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Muscle terms
Written by Tim Sheppard MBBS BSc. Created 15/6/10; last updated 20/6/10

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What are the origin and insertion of a muscle?

The origin and insertion of a muscle refer to where the ends of the muscle attach to the bone. The origin is the start of the muscle - the proximal end, where the muscle arises from. If the origin is fixed, then the other end of the muscle will be pulled towards it. The insertion is the end of the muscle - the distal end, where the muscle finishes. If the origin is fixed, this is the bit that will move when the muscle contracts.

It's worth remembering that it won't always be the distal, insertion end that moves. If that end is fixed, then it will be the proximal, origin end that moves. For instance, there are muscles that connect the top of your arm to the bottom. If you bend your arm to lift up a book, it is the insertion end that will move. However, if you fix your hand on the ground to do press-ups, it's the origin end that will move.

In fact the whole muscle contracts; the origin and insertion are just helpful ways of referring to a particular end.


What is a tendon?

A tendon is a tissue that connects muscle to bone. If you follow a muscle along its course from the middle up to the point that it connects to a bone, eventually it focuses into a particular strand of tissue that is tough, and which will securely attach to the bone at the insertion or origin. Tendons do not contract when the muscle contracts - they are there to pull the bone when the muscle contracts, although they can act a little bit like springs to soften the blow.

Tendons are made up mainly of collagen, which is tough to make sure that it is able to cope with the pulling force (tension) of the muscle. If this tendon breaks, the muscle will stop working - and if the tendon becomes inflamed, contraction of the muscle will cause pain as it pulls on the inflamed tendon.


What is a ligament?

A ligament is a tissue that connects bone to bone (or bone to cartilage). Ligaments are there to stabilise the body and make sure that bones that are supposed to stay together, stay together. At joints that are at risk of coming apart (e.g. the knee), there are lots of ligaments to make sure that the joint is able to move without falling into pieces. Ligaments strength joints because they make sure that movement only happens where it supposed to.

Ligaments are also made of collagen, because they need to be very tough, but unlike tendons, they have a lot more spring in their step! You can stretch ligaments more easily than tendons, and the more stretched your ligaments are, the more easily joints can move. This is a good thing if you want to do some exercise, but if the ligaments are stretched too much, they will stay stretched - which is a problem if it means they don't support the joint anymore.


What is an agonist muscle?

An agonist muscle refers to the muscle that you are looking at, which achieves the job you are thinking of. The more muscles that do what you're looking for, the more powerful the movement is going to be.

If we take the example of bending the arm, the agonist is a muscle which leads to arm flexion.

Another name for agonist is prime mover. Whether something is called the agonist or not depends on the movement you are looking at. If you start to look at the opposite movement (in this case, straightening the arm), this muscle would be moving in the opposite direction - so it becomes the antagonist.


What is an antagonist muscle?

An antagonist muscle refers to the muscle which tries to stop the agonist doing what it's trying to do - which contracts in the opposite direction. If an antagonist contracts, it will stop an agonist doing its job. If an antagonist relaxes, it will allow the movement. Antagonists are therefore important for controlling movement.

If we take the example of bending the arm, the antagonist is a muscle which leads to arm extension, which opposes flexion.

Remember that agonist and antagonist refer to a specific movement. If you were talking about straightening the arm, then the antagonist becomes the agonist. Whether something is the antagonist or not depends on the movement you are looking at.


What is a fixator?

A fixator is a muscle which keeps a joint in place, to help the other muscles involved in movement to do their job. They basically act like stabilisers, keeping the body steady and stable so that it is as easy as possible for the movement to take place.


What is a synergist?

A synergist is a muscle which acts with the agonist to make the movement as smooth as possible. When a joint can move in a number of directions, contracting a muscle won't always lead to the joint moving in the right direction - the joint should move in roughly the right direction, but there might be some temptation for it to move in one direction or the other. The synergist acts to make sure that the agonist only moves the joint in the direction that it is supposed to, preventing movement that you don't want, and making the movement you do want smooth.


What are parallel and converging fibres?

You can talk about muscles in a number of ways. One of the ways that you can talk about them is the way in which the fibres are arranged - because not all of the fibres are just arranged in rows. There are two major ways that the fibres can be put together; one is with fibres that are parallel or converge; the other is in fleshy bellies that are attached to central tendons.

Parallel and converging fibres allow a big range of movement, but they are not as powerful as pennate muscles. They come in three main types: strap, fusiform, or fan-shaped. Strap involves all of the fibres running alongside each other in parallel; fusiform involves a bulge in the middle, and fan-shaped involves each successive fibre changing direction so that you get a fan-arrangement.


What are pennate muscles?

When you are talking about types of muscle, you can talk about the way that the fibres are arranged. If they are separated into separate bellies, then you talk about pennate muscles.

Pennate muscles have bellies that are attached to a central tendon. The number of bellies that are attached determines what sub-type of pennate muscle you're looking at. If it's just one belly, it's a unipennate muscle; two bellies is called bipennate. If you have anything more than two, it is called multipennate.


Further Reading