ANATOMY

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: -Muscles can only pull and cannot push. This would be a problem if a joint was controlled by just one muscle. As soon as the muscle has contracted and pulled on a bone, that would be it, with no way to move the bone back again.

Detailed explanation-3: -Muscles always PULL they never PUSH-this is why we call it muscle contraction. When the muscle contracts it shortens, which applies force on the bones it is attached to by PULLING on them.

Detailed explanation-4: -Muscles can only contract. They cannot actively extend, or lengthen. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps muscles of the upper arm work in opposition to bend and extend the arm at the elbow (see Figure below).

Detailed explanation-5: -Muscles are believed to contract through a process called the Sliding Filament Theory. In this theory, the muscles contract when actin filaments slide over myosin filaments resulting in a shortening of the length of the sarcomeres, and hence, a shortening of the muscle fibers.