Except for thinking, everything we do depends on our muscles, whether we realise it or not. Without muscles we wouldn't be able to sit, move talk, write, read, breathe or even digest our food. A muscle is a bundle of elastic tissue which contracts in other words, it becomes shorter and thicker) when it receives a signal from our nerves or hormones. There are three kinds of muscle skeletal muscle, smooth muscle and cardiac muscle. skeletal muscles are also called voluntary muscles, because we can control their movement when we want to. These muscles vary greatly in size and strength, from the small muscles around our eyes to the large, strong muscles of our thighs. They are attached to the bones of our skeleton, and each muscle can move a part of the body in a certain direction. Even quite simple actions, such as kicking a ball or smiling, may involve several muscle groups working together smooth muscles are found in most of the body's internal organs. For example, smooth muscles in our stomach and intestine move food through our digestive system. Tiny muscles in our eyes make the pupil bigger or smaller in response to light. Ali muscles contract and relax automatically,without our thinking about it, and so they are called involuntary muscles. The third group, cardiac muscle, is found only in the walls of the heart. These muscles contract and relax automatically in a powerful, regular rhythm to pump blood around our body. Our heart beats an average of about 70 times a minute, every single minute of our lives, without ever resting.