Your body is a highly adaptable and complex organism with an enormous capacity for energy production. During exercise, you burn fats and carbohydrates for fuel to keep your muscles pumping using both aerobic and anaerobic metabolism. Understanding the basics of energy metabolism will help you build a training program for optimal function and peak performance.
Energy and ATP
Energy for muscle contractions comes from adenosine triphosphate, or ATP, a chemical molecule that is made and stored in your muscle cells. Because your stores of ATP are limited, and because they are rapidly consumed during physical activity, you must perpetually manufacture ATP as you exercise. ATP is mostly made from glucose that is stored in your muscles in the form of glycogen, and fat that is stored in both your muscles and adipose tissue. The intensity of your exercise determines which fuel source is most dominant, although all energy pathways contribute simultaneously, according to exercise scientist Jason Carp, PhD of the IDEA Health and Fitness Association.
During very intense exercise lasting less than 10 seconds, you rely on muscle stores of creatine phosphate, or CP, to resynthesize ATP. This energy pathway, known as the ATP/CP system, is anaerobic, meaning it is not reliant on oxygen. While the ATP/CP system can produce large amounts of force for several seconds, its stores are limited and quickly exhausted. The ATP/CP system is used in powerful bursts of movement, like explosive jumps during basketball, soccer or tennis. Like ATP, CP is regenerated in your muscle cells during lower intensity rest periods. To enhance your capacity to store and regenerate ATP/CP, Carp recommends performing short intense sprints of five to 15 seconds, alternating with three to five minutes of lower intensity rest. Repeat this cycle five to eight times.
Quick and Intense
You can also use glucose to make ATP anaerobically, a process known as glycolysis. According to Carp, glycolysis satisfies energy needs for all-out exercise lasting from 30 seconds to two minutes, as in sprints or resistance training sets. Lactic acid is a byproduct of glycolysis that is released into your bloodstream for elimination. During aerobic endurance activities, you make a limited amount of ATP with glycolysis. As you speed up to a more challenging pace, your reliance on glycolysis increases, and your blood lactic acid begins to accumulate faster than it can be removed, a point called the lactate, or anaerobic, threshold. When you reach your threshold, you will not be able to continue at that pace for more than a minute or two. You can train your body for faster lactate removal and raise your lactate threshold by alternating all-out sprints with longer bouts of aerobic activity.
Slow and Steady
During lower-intensity rhythmic activities like walking, running, swimming or cycling, you use glucose and fat to make ATP in your muscle mitochondria, tiny organelles where aerobic metabolism occurs. According to Carp, your aerobic system produces 18 times the amount of ATP as glycolysis, making it the ideal pathway for long-duration activities. Because your glycogen stores are more limited than your fat stores, you use an increasing percentage of fat for fuel as time wears on and your glycogen stores become depleted. While glucose can be used both aerobically and anaerobically, fat is a strictly oxidative fuel, and can only make ATP in the mitochondria. Your aerobic energy system can be enhanced with both endurance training and sprint intervals.