All movements in our body are controlled by muscles, which are living tissues and work by pulling against the skeleton. Muscles are made of bundles of fibers, which contract when they receive a signal from the nerves. The power of muscles depends on the number of fibers.
These increase with exercise, which is the reason why body builders develop big muscles.
There are three types of muscles in the body. The first is skeletal or voluntary muscle. Together with the bones and tendons, it is responsible for all forms of conscious movements. The second is the smooth muscle, which is concerned with the involuntary movement of internal organs, such as the guts and bladder; the third is cardiac muscle which makes up the main bulk of the heart.
This type of muscle works throughout life.