The action at both the knee joint and the hip joint (rear leg) during the up phase of a lunge:

During the up phase of a lunge, the knee joint and hip joint undergo specific movements that are significant for understanding lower body mechanics. When you push up from the lunge position, the rear leg (the one behind you) experiences hip flexion as the thigh is brought forward towards the torso. At the same time, the knee joint in the rear leg extends as the leg pushes off the ground to help propel the body back to a standing position.

The correct choice indicates that while the knee joint is extending, the hip joint is flexing. This dual action is crucial for executing the lunge effectively, as it allows for powerful propulsion while maintaining balance and stability. Understanding these movements can help in designing effective training programs and improving lunge techniques.