Hard and impossible are different
A set feels demanding, but you think you could still complete another repetition. Did you stop too soon? Not necessarily. ACSM’s 2026 guidance does not make momentary muscle failure a requirement for the average healthy adult. [1] That does not mean effort is irrelevant.
Here, failure means being unable to complete another repetition under the exercise conditions you chose, despite trying. Feeling tired, reaching your planned rep count, and being unable to continue are different reasons to end a set. Write down which one happened rather than treating them as the same result.
What a small experiment can tell us
Fisher and colleagues studied nine men doing knee extensions for six weeks. Both failure and non-failure conditions increased measured strength; the between-condition difference was not statistically significant. [2] The failure condition felt harder and took less training time.
That is a small, specific study, not proof that all approaches are equivalent. It used a machine, matched total repetitions and load, and assigned the two conditions to different legs. It cannot tell you the ideal stopping point for your push-ups, or establish that easy sets and demanding sets produce the same results.
Give the set a stopping rule
Our practical suggestion is to decide what the set is for before starting: completing a planned amount of work, practicing a familiar movement, or following a coach’s effort target. Reaching the planned endpoint can be a completed set; you do not need to add a failed attempt as a receipt.
An invented example: you finish the repetitions in your plan and record “demanding, could probably do more.” Keep that observation for the next review. It is an estimate, not a measured reserve or a reason to automatically add repetitions. Pain is not a target to reach; stop rather than push through it.
Review more than the last repetition
Look at the exercise variation, work completed, rest periods, and effort across several comparable sessions. Was the planned work manageable? Could you complete the rest of the session as intended? Those questions give you more to discuss than whether every final repetition failed.
If you follow an individualized program, use its stopping criteria and discuss adjustments with the person guiding you. This article does not choose a universal number of repetitions to leave in reserve. The useful change is to make the reason for stopping explicit.