Humans evolved to live on Earth, and to move around under the influence of Earth's gravitational force. The acceleration of gravity is defined as 1 g and equals 9.81 m/sec². It is technically known as 1 g_z, because the force acts through the z axis — through the head and feet for an upright human (Fig. 1). Humans (like other animals) can easily cope with the changed patterns of the various accelerative forces involved in running, jumping and swimming. Mechanised travel causes more difficulty, because it involves large variations in accelerative forces in one or more axes. Humans can adapt to these changes to some extent, but they usually show an initial impairment in motor skills, and may suffer from motion sickness.