By Brian B. Parr, Ph.D., ACSM Certified Clinical Exercise Specialist
Imagine you are out running or cycling, cruising along at a speed that is intense but sustainable. Then imagine you start up a long hill. At first you can keep your pace pretty easily, but soon the hill starts to get your attention as it gets more difficult to maintain your speed. Then you feel it… the fatigue, heaviness, and weakness in your legs that is commonly called the “burn.”
Almost everyone who exercises has experienced this at least once and it can be a regular part of working out for people who are engaged in intense training. If you end your training sessions with a hill or a sprint it can even persist for a short time after exercise.
But what causes it? Conventional wisdom holds that lactic acid is responsible for the “burn” you feel during and immediately following intense exercise. Lactic acid is also the usual suspect for the muscle soreness that can occur the next day after exercise. In reality there are a host of factors that can contribute to this feeling and the associated fatigue and weakness that occurs during and after exercise.
During exercise, especially intense exercise, there are a number of changes that occur inside the muscle that are related to the energy systems used to produce ATP, the energy used by the muscle. They include lactic acid as well as changes in muscle oxygen and carbon dioxide levels and lower pH (the muscle becomes more acidic). Other factors including free radicals, intramuscular phosphates (by-products of ATP use), and calcium may also play a role. Additionally, during exercise there are large changes in muscle temperature and blood flow. Any of these factors can contribute to the “burn” that people feel and it is likely a combination of these factors.
These metabolic and muscle changes are returned to normal shortly after exercise ends, more quickly with an active cool-down. Light exercise in recovery, speeds the clearance of lactic acid and other metabolic by-products, which is one reason why simply stopping exercise and sitting down is a bad idea.
Incidentally, lactic acid is an important cause of fatigue that occurs during prolonged, intense exercise. As exercise intensity increases the muscle relies more on anaerobic ATP production, which results in the production of lactic acid. The hydrogen ions released from the lactic acid can interfere with the ATP production in the muscle as well as with muscle contraction – a “double whammy” leading to fatigue.
While these muscle changes seem negative, they are actually part of the training response. For example, endurance training such as running, cycling, and swimming results in changes that make the muscle more oxidative, meaning it uses oxygen better and is more resistant to fatigue. It is consistent with a shift from fast, anaerobic muscle fibers to more slow, aerobic fibers, which leads to better performance in endurance events. The muscle adapts in this way in part as a response to the changes that occur during intense training. This is also why high-intensity interval training can be so effective for promoting muscle adaptations – the repeated intense effort stimulates the muscle better than prolonged training at a lower intensity.
It is important to note that the “burn” that is felt during and immediately after intense exercise is different from the muscle soreness felt a day or two after an intense training session. This is called delayed-onset muscle soreness (DOMS) and typically occurs 24-48 hours after exercise. It is most common after weight training, especially in people who are just starting out or after a session in which they increase their weights. As runners, cyclists, and rowers know, it can also occur after an intense endurance training session.
Contrary to what many people believe, lactic acid is not the cause of DOMS. In fact, lactic acid is cleared from the muscle shortly after even intense exercise, so it is not even present in high levels when the muscle soreness occurs.
DOMS is actually caused by microscopic damage to muscle fiber, which leads to inflammation, edema (swelling), and pain. Many people also notice weakness of the involved muscles. (This is one reason why intense training right before a big event is a bad idea.) Again, this sounds bad but the response to this damage is to build more muscle fibers, which, over time, leads to increased strength.
This is the foundation of the “no pain, no gain” philosophy of training which, unfortunately, can lead to more pain than gain. Well-designed training programs that progress slowly, minimize the pain and maximize the gains in strength. Many people are familiar with the “10% rule” which suggests that you can avoid injury if you limit your increases in training volume to 10% at a time.
So, the next time you sprint to the finish or ride up a long hill keep in mind that “the burn” you feel is promoting muscle adaptations that will improve your performance over time. That feeling can also serve as a signal that the intensity is not sustainable, so slow down if you have more distance to cover. You can minimize muscle soreness after exercise (DOMS) by progressing slowly and not doing too much too soon.
Brian Parr, Ph.D. is an Associate Professor of Exercise and Sports Science at the University of South Carolina Aiken where he teaches courses in exercise physiology, nutrition, and health behaviors. He also conducts research related to physical activity and weight loss. Dr. Parr is an ACSM Certified Clinical Exercise Specialist.