Why does muscle soreness fade when you repeat the same workout?

Short answer

Because muscle and nerves adapt to the first session and get damaged less by the same stress. The second time, soreness and damage markers were lower, and this protection lasted up to 6 months and was gone by 12 months. Even one light session at 10% of maximal strength gave protection for up to 1 week.

Why does muscle soreness fade when you repeat the same workout?
'Sore at first → the body adapts → less sore next time': how far is it true?
The first time you do a new exercise, your muscles ache a day or two later, and repeating it hurts less. We checked step by step how big this protection is, how long it lasts and why it happens.
The second time clearly hurt less
Both soreness and damage markers were lower
In an analysis that pooled 20 studies of exercises using several joints, the second session brought less soreness and lower muscle damage markers one and two days later, and strength and jumping dropped less. When 34 people did sprints and depth jumps and repeated them 2 weeks later, every measure also recovered faster.
How long does it last?
It held at 6 months and was gone at 12 months
35 people did a hard session of slowly lowering weights with the elbow-bending muscles and repeated it 6, 9 or 12 months later. At 6 months, soreness, swelling and damage markers were all lower; at 9 months only the faster return of strength remained. At 12 months there was no protection.
Even a session too light to hurt worked
30 reps at 10% of maximal strength, protection for up to 1 week
In an experiment with groups of 13, slowly lowering a dumbbell at 10% of maximal strength 30 times left no muscle damage. Yet done 2 days or 1 week earlier, it reduced the damage from a hard session. At 2 weeks the protection was weaker, and at 3 weeks only soreness was slightly lower.
Why does it fade? (1) The muscle itself changes
In the same muscle, the second injury was almost blocked
In a rat experiment, when the same muscle was injured twice 2 weeks apart, cell-membrane damage and protein breakdown were almost blocked the second time and strength came back within a day. This matches a review that describes muscle elasticity, the connective tissue around muscle, cell signals and nerves changing together.
(2) The nerves change too
The untrained opposite limb also weakened less
When 23 people did the same amount of exercise twice, 3 weeks apart, strength and soreness recovered faster the second time and the wobble in nerve signals to the muscle dropped. In an analysis of 14 studies, training one side made the opposite side lose less strength after a later session. In rats, this opposite-side protection appeared with no change in the muscle.
From the third time on, it did not grow
Most protection comes from the very first session
In an analysis that pooled 11 studies (72 people plus animals), strength loss and recovery speed after a third session showed no difference from the second. Measured by strength, most of the protection forms after the first session, and later repeats did not add more.
Summary: muscle and nerves change to get hurt less
For a new exercise, one light session first is enough
Once you have done an exercise, muscle and nerves change together, so next time you get damaged and sore less. This protection lasted up to 6 months and was gone by 12 months. For a new exercise, or one you have skipped for close to 1 year, one light session at about 10% of maximal strength 2 days to 1 week before is enough (based on an arm exercise study).