Muscles are organs composed of muscle tissues which cause movement of some parts of the individual muscle fiber or cells which are specialized for contraction. Contractions cause movement. Muscles make possible the movement from place to place. They also make the heart beat, force blood to circulate, and push food through the digestive system. In order to do their work, muscles require a great deal of energy. They get energy from food, with the most important sources being fats and carbohydrates. The three types of muscles found in the human body are skeletal, smooth, and cardiac muscles.
Source: My sixth grade super workbook: Bearl Brook
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