Why Do Your Thighs Burn When You Are Working Out & Walking?

When the body exercises anaerobically, it produces energy from the breakdown of carbohydrates. This process, called glycolysis, uses stored sugar to make energy, with lactic acid the byproduct. Activities from brisk walking that leaves you out of breath to strenuous squats, lunges and curls produce lactic acid in the muscles, causing a burning sensation. The less accustomed you are to a certain activity, the faster and more easily your muscles will burn.

Fitness professionals sometimes refer to glycolysis as the lactic acid system because its production results from the muscle’s reduced capabilities during exercise. As the muscle becomes more acidic from lactate production, its nerve endings become aggravated, exercise physiologist Joe Cannon notes. This results in a nearly painful burning sensation. As lactic acid levels rise, the muscle’s ability to make energy and exert force fall. This is why thigh muscles feel tired after a long brisk walk or following an intense session of strength training your legs.

A popular misconception among amateur athletes is that lactic acid production causes muscle soreness and reduced range of motion in the days following an intense workout. Cannon explains that the body removes most of the lactic acid from the muscle in the hour following exercise. In fact, some of the lactic acid converts to glucose and is stored by the body for later use. Although health professionals theorize as to why people experience delayed onset muscle soreness -- commonly called DOMS -- no one theory provides a complete explanation as to why it occurs.

As you exercise and strength-train the legs, you work the quadriceps and hamstrings of the thigh, the calves of the lower leg and the glutes (your rear end). The exercise “burn” that people report feeling builds up over time and does not suddenly occur; any immediate, sharp pain may be the result of an injury such as a strain or tear. Although working through a slow burn helps you increase strength and endurance, working through a sharp pain is not recommended, as it may prolong and worsen a potentially serious injury.

Once you stop exerting force with the burning muscle, the painful sensation should cease. You can improve muscular endurance by lifting lighter weights but for more repetitions -- for example, instead of squatting 10 times while holding 16 pounds, try squatting 20 times as you hold 12-pound weights; limit rest between sets to less than 30 seconds. Also, try exercising aerobically at slower paces but for longer periods; a 45-minute jog at 5.5 mph increases your endurance more than running at 7.5 mph for 20 minutes. As your body exercises through the slow lactic acid burn, you develop a tolerance for it -- and improve your endurance at the same time.