Muscles are tissue that contract to move parts of your body. You have different types of muscle:
Skeletal muscles are attached to bones and come in pairs—for example, your biceps muscles bend your elbows, and your triceps muscles straighten them. Skeletal muscles are voluntary (meaning you move them when you decide to).
Smooth muscles surround your arteries, veins, and intestines. The smooth muscles in your blood vessels contract and relax to adjust blood flow. The smooth muscles in your intestines contract to move food and stool through your digestive system. You can't control your smooth muscles. They do their job without you thinking about it.
Heart muscle is a special type of muscle that never needs to stop for a rest, and it's not under your voluntary control.
Muscles generate energy from food and oxygen that come through your bloodstream. Muscles use this energy to contract (shorten). The bigger the muscles are and the more blood supply they have, the stronger they can contract.
Without a good blood supply, your muscles can't do much work.
Certain kinds of exercise, for example, lifting weights, can make your muscles get bigger and stronger. Other kinds of exercise, such as running, can give your muscles more endurance (they can work longer).
To make your body move, muscles must connect one bone to another across a joint.
When a muscle contracts, the tendons pull on both bones and move them in the direction the joint allows. Because muscles can only contract, if one muscle bends a joint, you need to contract another muscle on the other side of the joint to straighten it.
Problems with muscles include:
Tendons can become inflamed too (a problem called tendinitis).