Objectives: To evaluate the electrical activity of the vastus medialis obliquus (VMO) and vastus lateralis longus (VLL) with the tibia in neutral, medial and lateral rotation on the horizontal leg press.Design: Repeated measures analysis of the effects of tibia rotation on VMO and VLL activity.Setting: Evaluation and intervention in orthopaedics and traumatology laboratory.Participants: Fifteen healthy participants, no previous musculoskeletal damage of the lower limb.Main outcome measures: Electrical activity (Root Mean Square) of the VMO and VLL was measured during submaximal isometric contractions (SIC) with the knee at 90 o of flexion. The effect of tibia rotation in electrical activity of the VMO and VLL was measured.Results: VLL activity was significantly higher than VMO activity with the tibia in medial rotation (p=0.03). Tibia rotation did not influence the activity of the VMO muscle (p=0.26). VLL activity was significantly higher with medial than neutral tibia rotation (p=0.005).Conclusions: The results suggest that tibia rotation does not strengthen selectively the VMO muscle during isometric leg press at 90 o knee flexion.