Increased vigilance with vertical vibration at 5 Hz: an alerting mechanism.

The human body resonates at a vertical frequency of 5 Hz. Yet in vigilance tasks when motivation is low, performance improves reliably with vertical vibration at 5 Hz. This is because the vertical vibration of the shoulders at 5 Hz can be reduced by tensing the trunk muscles. The increased muscle tension helps to maintain alertness. The alerting mechanism is not mentioned in the international standard for human exposure to vibration. Drivers who travel long distances on deserted motorways at night with modern well-sprung car bodies and seats are deprived of the alerting mechanisms. They would be well advised to use some other form of stimulation to help them to stay alert.