The build-up of metabolites during a set, felt as the burn and the pump, once proposed as a growth driver in its own right.
It accumulates in high-rep sets, short-rest work and anything with occlusion, such as continuous tension sets where the muscle never fully relaxes. Blood flow restriction training is built on it.
The current view is more modest than the 2010s version. Metabolic stress probably contributes indirectly, largely by driving fatigue that forces higher recruitment within a set, rather than by signalling growth on its own. Chasing the pump for its own sake is not a program.
Where it is genuinely useful is when load has to stay low: a client recovering from injury, or one training around a sore joint, can grow with light loads and short rest in a way that heavy-load logic would not predict.