Muscles
Muscles work by getting shorter. We say that they Become shorter., and the process is called contraction. Muscles are attached to bones by strong Tough material joining a muscle to a bone.. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.
Antagonistic muscles
Muscles can only pull and cannot push. This would be a problem if a joint were controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. This problem is solved by having muscles in pairs, called A pair of muscles that act on a joint. As one contracts, the other one relaxes..
For example, your elbow joint has two muscles that move your forearm up or down. These are the biceps on the front of the upper arm and the triceps on the back of the upper arm:
- to raise the forearm, the biceps contracts and the triceps relaxes
- to lower the forearm again, the triceps contracts and the biceps relaxes
Image caption, The biceps contracts and raises the forearm as the triceps relaxes
1 of 2
Another example of a pair of antagonistic muscles is found in your legs. Here your quadriceps and hamstrings work together.
Sports competitors and athletes are seen in action using their muscles
Muscles exert a force on bones when they contract. This happens for example when you lift or hold an object, or when you move a part of your body. If you hold an object weighing 10 N, keeping it still and with your forearm horizontal and your upper arm vertical, you would be exerting an upwards force of 10 N on the object. You could work out the force exerted by the biceps muscle to do this using the idea of A turning effect of a force.. The way in which muscles and bones work together to exert forces is called biomechanics.