Influence of varying proximity-to-failure on muscular adaptations in resistance-trained individuals

Abstract

Resistance training (RT) to failure may not elicit greater gains in muscular adaptations compared to training protocols leaving few repetitions in reserve (RIR). However, it remains unanswered if varying proximity-to-failure during RT offers benefits in contrast to keeping it unaltered. Hence, we aimed to compare the influence of varied RIR training to constant RIR training on muscle size and strength.

31 healthy participants (27 ± 4 years) with a minimum of two years of resistance training experience were randomized to RIR1-4, (n=16) or RIR1, (n=15) groups. Both groups completed a 10-week RT period. In two 5-week mesocycles, RIR1-4 gradually decreased the weekly RIR target from 4 to 1. Instead, RIR1 kept the RIR target constant. RT was conducted four times a week with an upper-lower split. For major muscle groups participants conducted 12 sets with a repetition range of 7-9 per week for the 1st mesocycle and 14 sets with a repetition range of 4-6 per week for the 2nd mesocycle. Measurements of muscle cross-sectional area of vastus lateralis (VLCSA), muscle thickness of triceps brachii (TBMT) using ultrasound, barbell back squat (squat) and barbell bench press (bench press) one repetition maximum (1RM) were taken before and after the intervention. Session ratings of perceived exertion (sRPE) were collected weekly. The accuracy of RIR estimation at RIR3 and RIR1 calculated as RIR errors were also obtained before and after the intervention in squat and bench press.

No differences were observed between groups in changes of muscle size or strength. Both groups significantly (p < 0.05) increased VLCSA (RIR1-4: 6 ± 4%; RIR1: 6 ± 4%). TBMT increased (RIR1-4: 6 ± 6%; RIR1: 2 ± 5%) only in the RIR1-4 group (p<0.01). 1RM in bench press (RIR1-4: 4 ± 2%; RIR1: 6 ± 5%) and squat (RIR1-4: 11 ± 5%; RIR1: 9 ± 9%) also increased significantly (p<0.05) in both groups. Average session RPEs were lower (p<0.05) in RIR1-4 (6.9 ± 1) than in RIR1 (7.7 ± 1.1). The RIR accuracy improved significantly (p<0.05) only in the RIR1-4 group for the squat (pre: -1.5±1.7, post: -0.6±1.4) and the bench press (pre: -1.1±1.8, post: -0.1±0.9) RIR3 estimation.

Varying proximity-to-failure did not increase RT adaptations compared to constant RIR training. However, on average RIR1-4 group’s session RPEs were lower, thus potentially allowing higher training volumes or longer progressive training periods. RIR1-4 may also better suit sports training for goals other than solely muscle strength or size.