Final answer:
During the concentric phase of a seated leg extension, the hamstrings are the primary antagonistic muscle group, working against the quadriceps, which are the agonists in knee extension.
Step-by-step explanation:
When performing a seated leg extension, which muscle group is primarily antagonistic during the concentric phase? In the context of the seated leg extension exercise, the primary agonists are the quadriceps femoris muscles located in the anterior compartment of the thigh. These muscles are responsible for extending the knee. On the other side, the hamstring group is the primary antagonist during the concentric phase of this exercise, where the quadriceps are actively contracting to extend the knee.
The hamstring group consists of the biceps femoris, semitendinosus, and semimembranosus. These muscles are located in the posterior compartment of the thigh and their primary action is to flex the knee joint. Hence, during the concentric phase of a seated leg extension, which involves the active extension of the knee, the hamstrings serve as the antagonists since they provide the opposite action of knee flexion.