effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise

This study compared the effects of resistance training on quadriceps femoris hypertrophy while sitting upright (90° hip flexion) versus recumbent (40° hip flexion) when performing the leg extension exercise with similar knee flexion range of motion. We hypothesised that ten weeks of resistance training with 40° hip flexion in the leg extension would cause greater muscle hypertrophy of the rectus femoris but not vastus lateralis compared with 90° hip flexion. Twenty-two untrained men completed a ten-week intervention comprising two resistance training sessions per week with four sets of leg extension to momentarily concentric failure. A within-participant design was used, with lower limb side randomly allocated to the 40 or 90° condition. Muscle thickness of distal and proximal rectus femoris and vastus lateralis were quantified via ultrasound. Data were analysed within a Bayesian framework including univariate and multivariate mixed effect models with random effects to account for the within participant design. Differences between conditions were estimated as average treatment effects (ATE) and inferences made based on posterior distributions and Bayes Factors (BF). Results were consistent with the a-priori hypotheses, with 'extreme' evidence in support of a hypertrophic response favouring the 40° hip angle for the rectus femoris (BF>100; p(Distal/ATE & Proximal/ATE >0)>0.999), and 'strong' evidence in support of no difference in hypertrophic response for the vastus lateralis (BF = 0.07). Therefore, when the goal is to increase overall quadriceps femoris hypertrophy we suggest training with reduced hip flexion in the leg extension exercise.