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What Happens to Bones When Muscles Contract?

Neurobiology, a branch of Biology Topics, investigates the structure and function of the nervous system.

How do Muscle Contractions Move Bones at Joints ?

Muscle is the fibrous tissue in the body that has the ability to contract (when asked to do so by the brain). When a muscle contracts, it shortens (becomes smaller). Muscles are attached to the bones of our skeleton (through strong fibres called tendons). So, when the muscle attached to a bone contracts, it pulls the bone due to which the bone moves at the joint. A muscle can only pull a bone, it cannot push a bone. So, another muscle attached to the same bone has to contract to move it in the opposite direction (so as to bring it back to its original position). This means that two muscles (or a pair of muscles) work together to move a bone.

The muscles joined to our bones (or skeletal muscles) work in pairs. When one muscle of the pair contracts (or shortens), then the other muscle of the pair is stretched (or relaxed) (The stretched state of a muscle is called its relaxed state). A contracted muscle (or shortened muscle) cannot come back to its original stretched state or relaxed state on its own. A contracted muscle can come back to its original state only when the other muscle of the pair pulls it back by contracting itself. Please note that when a muscle contracts, it shortens; but when a muscle stretches or relaxes, it lengthens.

The movement of body parts (and locomotion) in human beings is brought about by the alternate contraction and stretching of the muscles attached to the movable bones of the skeleton. When muscles pull on the bones (by contracting), they produce movements such as moving the head, bending of arm, straightening of arm, walking or running, etc. The contraction of muscles (which brings about movement) is controlled by the brain. As an example of how our body parts move by the action of muscles joined to the bones, we will describe the bending and straightening of our arm.

Bending and Straightening of Arm

Our arm consists of two parts, upper arm and lower arm, which are joined at the elbow. The upper arm has one bone. The top end of upper arm bone is joined to shoulder blade by a ball and socket joint. The lower arm has two bones. The upper arm bone and lower arm bones are joined at the elbow by a hinge joint. The upper arm bone has a pair of muscles called ‘biceps’ and ‘triceps’ on its two sides [see Figure (a)]. The upper end of both the muscles is attached to the fixed bone called shoulder blade by tendons. The lower ends of both the muscles are attached to the bones of lower arm by tendons [as shown in Figure (A)]. We will now describe how the pair of muscles, biceps and triceps, moves the arm.

(i) When we want to bend the arm, the biceps muscle contracts (it becomes shorter and fatter) [see Figure (a)]. The contraction of biceps muscle pulls the lower arm bones due to which the lower arm moves up (or bends) [see Figure (a)]. Biceps muscle is called flexor muscle because it flexes (bends) the arm. Please note that when biceps muscle contracts and bends the arm, the triceps muscle stretches or relaxes (becomes long and thin) [see Figure (a)].
Muscle Contractions Move Bones at Joints 1

(ii) When we want to straighten the arm, the triceps muscle contracts (it becomes shorter and fatter). The contraction of triceps muscle pulls the lower arm bones at the end due to which the lower arm moves out and straightens [see Figure (b)]. Triceps muscle is called extensor muscle because it extends (straightens) the arm. Please note that when the triceps muscle contracts and straightens the arm, then the biceps muscle stretches or relaxes (becomes long and thin) [see Figure (b)].

Thus, by the alternate contraction and relaxation of biceps and triceps muscles, we can bend or straighten the arm, that is, we can move the lower arm (or forearm) up and down. Please note that biceps and triceps muscles just bend and straighten the arm. Other muscles move the arm in other directions. We will now describe how the various animals move from one place to another.

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