Answer is Muscles.
Muscles work on contraction. Muscles can only pull and cannot push. This means that a muscle can only move a bone in one direction and there is no way to move the bone back again. This problem is solved by having muscles in pairs, called antagonistic muscles. The antagonistic muscles work in pairs. One muscle contracts while the other relaxes.
For example, the lower arm is moved upwards when the biceps muscle contracts and the triceps muscle is relaxed. To lower the forearm again, the triceps contracts and the biceps relaxes.
