As
soon as the subject lifts his bottom from the seat, the base of support
decreases to the feet and the area between them. Since the gravity line
is far behind the base of support, the subject cannot stand up. 
By
leaning the head, arms, and trunk (HAT) forward, the center of gravity
moves forward. By sliding the feet back under the chair, the gravity line
falls within the base of support. The subject stands easily
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