Final answer:
During the concentric phase of the back squat, the quadriceps femoris muscle group in the anterior compartment of the thigh contracts to extend the knee joint, which lifts the body back to a standing position.
Step-by-step explanation:
The quadriceps femoris muscle group is responsible for the action of knee extension during the concentric phase of the back squat exercise. This action occurs when you rise from the bottom of the squat to a standing position. The quadriceps femoris, located in the anterior compartment of the thigh, is made up of four muscles: the rectus femoris, vastus lateralis, vastus medius, and vastus intermedius. When you perform a back squat, these muscles work together to extend the knee joint and lift your body upwards.
During the back squat, as you ascend, your quadriceps femoris takes on the role of the agonist, meaning they are the primary muscles involved in the knee extension action. In contrast, the muscles in the posterior compartment of the thigh, known as the hamstring group, act as antagonists during this phase of the exercise. The hamstrings, consisting of the biceps femoris, semitendinosus, and semimembranosus, provide controlled resistance to knee extension, ensuring stability and proper joint function.