Validity of repetitions in reserve for prescribing resistance exercise in older adults

We aimed to assess the validity of predicted repetitions in reserve (RIR) during resistance exercise (RE) in community-dwelling older adults (n = 25; 68 ± 4 yrs.; body mass index [BMI] = 28.1 ± 4.6 kg·m^-2). Prior to data collection, participants were familiarized with resistance training as well as with the use of RIR and rating of perceived exertion (RPE). Participants completed a one-repetition maximum (1RM) test in the chest press exercise. The repetitions-to-failure test, used to determine actual RIR, was then performed at 65 %-1RM. On a subsequent visit, participants performed 3 sets (65 %-1RM) at the maximum intended velocity (quantified via linear position transducer) until attaining 3 different predicted RIR (2, 4 and 6; in randomized order). After each set, participants reported their RPE. Velocity loss in the last repetition of the predicted RIR-2 (16 % compared to the corresponding repetition in the repetitions-to-failure test; p < 0.001) and RIR-4 (10 %; p = 0.009) was significantly greater, whereas no difference was found for RIR-6 (0 %; p = 0.989). Participants underestimated the number of repetitions for predicted RIR-2 (-2.1 ± 0.3 reps vs. actual RIR-2; p < 0.001) and RIR-4 (-1.6 ± 0.6 reps; p = 0.003), but not for RIR-6 (0.1 ± 0.8 reps; p = 0.823). Predicted RIR-2 was associated with greater velocity loss and number of repetitions compared to predicted RIR-4 and RIR-6 (p < 0.05). RPE values did not significantly differ between predicted RIR-2, RIR-4 and RIR-6 (8.0 ± 0.2, 7.6 ± 0.2, and 7.3 ± 0.3, p > 0.05), suggesting limited sensitivity. Our findings suggest that using predicted RIR may lack precision for accurately prescribing RE in older adults, but might be relatively useful for volume monitoring, especially when interpreted alongside RPE. Trial registration: ClinicalTrials.gov ID: NCT05619250.