extend. extend hand and put knuckle on ground Put arm forward and place knuckle on ground. Often performed to solicit grooming as in extend hand to hold tree. Seen in bonobos. extend hand, palm downward Goodall Arm stretch: The arm or arms are extended toward another. The palm of the hand usually faces down. Seen in courtship or greeting. Also as solicitation of play. Mother may invite infant to riding on her or to go ahead. van Hooff Stretch over, Plooij Extend hand, palm downward, Pika Reach, Nishida Arm-stretch. Seen in bonobos. See video. extend hand, palm sideways Arm extended to another with elbow bent, palm sideways, and fingers straightened or flexed, occasionally accompanied by panting. Greeting and when mother seeks to retrieve infant from rough social play.