Trainability of repetitions-in-reserve estimation in younger and older adults: a parallel-group repeated-measures study

Abstract Background Effort-based resistance training (RT) approaches, such as repetitions (reps) in reserve (RIR), aim to enhance RT individualization, which is of particular relevance for older adults. However, evidence regarding the accuracy of RIR estimation and its trainability in this population remains limited. This study aimed to (I) compare RIR estimation accuracy between younger and older adults, and (II) determine whether six training sessions improve estimation accuracy in both groups. Methods The study followed a parallel-group repeated-measures-design. Thirteen younger adults (33 ± 7 years; 9 females, 4 males) and thirteen older adults (67 ± 4 years; 7 females, 6 males), completed six sessions across six weeks. Each session comprised three sets of bench press and leg press at randomized loads (75%-85% one rep maximum). During each set, participants verbally indicated the rep at which they estimated having 4 and 2 RIR (4RIR, 2RIR) and then continued the set to concentric muscle failure. Results Linear mixed-effects models showed a significant main effect of training session on mean absolute error for both exercises at 2RIR and 4RIR, with MAE decreasing by up to 2.3 reps from Session 1 to Session 6, and no significant effects of age group or session x group interactions. Conclusions The present findings suggest that RIR estimation accuracy is a trainable skill for both younger and older adults, with no evidence of age-related differences in baseline accuracy or trainability. These findings extend the limited evidence on RIR-based autoregulation to older adults and suggest that, after brief familiarization, RIR-based resistance training may represent a feasible strategy to support individualized exercise prescription in older adults.