Motion sickness and tilts of the inertial force environment: active suspension systems vs. active passengers

Background: Maneuvering in vehicles exposes occupants to low frequency forces (<1 Hz) which can provoke motion sickness. Hypothesis: Aligning with the tilting inertial resultant (gravity + imposed horizontal acceleration: gravito-inertial force (GIF)) may reduce motion sickness when tilting is either 'active' (self-initiated; Experiment 1) or 'passive' (suspension machinery; Experiment 2). Methods: Exp 1: Twelve seated subjects were exposed to continuous horizontal translational oscillation through the body x-axis (3.1 m · s-2 peak acceleration, 0.20 Hz) while making head tilts which were either aligned or misaligned (180° out of phase) with respect to GIF. The two sessions were one week apart at the same time of day, counterbalanced for order. Head tilts were controlled by tracking a moving LED display and head trajectory was verified by accelerometry. Motion continued until moderate nausea was achieved (motion endpoint) or until a 30 min cut-off. Exp 2: A different group of 12 subjects were exposed to continuous horizontal translational sinusoidal oscillation through the body x-axis (2.0 m · s-2 peak acceleration, 0.176 Hz) while seated in a cab which was tilted by suspension machinery around the y-axis of the ears so that GIF was aligned or misaligned (180° out of phase) with the body z-axis. Results: Exp 1: Mean ± SD time to motion endpoint was significantly longer for aligned (19.2 ± 12.0 min) than for misaligned (17.8 ± 13.0 min; p < 0.05, two-tail). Exp 2: Mean ± SD time to motion endpoint was significantly shorter for aligned (21.8 ± 10.9 min) than for misaligned (28.3 ± 5.8 min; p < 0.01, two-tail). Conclusions: Whether or not compensatory tilting protects against (Exp 1) or contributes to (Exp 2) motion sickness may be influenced by whether the tilting is under the active control of the person (e.g., car driver) or under external control (e.g., passenger in a high-speed tilting train)