Body Movements — Science STD 6 — Question

Make a fist with one hand, bend your arm at the elbow and touch your shoulder with the thumb. Touch it with the other hand. The muscle bulged due to contraction. Now, bring your arm back to its normal position. When contracted, the muscle becomes shorter, stiffer and thicker. It pulls the bone.
When one of them contracts, the bone is pulled in that direction. The other muscle of the pair relaxes. To move the bone in the opposite direction, the relaxed muscle contracts to pull the bone towards its original position, while the first relaxes. A muscle can only pull. It cannot push. Thus, muscles help in moving bones.