Total body muscle tone is reflected in the infant's preferred posture at rest and resistance to stretch of individual muscle groups. As maturation progresses, the fetus gradually assumes increasing passive flexor tone that proceeds in a centripetal direction, with lower extremities slightly ahead of upper extremities. For example, very early in gestation only the ankles are flexed. Knees will flex as wrists just begin to flex. Hip flexion, then adduction are just ahead of elbow, then shoulder girdle flexion. The preterm infant primarily exhibits unopposed passive extensor tone, while the infant approaching term shows progressively less opposed passive flexor tone.
To elicit the posture item, the infant is placed supine (if found prone) and the examiner waits until the infant settles into a relaxed or preferred posture. If the infant is found supine, gentle manipulation (flex if extended; extend if flexed) of the extremities will allow the infant to seek the baseline position of comfort. Hip flexion without adduction results in the frog-leg position as depicted in posture square #3. Hip adduction accompanying flexion is depicted by the acute angle at the hips in posture square #4. The figure that most closely depicts the infant's preferred posture is selected.
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