The term “lactic acid” should never be used in health and fitness settings. While growing up as an athlete, this was one of the most frequently used terms in training. If pitchers were sore, it had to be caused by lactic acid. If your legs hurt after lifting, it was the same story. In reality, no human ever produces lactic acid as a byproduct of any metabolic process. You will never find lactic acid inside of a cell. Most likely, the only time you’ll ever find lactic acid in your body is if you ate food with the compound in it.
Then what causes the soreness?
Soreness may stem from multiple factors. In high-intensity exercise, however, it is assumed that metabolic acidosis plays a role. In some cases, acidosis is caused by ATP hydrolysis, which is the process of converting ATP (the body’s energy molecule) into functional cellular output like lifting a barbell. The breakdown of ATP releases hydrogen ions, which make the cellular environment more acidic. A more acidic environment can interfere with the mechanisms of muscle contraction and inhibit glycolysis, which is one of the main pathways for producing more energy in high intensity exercise. In other words, the more high intensity work you do, the more acidic your muscles become. The more acidic your muscles become, the less high intensity work your muscles can perform.
How does your body naturally combat acidosis from high intensity anaerobic training?
Glycolysis is the breakdown of carbohydrates for energy. The result of the glycolytic pathway is a compound called pyruvate. The utilization of pyruvate depends on the type of training an athlete is performing. If performing high intensity anaerobic exercise, pyruvate is converted by an enzyme into lactate. Lactate is a weak base (opposite of acidic) that ‘counteracts’ the effects of a more acidic muscle environment, and it is almost certainly the basis for the improper use of the term “lactic acid” in training settings. Lactate is not lactic acid.
Lactate build up correlates with muscular fatigue, but lactate is not the root cause of this fatigue. In fact, lactate enables your muscles to perform longer than they would otherwise by combatting the acidosis caused by energy utilization. Further, lactate can be transported to the liver where it is converted into glucose. More glucose helps your body produce more energy for exercise.
How does lactate fit in training?
The point in a training episode at which lactate begins to abruptly rise is called the lactate threshold. The lactate threshold corresponds with increased reliance on anaerobic energy systems. Specifically, this means that your body can only use remaining ATP and carbohydrate sources to refuel. If you’re performing high intensity anaerobic training (i.e. ranging from 6 second to 2 minute max effort work, depending on fitness level), take this into consideration with refueling techniques.
Fortunately, the lactate threshold can be trained. Over time, performing more high intensity training can delay the abrupt rise in lactate, which again, coincides with the underlying acidity of the muscular environment. Furthermore, type II fast-twitch muscle fibers are well-suited for anaerobic training and are the largest producers of lactate. Training these fibers will naturally produce more lactate than type I slow-twitch fibers. Over time, increasing the amount of fast-twitch fibers in your muscle may help to push your lactate threshold further as you make training adaptations.
In summary, high intensity training builds lactate in your muscles. High lactate corresponds with fatigue and decreased performance. To better anaerobic performance and lessen fatigue, you must improve your lactate threshold.
In order to increase your lactate threshold, you can train high intensity exercise in the 6 second to 2 minute range. If you are performing resistance exercises (i.e. strength training), increasing the total volume of work will help push this threshold higher. Other ways to improve this threshold include sprints and 400 m high intensity runs.
- Herda et al. Bioenergetics of Exercise and Training. Essentials of Strength Training and Conditioning 4th ed. NSCA.