As you learn different muscles, you'll see that different muscles can do different things depending on the movement. And so we're going to say that muscles can take on different roles to perform a movement, and we have 4 different roles here that we're going to talk about. The first one we are going to call the agonist or the prime mover. And you're likely to see both words there, so you should remember both agonist and prime mover for this one. The agonist or the prime mover, we're going to say is going to be the main muscle responsible for a move. And to highlight these different roles, we have the same sort of diagram drawn 4 times, each time focusing on different muscles. And this diagram shows someone from the back showing the bones and the muscle, and they are flexing at the elbow. So they're bending the arm like this. The muscle that is the prime mover for that is going to be the biceps brachii, that sort of major muscle of the upper arm. So we can see that here, the origin is going to be up in the shoulder, and the insertion is down in the forearm there. When that muscle contracts, your forearm is going to bend. You're going to flex at the elbow. So we are going to say that biceps contracts, and when that happens, that flexes the elbow joint. The biceps brachii is an agonist or a prime mover of that movement.
Alright. For every agonist, there's an antagonist. So the antagonist is a muscle that opposes the action of the prime mover. And in this case, for flexing the elbow, that's going to be the triceps brachii. So we can see here the triceps brachii inserted in the back, a little bit on the side of your arm. We can see the origin's also up in the shoulder here, but the insertion here sort of goes back around the back of the elbow to the forearm. So when it contracts it straightens the arm. This means to flex the arm, the tricep has to relax. So we're going to say the triceps relaxes and that allows for flexion at the elbow. Remember, muscles can only contract. So for a prime mover to do something, to do the opposite movement, it must have an antagonist. And you can kind of test these out too on your own body. So for example, if you put your arm underneath the desk and you try to bend your elbow, you sort of lift up on the desk or the table that you're at, you should feel your bicep is sort of bulging. Right? It's firm. If you feel your tricep, it probably feels pretty soft. It has to relax to allow that movement. Now you can do the opposite. You can put your arm on top of the table or the desk and push down. Again, don't do this with your whole arm. Just really try and straighten or bend at the elbow. As you push down, you should feel the tricep feels pretty firm, it's bulging. It's a prime mover for extension at the elbow. Right now the bicep, it should be pretty soft. Now just to be clear, the antagonist doesn't always relax completely. Sometimes it opposes a little bit just to sort of control and stabilize the movement. But agonists and antagonists work in opposition.
Alright. We have antagonists, we can also have synergists. And synergist is something that is going to aid the prime mover. Muscles don't work by themselves. A lot of times a lot of muscles work together to make a movement happen. So if we're talking about the biceps brachii, it's a prime mover for flexing at the elbow. Well, another muscle that flexes at the elbow is the brachialis, and we can see here brachialis has an origin on the humerus there and an insertion, down on the forearm as well. So we are going to see that the brachialis adds to the amount of force. Now sometimes determining which is the prime mover and which is the synergist doesn't really make sense. Sometimes they're just both working on a movement. So distinguishing between those two isn't always helpful. Things work together as synergists. The brachialis also works as a prime mover for this movement, but it is a synergist to the biceps brachii.
Alright. Our final type of movement here we're going to say is a fixator. And you'll see we have fixator color coded the same as synergist. They're both purple here because sometimes fixators can just be considered a type of synergist. Sometimes its own category. Sometimes it's considered sort of a subgroup of the synergist, but we'll talk about them separately here. A fixator is going to stabilize the origin of the prime mover. Remember, the origin doesn't move in a muscle contraction, the insertion does, but a lot of your body parts move. So sometimes you have to hold the origin steady. That's the job of the fixator. So for this we have the rotator cuff illustrated, and the rotator cuff is a group of muscles on the scapula that all insert sort of at the top of the humerus here. And we're just going to say that the rotator cuff is going to brace the humerus. If you're doing this movement, you need the humerus to stay in place so that you can actually lift something. That's the job of a fixator.
Okay. Learning these, again, sometimes it just feels like another thing that you need to add on to the list of facts. But remembering these things together, understanding what an agonist is and what an antagonist is, and learning the biceps brachii with the triceps brachii, understanding that they work in opposition, is going to help you understand all these movements much better, and it's going to help you understand it much more deeply. Alright. With that, we have an example to follow and more questions after that. I'll see you there.
