Why am I sore after exercises sometimes?

Why Does Exercise Sometimes Cause Muscle Soreness? (HINT: It’s NOT Lactic Acid!)

For many years, it was commonly believed muscle soreness was from lactic acid buildup, but this has now been thoroughly debunked by science. The burn you feel while exercising is indeed lactic acid, however, your body flushes it out very quickly — within an hour of exercising.

In actuality, lactic acid is a muscle fuel, not a caustic waste product. The myth that lactic acid causes muscle soreness stems from a century-old misinterpreted frog experiment.²

Your muscles produce lactic acid from glucose, which is then taken up by your mitochondria. The more fit you are, the better adapted your muscles are at using it.

The larger your muscles become, the more mitochondria you have, and the more efficient your “lactic acid furnace” will be. Mitochondrial mass (and therefore, athletic performance) is further increased by high-intensity burst type training.

Delayed onset muscle soreness (DOMS), or the muscle soreness you’ve experienced one to two days after exercise, is actually caused by inflammation stemming from microscopic tears in your muscle fibers, or more specifically, microtears between your muscles and their surrounding tissues.

This most often occurs when you start a new exercise program, change it in some way, or resume exercising after a period of inactivity. Eccentric contractions seem to cause the most soreness, meaning movements that cause your muscle to forcefully contract while lengthening, such as the downward motion of squats or pushups.

These damaged muscles release chemical irritants that trigger mild inflammation, which awakens your pain receptors. Other theories about DOMS attribute the phenomenon to changes in osmotic pressure, muscle spasms, or differences in how your muscle cells regulate calcium.

Although science has not yet pinned down the exact process, post-workout soreness is a normal response to exertion and part of an adaptive physiological process that leads to increased strength and stamina.