Define the stopping point
Momentary muscular failure means being unable to complete another repetition under the required conditions. Stopping because your technique is deteriorating is a different decision. Repetitions in reserve, or RIR, describe how many more repetitions you estimate you could perform. That estimate is useful for discussing effort, but it is not a perfectly precise measurement, especially when you are still learning an exercise.
What the research supports
A 2024 meta-regression by Robinson and colleagues found that training closer to failure was associated with greater muscle growth, while the relationship with strength gains was much weaker. The authors caution that RIR was estimated and the analysis was exploratory. It does not establish one exact ideal number for everyone. ACSM's 2026 guidance likewise says that reaching failure does not consistently improve outcomes for the average healthy adult. Together, these sources do not support making failure compulsory in every set.
Judge the whole programme
A practical starting point is to record the exercise, load, repetitions and perceived remaining capacity, then discuss the pattern with a qualified coach. Consider the goal of the session and whether you can maintain the intended movement. A demanding set can still stop before an unsuccessful repetition. Do not treat pain, dizziness or a loss of control as proof of productive effort. The useful question is whether your programme produces manageable, repeatable progress, not whether every set ends in a dramatic struggle.