Feeling the burn while exercising is another way to describe the chemical reaction that occurs in your muscles when they exercise in the absence of oxygen. There are a number of factors that contribute to the oxygen in your muscles, so it is not true that the burn is directly related to the optimal workout. However, there are ways to adjust your workout so that you do feel the burn.
What is the Burn?
The burn comes from your muscles' production of lactic acid. Lactic acid is produced when you exercise at an intensity high enough to exhaust the muscle's supply of oxygen. The muscles need oxygen to break down the glucose that produces energy. Without the oxygen, it uses enzymes to break down the glucose, and lactic acid results as well. With the muscles overwhelmed with the exercise and trying to get more oxygen, the bloodstream can become overrun with lactic acid. The amount of lactic acid begins to mount, causing the burn you feel.
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Anaerobic exercise is exercise that occurs in the absence of oxygen. It is the type of exercise — as opposed to aerobic exercise, which occurs in the presence of oxygen — that leads to the burn feeling. Anaerobic exercise is not, however, the optimal type of exercise for everybody. Anaerobic exercise is high-intensity exercise, and some people do not want to exercise at such intensity. For this reason, you do not need to feel the burn when exercising.
Aerobic exercise may be the best exercise option for some people; in which case, not feeling the burn is actually the goal of the exercise. Aerobic exercise occurs in the presence of oxygen, so the body creates less lactic acid and you do not feel the burn. Aerobic exercises include long duration, low-intensity exercises such as walking, jogging, cycling and swimming.
What if There is No Burn?
When you do not feel the burn, you are exercising at an intensity at which your muscles have sufficient oxygen. If you would like to feel the burn, that means that you are seeking an exercise intensity at which your muscles lack oxygen and therefore begin producing lactic acid. In order to feel the burn, increase the intensity of your workout until you feel the burn or consider switching to an anaerobic exercise such as weightlifting.