Why do I get sore a day or two after hard exercise?
Scientists have made great progress in discovering what doesn’t cause soreness after exercise. It’s not a low-grade persistent muscle spasm, and it’s not an accumulation of lactic acid. Instead, most researchers now agree with a theory first proposed over a century ago that blames post-exercise soreness on microscopic “tears” in your muscles. But that leaves an important mystery: if muscle damage is the cause, why does the pain peak 24 to 48 hours after you stop exercising?
You may know from bitter experience that the harder you work out, the more likely you are to be sore afterwards. But intensity isn’t the only factor, as numerous experiments with hill running and stair climbing have shown. For example, Swedish researchers compared three groups of volunteers who ran for 45 minutes on a treadmill, either on an uphill slope of four degrees, a downhill slope of four degrees, or a downhill slope of eight degrees. Even though the uphill group had to work the hardest to maintain pace, it was only the downhill groups that developed “delayed-onset muscle soreness,” or DOMS.
The reason is that downhill running involves “eccentric” muscle contrations, which occur when the muscle is trying to shorten but is being forced by an external load to lengthen. Typical examples include lowering the weight in a biceps curl or the braking action of your quadriceps (front upper leg) muscle as you run downhill. During eccentric contractions, your muscle filaments are stretched to their limits – and sometimes beyond. The resulting damage effectively weeds out the weakest links in your muscles, so that they will be stronger once they’re repaired.
Ironically, it’s the repair process, rather than the damage itself, that is thought to cause pin in the day or two after exercise. The body sends cells called neutrophils and macrophages to clear out the damaged tissue and mobilizes a host of other types of cells to begin the rebuilding process. The outer membranes of nearby muscle cells get damaged in the process, allowing fluid to rush in and cause the muscle to swell. Meanwhile, another substance called bradykinin is released by the damaged muscle, which after a delay of about 12 hours, causes an increase in levels of “nerve growth factor” that lasts for about two days. Nerve growth factor, which is associated with chronic pain conditions, makes your nerve endings more sensitive – so that any movement of your inflamed muscles presses against these hypersensitive nerves and causes pain. Recent studies in Japan and elsewhere have suggested that nerve sensitivity caused by bradykinin is enough to explain the delayed response of DOMS, and any inflammation is purely coincidental. This debate hasn’t yet been resolved.
Still, the practical message is clear: once you’ve done the crime, you’ll have to serve the time. The pain results from muscle damage, and once the workout is over you can’t undamaged” the muscles, despite the promises of various locations, creams, and pills. The good news, though, is that the damaged muscle comes back stronger once it’s repaired. In fact, without this damaged-repair cycle, you wouldn’t get any benefit from training – so ideally, you want your workout to fall in that sweet spot where you’re doing enough microscopic damage to stimulate adaptation, without doing so much damage that you have to skip the next few workouts. As you weed out the weak muscle fibers, you’ll become less an less susceptible to DOMS. And you don’t have to suffer the full effects of DOMS to get this protective effect so a little moderation when you’re getting back into workout out or trying a new exercise can allow you to avoid DOMS entirely.