We know that when bones move at joints, they produce movements. The movements in bones are brought about by the contractions of skeletal muscles, which are attached with them by tendons. The role of skeletal muscles is as follows.
It is important to remember that muscles can only pull or contract, not push.
One end of a skeletal muscle is always attached with some immoveable bone. This end of muscle is called the origin.
Other end of muscle is attached with a moveable bone and is called the insertion. When a muscle is stimulated by a nerve impulse, it contracts to become shorter and thicker. Due to this contraction, it pulls the moveable bone (at insertion).
Most activities in our body like standing, walking, running, playing etc. require combined action of several muscles.
Skeletal muscles are usually in pairs of antagonists. In an antagonistic pair, both muscles do opposite jobs. When one muscle contracts the other relaxes and this phenomenon is known as antagonism (antagonistic action). When a muscle contracts and bends the joint, it is known as flexor muscle and the movement is called flexion.
When a muscle contracts and straightens the joint, it is known as extensor muscle and the movement is called extension. Following is an example of the antagonistic action of a pair of skeletal muscles.
Biceps is a flexor muscle on the front of the upper arm bone while Triceps is an extensor muscle on the back of arm. Both these muscle have their origin at pectoral girdle and insertion at one of the two bones of forearm. When biceps contracts, the forearm (insertion end) is pulled upward. It is the flexion of elbow joint. During this flexion, triceps muscle relaxes. When triceps muscle contracts, forearm is pulled down. It is the extension at elbow joint. During it, biceps muscle relaxes.
In this way, biceps and triceps make up an antagonistic pair of muscles. Similar pairs, working antagonistically across other joints, provide for almost all the movements of skeleton.
Can you do it?
Aquatic animals need less skeletal support than land animals of similar size. Propose an explanation for this fact.