Many people think muscle pain after exercise comes from lactic acid. But this is not true. Lactic acid goes back to normal about one hour after exercise. Muscle pain is worst one or two days later.

In a 1983 study, running downhill caused stronger muscle pain than running on flat ground. When you go downhill, your muscles work like brakes. This causes tiny damage, and the body repairs it.

For a long time, people believed that muscle soreness was caused by lactic acid building up in the muscles. However, scientists now reject this idea. Lactic acid in the blood returns to normal about an hour after exercise, but muscle soreness usually peaks one or two days later.

In 1983, American researchers compared running on flat ground with running downhill. Although downhill running was done at a lower intensity, it caused clearly stronger soreness. The key is the type of movement. When muscles produce force while being stretched, as in going downstairs, they act like brakes. This causes tiny damage, followed by inflammation as the body repairs it.

Exercise scientists have largely abandoned the popular idea that muscle soreness comes from lactic acid, partly because the timing does not fit. Blood lactate returns to resting levels within about an hour after exercise, whereas delayed onset muscle soreness typically peaks one to two days later. A 1983 experiment by Schwane and colleagues added further evidence.

Running downhill produced clearly greater soreness than running on level ground, even though it was done at a lower intensity. That comparison involved only a few participants, but its pattern points to eccentric contraction, in which muscles generate force while lengthening, as when descending stairs or lowering a weight. Such braking movements are thought to cause microscopic damage to muscle fibers and surrounding tissue, which triggers inflammation as part of the repair process.

Animal studies in Japan suggest that substances such as nerve growth factor make the muscle sensitive to pressure, though the full chain of events has not been worked out. Repeating the same exercise reduces soreness, a phenomenon called the repeated bout effect. Lactate, meanwhile, is now regarded less as waste than as a fuel the body reuses.