Dynamic Muscle Actions
We have two, our muscles have two basic types of muscle actions. The first one is concentric, or shortening of the muscle, and the second is eccentric, or lengthening of the muscle. So if you look at the picture above, the book is providing the resistance here, and you can think about this as a bicep curl movement. In the picture on the top left, the book is being lifted up towards the shoulder. As the person does that, you see that the biceps muscle is getting shorter, so we're going to call that a concentric contraction. In the picture on the bottom right, again the book is the resistance here, and the person is lowering the book down towards their leg or towards the floor, and you can see that the biceps muscle is getting longer and it's doing an eccentric contraction where it's lengthening the muscle. So concentric is shortening, eccentric is lengthening of the muscle. And although it isn't pictured here, you should also know that most of our muscles work together in pairs. In this case, the triceps muscle is not pictured, but the triceps and biceps work together as a pair. In the picture on the left, the biceps muscles are contracting and the triceps muscle is resisting that movement. We call the contracting muscle, in this case the biceps, the prime mover or the agonist. The muscle that resists the movement is called the antagonist; in this case that's the triceps muscle.