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The muscles in the body can be divided into three distinct groups:
Voluntary muscles Voluntary muscles are attached to the skeleton and move the bones at joints when the voluntary muscles contract and relax. Our brain sends messages to the muscle through the central nervous system when we consciously decide that we want the muscle to contract. Voluntary muscles allow us to walk, throw a ball, grip a pen, play the piano, in fact any action where the skeleton is moved. Voluntary muscles also contract to keep the body stable when we are sitting and standing. The muscles are always in a state of slight tension. This state is called muscle tone. Involuntary (Smooth) muscle These muscles are found in the walls of our internal organs. Their contraction is not under our conscious control. We have involuntary muscles in the eyes, diaphragm, intestines, blood vessels, bladder and the stomach. It is sometimes called smooth muscle because it lacks the characteristic stripes that are apparent on the above diagram. It is these muscles which line the walls of veins to push blood back to the heart from the lower body. These muscles line the uterus and are used during childbirth. Cardiac muscle This involuntary muscle is found only in the walls of the heart. Cardiac muscle contracts the heart to pump the blood through it. The average heart rate is 72 bpm (beats per minute), but the heart can increase this when extra demands are placed upon it - for example, when we are exercising. It is different from other involuntary muscles in that it contracts rhythmically. When the heart muscles do not contract in rhythm, it results in a condition called arrhythmia. We can tone cardiac muscle through regular exercise.
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