**3.1 Muscular system**

Contrary to the bones, which provide the structural strength and stability to the human body, muscles are responsible to hold the bones with the possible movement of each joint with the application of forces by contracting themselves. Muscles are not simply the force generators in the body; the force developed in the muscle depends on the level of neural excitation at the central nervous system (CNS) with the speed and length at which the muscle contracts. Almost all the muscles are arranged in pairs as agonistic and antagonistic muscles, and these pairs work against each other, as one muscle pull then the other muscle relaxes and stretched in response. There are almost 320 muscle pairs that exist in a human body which make up almost 40% of the whole body mass.
