Almost half of the body’s weight is muscle. It is a body part specialized to get shorter, or contract, when it receives nerve signals from the brain. Most muscles are long and strap-shaped. They taper at each end into ropelike tendons which attach firmly to bones. When it contracts, it pulls the bone and moves that part of the body. This sounds simple, but the process of moving is incredibly complicated. There are more than 640 muscles, and they hardly ever work alone. They usually work in teams to pull, tilt, and twist several bones at once. Also, as one part moves, such as when you hold your arm out sideways, muscles in the other parts need to work too. Your back and front muscles tense to take the strain, and your leg muscles shift weight to keep you balanced. The result is a smooth, coordinated movement, without your falling over!
A typical muscle is made of bundles of muscle fibers, or myofibrils. Each fiber is a bundle of even thinner parts, muscle fibrils or myofibrils. And in turn again, each fibril contains bundles of long thin stringy substances known as actin and myosin. When a muscle contracts, the actins slide past the myosin’s, like rows of people pulling ropes. As million of actins and myosins do this, the whole muscle gets shorter.
About 60 muscles in the face, head, and neck produce our huge range of facial expressions. Some of these them are joined, not to ones, but to other ones. For example, the frontalis muscles in your forehead can raise your eyebrows in a questioning way. Smiling is easier than frowning. A grin requires 20 muscles, while a grimace uses more than 40.
A muscle can get shorter and pull. But it cannot make a pushing force, so most of them are arranged in opposing teams, one team pulls the body part one way. Then the other team pulls it back again. As each team pulls, the other relaxes and gets stretched. For example, muscles in the rear of the thigh pull the leg back at the hip and knee. Then opposing muscles in the front of the thigh quickly swing the leg forward and straightened the knee; KICK!
Many muscles, especially those in the arms and legs, are long and slim. As they contract, they bulge in the middle, at the part called the belly. But there are many other muscle shapes. The movements they produce depend on which other muscles are working at the same time, to tense or stabilize other parts of the body.