Let’s just start this off immediately by addressing that lactic acid does not cause muscle soreness (Bebbert et al 1986). We’ve known that longer than I’ve been alive, but this myth still persists. Now that we got that out of the way, we can discuss what is really going on those days after a strenuous workout.
The predominate theory in the field blames delayed onset muscle soreness (DOMS) on microscopic “tears” in your muscles. Think of it almost as a small, controlled muscle strain of sorts (don’t’ worry, it’s a good thing). However, if you’ve ever strained (“pulled”) a muscle, you know that it hurts immediately, and DOMS really kicks in about 24-48 hours after exercising. So why the difference?
Ironically, it’s the repair process, rather than the damage itself, that is thought to cause DOMS. Since these micro-tears in the tissue exist, the body sends in specialized cells to clear out damaged tissue and mobilize a host of other types of cells to start the healing process. During this chaos of cell removal, repair and rebuilding nearby healthy cells are also damaged, allowing fluids to enter the cells and they begin to swell (inflammation). While all of this is happening other signaling chemicals are released to facilitate healing, but one in particular, called bradykinin, ends up causing an increase in nerve growth factor (NGF). This increase in NGF is what is associated with chronic pain conditions because it makes your nerve endings more sensitive to pain. NGF is predominately what is responsible for the pain during DOMS, while inflammation of the muscle, while occurring simultaneously, is only a fraction of what contributes to the pain you’re feeling (Murase 2010). This is still being debated in the exercise physiology world, but the amount of evidence is growing to support this claim.
Knowing this, once that workout is in the books, you’re doomed for DOMS. Additionally, there’s no shortage of promises for relief from DOMS, most of which actually have little to no effect on reducing DOMS. I’ll have a different post addressing many of these “remedies” more specifically. Knowing this, embrace the soreness! It’s a good thing that you pushed your body to that point, however uncomfortable it might feel at the time. In order for muscles to adapt and become bigger, stronger, and faster they need to be stressed, and DOMS is a normal reaction to that stress, indicating you’ve done enough to challenge that tissue.
So next time you’ve done all those sets of squats, and you’re cursing anytime you go down stairs, sit on a toilet, or get out of your car, remember this: you’re in the process of becoming a more awesome version of you.
Bobbert, Maarten et all, 1986. “Factors in delayed onset muscular soreness of man.” Journal of Science and Medicine in Sports & Exercise. 18(1), 75-81.
Murase, Shiori et al., 2010. “Bradykinin and nerve growth factor play pivotal roles in muscular mechanical hyperalgesia after exercise (delayed-onset muscle soreness).” Journal of Neuroscience. 30(10), 3752-3761
American College of Sports Medicine Resources for the Personal Trainer, 4th Edition, 2014. Lippincott Williams & Wilkins. Pp. 366.