Lactic acid gets blamed for the burning during a hard set and for the soreness two days later. I have since read that both attributions are wrong, and that lactate is a fuel rather than a waste product.
What is actually causing the burn?
Accepted Answer
Two separate corrections, and it is worth keeping them apart.
First, lactic acid as such barely exists at body pH. It dissociates almost completely, so what accumulates is lactate plus hydrogen ions. The acidity comes from the hydrogen ions, and lactate is produced alongside rather than being the acid.
Second, lactate is not waste. It is shuttled to other muscle fibres, the heart and the liver and used as fuel, which is the Cori cycle.
The burning sensation tracks hydrogen ion accumulation and the metabolites that build up in hard anaerobic work, sensed by metabolically sensitive afferents. Lactate is the marker that rises alongside, which is how it got the blame.
And the soreness two days later is a completely different phenomenon, which is the more useful half of this to know.
Lactate clears within an hour of stopping. Delayed onset soreness peaks at 24 to 72 hours, long after there is any lactate left to blame.
It is associated with microscopic damage from eccentric contractions, the lowering part of a movement, plus the inflammatory response to it. That is why walking downhill or lowering weights slowly produces far more soreness than the concentric equivalent.
The practical consequence is that soreness is a poor guide to whether a session was useful. A new movement makes you sore because it is new, not because it was productive.
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