Effect Of Proximity To Failure During Resistance Training On Vastus Lateralis Hypertrophy

PURPOSE: To compare changes in muscle thickness (MT) of the vastus lateralis at both 50% (VL50) and 70% (VL70) of the length of the muscle between training programs with different proximities to failure using the repetitions in reserve (RIR)-based rating of perceived exertion (RPE) scale. METHODS: Twenty (height: 174.60 ± 6.23 cm, body mass: 80.43 ± 10.97 kg, back squat one-repetition maximum-1RM relative to body mass: 1.75 ± 0.26) with ≥2 yrs of training experience were assigned to one of two groups: 4-6 RPE, n = 10 (4-6 RIR), or 7-9 RPE, n = 10, for 8wks. At pre and post, VL50 and VL70 were assessed 48 hours before the first training session and 48 hours after the last training session, respectively. All groups performed the back squat 3x/wk on non-consecutive days (e.g., Mon., Wed., Fri.) using the same number of sets and repetitions on an undulating resistance training program, which linearly decreased repetitions throughout. Week 1 served as an introductory week in which a lower set volume was performed at a lower RPE. Week 8 was a taper with low repetitions in sessions 1 and 2, followed by post-testing in session 3. In weeks 2-7 (i.e., the main training period), 10 weekly sets were performed. Subjects were instructed to select a load in which the set ended within the desired RPE range. A general linear model fit in an analysis of covariance structure was fit to analyze changes in MT between RPE groups. The baseline value (pre-test MT) was set as a covariate to adjust the estimated marginal means for further comparisons. Additionally, equivalence testing was performed with equivalence bounds set at g = ±0.25 to test for the statistical similarity between groups. RESULTS: For VL50, changes in MT was observed in both groups (4-6 RPE: -1.06 mm [-3.43-1.32], 7-9 RPE: -1.13 mm [-3.38-1.12]), with trivial between-group effect size (g = 0.02 [-0.45-0.48]) and ambiguous findings from equivalence testing. For VL70, changes in MT was observed in both groups (4-6 RPE: 1.20 [-1.05-3.45], 7-9 RPE: -0.08 [-2.22-2.05]), with small between-group effect size (g = 0.21 [-0.13-0.56]) and ambiguous findings from equivalence testing. CONCLUSIONS: Our findings indicate that resistance training with 4-6 RPE and 7-9 RPE produces statistically ambiguous differences in MT changes at VL50 and VL70.