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The present findings suggest that walking among community-dwelling older adults can be more effective for fall prevention than balance training. However, because walking can induce more trips, walking should not be recommended for older adults who are susceptible to falling or frailty.

The fractal scaling of the long-term heart rate variability (HRV) reflects the ‘intrinsic’ autonomic regulatory system. Herein, we examine how microgravity on the ISS affected the power-law scaling β (beta) of astronauts during a long-duration (about 6 months) spaceflight. Ambulatory electrocardiographic (ECG) monitoring was performed on seven healthy astronauts (5 men, 52.0±4.2 years of age) five times: before launch, 24±5 (F01) and 73±5 (F02) days after launch, 15±5 days before return (F03), and after return to Earth. The power-law scaling β was calculated as the slope of the regression line of the power density of the MEM spectrum versus frequency plotted on a log10–log10 scale in the range of 0.0001–0.01 Hz (corresponding to periods of 2.8 h to 1.6 min). β was less negative in space (−0.949±0.061) than on Earth (−1.163±0.075; P<0.025). The difference was more pronounced during the awake than during the rest/sleep span. The circadian amplitude and acrophase (phase of maximum) of β did not differ in space as compared with Earth. An effect of microgravity was detected within 1 month (F01) in space and continued throughout the spaceflight. The intrinsic autonomic regulatory system that protects life under serious environmental conditions on Earth is altered in the microgravity environment, with no change over the 6-month spaceflight. It is thus important to find a way to improve conditions in space and/or in terms of human physiology, not to compromise the intrinsic autonomic regulatory system now that plans are being made to inhabit another planet in the near future.

SummaryBackgroundSpaceflight alters human cardiovascular dynamics. The less negative slope of the fractal scaling of heart rate variability (HRV) of astronauts exposed long-term to microgravity reflects cardiovascular deconditioning. We here focus on specific frequency regions of HRV.MethodsTen healthy astronauts (8 men, 49.1 ± 4.2 years) provided five 24-hour electrocardiographic (ECG) records: before launch, 20.8 ± 2.9 (ISS01), 72.5 ± 3.9 (ISS02) and 152.8 ± 16.1 (ISS03) days after launch, and after return to Earth. HRV endpoints, determined from normal-to-normal (NN) intervals in 180-min intervals progressively displaced by 5 min, were compared in space versus Earth. They were fitted with a model including 4 major anticipated components with periods of 24 (circadian), 12 (circasemidian), 8 (circaoctohoran), and 1.5 (Basic Rest-Activity Cycle; BRAC) hours.FindingsThe 24-, 12-, and 8-hour components of HRV persisted during long-term spaceflight. The 90-min amplitude became about three times larger in space (ISS03) than on Earth, notably in a subgroup of 7 astronauts who presented with a different HRV profile before flight. The total spectral power (TF; p < 0.05) and that in the ultra-low frequency range (ULF, 0.0001–0.003 Hz; p < 0.01) increased from 154.9 ± 105.0 and 117.9 ± 57.5 msec2 (before flight) to 532.7 ± 301.3 and 442.4 ± 202.9 msec2 (ISS03), respectively. The power-law fractal scaling β was altered in space, changing from -1.087 ± 0.130 (before flight) to -0.977 ± 0.098 (ISS01), -0.910 ± 0.130 (ISS02), and -0.924 ± 0.095 (ISS03) (invariably p < 0.05).InterpretationMost HRV changes observed in space relate to a frequency window centered around one cycle in about 90 min. Since the BRAC component is amplified in space for only specific HRV endpoints, it is likely to represent a physiologic response rather than an artifact from the International Space Station (ISS) orbit. If so, it may offer a way to help adaptation to microgravity during long-duration spaceflight.