extensor

ojentaja puhekieltä A muscle whose contraction extends or straightens a limb or body part.
2004, Jean-Pierre Hourdebaigt, Canine Massage: A Complete Reference Manual

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.