Activity of quadriceps muscles at different trunk tilt angles while squatting

This study aimed to clarify the relationship between different trunk inclination angles and knee joint positions while squatting and the activity of the vastus medialis obliquus (VMO). The participants were 24 healthy young males and females. The task was performed in the squatting position with the knee joint in 60° flexion. Group F (knee forward group) allowed the apex of the knee to move over the toes in the forward direction, and group B (knee backward group) allowed it to be aligned. Furthermore, group F was set to 0° forward tilt (F0), 20° forward tilt (F20), and 40° forward tilt (F40) of the trunk, whereas group B was set to 20° forward tilt (B20) and 40° forward tilt (B40). Surface electromyography (sEMG) was used to detect action potentials from the VMO, vastus medialis longus (VML), and rectus femoris (RF). sEMG analysis showed that the vertical trunk position was significantly higher than the anterior trunk position in all muscles (p < .05). F0 had the smallest displacement of the anterior-posterior centre of pressure. F0 with knee flexion of around 60° and body supported by both upper limbs is a suitable squatting limb for quadriceps training, particularly in the VMO.