From byproduct buildup to a growth signal
This view of metabolic stress traces a proposed chain of events during high-rep, short-rest training. As the set continues, anaerobic glycolysis produces lactate and hydrogen ions faster than they can be cleared, lowering local pH. Occluded blood flow during contraction holds fluid in the muscle, causing cells to swell. The hypothesis is that this swelling and the altered chemical environment are detected by the fiber and trigger downstream signaling that can release growth factors and support protein synthesis.
Cell swelling in particular is sometimes framed as a perceived threat to membrane integrity that promotes an anabolic response.
A secondary pathway, not the main event
Most current thinking treats metabolic stress as a contributory, secondary pathway rather than the dominant cause of growth, which is attributed primarily to mechanical tension. High-rep sets that generate strong metabolic stress also recruit many motor units as fatigue mounts, so part of the effect may simply reflect that recruitment.
The exact mechanistic steps are still being investigated and should not be read as settled fact. This is general educational information about a proposed mechanism, not medical or training advice.