Metabolic stress
The build-up of metabolites during hard training: the source of the burn and the pump.
Metabolic stress is the accumulation of metabolic by-products during hard exercise: hydrogen ions, inorganic phosphate, lactate and the cell swelling that comes with them. It is the burning, congested sensation of a high rep set, and the physiological basis of the pump.
It was for years proposed as one of three drivers of muscle growth, alongside mechanical tension and muscle damage. That framework is now viewed more sceptically, and the evidence has consolidated around mechanical tension as the primary signal.
The reason metabolic stress looked causal is that it travels with hard work. Sets taken close to failure produce both high tension in the recruited fibres and a great deal of metabolic disturbance, so studies that increased one usually increased the other.
Blood flow restriction training is the closest thing to a clean test. Training with light loads and restricted venous return produces substantial growth with very little mechanical load, which suggests metabolic conditions matter, and the most common interpretation is that they accelerate the recruitment of larger motor units rather than acting as an independent signal.
The practical implication is unromantic. Chasing the pump for its own sake, with endless light sets and short rests, produces plenty of sensation and less growth than fewer harder sets with adequate rest.
Muscle damage has travelled the same road. Soreness reflects novelty and mechanical disruption rather than stimulus, and a session that leaves you wrecked is not thereby a better session.
In practice, judge a set by how close to failure it was and by whether the logbook moves over months, not by how much it burned.