Of the 40 subjects studied: 11 were from short duration (4-7 day) missions, 18 from medium duration (8-10 day) missions, and 11 from long duration (11-16 day) missions. Seventeen of the subjects were first time (rookie) fliers, and 23 were experienced (veterans). All testing was performed using a modified version of the Equitest computerized dynamic posturography system developed for clinical assessment of disorders in balance control. The posturography system consisted of a computer controlled, motor driven dual foot plate capable of both rotational and translational movements, and a computer controlled, motor driven visual surround capable of rotational movements about an axis colinear with the subject’s ankles. Force transducers located beneath the dual foot plate were used to monitor and record the subject’s weight distribution and reaction torques during testing. To improve the sensitivity of the posturography system, it was modified to monitor and record the EMG activity of various antigravity muscles as well as dynamic changes in sagittal plane hip position, shoulder position, and head angular velocity throughout the testing periods. Also, to eliminate auditory spatial orientation cues from external sources, the subjects were required to don headphones, through which wide-band masking noise was provided.

RESULTS:
Sensory Test Performances
Compared to preflight, significant sway amplitude increases were observed early after flight (2.72 ± 0.13 hrs) in all six test conditions. Under the standard Romberg conditions, the sway amplitude increased by only 0.27 degrees (35%) with eyes open and 0.35 degrees (25%) with eyes closed. Under sensory conflict conditions, the sway amplitude increased by 0.60 degrees (60%) when the visual surround was sway referenced (test 3), by 0.94 degrees (69%) when the support surface was sway referenced and eyes were open (test 4), by 1.97 degrees (63%) when the support surface was sway referenced and eyes were closed (test 5), and by 3.12 degrees (104%) when both the visual surround and the support surface were sway referenced (test 6). While the sway was increased on all sensory organization tests after flight, the increased sway was only stability threatening under the postflight conditions during which vestibular inputs provided the only theoretically accurate sensory feedback (tests 5 and 6).

Sensory Analyses
For all subjects and test sessions combined, altering visual cues approximately doubled sway amplitude, from 1.31 degrees with eyes open to 2.61 degrees with eyes closed, or 2.49 degrees with vision sway referenced. There was no significant difference between the eyes closed condition and the sway referenced vision condition. Mechanically altering proprioceptive cues nearly tripled sway amplitude, from 1.24 degrees with a fixed support surface to 3.25 degrees with a sway referenced support surface. Altering vestibular inputs by 4 to17 days adaptation to microgravity increased sway amplitude by 60%, from 1.61 degrees before flight to 2.56 degrees after flight.

Conclusions:
DSO 605 represented the first large n study of balance control following space flight. Data collected during DSO 605 confirmed the theory that postural ataxia following short-duration space flight is of vestibular origin. The results demonstrate unequivocally that balance control is disrupted in all astronauts immediately after return from space. The most severely affected returning crew members performed in the same way as vestibular deficient patients exposed to this test battery. The investigators concluded that otolith mediated spatial reference provided by the terrestrial gravitational force vector is not used by the astronauts’ balance control systems immediately after space flight.