Antagonist

Antagonist is the correct answer. The antagonist works to counter the torque of the agonist, often used in slowing down a motion. Occurring in pairs, the agonist and the antagonist often balance each other, one contracts and the other relaxes. 

Abductor is a movement away from the midline. Bipennate is a type of muscle fiber, where two rows of opposite diagonal fibers come together at a single tendon creating more strength but less shortening. Sarcomere is the smallest and most basic building block of a muscle fiber.