So the tricep muscle thickens, it basically shortens as a result of that contraction of the contraction of the sarcomeres inside the skeletal muscle. At the same time the bicep is stretching out, it's basically elongating. And you can see that as this motion takes place, this muscle basically stretches out. This muscle is the muscle that contracts. In this case, this muscle is our antagonist, while this muscle, our contracting muscle, is our agonist. So basically the bicep can act as the agonist but it can also act as our antagonist.