Posture is maintained by

The dynamic stretch reflex is elicited by the potent dynamic signal transmitted from the primary sensory endings of the muscle spindles, caused by rapid stretch or unstretch. Thus when a muscle is stretched or unstretched, a strong signal is transmitted to the spinal cord which causes an instantaneous strong reflex contraction(or decrease in contraction) of the same muscle from which the signal originated. Thus, reflex functions to oppose sudden changes in muscle length, which maintains the posture. REF: TEXTBOOK OF MEDICAL PHYSIOLOGY GUYTON AND HALL, 12e Page no 771