The ache that shows up a day or two after leg day isn’t caused by what most people blame it on. For decades, “lactic acid buildup” was the go-to explanation for post-workout soreness. It’s a tidy story, but it doesn’t match the timeline — and the real explanation says more about how muscles grow stronger than most people realize.

The Lactic Acid Myth, Debunked by Timing Alone

Lactic acid does build up during intense exercise, but it clears out fast. Dr. Corbin Hedt, a physical therapist at Houston Methodist, explains that “generally all of this lactic acid is gone within about an hour afterwards” of a workout. Delayed onset muscle soreness, or DOMS, typically doesn’t show up until 24 to 72 hours later — long after any lactic acid has cleared the muscle. The timing mismatch alone rules it out as the cause.

What’s Actually Happening: Microscopic Muscle Damage

The real driver is mechanical, not chemical. The Cleveland Clinic explains that muscles are made of thousands of fibers that stretch and move, and that exercise “can sometimes cause tiny tears in those fibers.” Dr. Hedt describes a similar process: “muscle soreness occurs because muscle and the connective tissue around it get damaged during exercise.” That microscopic damage triggers an inflammatory response as the body works to repair and rebuild the tissue — and that repair process is what you feel as soreness.

Why Some Exercises Cause More Soreness Than Others

Not all muscle contractions are equally likely to trigger DOMS. The Cleveland Clinic identifies eccentric exercise — movements where a muscle lengthens while under tension, like the lowering phase of a bicep curl, downhill running, or landing from a jump — as the most common trigger. Concentric movements, where the muscle shortens, tend to cause less soreness. This is also why trying an unfamiliar exercise, or suddenly increasing intensity, often produces more soreness than a routine you’re used to.

What Lactic Acid Actually Does

Lactic acid isn’t villainous, either — it may even help. Dr. Hedt suggests that lactic acid buildup during a hard workout “might actually cue muscle regeneration — acting as a signal to your body that your muscles have worked really hard and need to be repaired and rebuilt.” In other words, the burning sensation during a tough set and the soreness that follows days later are two separate biological processes, not two stages of the same one.

How Long It Lasts, and What Helps

According to the Cleveland Clinic, DOMS symptoms build over several hours, peak around one to three days after exercise, and rarely last more than five days. Rest is the primary treatment, though ice can reduce pain and inflammation, heat can relieve stiffness, and light movement like walking or gentle stretching can support recovery by improving blood flow to the area. Warming up before exercise and increasing intensity gradually are the most reliable ways to reduce how sore a new or harder workout leaves you.