The Collaborative International Dictionary of English v.0.48:

Flexor \Flex"or\, n. [NL.] (Anat.)
   A muscle which bends or flexes any part; as, the flexors of
   the arm or the hand; -- opposed to extensor.
   [1913 Webster] flextime
   flexitime

WordNet (r) 3.0 (2006):

flexor
    n 1: a skeletal muscle whose contraction bends a joint [syn:
         flexor muscle, flexor] [ant: extensor, extensor
         muscle]