of its joint. To understand muscles and joint movements, there are four things to keep in mind:
Movement happens at joints, where one bone acts freely as the other remains relatively stationary.
The bones can and do switch roles depending on the action you're making and the position you're in, but such is knowledge is a bit more advanced. (The Pilates method takes advantage of this phenomenon in the design of exercises that work your "anti-gravity" muscles.)
That said, a brief example is the comparison between walking and bending over while in a standing position. Both use the same hip action — flexion — but when you walk, your leg is freely mobile. When you bend over at the hips, your pelvis is the bone that is performing the movement.
Muscle attachments are named with this in functionality mind, so the label is given to an insertion (one of the two types of attachments) generally includes a reference to the bone that does the moving most of the time. In our walking versus bending at the hip example above, we walk way more than we bend over at the hips. This means that some of the muscles that go from pelvis to thigh bone reflect the fact that the thigh bone is freely movable during walking. An example of this is the rectus femoris, that big bulky muscle that lives at the front of your thigh. The word "femoris" refers to your thigh bone.
