Lactic Acid & Muscle Soreness From Endurance Exercise: Fact vs. Fiction

A belief among some athletes and coaches is that lactic acid production during endurance exercise is a primary cause of delayed onset muscle soreness (soreness occurring 24 to 48 hours after exercise). Nonetheless, physiological evidence indicates that lactic acid is not a primary cause of this type of muscle soreness (click here to read what is.)

Several lines of physiological reasoning can be used to support this position:

  1. First, although lactic acid production occurs in active skeletal muscle during high-intensity exercise, lactic acid removal from the muscle and blood is rapid following an exercise session. In fact, blood levels of lactic acid return to resting levels within 60 minutes after exercise. Therefore, it seems unlikely that lactic acid production during a single bout of endurance exercise would result in muscle soreness one or two days later.
  2. The second argument against lactic acid causing delayed onset muscle soreness is that if lactic acid production caused muscle soreness, power athletes would experience soreness after each workout. Clearly, this is not the case. Indeed, well-conditioned power athletes like sprinters rarely experience muscle soreness after a routine training session.

If lactic acid is not the cause of delayed onset muscle soreness, what is the cause? Growing evidence suggests that this type of muscle soreness originates from microscopic injury to muscle fibers. This kind of injury results in a slow cascade of biochemical events leading to inflammation and swelling within the injured muscles. Because these events are slow to develop, the resulting pain generally doesn’t appear until 24 to 48 hours after endurance exercise.