Every skeletal muscle basically has two ends where it attaches to the skeleton, and when the muscle contracts, those two ends are going to get closer to each other. We're going to define those two ends as the origin and the insertion. So you can kind of think of the job of a muscle is to make two parts of the body get closer to each other. And when we do that, we're going to get the origin and the insertion closer. And specifically, we can say that the insertion, we're going to say the origin is the stationary attachment site. It doesn't move. The insertion, that's the mobile attachment site. That's the attachment site that moves. To see what we're talking about here, we have a diagram showing the bicep brachii. Now, bicep brachii, that's that sort of major muscle of your upper arm and has two attachment points. One's up in the shoulder and one's down here on the forearm. Well, if I flex my bicep brachii, my forearm is going to come up, my elbow is going to bend, and my forearm is going to keep on getting closer to my shoulder. The insertion moved closer to the origin. That means that the insertion must be down here on my forearm, and the origin didn't move, that's up in my shoulder. A little memory tool to keep these straight in your head, you can say the origin observes and the insertion inches closer. Now, learning the origin and insertion can sometimes just feel like another thing on the list of facts you got to memorize, but if you understand how a muscle works, they kind of just make sense. If you know the origin and the insertion, you should be able to figure out the movement of the muscle. If you know the movement of the muscle, you can often figure out the origin and the insertion. That's because the origin and the insertion are going to determine the movement. And to see what I mean, we have some examples here. We're going to show flexion at the knee. So here, this person is bending their knee, and their lower leg is sort of bending up, and they're using their bicep femoris. This is a muscle of a hamstring. You can see this has some attachment points up here on this end. One attachment point's on the ischium, and the other's on the back of the femur there. And then on the other end, it has an attachment point on the tibia and the fibula on the other side of the knee there. So, one way you can think of this movement of flexion is, well, you're getting your tibia and your fibula closer to your rear end. So, which end is moving? Well, down here on the tibia and fibula, that's moving. That's my insertion. I'm going to mark it with an I. The part that isn't moving, that's the origin. I'm going to mark it with an O. Next, we can look at extension. Extension is straightening the leg here. Sort of, straightening the leg at the knee. And we can see here we have a muscle that does that. That's the vastus intermedius. We have two attachment points for this muscle. One is on the front of the femur, and the other one sort of comes over the kneecap there, and attaches to the tibia at the tibial tuberosity. So, one way to think of extension is you're sort of getting the front of your tibia closer to the front of your femur. So, which part is moving? Well, that tibial tuberosity is moving, so that's going to be my insertion. The front of the femur is not moving, so that's going to be my origin. And I'll mark those. We can look at abduction. Abduction is taking an arm or a leg away from the midline of the body. So here we have the muscle, the gluteus medius, and this is attached up on the ilium and it's also attached on the greater trochanter. So, to pull the leg away like this, we can see here the greater trochanter of the femur. You can think of it as getting closer to the ilium. That's the job of this muscle. So, which part's moving? Well, the insertion moves. That's gotta be on the greater trochanter. I'll mark it with an I. The ilium does not move, so that's gotta be my origin. I'll mark it with an O. For adduction, adduction is bringing an arm or leg closer down into the midline of the body. Here, we're looking at the adductor longus muscle, and this attaches to the pubis and also to the femur down here. So, to bring this leg in towards the body, one way you can think of that, you're kinda taking the inside of your femur and moving it closer to the pubis. Well, which part's moving? The femur is moving, so that is going to be my insertion. The pubis isn't moving, so that's going to be my origin. Alright. Our last example here is going to be rotation. And for rotation, we're taking this leg and we're turning it out. So, you're sort of taking your foot and turning it out. And the muscle we're looking at here is the piriformis. And the piriformis has one attachment point on the sacrum and another on that greater trochanter, on the outside of the femur. So, one way you can think of this movement, that outside of the femur, that greater trochanter, is getting closer to the sacrum when you turn your leg out. That means that the insertion must be on that greater trochanter, and the origin must be on the sacrum. Alright. Again, trying to learn your origins and insertions of muscles on their own can sometimes just feel like a really unrelated list of facts. Remember, they cause the movements. So, if you know the origin and insertions, you know the movement. If you know the movement, you can usually figure out the origin and insertion.