extensor

from WordNet (r) 3.0 (2006)

extensor
    n 1: a skeletal muscle whose contraction extends or stretches a
         body part [syn: {extensor muscle}, {extensor}] [ant:
         {flexor}, {flexor muscle}]

from The Collaborative International Dictionary of English v.0.48

Extensor \Ex*ten"sor\, n. [L., one who stretches. See {Extend}.]
   (Anat.)
   A muscle which serves to extend or straighten any part of the
   body, as an arm or a finger; -- opposed to {flexor}.
   [1913 Webster]