Muscle · Thigh

Vastus medialis

Musculus vastus medialis

Vastus medialis is the quadriceps on the inner side of the thigh, and it is the one people notice — the teardrop-shaped bulge that appears just above and inside the kneecap when the knee straightens fully. It wraps around the femur from the back of the bone to the inner edge of the patella, and its lowest fibres arrive at the kneecap on a marked slant. Those slanted fibres (often called the VMO) are the main thing pulling the kneecap towards the midline, balancing the outward pull of vastus lateralis on the other side. It is a single muscle, not two, and the oblique part cannot be trained in isolation — but the direction of its pull is why it comes up in almost every conversation about how the kneecap tracks.