Three-Dimensional Shoe Kinematics During Unexpected Slips: Implications for Shoe–Floor Friction Testing

Abstract

OCCUPATIONAL APPLICATIONS This study described the three-dimensional kinematics of the shoe during slipping and compared them to shoe kinematics specified by standard methods for coefficient of friction testing. At the time of slip initiation, substantially higher sagittal-plane shoe–floor angles and more medial shoe velocity occur than what have previously been reported. These results suggest that standard slip-testing methods should be reexamined so they better align with the state of the shoe when it begins to slip. The incongruence between actual slips and testing methods could lead to shoe designs that perform well during friction testing but are sub-optimal during an actual slip.TECHNICAL ABSTRACT Background: Shoe design is an important component of slip and fall prevention efforts. Evaluating the slip resistance of shoes in a way that is relevant to slipping accidents requires a comprehensive understanding of the shoe biomechanics during slipping. Limitations in previous studies on this topic include omission of kinematics outside the sagittal plane, which may impact coefficient of friction measurements, and the use of multiple slip perturbations, which can lead to kinematic changes due to anticipation and adaptation. Purpose: The purpose of this study was to describe the three-dimensional kinematics of the shoe during unexpected slips to better inform shoe–floor coefficient of friction testing. Methods: Thirteen subjects were exposed to a low friction fluid contaminant while wearing shoes without tread. The sliding speed, direction of sliding, sagittal-plane shoe–floor angle, and frontal plane shoe–floor angle were described at the moment of slip initiation, peak slipping speed (PSS), and 50% of the peak slipping speed (½ PSS). Statistical comparisons assessed whether the kinematics obtained from standard shoe coefficient of friction methods fell within the 95% confidence interval of the measured shoe kinematics at each time point. Results: At least one of the kinematic variables used during standard friction testing methods deviated from the observed kinematics at each time point. Specifically, the central tendency of the observed slips was characterized with a higher sagittal plane shoe angle at slip initiation, a more medial sliding direction at slip initiation, and a higher sliding speed at ½ PSS and PSS than those used during standard shoe friction testing methods. Conclusions: Shoe kinematics in friction testing standards exhibit differences with shoe kinematics during actual slips. Thus, a need exists for revisiting the kinematic conditions used in slip testing based on rigorous biomechanical studies of slipping.