A biomechanical study of slipping accidents with load carriage
Li, Kai Way
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The slipping behaviors of ten male subjects under load carrying conditions were studied. The subjects carried 0, 10%, 20%, 30%, and 40% of body weight during different trials while walking on greasy steel plate with the speed of 6.4 km/hr (or 4 mph). The three-dimensional coordinates of the anatomical joints and the ground reaction forces of the subjects were measured and analyzed. The non-parametric Friedman test showed that the subjects were in a more dangerous situation when extra loads were carried. The heavier the load carried, the more severe the slip was. This implies that manual load carriage should be avoided whenever possible if there is any chance of slipping accidents. Both biomechanical and tribological effects were significant on the severity of slipping incidents. The biomechanical effects were the results of the changes of the gait parameters such as heel sliding velocity, stance time, time to slip-start, and so on. The tribological effects were inferred from the alterations of the relative change of the rate of increase of horizontal force versus vertical force and forward impulse. By comparing the time to slip-start and the time to the peak of friction use, it was concluded that friction use did not necessarily indicate the location that slips were most likely to start. The Index for Pedestrian Safety (IPS), or friction use/dynamic coefficient of friction, may be more appropriate than friction use alone to explain the occurrence of slipping since IPS identifies not only the friction demand but also the friction available during the stance phase of gait. Some parameters, based on human sliding patterns, were recommended as the set-up values for dynamic friction measurement devices. It is believed that a good friction measurement device should be able to increase the vertical force from zero to up to 10 KN in a very short period - less dian 0.5 second. A tester that applies constant force on the floor is not recommended to be used for dynamic friction measurements.