What Causes the Burn in Muscles During Exercise?

You may think lactic acid is to blame for the burning sensation in your muscles that makes you stop short of winning a race. Turns out lactic acid is not the culprit. Lactic acid is actually a valuable fuel source for your system, but when your body’s production of lactic acid goes into overdrive, it releases a byproduct into your muscles. When this byproduct builds up, your muscles burn and you must slow down.

Your body has different ways of producing energy for muscles, depending on the demands of your effort. Adenosine triphosphate is the ultimate supplier of energy for your cells. You have a limited amount of it stored in your muscles, and this can get you through six to 10 seconds of hard work. After you’ve used up this stored ATP, your body starts transforming carbohydrates into ATP during a process called glycolysis. When the conversion happens, the ATP is created, but so are extra hydrogen ions. An excessive buildup of these ions inhibits muscle contraction and leads to the burning sensation in your muscles.

How soon you experience the burn depends on the intensity of your workout. Your body can run off the glycolytic system of converting carbs into immediate energy for 30 to 120 seconds. A max set on the bench press or a 400-meter dash usually defines your limit. At lower intensity levels, such as during a long jog, your body can wait for the oxidative system to kick in and produce ATP with the assistance of oxygen. ATP is created at a slower rate, but this system is more efficient. The byproduct builds up more slowly, so you can go longer before feeling the burn. Highly trained athletes, such as marathon runners, can go hours using the oxidative system.

You can train your body to sustain high levels of intensity for longer, delaying the burning sensation. Your body becomes more adept at buffering the excess ions, leading to faster recovery. Two to four sets of alternating intervals of 20 to 40 seconds of high-intensity exercise with one to two minutes of light effort or complete rest helps your body become more efficient in using glycoloysis. Examples of high-intensity effort include a 50-meter freestyle swimming sprint, a 200-meter running sprint or strength training for eight to 12 repetitions to fatigue. Participating in some training just below the point where you muscles burn, called the lactate threshold, also improves your body's ability to sustain high-intensity exercise. This level is called maximal steady-state training, for runners this is the “tempo” run. Start by going at a pace that feels hard for about 20 minutes. Over several weeks, as your system increases in efficiency, you can extend this period to up to an hour or longer.

The deconditioned athlete may feel the burn at a point equal to 50 to 60 percent of max heart rate. A highly trained athlete can push the point to 80 or 90 percent of max heart rate. Your ultimate ability to delay muscle burning depends on factors outside of training, however. Your age, gender and genetics all play a role.