extensor

noun

1. A muscle whose contraction extends or straightens a limb or body part.

   “Starting at the point of shoulder, use muscle squeezings, picking-ups, kneadings and gentle frictions, interspersed with effleurages, over the triceps muscle as well as the fleshy part of the flexor and extensor muscle groups.”

   “The bird is then off on a mad dash like an aquatic Roadrunner cartoon: It spreads its legs wide with strong extensor thrusts and long forward strides, and it then steps in the water four or five times, increasing its speed and flapping its wings until it achieves liftoff with a final drag of its toes in the water.”