If you've run, biked, swum or lifted weights at high intensity, you're familiar with the sensation of lactic acid flooding your working muscles -- the "burn" that some exercise impresarios and personal trainers exhort you to strive for. Lactic acid is a by-product of anaerobic metabolism, or the burning of fuels by muscles and tissues that doesn't require oxygen. You may have wondered why lactic acid accumulates in your system even when you're breathing as hard as you can.
Anaerobic vs. Aerobic Fuel Metabolism
The primary fuels you use while exercising are carbohydrate and fat, with protein making a minimal contribution. At low to moderate exercise intensities, glucose is converted in muscle cells in a series of steps to pyruvate, which then enters the mitochondria of the cells and is used to generate ATP, the direct source of energy for working muscle, in the presence of oxygen.
During high-intensity exercise, the pyruvate converts to lactate, which begins to accumulate locally as lactic acid. During the first 10 or so seconds of very-high-intensity exercise, phosphocreatine is also used for fuel, but its stores are exhausted in short order. Although you breathe hard during the first ten seconds of an all-out sprint, the oxygen you take in is not used to convert glucose to energy during this time.
Lactate Threshold
As an endurance athlete ramps up the intensity of her workout, she begins to approach what exercise scientists call that lactate threshold, or anaerobic threshold. Even at low intensity, your body produces small amounts of lactate but you clear it more quickly than you produce it under these conditions, so it does not accumulate. Once you reach a certain high intensity -- about 85 to 90 percent of maximum heart rate for trained athletes -- you cross the lactate threshold, and begin to derive most of your fuel through anaerobic metabolism. Lactate thus accumulates more quickly than it can be cleared and no matter how hard you breathe, muscle exhaustion looms on the near horizon.
Muscle Exhaustion
It was long believed in the exercise-science community that the accumulation of lactic acid in muscle during anaerobic exercise and the accompanying muscle-cell acidosis, manifested as a lower pH, was directly responsible for the inevitable muscle failure under these conditions. In recent years, however, scientists have learned that while lactic-acid levels correlate strongly with fatigue, they do not cause it.
Instead, an accumulation of inorganic phosphate during high-intensity exercise interferes with muscle cells' interaction with calcium ions, which are critical in the generation of forceful muscle contractions. No amount of hard breathing can negate this process; oxygen is simply not a player in the anaerobic game.
Anaerobic Threshold Training
Once the concept of an anaerobic threshold became known, athletes and coaches naturally set about discovering training methods that would push the threshold back and allow for greater workloads in a purely aerobic zone. According to Len Kravitz Ph.D. of the University of New Mexico, this training involves a combination of high-volume endurance training, maximal steady-state training just below an athletes current lactate threshold and high-intensity interval workouts with repetitions of several minutes in the anaerobic zone separated by very-low-intensity recovery periods.
References
- UCLA Center for Human Nutrition: Fuel
- Brian Mac Sports Coach: Anaerobic Threshold Testing
- University of New Mexico; Lactate Threshold Training; Len Kravitz, et al.
- "Human Muscle Fatigue"; Craig Williams and Sébastien Ratel; 2009
