Muscle does not enlarge because it has been used. It enlarges when the demand placed on it exceeds what it currently handles comfortably, which is why training programmes are built around progression rather than repetition.
Tension is the primary signal
Muscle fibres contain proteins that respond to mechanical strain by triggering chemical signalling inside the cell. The tissue is directly sensitive to how hard it is being pulled.
Those signals converge on pathways that increase the rate at which new contractile protein is assembled, which over repeated exposures increases fibre size.
This is why load matters more than the sensation of effort. A movement that feels demanding but produces little tension in the target muscle generates a weak signal.
The stimulus must exceed the current baseline
Tissue adapts to what it routinely encounters. Once a given load is comfortably handled, it no longer represents a demand for additional capacity.
Continuing at that level maintains what exists. This is useful, and it is what most physical activity achieves, but it is not a growth stimulus.
Progression can come from more load, more repetitions at the same load, or shorter recovery, but something must exceed what the tissue has already accommodated.
Growth happens during recovery, not during training
Training initiates the signal and simultaneously causes damage and fuel depletion. The synthesis of new protein occurs over the following day or two.
If the next demanding session arrives before that process has run, the tissue is being asked to adapt while still recovering from the previous stimulus.
This is the reasoning behind spacing sessions for the same muscle group rather than treating more frequent training as automatically better.
Adaptation is not only in the muscle
Early strength gains in an untrained person are largely neural. The nervous system recruits more motor units and coordinates them better, with little change in fibre size.
This is why measurable strength can improve within weeks while visible change takes considerably longer, and why the two do not move in step.
Tendon and bone adapt on slower timescales again, which is one reason progression that outpaces connective tissue is a common route to injury.
Why the response has limits
The rate of possible growth declines as a person approaches their trained ceiling, and that ceiling differs substantially between individuals for reasons largely outside their control.
Protein availability, total energy intake and sleep all constrain the response, so a strong stimulus applied without adequate recovery produces less than expected.
Anyone with an existing injury or medical condition should establish what loading is appropriate with a clinician or qualified professional before progressing.