The purpose of this study was to determine whether elderly adults exhibit deficits in the performance of multi-joint movements. Two groups of subjects (mean ages 68.9, 30.1 years respectively) participated in this experiment. Subjects performed planar arm pointing movements to various targets. One target could be achieved via elbow extension only, while the remaining three required both elbow extension and horizontal shoulder flexion, thus requiring coordination at the two joints. In contrast to the young adults, the elderly adults produced movements that became less smooth and less accurate with increasing shoulder joint contribution. The results imply a selective coordination deficit for the elderly adults. In addition, the elderly adults coactivated opposing muscles more than the young adults for the single-joint movement. The elderly adults reduced coactivation at both joints for the two-joint actions, however, while the young adults did not. These data suggest a relationship between high coactivation levels and good performance for elderly adults. It may be more difficult for the elderly to implement high coactivation levels for multi-joint movements because of the increased energy costs and complexity of planning required in comparison to the single joint actions. Thus, elderly persons appear to use coactivation in a manner that is fundamentally different than young adults to achieve motor performance.

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As plans develop for Mars missions, it is important to understand how long-duration spaceflight impacts brain health. Here we report how 12-month (n = 2 astronauts) versus 6-month (n = 10 astronauts) missions impact brain structure and fluid shifts. We collected MRI scans once before flight and four times after flight. Astronauts served as their own controls; we evaluated pre- to postflight changes and return towards preflight levels across the four postflight points. We also provide data to illustrate typical brain changes over seven years in a reference dataset. Twelve months in space generally resulted in larger changes across multiple brain areas compared to 6-month missions and aging, particularly for fluid shifts. The majority of changes returned to preflight levels by six months after flight. Ventricular volume substantially increased for one of the 12-month astronauts (left:+25%, right:+23%) and the 6-month astronauts (left:17 ± 12%, right:24 ± 6%) and exhibited little recovery at six months. Several changes correlated with past flight experience; those with less time between subsequent missions had larger preflight ventricles and smaller ventricular volume increases with flight. This suggests that spaceflight-induced ventricular changes may endure for long periods after flight. These results provide insight into brain changes that occur with long-duration spaceflight and demonstrate the need for closer study of fluid shifts.

Astronauts return to Earth from spaceflight missions with impaired mobility and balance; recovery can last weeks postflight. This is due in large part to the altered vestibular signaling and sensory reweighting that occurs in microgravity. The neural mechanisms of spaceflight-induced vestibular changes are not well understood. Head-down-tilt bed rest (HDBR) is a common spaceflight analog environment that allows for study of body unloading, fluid shifts, and other consequences of spaceflight. Subjects in this context still show vestibular changes despite being in Earth's gravitational environment, potentially due to sensory reweighting. Previously, we found evidence of sensory reweighting and reduced neural efficiency for vestibular processing in subjects who underwent a 70-day HDBR intervention. Here we extend this work by evaluating the impact of HDBR paired with elevated carbon dioxide (CO 2) to mimic International Space Station conditions on vestibular neural processing. Eleven participants (6 males, 34 ± 8 years) completed 30 days of HDBR combined with 0.5% atmospheric CO 2 (HDBR + CO 2). Participants underwent six functional magnetic resonance imaging (fMRI) sessions pre-, during, and post-HDBR + CO 2 while we measured brain activity in response to pneumatic skull taps (a validated method of vestibular stimulation). We also measured mobility and balance performance several times before and after the intervention. We found support for adaptive neural changes within the vestibular system during bed rest that subsequently recovered in several cortical and cerebellar regions. Further, there were multiple brain regions where greater pre-to postdeactivation was associated with reduced pre-to post-balance declines. That is, increased deactivation of certain brain regions associated with better balance post-HDBR + CO 2. We also found that, compared to HDBR alone (n = 13 males; 29 ± 3 years) HDBR + CO 2 is associated with greater increases in activation of multiple frontal, parietal, and temporal regions during vestibular stimulation. This suggests interactive or additive effects of bed rest and elevated CO 2. Finally, we found stronger correlations between pre-to post-HDBR + CO 2 brain changes and dependence on the visual system during balance for subjects who developed signs of Spaceflight-Associated Neuro-ocular Syndrome

Long duration head down tilt bed rest (HDBR) has been widely used as a spaceflight analog environment to understand the effects of microgravity on human physiology and performance. Reports have indicated that crewmembers onboard the International Space Station (ISS) experience symptoms of elevated CO2 such as headaches at lower levels of CO2 than levels at which symptoms begin to appear on Earth. This suggests there may be combinatorial effects of elevated CO2 and the other physiological effects of microgravity including headward fluid shifts and body unloading. The purpose of the current study was to investigate these effects by evaluating the impact of 30 days of 6° HDBR and 0.5% CO2 (HDBR + CO2) on mission relevant cognitive and sensorimotor performance. We found a facilitation of processing speed and a decrement in functional mobility for subjects undergoing HDBR + CO2 relative to our previous study of HDBR in ambient air. In addition, nearly half of the participants in this study developed signs of Spaceflight Associated Neuro-ocular Syndrome (SANS), a constellation of ocular structural and functional changes seen in approximately one third of long duration astronauts. This allowed us the unique opportunity to compare the two subgroups. We found that participants who exhibited signs of SANS became more visually dependent and shifted their speed-accuracy tradeoff, such that they were slower but more accurate than those that did not incur ocular changes. These small subgroup findings suggest that SANS may have an impact on mission relevant performance inflight via sensory reweighting.New And NoteworthyWe examined the effects of long duration head down tilt bed rest coupled with elevated CO2 as a spaceflight analog environment on human cognitive and sensorimotor performance. We found enhancements in processing speed and declines in functional mobility. A subset of participants exhibited signs of Spaceflight Associated Neuro-ocular Syndrome (SANS), which affects approximately one in three astronauts. These individuals increased their visual reliance throughout the intervention in comparison to participants who did not show signs of SANS.