Sore muscles? Don’t blame lactic acid.

Image Credit: mrflip

With Tim Burton’s film adaptation of Dark Shadows* currently in theaters, it seems fitting to begin this post with a classic trope from vampire comedy, “I just flew in from Transylvania…and boy are my arms tired!” Get it? Arms? Oh, never mind. For me it’s presently more the legs anyway. And it’s not so much fatigue as an excruciating soreness and stiffness of the muscles. An unsolicited preview of old age. Superficially, my suffering was created by an activity called “yard work”, which I discovered only recently and which resulted in several hours crawling around on the ground obsessively uprooting every weed in the vicinity, all in a configuration to which my limbs were apparently unaccustomed.

But what is the actual physiological cause of such aches? If you’d asked me this question a week ago, I would have answered that lactic acid was the culprit, thus making myself look like an imbecile. In case you’re laboring under similar misconceptions, let’s remedy this before any of us has the opportunity to embarrass ourselves in public.

Somewhere along the line many of us learned that lactic acid builds up in the muscles during strenuous exercise and that this causes muscle aches. The basic idea is that lactate (the predecessor of lactic acid) is a byproduct of anaerobic respiration, which is the kind of cellular metabolism that occurs when you’re pushing yourself hard enough to run out of oxygen (on a cellular level, that is, if you’re actually hyperventilating that’s a separate issue). But while lactic acid may be to blame for immediate pain, the proverbial “burn” felt during an extreme workout, it’s long gone from your system by the time the real muscle soreness sets in. This second wave pain typically shows up the next day, reaching the apex of ouch somewhere between 24 and 72 hours after you overdid it at the gym.

The phenomenon is well documented enough to be christened with the badass acronym DOMS (Delayed Onset Muscle Soreness), and it doles out pain as efficiently as its leather-clad namesakes. Science is still working to get a handle on what exactly is transpiring on a molecular level, but the most popular explanation is that DOMS is caused by damage to muscle cells. Sort of like when you sprain your ankle, except instead of one big injury you’re incurring a slew of teeny tiny injuries. As with any other injury, this triggers an inflammatory response, in which your body sends various repair-performing metabolites to the site of the problem, creating a sea of swelling, stiffness and soreness in the process.

DOMS occurs when a person is using their muscles in a way that somehow deviates from the normal routine, either by engaging muscles that typically don’t see much action (as with painting a ceiling or moving a lot of unwieldy boxes) or by ramping up the intensity of one’s existing exercise regimen (i.e., being a competitive jackass in the weight room). But you probably already noticed this trend from personal experience. What you may not be aware of is that some types of muscle movements are more likely to cause DOMS than others. So-called “eccentric” muscle contractions make for the most aches. Eccentric contractions are those in which an elongated muscle braces against an opposing force. This is the converse of concentric contractions, in which shortening of muscles does the work. Imagine that the arm protruding from that building in the photograph suddenly came to life. If it continued curling that weight toward the roof, its biceps muscle would shorten (concentric contraction), but if it lowered the weight toward the sidewalk (in a controlled way, without dropping it on pedestrians), its biceps would elongate (eccentric contraction).^† Running downhill is a form of exercise notably rife with eccentric contractions.

The amount of lactic acid produced during the activity does not predict the severity of DOMS, so just because you aren’t in pain while in the midst of novel physical exertion, don’t assume that you won’t feel crappy the next day. The best way to avoid the late-blooming agony of DOMS is to make only incremental increases in muscle usage, allowing your body to acclimate to new demands before pushing onward. (Pretty useless advice if your main sources of soreness are all-or-nothing activity binges that you’re unlikely to repeat… weeding a lawn, for instance.)

In any event, lactic acid is the least of your problems. In fact, the human body can even use it as an energy source, at least according to this New York Times article, which explains how certain methods of exercise condition the body to use lactic acid more efficiently. Apparently, if you train like a professional athlete, you can grow massive mitochondria that suck up lactic acid like it's Gatorade, while other folks’ cells are just sitting there gasping for breath. You’ll be the envy of the marathon. Though you might still want to avoid running downhill.

* I haven’t seen it. I probably won’t see it. And I don’t need to know any additional details. Just let me have my fantasy that Tim Burton actually managed to successfully adapt one of my favorite TV shows.

^† A note on muscle motion: muscles only actively shorten, the lengthening is passive. When the biceps elongates, it’s due to the shortening of a complementary muscle (the triceps). So you always have both shortening and elongation happening simultaneously. But the important factor is which of the muscles is under tension. This may be one case where feeling beats thinking in terms of comprehension. Grab something heavy, then raise and lower it a few times and note which muscles feel most engaged. See what I mean?