When performing a seated leg extension, which muscle group is the primary antagonist during the concentric phase?

The hamstrings cross the posterior knee joint and are responsible for knee flexion, which is the direct anatomical opposite of the knee extension performed during the concentric phase of a leg extension. As the agonist quadriceps contract concentrically, the hamstrings lengthen and act as the primary antagonist, producing reciprocal inhibition to allow smooth movement.

The quadriceps are the agonist during this movement, not the antagonist, as they are the primary muscle group concentrically contracting to produce knee extension.

Hip extensors such as the gluteus maximus act at the hip joint, not the knee joint, and are not the primary antagonists during knee extension in a seated position.

Back extensors act on the spinal column and do not cross the knee joint, so they play no direct antagonist role in the seated leg extension movement.