Resistance training does not induce uniform adaptations to quadriceps.

Gerald T Mangine,Michael J Redd,Adam M Gonzalez,Adam R Jajtner,Jeffrey R Stout,Kyle S Beyer,David H Fukuda,Nicholas A Ratamess,Jeremy R Townsend,Michael B La Monica,Jay R Hoffman,Adam J Wells,Carleigh H Boone,Celena Scheede-Bergdahl

doi:10.1371/journal.pone.0198304

Abstract

Resistance training may differentially affect morphological adaptations along the length of uni-articular and bi-articular muscles. The purpose of this study was to compare changes in muscle morphology along the length of the rectus femoris (RF) and vastus lateralis (VL) in response to resistance training. Following a 2-wk preparatory phase, 15 resistance-trained men (24.0 ± 3.0 y, 90.0 ± 13.8 kg, 174.9 ± 20.7 cm) completed pre-training (PRE) assessments of muscle thickness (MT), pennation angle (PA), cross-sectional area (CSA), and echo-intensity in the RF and VL at 30, 50, and 70% of each muscle’s length; fascicle length (FL) was estimated from respective measurements of MT and PA within each muscle and region. Participants then began a high intensity, low volume (4 x 3–5 repetitions, 3min rest) lower-body resistance training program, and repeated all PRE-assessments after 8 weeks (2 d ∙ wk-1) of training (POST). Although three-way (muscle [RF, VL] x region [30, 50, 70%] x time [PRE, POST]) repeated measures analysis of variance did not reveal significant interactions for any assessment of morphology, significant simple (muscle x time) effects were observed for CSA (p = 0.002) and FL (p = 0.016). Specifically, average CSA changes favored the VL (2.96 ± 0.69 cm2, p < 0.001) over the RF (0.59 ± 0.20 cm2, p = 0.011), while significant decreases in average FL were noted for the RF (–1.03 ± 0.30 cm, p = 0.004) but not the VL (–0.05 ± 0.36 cm, p = 0.901). No other significant differences were observed. The findings of this study demonstrate the occurrence of non-homogenous adaptations in RF and VL muscle size and architecture following 8 weeks of high-intensity resistance training in resistance-trained men. However, training does not appear to influence region-specific adaptations in either muscle.

Highlights

Exercise selection and modality influence the degree to which specific muscles are activated during training
Following 8-weeks of training, a significant main effect for time was observed for cross-sectional area (CSA) (F = 19.9, p < 0.001, ɳ2P = 0.59), where average muscle size increased by 1.78 ± 0.40 cm2 (95% C.I. = 0.92–2.63 cm2)
Exploratory post-hoc analysis revealed a significant increase in rectus femoris (RF) echo intensity (EI) at 70% (3.70 ± 1.09 au, p = 0.004) but not at any other location

Summary

Introduction

Exercise selection and modality influence the degree to which specific muscles are activated during training. Activation increases when exercises become more complex [1], while the range of motion may alter the percent contribution of various muscle groups associated with the exercise [2] These differences appear to be modulated by each muscle’s specific role during movement. Vastus lateralis (VL) possess a similar function during knee extension, and are activated during that exercise [3] Their functional roles are different when an exercise requires simultaneous motion at the hip and knee joints (e.g., back squat or deadlift) [4]. During the descent phase of the squat or deadlift, proximal RF fibers shorten to flex the hip, while VL and distal RF fibers lengthen to flex the knee This process is reversed during the ascent.

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