Motor behavior underlying the control of an upside-down vertical posture.

The present study examines the control of equilibrium by trained acrobats standing balanced upside-down on their hands. The focus of the experiment was to analyze the postural attitudes of various body segments and the underlying motor activity, and to evaluate the influence of vision. We found a strong correlation between the motor activity of two antagonist muscles of the forearm and the anteriorposterior position of the center of gravity. The upside-down standing posture was most stable when the neck was dorsiflexed, indicating that the tonic neck reflex and the tonic labyrinthine reflex play an important role in the stability of this posture. Comparison between known data about the normal standing posture, on the feet, and the upside-down standing posture, on the hands, shows that motor behavior of the two postures is similarly organized.