Muscles (actually the muscles’ cells) use a variety of energy sources to power their contractions. For quick energy, the cells use their stores of ATP and creatine phosphate, which is another phosphate-containing compound (for more about ATP, see the chapter “Basics of Biology”). These stored molecules are usually depleted within the first twenty seconds of activity. The cells then switch to other sources, most notably glycogen, a carbohydrate that is made of glucose molecules strung together in long-branching chains. The following lists the sources and where energy is stored in the human body: