Meeting the demands of working muscles involves nearly every system in the body. The cardiovascular system ramps up heartbeat to move blood to the muscles. The respiratory system replenishes that blood with oxygen when lungs fill with air. The digestive system shuts down to preserve energy for where it's most needed. And the skin processes heat from ongoing chemical and metabolic reactions that might otherwise build up to dangerous levels.

Muscles contain a limited supply of a chemical called adenosine tri-phosphate, or ATP. When a muscle cell needs energy, it breaks the bond holding ATP's nucleotide and phosphate groups together. This bond releases energy that can be used to fuel chemical reactions that in turn keep muscles moving. However, once the limited supply is exhausted—typically after a short burst of exercise—a new supply of ATP must be produced if exercise is to continue.

Mitochondria, the "powerhouses" of cells, transform food, in the form of stored carbohydrates and fats, into chemical energy, in the form of more ATP. To do this, they require oxygen. The body has several ways of increasing the flow of oxygen to working muscles, including increasing the flow of blood from the heart and increasing the rate and depth of breathing. The body can be trained so that these systems operate more efficiently, beginning at the cellular level.

So-called aerobic endurance training—"aerobic" meaning "in the presence of oxygen"—improves the efficiency with which oxygen is delivered to a working muscle, making that muscle perform better. By increasing blood flow through training, hemoglobin, a protein found in red blood cells, can deliver more oxygen to the working muscle. Hemoglobin carries oxygen from the lungs to the muscle and then binds with carbon dioxide waste generated by the muscle. Exercise also causes blood vessels and capillaries to expand, which in turn helps maximize the volume of blood being pumped by the heart. One composite measurement of how well a body utilizes oxygen during exercise is called VO₂ max (maximal volume of oxygen uptake). A high VO₂ max reading means the muscles are producing more and larger mitochondria, which are using all the available oxygen to generate energy.

To improve VO₂ max readings, training should be focused on increasing both the duration and intensity of activity. For example, distance runners may gradually increase their weekly mileage program, all the while sustaining their running pace. To safely increase training intensity, which may enhance VO₂ max even more than duration, a runner may engage in what's known as interval training. This means incorporating shorter bursts of higher-intensity running into certain workouts. For example, instead of running an entire workout at 75 to 80 percent of one's maximum heart rate, a runner could perform several 3- to 5-minute intervals at 100 percent of his or her maximum heart rate, allowing recovery time between intervals.

Increased muscle strength and endurance are two obvious benefits of regular exercise. However, there are several others. Doctors consider obesity a silent killer that affects millions upon millions throughout the world and recommend exercise to help combat the condition. Also, while exercise cannot restore lung function that has been lost, it can improve the lung's ability to take in oxygen, which can benefit patients with respiratory diseases such as emphysema. Lastly, exercise stimulates various brain chemicals, which may produce relaxation and relief from depression or anxiety.

Determine recovery times after exercise in this NOVA classroom activity.

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