Exercise can be uncomfortable, even painful. While some discomfort is all but unavoidable, it's important that you know the difference between good pain and bad pain, bad pain being an injury. One of the most common forms of discomfort during activities such as running is burning, and as your thighs are working very hard when you run, they are a common area of discomfort. Many circumstances can lead to burning thighs.
Lactic acid is the byproduct of anaerobic activity. Running fast and sprinting are predominantly anaerobic activities, so a high amount of lactic acid is produced. Running more slowly is an aerobic activity, but lactic acid is still produced, albeit in smaller amounts. Lactic acid alters the pH levels within your body and causes a deep, burning sensation when it accumulates in sufficient amounts. To stop the burning sensation, you need to slow down or stop running.
Your body requires a constant supply of fuel to create energy in your muscles. This energy can come from fat, protein or carbohydrates, and the fuel used depends on your nutritional status and how hard you are working. Carbohydrates stored in your muscles are called glycogen, which is simply glucose molecules bound to water molecules. If you have not eaten enough carbohydrates prior to your workout or have run a very long way, these glycogen stores can empty, and this will leave your thighs tired and burning. In long-distance running, this is often called hitting the wall.
Running is a strenuous activity in which your muscles have to support your entire body weight one leg at a time. Your thigh muscles, properly called your quadriceps, act like shock absorbers while they push you up and off the ground. If you are unaccustomed to running or have run farther or faster than normal, this can cause a burning sensation as these big but not indefatigable thigh muscles begin to tire.
Delayed Onset Muscle Soreness
After intense exercise, many people suffer from sore, stiff muscles. This post-exercise discomfort is called delayed onset muscle soreness, or DOMS for short. It's unclear exactly what causes DOMS, but a buildup of debris from energetic chemical reactions, lactic acid and muscle micro-trauma are all likely suspects. DOMS can leave your muscles feeling tired and weak and also make them burn even when you are performing light activities. Warming up properly, cooling down after exercise and stretching as well as not doing too much running too soon will limit the occurrence of DOMS.
- Fitness and Health with Dr. Gabe Mirkin: What Causes Muscle Fatigue in Endurance Events?
- Sports Injury Clinic: Delayed Onset Muscle Soreness
- Physiology of Sport and Exercise; W. Larry Kenney et al.
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