What is Reps in Reserve (RIR)?
Quick Definition
A measure of intensity defined by how many reps you could do before muscle failure.
Reps in Reserve (RIR) is an intensity-regulation metric that represents the number of additional repetitions a lifter could have performed before reaching muscular failure in a given set. RIR is inversely related to the Rate of Perceived Exertion (RPE) scale; for example, 0 RIR corresponds to RPE 10, 1 RIR corresponds to RPE 9, and 2 RIR corresponds to RPE 8. Prescribing training by RIR allows lifters to control the proximity to failure.
Research in exercise science indicates that training to absolute muscular failure is not always necessary for hypertrophy and can be counterproductive for absolute strength development due to the excessive fatigue it generates. By maintaining a buffer (typically 1 to 3 RIR), lifters can maximize mechanical tension and recruit high-threshold motor units while managing fatigue, allowing for higher training frequency and volume. Utilizing RIR requires a degree of training experience, as beginners often struggle to accurately predict how close they are to true muscular failure.