Comparing vastus lateralis:vastus medialis activation ratios in the concentric and eccentric phases of unilateral multi-joint exercises

Background: Activation imbalances between the vastus lateralis (VL) and vastus medialis (VM) may contribute to patellar maltracking and dynamic knee valgus. Given the VL’s greater force production capacity, rehabilitation programs that employ multi-joint bodyweight exercises promoting relatively higher VM activation are considered beneficial. However, the behavior of the VL:VM activation ratio across concentric and eccentric phases remains underexplored. Aim: To compare the VL:VM activation ratio during the concentric and eccentric phases of three unilateral multi-joint exercises: the single-leg squat (SLS), lunge (LUN), and Bulgarian split squat (BUL). Methods: Twenty-five healthy volunteers (15 men) were monitored for VL and VM electromyographic (EMG) signal, while performing ten repetitions of each exercise. EMG data acquired during concentric and eccentric phases were normalized by the knee extensors’ maximum voluntary isometric contraction. VL:VM ratio was determined by dividing the VL’s EMG amplitude by the VM’s EMG amplitude. Ratios greater than 1.0 indicated that the VL was more active than the VM. Results: No significant differences were found between-exercises during the concentric phase (p=0.33). However, during the eccentric phase, a significant difference was observed between exercises (p=0.043), where a higher VL:VM ratio was observed in the SLS (0.83 ± 0.41) compared to the LUN (0.70 ± 0.32) (p=0.041) exercise. Conclusion: Although SLS had a higher ratio than the LUN exercise during the eccentric phase, as the evaluated exercises showed ratios below 1, practitioners can consider these three exercises effective for muscle strengthening without placing greater demand on the VL relative to the VM.