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Intensity & Effort

RIR (Reps in Reserve)

Also known as: Reps Left, Buffer

The estimated number of additional repetitions you could have performed before reaching muscular failure. RIR is the inverse anchor for RPE: RIR 0 = RPE 10 (failure), RIR 2 = RPE 8, RIR 4 = RPE 6.

Formula

RIR = 10 - RPE

Example

If you squat 100 kg for 5 reps and feel you could have done 2 more, your RIR is 2 (RPE 8).

How Afitpilot Uses This

RIR feeds directly into the e1RM formula (Epley with RPE adjustment). A higher RIR (lower RPE) increases the estimated 1RM because it implies the weight was submaximal. We don't ask athletes to log RIR separately — it's derived from RPE.

Recommended RIR by context

Who / ContextValueNote
Hypertrophy trainingRIR 1-3 (RPE 7-9)Captures ~90% of growth stimulus
Strength / peakingRIR 0-1 (RPE 9-10)Near failure; high fatigue cost
Active aging (60+)RIR 2-4 (RPE 6-8)Lower injury risk; consistent > heroic
Sets of 15+ repsRIR accuracy dropsMetabolic burn mimics failure — hard to gauge

Known Limitations

  • •RIR accuracy depends entirely on RPE accuracy. The same issues with subjective reporting apply.
  • •RIR estimation is particularly unreliable for sets of 15+ reps, where fatigue accumulates non-linearly and the sensation of "reps left" becomes harder to gauge.

What We're Improving

  • Future

    Effective Sets (Hard Sets) Tracking

    Track sets taken within 0-3 RIR of failure separately from total sets. Research increasingly suggests that "hard sets" are a better predictor of hypertrophy stimulus than total volume. This would give a more nuanced view of training dose.

  • Future

    Prilepin's Table as LLM Strength-Prescription Guardrail

    Validate LLM-generated strength sessions against Prilepin's reps-per-set / total-reps ranges for the implied %1RM (derived from targetRpe → RIR → e1RM). Two surfaces: a plan-generation guardrail that flags or corrects out-of-range prescriptions, and a post-session signal that compares actual sets × reps × estimated %1RM against the optimal range. Deferred until we have 20+ real LLM-generated weeks audited; a ~8% high-intensity overshoot rate on a small fixture set was borderline-but-too-small to act on.

Science Context

RIR-based RPE scales have become the standard in evidence-based strength programming. Research by Helms et al. (2016) validated that trained lifters can estimate RIR within 1-2 reps for compound movements, though accuracy decreases with higher rep sets and isolation exercises.