The primary muscles or muscle group responsible for extension of the hip joint is the:

The gluteus maximus is primarily responsible for the extension of the hip joint. This large muscle, located in the buttock region, plays a critical role when the hip moves from a flexed position to an extended position. It is engaged during activities such as standing up from a sitting position, climbing stairs, and running, where powerful hip extension is required.

The gluteus maximus is particularly valuable for its strength and ability to generate force. Its fibers are oriented in a way that maximizes leverage for hip extension, making it essential for both functional movements and athletic performance.

Other muscles involved in hip movement, like the iliopsoas or sartorius, primarily assist with hip flexion or are involved in stabilization and other actions; thus, they do not significantly contribute to hip extension. The gluteus medius, while important for hip stability and abduction, does not primarily function to extend the hip. Therefore, the gluteus maximus stands out as the key muscle for hip extension.