Microscopic damage from eccentric contraction, not lactate
Expansion
Mechanism
- Eccentric (lengthening) contractions generate high force in few active fibres, causing microscopic damage to sarcomeres and the cytoskeleton
- Damage triggers an inflammatory response with swelling and sensitisation of nociceptors
- Peak soreness at 24 to 72 hours, resolving over 5 to 7 days
- Creatine kinase rises, peaking at 24 to 48 hours
Not caused by lactate, which returns to baseline within about an hour of stopping exercise, long before soreness develops.
Repeated bout effect: a single bout confers protection for weeks to months, through neural adaptation, strengthening of connective tissue and addition of sarcomeres in series. This is why the same session hurts far less the second time, and why training should progress gradually.
Downhill running, lowering weights and plyometrics are the classic provocations.
Rhabdomyolysis is the pathological extreme, with creatine kinase in the tens of thousands, myoglobinuria and a risk of acute kidney injury, and should be considered when soreness is severe, the urine is dark, or the exertion was unaccustomed and extreme.