Intensity, RPE, RIR – How Hard Should Sets Feel?

TL;DR

Training intensity is how hard or heavy a set is—either as a percentage of your 1-rep max or how close you are to failure. RPE (1–10) describes effort: RPE 8 ≈ 2 reps left, or 2 RIR (reps in reserve). It essentially means you can still perform 2 controlled reps with the correct form before failure. 

For muscle growth, most sets should land around 1–3 RIR (RPE 7–9). If a set feels like you could have done 5+ extra reps, increase the weight. If you hit failure early or technique breaks, reduce the weight.

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Volume tells you how much work you’re doing. Intensity tells you how hard that work is. Both matter.

In strength and hypertrophy training, intensity has two related meanings:

• Load intensity – the percentage of your 1-rep max (%1RM). For example, 80% of 1RM is heavier intensity than 60%.
• Effort intensity – how close you are to failure on a given set, regardless of load.

Effort is where RPE and RIR come in. They’re measurements/indicators of "feel" to gauge how hard a set felt and how close you were to technical failure.

• RIR (Reps in Reserve):

   

  • 3 RIR = you could have done 3 more reps

  • 1 RIR = you had 1 rep left in the tank

  • 0 RIR = you hit failure on that rep

• RPE (Rate of Perceived Exertion), strength version (1–10):

  • RPE 10 = 0 RIR (max effort)

  • RPE 9 = 1 RIR

  • RPE 8 = 2 RIR

  • RPE 7 = 3 RIR

For muscle growth, for the less hardcore lifters, follow where research and practical experience align: you generally want to train close to failure, but not always to failure. A good target is:

• RIR 1–3 on most working sets
• This usually corresponds to RPE 7–9
• If you want to maximize results, try stay in the RIR 1-2 range, and hit failure in your last set of a movement.

Why not go to all-out failure every time? Because failure massively increases fatigue and joint stress, often without matching extra growth. You want maximum stimulus with manageable fatigue.

How to use RPE/RIR in practice:

1. Choose a rep range and target RIR.
   Example: 3 sets of 8–12 reps at ~2 RIR.
   You pick a weight, perform a set, and ask, “How many more good reps did I have?” Adjust the load so the set ends with 1–3 reps in reserve.
2. Adjust load set to set.
   If your first set was way too easy (5+ RIR), go heavier next set.
   If your form collapses or you fail before your rep target, reduce the weight.
3. Use heavier intensities for strength emphasis.
   For strength, you’ll spend more time in heavier %1RM ranges (e.g., 75–90%) with lower reps (3–6) while still keeping 1–3 RIR on most sets.
4. Save true failure for select sets or isolation work.
   Going to 0 RIR is best reserved for machine or isolation lifts (leg curls, lateral raises) where failure is safer. Big compounds (squats, deadlifts) are usually better kept at 1–3 RIR.

RPE and RIR help you auto-regulate training day-to-day. If you slept poorly or are stressed, the same weight might feel like higher RPE; you’ll back off a bit. If you’re fresh and strong, you might push closer to RPE 9. The goal is to keep intensity high enough to stimulate gains, but controlled enough that you can recover and progress week after week.