Elbow Flexor Adaptations to Failure Versus Repetitions-in-Reserve Training: A Unilateral Within-Participant Study in Resistance-Trained Adults.

This study compared elbow flexor hypertrophy and maximal strength adaptations after eight weeks of resistance training performed either to momentary muscular failure or with 1-3 repetitions in reserve (RIR). Nineteen resistance-trained adults completed a unilateral within-participant protocol in which one arm trained to failure and the contralateral arm trained with 1-3 RIR. Participants trained twice weekly using the unilateral preacher curl exercise at 70-80% one-repetition maximum (1RM). Elbow flexor muscle thickness and 1RM strength were assessed before and after the intervention. Bayesian mixed-effects models estimated between-condition differences. Both protocols increased muscle thickness and 1RM strength. The estimated between-condition differences for muscle thickness were 0.032 cm (95% highest density interval [HDI]: -0.044 to 0.113) while the estimated difference in 1RM strength was 0.588 kg (85% HDI: -0.491 to 1.66), with both 95% HDIs spanning zero and including values favoring either protocol. Sex-specific analyses showed the same general patter of responses. Overall, resistance training performed with 1-3 RIR produces similar elbow flexor hypertrophy and maximal strength adaptations to training performed to momentary muscular failures under the loading and volume conditions examined.