You train hard.Nothing moves.Find out why.

Muscle doesn't grow because a set happened; it grows because the last few reps of a set were hard enough to recruit the biggest, highest-threshold motor units and make them work slowly under load. In the studies that compared sets stopped four or more reps short of failure with sets taken close to it, the close-to-failure sets produced more growth per set, and the gap widened as sets got easier. For muscle, most of the useful stimulus in a set lives in roughly the last zero to three reps before failure. Stop at rep six of a possible twelve and you've done the easy half. Strength works differently: it rises mainly from regular contact with weights close to your limit, because a heavy bar recruits the big motor units from the first rep, and how close each set gets to failure matters far less. So for a strength lifter the problem isn't avoiding failure — it's a week in which nothing is both heavy and hard.

The reason is recruitment. Early in a set the small, fatigue-resistant fibres do the job. As they tire, the nervous system is forced to bring in the larger fibres — the ones with the most room to grow and the most to contribute to a heavy single. If you leave with six reps in the tank, those fibres were barely asked. You still feel the set — the burn, the breathing — which is why it's convincing. But the sensation of work and the recruitment that drives change are not the same thing.