The vastus medialis is one of the four muscles that make up the quadriceps group on the front of your thigh. It sits on the inner side of the thigh, close to the knee, and you can often see it as the teardrop shaped muscle just above the inside of the kneecap when it is well developed. Like the rest of the quadriceps, its main job is to straighten the knee, which you do when you stand up, climb stairs, sprint, jump, or drive out of the bottom of a squat or lunge.
A second, very important role is helping control how the kneecap tracks as your knee bends and straightens. The vastus medialis pulls the kneecap slightly inward and helps keep it centered in its groove. When it is weak, slow to activate, or fatigued compared with the outer quad, the kneecap can drift outward, which may contribute to irritation around the front of the knee in some people.
The final portion of the muscle near the knee has fibers that run more diagonally and is sometimes called the vastus medialis oblique. You do not need to obsess over isolating that part, but you do want strong, well coordinated quads and hips so the knee moves smoothly.
You train the vastus medialis through full range knee bending movements done with good control. Squats, split squats, step ups, leg presses, and controlled leg extensions all challenge it. Deep knee flexion, a stable foot tripod, and tracking the knee over the middle toes tend to load it well. Strong knees support harder lower body training, which helps you burn more calories and keep performance high while you lean down for visible abs. If you feel knee pain, scale range, slow the tempo, and strengthen glutes and calves to share the work.