【Flex】 ,  【crook】 ,  【bow】 ,  【buckle】  mean to bend, but because of special implications and applications they are not freely interchangeable.

【Flex】  is used chiefly of the bending of a bodily joint especially between bones of a limb by which the angle between the bones is diminished, or it may apply to the contraction of muscles by which the bending is accomplished; in either case the word is usually opposed to extend.

【Crook】  may replace  【flex】 , but it is also used to convey an implication of circuitousness and hence of contortion or distortion.

【Bow】  (see also YIELD ) may denote to bend as in reverence or submission, but it often means explicitly to incline downward the head and usually the part of the body above the waist, especially as a gesture of greeting, of recognition, or of reverence. In a related use  【bow】 retains some suggestion of submission and implies a bending under something (as a heavy weight) that wears or oppresses.

【Buckle】 implies a bending under stress (as from undue pressure, weight, heat, or fright) that loosens or weakens what supports and that brings on collapse, often to the point of permanent distortion.