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Jarchow, T., M. Wirz, T. Haslwanter, V. Dietz, and D. Straumann. Perceived horizontal body position in healthy and paraplegic subjects: effect of centrifugation. J Neurophysiol 90: 2973-2977, 2003. First published July 23, 2003 10.1152/jn.01129.2002. The perception of body position is mainly mediated by otolith information and visual cues. It has been shown, however, that proprioceptive sources are also involved. To distinguish between the contributions of the vestibular and nonvisual extra-vestibular information to graviception, we tested the effects of a stimulus that leaves the vestibular input unchanged but modifies the information from sense organs located more caudal along the trunk. This was achieved by bringing subjects into a horizontal ear-down position and rotating them around an earth-vertical axis that coincided with the interaural axis. In this paradigm, through centrifugal force, the stimulation of the vestibular and the putative extravestibular graviceptive organs in the body becomes dissociated. Healthy subjects (n ϭ 14) and paraplegic patients with lesions between T4 and T8 (n ϭ 7) adjusted themselves to the perceived horizontal right-ear down body position under two conditions: one with constant velocity rotation (ROT, velocity ϭ120°/s) around the earth-vertical axis of the turntable, and one without rotation (BASE). Among healthy subjects, the individual differences between BASE and ROT varied widely in both the feet-up or feet-down direction. In contrast, adjustments in paraplegic patients during ROT were always in the feet-down direction compared with BASE. A model with two extravestibular graviceptive sensors could explain our results: one sensor is located rostral to T4, and the other is caudal to T8. A load on the rostral graviceptor is interpreted as a tilt of the body in the feet-up direction and shifts the adjustments of perceived body position feet-down; a load on the caudal receptor is interpreted as a tilt in the feet-down direction and shifts the perceived body position feet-up. During ROT, healthy subjects solve the discrepant inputs of both extravestibular graviceptors in a highly variable manner, while paraplegic subjects show less variability because they are restricted to only the rostral graviceptor.

Artificial gravity (AG) created by short-radius centrifugation is a promising countermeasure to the physiological de-conditioning that results from long-duration spaceflight. However, as on Earth, gravity alone does not ensure fitness. We will need to supplement passive exposure to AG with physical exercise to achieve a comprehensive countermeasure. Before AG exercise can be deemed safe and effective, we must understand how Coriolis accelerations and a gravity gradient affect our biomechanics and how centrifuge-based exercises differ from Earth-upright ones.Two experiments were designed to investigate the squat biomechanics while upright in the laboratory and while lying supine on a horizontal, clockwise-rotating short-radius centrifuge at speeds up to 30 revolutions per minute. Constant force springs provided additional resistive force up to 25% of body weight. Dependent measure included the three-dimensional position of the left and right knee, left and right foot reaction forces, and muscle activity. We investigated the Coriolis-induced mediolateral knee perturbations and the sensory-motor after-effects from a multiple repetition protocol. The upright and centrifuge biomechanics were compared for similarities and differences between them. In addition, a two-dimensional kinematic model was developed to predict foot reaction forces, Coriolis accelerations, and joint torques.Our results show that mediolateral knee travel during the AG squats was 1.0 to 2.0 centimeters greater than Earth-upright squats. Increasing the rotation rate or adding resistive force did not affect the results. The peak foot forces increased with rotation rate, but rarely exceeded 200% body weight. The ratio of left-to-right foot force during centrifugation was non-constant and approximately sinusoidal, suggesting a postural correction for the Coriolis accelerations. There was a qualitative difference in the foot force vs. knee angle profile between upright and centrifuge-supine because of the centripetal acceleration. Muscle activity, however, was qualitatively similar between the conditions. The kinematic model was used to evaluate the exercise safety and extend the results to larger-radius centrifuges. We conclude that centrifugation provides a unique and challenging environment for exercise and that a brief artificial gravity squat can be carried out safely. The results are extended to cycle ergometry, when possible, and recommendations are made for future AG squat protocols.