The effort number that gives the weight its meaning.

Reps in reserve — RIR — is the number of clean reps you could still have done when you ended the set. Stop a set of ten when an eleventh and twelfth were there? Two in reserve. It's a simple idea with a large consequence: two lifters can lift the same weight for the same reps and have done completely different workouts, because load matters, but effort tells you what that load means to the person lifting it.

Why not just train to failure?

Because failure is expensive. (Failure here means the point where the rep stalls; form breakdown is your earlier stop line.) The last rep before breakdown recruits everything, yes — but research keeps finding that sets taken a rep or two shy of failure build comparable muscle for most lifters, while costing less recovery and teaching better technique. The rep that breaks your form also practices broken form. Do that often enough and you've trained the mistake.

Effort-based prescriptions also self-correct. On four hours of sleep, the same RIR target hands you a lighter weight automatically. No negotiation with a percentage chart, no hero day you'll pay for on Thursday. The program bends with the week instead of breaking against it.

Learning the feel

Everyone misjudges RIR at first — usually by stopping far too early. Two drills fix it. First, calibrate once: on a safe machine exercise — a leg press or machine chest press, not a free barbell lift — estimate two-in-reserve, then keep going and count what was actually left. Most people find three or four extra reps. Second, watch bar speed: it often slows as failure approaches — treat it as one clue rather than a fixed rule. Film a set and look.

Within a few weeks the feel starts to arrive, and you own a skill that makes every program you ever run — anyone's program — work better.

References

1. Grgic J et al. (2022). Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: a systematic review and meta-analysis. Journal of Sport and Health Science, 11(2):202-211. Source
2. Refalo MC et al. (2024). Similar muscle hypertrophy following eight weeks of resistance training to momentary muscular failure or with repetitions-in-reserve in resistance-trained individuals. Journal of Sports Sciences. Source
3. Helms ER et al. (2016). Application of the repetitions in reserve-based rating of perceived exertion scale for resistance training. Strength and Conditioning Journal, 38(4):42-49. Source

Keep reading: How to choose your first lifting program · What actually changes after 40 — and what doesn't · or all of them at the guides. Not sure which program fits? The 60-second finder.