Load-dependent Relative Muscular Effort of the Knee Extensor Muscles During Back and Front Squats

Abstract

INTRODUCTION: The back squat (BS) and the front squat (FS) are mainstay exercises of strength training programs. However, not much is known about joint-specific kinematic and kinetic changes during the execution these two exercises as the external load is varied. In addition, the Relative Muscular Effort (RME), which quantifies a muscle groups operating level with respect to its maximum capacity, of the knee extensor muscle group during both exercises is not well characterized. PURPOSE: To investigate load-dependent RME of the knee extensor muscles during the BS and FS. METHODS: Seven collegiate athletes (4 male, 3 female) participate in this study. Each athlete completed motion analysis and isometric muscle strength testing. During motion analysis testing each athlete performed, in counterbalanced order, both the BS and FS at loads of 40, 60, and 80% of their FS one-repetition maximum (1-RM). Kinematic and kinetic data were captured from markers placed on anatomical landmarks (Plug-in Gait marker set) and from two force plates underneath the athletes’ feet. These data were used to calculate the net joint moments (NJM) during each exercise and at each load. During the isometric strength testing sessions each athlete performed maximal voluntary isometric contractions (MVIC) at 30, 60, and 90 degrees of knee flexion. A moment-angle curve was fitted to the MVIC data and used to calculate the theoretical peak NJM during the squats, which was then compared against the actual NJM during the execution of the BS and FS to calculate the RME. A 2x3 ANOVA ([Exercise: Front, Back] x [Load: 40, 60, 80]) was used to determine the effects of exercise and load on RME. RESULTS: : The interaction (p = 0.025) and load main effect were significant (p = 0.004). Post-hoc testing, however, indicated that only the exercise-pooled RME differed across loads (40% RME: 51.6±0.05, 60% RME: 61.3±0.06, 80% RME: 69.3±0.06). Specifically, the RME differed significantly between 40% and 60% (p = 0.049), 40% and 80% (p = 0.015), and 60% and 80% (p = 0.008). CONCLUSIONS: : RME did not differ between BS and FS at any load, but increased linearly from 40% through to 80% of FS 1-RM. These results suggest that at the same absolute load both exercises impose similar demands upon the knee extensor muscle groups, and that greater loads increase that demand.