AP BIOLOGY

Detailed explanation-1: -Muscles can pull bones, but they can’t push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint.

Detailed explanation-2: -Skeletal muscles only pull in one direction. For this reason they always come in pairs. When one muscle in a pair contracts, to bend a joint for example, its counterpart then contracts and pulls in the opposite direction to straighten the joint out again.

Detailed explanation-3: -Muscles work in pairs, when one muscle shortens the opposite muscle lengthens e.g. the biceps shortens to bend the elbow at the same time its opposite muscle, the triceps, lengthens to allow the movement to occur. Good working between pairs of muscle allows for smooth controlled movement.

Detailed explanation-4: -To move the bone in the opposite direction, the relaxed muscle contracts and the first one relaxes. Thus two muscles have to work together to move a bone.

Detailed explanation-5: -Muscles can only pull and cannot push. Therefore muscles have to work in pairs to move a joint. One muscle will contract and pull a joint one way and another muscle will contract and pull it the other.