Anaerobic and aerobic respiration are ways your body converts food into energy so that your brain, muscles and other organs can function normally. Exercises can be anaerobic, aerobic or a combination of both energy systems. The exercise type that you perform depends on your goals, health and fitness status and preferences.
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Exercise and Energy
Your body usually transitions from one form of energy system to another, which is similar to how your car accelerates when you drive a stick shift. When you run, your body relies primarily on anaerobic respiration in the first three to five minutes to break down carbohydrates that are stored in your muscles and liver for energy. Although your body hardly uses any fat for energy, the rate of fat oxidation -- or breakdown -- is gradually increasing like a log slowly burning in a fireplace. As you continue to run at a steady pace for the next hour, your body increases fat oxidation and reduces the rate of carbohydrate metabolism, allowing you to sustain exercise for a long period of time.
Anaerobic exercises require a huge amount of energy. Therefore, anaerobic respiration is the dominant energy system that breaks down carbohydrates into energy, allowing your nervous system and muscles to produce large amounts of force. However, this only lasts between a few seconds to less than three minutes because anaerobic respiration doesn't produce energy fast enough to meet the demands of your body. For example, if you continue to sprint longer than 30 seconds, lactate -- a byproduct produced by carbohydrate metabolism -- begins to accumulate in your muscles and fatigue sets in, slowing down your muscle contraction. This also explains why high-intensity anaerobic exercises, such as heavy weightlifting, box jumps, medicine ball throws and heavy kettlebell swings cannot be performed for as long as a 5K run.
Aerobic exercises uses fat -- with the help of oxygen and carbohydrates -- to provide a steady supply of energy to sustain muscle contractions. Unlike anaerobic exercises, the intensity in aerobic exercises is lower and doesn't require fast and powerful contractions to produce force. Examples of aerobic exercises include marathon running, long-distance cycling and walking. Even meditative exercises, such as yoga and tai chi, can be called aerobic because aerobic respiration is still the dominant energy system your body uses, even if the intensity is much lower than running.
Shades of Gray
Many recreational activities, sports and exercises are neither mostly aerobic or anaerobic because both systems are used interchangeably during the exercise. In soccer and basketball, athletes burst into a bout of sprinting and dodging -- anaerobic -- followed by a longer period of jogging, which is mostly aerobic. Exercise examples include most kettlebell exercises such as the snatch and press and moderate kettlebell swings, interval running or cycling, jumping rope and 1,000-meter freestyle swimming. Even anaerobic activities, like weightlifting, can transition your body's energy system from anaerobic to aerobic after a workout session because of excess post-exercise oxygen consumption, or EPOC. This condition uses aerobic respiration to replenish oxygen supply in your blood and energy for your cells, says exercise physiologist Len Kravitz. For example, the act of performing heavy squats is anaerobic. When you pace around the room between sets of squats, your body transitions to aerobic respiration.