Why Are Your Muscles Sore Two Days After a Workout?

You do a new workout or walk down a long hill, feel fine, and then wake up stiff and sore a day or two later. Researchers have a fairly clear picture of when this happens, and a still-debated one of exactly why.

Which exercise causes it

A review in The Journal of Physiology by Proske and Morgan says the soreness is tied to eccentric exercise. In eccentric exercise a contracting muscle is forcibly lengthened. In concentric exercise it shortens. Eccentric contractions slow or stop a movement. The review gives downhill walking as a common example: with each step down, the thigh muscle controls how fast the knee bends against gravity, so it works while being stretched.

The authors say untrained people become stiff and sore the day after this kind of exercise. There is no pain right after the exercise. It sets in several hours later and peaks at about 48 hours.

What is happening in the muscle

The review says the soreness is thought to come from damage to muscle fibres. Two early signs of damage are described: disrupted sarcomeres, the tiny repeating units inside muscle fibres, and damage to the system that links the nerve signal to the muscle contraction.

The authors favour the view that the damage starts when some sarcomeres are overstretched during the lengthening. They also say it is a point of controversy which of the two comes first. They describe a later stage in which damaged and dying fibres release breakdown products, leading to a local inflammatory response with swelling and soreness.

Why the second time hurts less

A second bout of the same exercise a week later leaves you much less stiff and sore, according to the review. This is an adaptation process. One proposed explanation is that muscle fibres add sarcomeres, which shifts the length at which the muscle produces its best tension. The authors say this quick adaptation raises the possibility of using mild eccentric exercise to help protect muscles against bigger injuries.

A note

Soreness that is severe, comes with dark urine, or does not improve after several days needs a doctor. This is general information, not medical advice.

Sources

• Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications, Proske and Morgan, The Journal of Physiology, December 1, 2001: https://pmc.ncbi.nlm.nih.gov/articles/PMC2278966/

Facts checked: October 14, 2026.