The tipping point: Friction and thigh length influence imminent falls from a tilting ski lift chair

Falls by subjects riding a ski lift while in transit are rare but can cause significant injury. Falls can occur at variable heights and can occur to small children through to adults of both genders. The current study was designed to investigate the mechanics associated with falls to elucidate this phenomenon. Five crash dummies served as surrogates for children (3-, 6-, and 10-year-old) and adults (5th percentile female and 50th percentile male). The subjects were seated in a simulated ski lift chair with a detailed seating procedure. The shorter length of the 3- and 6-year-old surrogates' thighs was associated with slouched posture while the remaining surrogates sat in an upright fashion. All subjects were fit with winter clothing, skis, boots, helmet, and goggles. The ski lift chair was slowly tilted forward to represent wind gusts, ski lift cable catenary, or activation of the lift emergency stop. As a worst-case scenario, a restraint bar was not used since field data suggests poor usage rates. The seat was covered with three different frictional interfaces to represent different seat designs and environmental conditions. The results showed that the 10-year-old and fifth percentile female surrogates required the greatest tilt angles to initiate a fall with increasing friction requiring greater tilt angles. The remaining surrogates exhibited significantly lower tilt angles with no significant effect observed for the different frictional interfaces. These data suggest that subject size and frictional interface create variable effects. As such, uniform use of restraint bars and footrests, where available, is recommended.