flexion /flek″shən/ [L, flectere, to bend] , 1. a movement allowed by certain joints of the skeleton that decreases the angle between two adjoining bones, such as bending the elbow, which decreases the angle between the humerus and the ulna. Compare extension. 2. a resistance to the descent of the fetus through the birth canal that causes the neck to flex so the chin approaches the chest. Thus the smallest diameter (suboccipitobregmatic) of the vertex presents.
