A flexor muscle.
The flex
The flexor is a muscle that bends part of your body.
Spine. Bone, not muscle.
knee
An extensor straightens part of your body.
A muscle which bends or flexes any part; as, the flexors of the arm or the hand; -- opposed to extensor.
The crystalline lens is the part of the eye that bends/refracts the light rays as it passes through it.
the ciliary muscle is the smooth muscle of the ciliary body
A flexor muscle is a muscle which bends or flexes a limb. Most flexor muscles are located on the anterior aspect of the body.
bends leg
Flexor