Helmet slippage during visual tracking: the effect of voluntary head movements

The influence of visual tracking on head movement and on the head/helmet system was investigated for two different helmets (a motorcycle helmet and a flying helmet) under static laboratory conditions. Subjects visually refixated between pairs of illuminated targets located at various horizontal (up to 160 degrees) and vertical (up to 90 degrees) distances apart while head position and helmet slippage were measured in azimuth, pitch, roll, X, Y, and Z using a double magnetic coil system. Results showed that for both helmets, root mean square (RMS) head movement increased with refixation distance, especially in the main axis of refixation, and that RMS helmet slippage can be a function of RMS head motion. Further, large individual differences in the degree of head motion were found. These findings may have implications for designers and investigators of helmet-mounted avionics.