Energy and Exercise

How does the body produce ATP during different stages of exercise?

Bang! The starter's pistol goes off, and the runners push off their starting blocks and sprint down the track, as seen in Figure 9–9. The initial burst of energy soon fades, and the runners settle down to a steady pace. After the runners hit the finish line, they walk around slowly and breathe deeply to catch their breath.

Let's look at what happens at each stage of the race in terms of the pathways the body uses to release energy. Humans have three main sources of ATP: ATP already in muscles, ATP made by lactic acid fermentation, and ATP produced by cellular respiration. At the beginning of a race, the body uses all three ATP sources, but stored ATP and lactic acid fermentation can supply energy only for a limited time.

FIGURE 9–9 Exercise and Energy During a race, runners rely on the energy supplied by ATP to make it to the finish line. Apply Concepts At the beginning of a race, what is the principal source of energy for the runners' muscles?

Quick Energy What happens when your body needs lots of energy in a hurry? In response to sudden danger, quick actions might make the difference between life and death. To an athlete, a sudden burst of speed might win a race.

Cells normally contain small amounts of ATP produced during cellular respiration. When the starting gun goes off in a footrace, the muscles of the runners contain only enough of this ATP for a few seconds of intense activity. Before most of the runners have passed the 50-meter mark, that store of ATP is nearly gone.