The effort number that matters more than the weight.

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 effort — not load — is what the body responds to.

Why not just train to failure?

Because failure is expensive. The last rep before breakdown recruits everything, yes — but research keeps finding that sets taken a rep or two shy of failure build essentially the same muscle, while costing far less recovery and teaching far 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, 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: at two in reserve the rep slows visibly; at one it grinds. Film a set and look.

Within a couple of weeks the feel arrives, 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.