Final answer:
The gluteus maximus works as a force couple with the hip flexors, as they collectively contribute to stabilizing and moving the hip joint, despite having opposite functions.
Step-by-step explanation:
The muscle that works as a force couple with the hip flexors is the gluteus maximus. This is because the hip flexors and the gluteus maximus have opposite roles at the hip joint. While the hip flexors, consisting of the iliopsoas group (iliacus and psoas major), are responsible for flexing the thigh at the hip, the gluteus maximus is a part of the gluteal group that extends, flexes, rotates, adducts, and abducts the femur. Force coupling refers to muscles working together to produce movement around a joint, and in this context, the gluteus maximus and hip flexors work together to stabilize and move the hip joint.