Mikko Hirvonen

Biomechanics of Slips

The biomechanics of slips are an important component in the prevention of fall-related injuries. The purpose of this paper is to review the available literature on the biomechanics of gait relevant to slips. This knowledge can be used to develop slip resistance testing methodologies and to determine critical differences in human behaviour between slips leading to recovery and those resulting in falls. Ground reaction forces at the shoe-floor interface have been extensively studied and are probably the most critical biomechanical factor in slips. The ratio of the shear to normal foot forces generated during gait, known as the required coefficient of friction (RCOF) during normal locomotion on dry surfaces or ‘friction used/achievable’ during slips, has been one biomechanical variable most closely associated with the measured frictional properties of the shoe/floor interface (usually the coefficient of friction or COF). Other biomechanical factors that also play an important role are the kinematics of the foot at heel contact and human responses to slipping perturbations, often evident in the moments generated at the lower extremity joints and postural adaptations. In addition, it must be realized that the biomechanics are dependent upon the capabilities of the postural control system, the mental set of the individual, and the perception of the environment, particularly, the danger of slipping. The focus of this paper is to review what is known regarding the kinematics and kinetics of walking on surfaces under a variety of environmental conditions. Finally, we discuss future biomechanical research needs to help to improve walkway-friction measurements and safety.

SLIPPERINESS OF THE SHOE-FLOOR INTERFACE: COMPARISON OF OBJECTIVE AND SUBJECTIVE ASSESSMENTS

Tribollogically and biomechanically valid slip-resistance measurement methods are needed urgently to facilitate new developments for improving the safety of footwear and floor coverings. Objective coefficient-of-friction measurements and subjective walking experiments were compared to scrutinize and validate anti-slip assessments of the shoe-floor interface. Four shoe types were tested on a smooth stainless steel floor contaminated with viscous glycerol. The friction utilization ratio during one step, the sliding distance, and the subjective rating of slipperiness were evaluated by seven test subjects in 420 walking trials. All subjects assessed the footwear in the same rank of slipperiness. The friction utilization ratios during the walking trial were a poor indicator for anti-slip assessments. The apparatus-based kinetic friction values mu(k1), however, were more valid and, in particular, more reproducible. The safe level of mu(k1) was approximately 0.22, which is in accordance with previous slip-resistance clasification of shoes and floors. Futher trials in other shoe-contaminant-floor conditions and walking tasks are needed to verify the findings in this study.

Detection of near accidents by measurement of horizontal acceleration of the trunk

The aim of the study was to develop a method for the detection and recording of sudden movements caused by a persons effort to regain balance during slipping and tripping. The method is based on the assessment of the horizontal acceleration of the trunk. A portable device was developed for recording sudden movements at work sites. The device was tested in laboratory conditions with 20 test persons. The experiments showed that the device is capable of detecting slips and even slight losses of balance which do not lead to actual falls. The acceleration levels of the trunk increased significantly in slipping incidents compared to normal walking, both in antero-posterior and medial-lateral direction.

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Validity and Reliability of Transitional Floor Friction Tests: The Effect of Normal Load and Sliding Velocity

The capability of a test method to estimate slip resistance depends on its validity and reliability, i.e., lack of systematic and random errors in the measurement process. A prototype portable slipmeter was used to study the sensitivity of transitional friction tests for predicting floor slipperiness. The effects of normal load and sliding velocity on the validity and reliability of the test method were assessed in particular. A lower sliding velocity (0.15 m/s) in combination with a higher normal load (200 N) led to optimum validity and reliability. For this optimum set of parameters, the Spearmans rank correlation coefficient with respect to subjective paired comparisons was 0.74 (p < 0.05) in the wet test condition.

The Effects of Cut-Off Length on Surface Roughness Parameters and Their Correlation with Transition Friction

Friction is widely used as an indicator of surface slipperiness in preventing accidents in slips and falls. Surface roughness affects friction, but it is not clear which surface roughness characteristics are better correlated with friction and, therefore, are preferred as potential interventions. The transition friction between quarry tiles and Neolite under three different mixtures of glycerol and water as contaminants was correlated with the surface parameters generated from the quarry tile surfaces. The surface roughness parameters were measured with three different cut-off lengths (0.8, 2.5 and 8 mm). The results showed that transition friction decreased as the glycerol content in the contaminant was increased due to the lubrication effect. The linear correlation coefficients between the surface roughness parameters and the measured friction increased as the cut-off length was increased from 0.8 to 8 mm. However, average of the maximum height above the mean line in each cut-off length (R pm), arithmetical average of surface heights (R a), mean height from third highest peak to third lowest valley in each cut-off length (R 3z) and the kernel roughness depth (R k) had the strongest correlation with transition friction across three cut-off lengths used.