The quadratus femoris is a small, deep muscle in the back of your hip. It sits underneath the larger glute muscles and close to other deep hip rotators. It runs from a bony point on your pelvis called the ischial tuberosity, which is basically the “sit bone,” to the upper part of your thigh bone near the hip joint. Because it’s short and tucked away, you won’t see it from the outside, but it plays an important role in how your hips move and how stable you feel during training.
Its main job is external rotation of the hip, meaning it helps turn your thigh outward. It also assists with bringing the thigh slightly toward the midline, and it contributes to hip joint stability by helping keep the ball of the femur centered in the socket. That stability matters when you squat, lunge, deadlift, sprint, and change direction, because a stable hip lets you transfer force more efficiently without the pelvis wobbling or the knees collapsing inward.
For visible abs, you might wonder why a deep hip muscle matters. Strong, well coordinated hips make it easier to brace your trunk and keep good alignment when you train your core and lower body. If the quadratus femoris and its neighboring muscles are tight or irritated, you may feel a deep ache in the back of the hip or a pinchy sensation with certain ranges, which can change your posture and reduce how well you can load the legs and train hard.
You train it indirectly through movements that demand hip control, like split squats, step ups, Romanian deadlifts, and controlled hip rotation work. Mobility work for the glutes and hip capsule, plus progressive strength training, usually keeps it doing its job without needing special isolation.