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We carried out clinostat and parabolic flight experiments to examine the effects of a microgravity (,uG) environment on human sperm motility.Semen samples were obtained manually from 18 healthy men (aged 27.4 ± 5.4 years) who had given their informed consent. In dinostat experiments, samples that were left stationary were used as a stationary control. Samples rotated vertically and horizontally were used as a rotation control and a dinostat rotation, respectively. In parabolic flight experiments using a jet plane, sperm motility was compared for each parameter at μG, 1G and 2G. The state of 1G during the flight was used as a control. Sperm motility was determined using an automatic motility analyzer HT-M2030 in a microgravity environment.All parameters of sperm motility tended to be lower in dinostat rotation compared with rotation control at both low-speed and high-speed, but the differences were not statistically significant. In parabolic flight, sperm motility and parameters of linear movement were decreased (P< 0.05). There was no significant difference between μG and 2G, but sperm motility was significantly decreased at μG than at 1G.Our findings suggest that sperm motility is reduced under μG.