Reconsidering Exercise Selection with EMG: Poor Agreement between Ranking Hip Exercises with Gluteal EMG and Muscle Force.

Abstract

PURPOSE Electromyography (EMG) is commonly used to inform exercise selection for injury prevention and rehabilitation. However, mechanical tension is more important for driving adaptation than muscle activation alone. Therefore, this study aimed to investigate the effect of ranking hip-focused exercises based on the magnitude of gluteal surface EMG compared to estimates of muscle forces and the implications for exercise selection. METHODS Fourteen healthy female footballers (18-30 years old) performed eight hip-focused exercises using body weight and 12-repetition maximum loads. Full body kinematics, ground reaction forces, and surface EMG from 12 lower limb muscles were recorded and used as inputs to a neuromusculoskeletal model to estimate gluteal muscle forces. Normalized EMG and normalized muscle force rankings were compared using Spearman's correlations and the linear relationship between normalized EMG and muscle force was compared using linear mixed effects models. RESULTS There was a weak relationship between exercise rankings based on EMG and muscle forces (Spearman's ρ = 0.29-0.51). Peak EMG amplitude alone explained 5% of gluteus maximus peak muscle force (R2 = 0.05) and 19% of gluteus medius peak muscle force (R2 = 0.19). However, when accounting for exercise and participant sources of variation, peak EMG amplitude explained 80-85% of gluteus maximus and medius peak normalized muscle force. CONCLUSIONS Ranking gluteal resistance exercises by EMG amplitude resulted in a different order to ranking exercises by estimated muscle forces. For exercise selection, EMG may only be useful when comparing within an individual and between biomechanically similar exercises. Caution is warranted when basing exercise selection on gluteal EMG amplitude alone.