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Introduction: An emerging topic in the resistance training literature are the outcomes between low repetitions in reserve (low-RIR) and high repetitions in reserve (high-RIR). Therefore, we wanted to measure muscular strength and skeletal muscle hypertrophic adaptations associated with low-RIR and high-RIR resistance training. Methods: Resistance-trained college-aged males (n=11) and females (n=8) participated in the study (24±3 years old, 173.4±10.9 cm, 79.0±22.7 kg, 1.60±0.33 squat: body mass ratio). Prior to the study, panoramic images of the vastus lateralis (VL) were obtained at 33% (proximal), 50% (mid), and 67% (distal) of the total femur length, and VL muscle cross-sectional area (VL mCSA) was assessed. Participants’ muscle strength was then assessed with a one-repetition maximum (1RM) test on the bench press, deadlift, and back squat exercises. Participants were then randomly assigned (based off Wilks score) into either low repetitions in reserve (low-RIR) or high-RIR groups. Both groups performed six weeks of resistance training (3 d/weekly). The low-RIR group was instructed to execute barbell squat, barbell bench press, and barbell deadlift loads that elicited a RIR of 0-1 at the conclusion of each set during the six-week period. The high-RIR group was instructed to execute this same paradigm for an RIR of 4-6 of the six-week period. Results: To account for potential sex differences, strength metrics were normalized by body mass. There were significant main effects of time for squat, bench press, and deadlift (p<0.001, p=0.002, p=0.001, respectively), but no condition x time (CxT) interactions for these variables (Squat: p=0.129, η2=0.130; Bench Press: p=0.794, η2=0.004; Deadlift: p=0.591, η2=0.018). No significant main effects of time or CxT interactions existed for changes in proximal VL mCSA or mid-thigh VL mCSA. However, there was a significant main effect of time for changes in distal VL mCSA (p=0.005), but there was no CxT interaction (p=0.061, η2=0.203). Conclusion: Our findings suggest low-RIR and high-RIR training elicit similar increases in strength and VL hypertrophy adaptations to all lifts. The distal VL mCSA decrease with training is unexpected and warrants further investigation.

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