Reps in Reserve (RIR) and Perceived Exertion are closely connected concepts used to measure how hard someone feels they are working during resistance training. RIR refers to the number of repetitions a person believes they could still perform before reaching muscular failure. For example, if you finish a set and feel you could do two more reps before failure, your RIR is 2. Perceived Exertion, often measured by the Rating of Perceived Exertion (RPE) scale, quantifies how intense the effort feels on a subjective scale, commonly from 1 to 10.
The relationship between RIR and RPE is that RPE can be expressed in terms of RIR to provide a more precise understanding of effort during strength training. On a 1 to 10 RPE scale, a rating of 10 means zero reps left in reserve (0 RIR), indicating maximal effort or failure. An RPE of 9 corresponds to about 1 rep left in reserve, 8 corresponds to 2 reps left, and so on. This method allows lifters to estimate their effort without necessarily reaching failure, which can help manage fatigue and recovery better[2][3].
Using RIR to gauge perceived exertion is valuable because it links subjective feelings of effort with an objective measure of how close one is to failure. This connection helps athletes and coaches adjust training intensity more accurately. For example, lifting a weight at 85% of your one-rep max (1RM) might feel different depending on how many reps you perform. One rep at 85% 1RM might feel like an RPE of 6 (with several reps left in reserve), while five reps at the same load might feel like an RPE of 10 (no reps left)[3][5].
This approach also helps avoid training to failure too often, which can impair recovery and performance. Studies show that training close to failure (high RPE, low RIR) can produce similar strength and hypertrophy gains as training to failure but with less fatigue and better recovery[3]. Therefore, monitoring RIR alongside RPE provides a practical way to balance training intensity and volume.
In summary, RIR is a way to quantify how many more reps you could do, and RPE translates that into a subjective effort scale. Together, they offer a reliable and scientifically supported method to measure and control training intensity based on how hard the exercise feels, rather than just the weight lifted or number of reps performed[2][3].
Sources
https://pmc.ncbi.nlm.nih.gov/articles/PMC12560393/
https://www.rpetraining.com/education
https://rippedbody.com/rpe/
https://betterlifefitness.net/rpe-to-1rm-calculator/
