Which muscle is primarily responsible for externally rotating the hip?

The gluteus maximus plays a principal role in externally rotating the hip due to its anatomical position and structure. As the largest muscle in the gluteal region, it has a broad origin from the posterior ilium, sacrum, and coccyx, and it inserts onto the femur and the iliotibial band. The orientation of the fibers allows for powerful contractions that facilitate not only hip extension but also external rotation when the hip is flexed.

In contrast, while other muscles may assist in external rotation, their primary functions do not revolve around this action. The adductor longus primarily acts to adduct the thigh, and the pectineus is mainly involved in hip flexion and adduction. The quadratus femoris, though it does contribute to hip external rotation, is not as significant or powerful in this movement as the gluteus maximus. Therefore, the gluteus maximus is identified as the primary muscle for the external rotation of the hip joint, showcasing its importance in various movements and activities requiring hip mobility.