You do not need to grind every set to failure to build muscle, and for pure strength, doing so may actually work against you. Recent meta-analyses converge on a clear split: training close to failure (roughly 0–3 reps in reserve) provides a modest edge for muscle growth, while stopping 3–5 reps short is just as effective for strength and leaves your body far fresher for the next session. The right choice depends on your primary goal.
Understanding Reps in Reserve (RIR) and Training to Failure
Reps in Reserve (RIR) is a tool for autoregulating training intensity by estimating how many repetitions you could still complete before failure, according to strength coach Travis Mash, who writes on RIR and velocity-based training. A set ending at 2 RIR means you believe you could have done two more reps with good form. Training to failure is simply 0 RIR: the point where you cannot complete another full repetition.
The distinction matters because proximity to failure is a dial, not a switch. You can approach it gradually, and where you stop each set shapes both the stimulus you deliver and the fatigue you accumulate. Notably, researchers cited by Florida Atlantic University found that training closer to failure also "enhances the accuracy of self-reported repetitions in reserve" — meaning your RIR estimates become more reliable the more you practice near the limit.
RIR for Muscle Growth (Hypertrophy)
If your goal is muscle size, proximity to failure matters — but only modestly. Refalo and colleagues' 2023 meta-analysis in Sports Medicine, pooling 15 studies, reported a small hypertrophy edge for sets taken closer to failure, with effect sizes of 0.15 to 0.21. Robinson and colleagues' 2024 meta-regression in Sports Medicine, covering 55 hypertrophy studies, found the same pattern: closer proximity helped growth, but the slope was positive yet flattening.
That flattening is the key nuance. As Domenic Angelino, a certified strength and conditioning specialist writing for FitCraft, summarizes it, 1 to 3 RIR is the practical sweet spot for hypertrophy work, because the benefit of going closer is small past 2 RIR — smaller, per Refalo's analysis, than the gain you would get from simply adding a set per session. The last set of your last exercise can drift to 0 RIR if you want, but it does not have to. The Florida Atlantic University write-up of the research similarly suggests working within 0–5 reps short of failure for optimized muscle growth.
