Postural reflexes, balance control, and functional mobility with long-duration head-down bed rest.

INTRODUCTION Spaceflight has functionally significant effects on sensorimotor behavior, but it is difficult to separate the effects of ascending somatosensory changes caused by postural muscle and plantar surface unloading from descending visual-vestibular neural changes. To differentiate somatosensory changes from graviceptor changes in post-spaceflight sensorimotor behavior, bed rest may serve as an exclusionary analog to spaceflight. METHODS Four separate tests were used to measure changes in sensorimotor performance: 1) the monosynaptic stretch reflex (MSR); 2) the functional stretch reflex (FSR); 3) balance control parameters associated with computerized dynamic posturography (CDP); and 4) a functional mobility test (FMT). RESULTS A mixed model regression analysis showed significant increases in median MSR start and peak latencies, while the median FSR latency showed no significant increase. Median MSR peak magnitude showed a significant increase during the middle bed rest period (19-60 d). There were no significant effects of bed rest on balance control, but some indication that dynamic head movements may affect posture after bed rest. Time to complete the course for the FMT increased significantly with bed rest. DISCUSSION The four primary tests indicate that long-duration head-down bed rest, through unloading and modification of the body's support surface, serves as an exclusionary analog for sensorimotor responses to spaceflight. Furthermore, the data suggest that procedures designed to alleviate modifications to the sensory substrate serving the soles of the feet may provide a countermeasure to help maintain support afferentation of the postural muscles.