Which muscle is an antagonist of the hamstrings?

The gastrocnemius crosses the knee and assists in knee flexion, making it a synergist (not antagonist) to the hamstrings' knee flexion action, and it primarily performs plantar flexion at the ankle.

An antagonist is a muscle that produces the opposite joint action to the agonist. The hamstrings perform hip extension and knee flexion as their primary actions. The iliopsoas - comprising the iliacus and psoas major - is the primary hip flexor, which is directly opposite to hip extension. When the hamstrings contract to extend the hip, the iliopsoas must lengthen as the antagonist, and vice versa.

The gracilis assists in hip adduction and knee flexion, functioning as a synergist to the hamstrings at the knee rather than performing an opposing action.

The sartorius assists in hip flexion, knee flexion, and lateral rotation of the hip; while its hip flexion component does oppose hamstring hip extension, it is a weak, secondary hip flexor and the iliopsoas is the principal antagonist designated in kinesiology for this pairing.