Muscle damageExercise-induced muscle damage

Exercise-induced muscle damage is small-scale disruption to muscle fibres and their surrounding structures, most pronounced after unfamiliar work and after heavily eccentric training. It shows up as soreness, temporary loss of strength and raised markers such as creatine kinase in the blood.

It matters because it was once assumed to be a necessary trigger for growth — the 'tear it down to build it up' story. Current evidence does not support that: damage appears to be a side effect of hard training rather than a driver of it, and the repair process largely restores what was there rather than adding to it.

In practice, soreness is a poor guide to whether a session worked. Chasing damage costs recovery and reduces the quality of the next session; the useful signal is whether you are progressing in load, reps or quality over weeks.