Required coefficient of friction during level walking is predictive of slipping

Effects of extension ladder fly configuration on climbing safety

Fall injuries often occur on extension ladders. The extendable fly section of an extension ladder is typically closer to the user than the base section, though this design is minimally justified. This study investigates the effects of reversing the fly on foot placement, frictional requirements, adverse stepping events (repositioning the foot or kicking the rung), and user preferences. Participant foot placement was farther posterior (rung contacted nearer to toes) in the traditional ladder compared to the reversed fly condition during descent, with farther anterior foot placements during ascent. The reversed configuration had similar friction requirements during early/mid stance and significantly lower frictional requirements during late stance. Increased friction requirements during late stance were associated with farther anterior foot placement and further plantar flexed foot orientation. The reversed fly had 5 adverse stepping events versus 22 that occurred in the traditional configuration. Users typically preferred the reversed fly. These results suggest that a reversed extension ladder configuration offers potential benefits in reducing fall-related injuries that should motivate future research and development work.

Curvilinear walking elevates fall risk and modulates slip and compensatory step attributes after unconstrained human slips

While much attention has been paid to understanding slip-related falls in humans, little has been focused on curvilinear paths despite their prevalence, distinct biomechanical demands and increased slipping threat. We determined the mechanics, compensatory stepping reactions and fall risk associated with slips during fixed-speed walking across ranges of path curvature, slipped foot and slip onset phase contexts possible in the community, which builds upon previous work by examining speed-independent effects of curvilinear walking. Twenty-one participants experienced 15 unconstrained slips induced by a wearable friction-reducing device as motion capture and harness load cell data were recorded. Falls were most likely after early stance slips to the inside foot and increased at tighter curvatures. Slip distance and peak velocity decreased as slips began later in stance phase, did not differ between feet, and accelerated on tighter paths. Slipping foot directions relative to heading transitioned from anterior (forward) to posterior (backward) as slips began later in stance, were ipsilateral (toward the slipping foot side) and contralateral (toward the opposite side) for the outside and inside foot, respectively, and became increasingly ipsilateral/contralateral on tighter curvatures. Compensatory steps were placed anteriorly and ipsilaterally after outside and inside foot slips, respectively, and lengthened at later onset phases for outside foot slips only. Our findings illustrate slip magnitude and fall risk relationships that suggest slip direction may influence the balance threat posed by a slip, imply that walking speed may modify slip likelihood, and indicate the most destabilizing curved walking contexts to target in future perturbation-based balance training approaches.

Prospective validity assessment of a friction prediction model based on tread outsole features of slip-resistant shoes

Shoe outsole design strongly influences slip and fall risk. Certain tread features that can be readily measured have been shown to predict friction performance. This research aimed to replicate those findings and quantify their ability to predict slipping. Participants (n = 34) were exposed to a low friction oil-coated floor surface, while wearing slip-resistant shoes. The coefficient of friction (COF) of each shoe were predicted based on tread surface area, the presence of a bevel, and hardness. The COF was measured, and the slip outcome was determined. Predicted and measured COF were correlated, and measured COF was a sensitive predictor of slip outcome. The relationship of predicted COF on slip outcome was weaker than anticipated and was not statistically significant. This study partially confirmed the ability of previous regression equations to predict COF. However, the effect size was weaker than previously reported and predicted COF was not sensitive for predicting slips.

During roof-to ladder transitions, walk-through extensions modify required friction direction

Severe and fatal falls involving ladders commonly occur during transitions across the ladder and another support surface. Slipping is a common initiating event in ladder falls. This study characterized the friction requirements and body kinematics of descending roof-to-ladder transitions with and without a walk-through extension. Healthy adults who regularly climb ladders (n = 17) completed descending roof-to-ladder transitions, while foot-rung kinetics and body kinematics were recorded. The peak required coefficient of friction (RCOF) with respect to the plane of the shoe sole was calculated. The RCOF and body angle were calculated using their resultant values and projections in the frontal and sagittal planes. Foot angle was calculated in the sagittal plane. Repeated-measures ANOVA determined that compared to a walk-through ladder, a traditional ladder was associated with a higher RCOF in the medial-lateral (ML) direction (F1,16 = 190.07, p < 0.001) and a lower RCOF in the anterior-posterior (AP) direction (F1,16 = 11.02, p = 0.004), but had no significant relationship with the resultant RCOF (F1,16 = 0.098, p = 0.76). Spearman's rho tests performed across all testing configurations identified significant associations between foot angle and overall RCOF (rs = -0.724, p < 0.001), foot angle and AP RCOF (rs = -0.871, p < 0.001), and frontal plane body angle and ML RCOF (rs = 0.782, p < 0.001). Clustering in the data suggests that ladder attachments reduced frontal plane kinematics, which altered the direction of RCOF by reducing the medial-lateral component. These results have implications for designing rungs with good friction in multiple directions and the potential for body position monitoring in ladder tasks.

Different Influencing Factors for Risk of Falls Between Men and Women while Descending from Mount Fuji

INTRODUCTION

Annually, approximately 250,000 people climb Mount Fuji in Japan. Nonetheless, only few studies have examined the prevalence of falls and related factors on Mount Fuji.

METHODS

We conducted a questionnaire survey of 1061 participants (703 men and 358 women) who had climbed Mount Fuji. The following information was collected: age, height, body weight, luggage weight, experience on Mount Fuji, experience on other mountains, presence or absence of a tour guide, single-day climber or overnight-stay lodger, information on the downhill trail (volcanic gravel, long distance, and the risk of falls), presence or absence of trekking poles, shoe type, shoe sole condition, and fatigue feeling.

RESULTS

The fall rate in women (174/358; 49%) was greater than that in men (246/703; 35%). A prediction model using multiple logistic regression (no fall, 0; fall, 1) indicated that the following factors decreased the risk of falls: male sex, younger age, previous experience on Mount Fuji, having information about long-distance downhill trails, wearing hiking shoes or mountaineering boots rather than other types of shoes (eg, running shoes, sneakers) or worn-out shoes, and not feeling fatigued. Additionally, the following factors may decrease the risk of falls in women only: experience hiking on any other mountains, not being part of a guided tour, and using trekking poles.

CONCLUSIONS

Women had a higher risk of falls on Mount Fuji than men. Specifically, having less experience on any other mountains, being part of a guided tour, and nonuse of trekking poles may relate to higher risks of falls in women. These results suggest that different precautionary measures for men and women are useful.

Validating the ability of a portable shoe-floor friction testing device, NextSTEPS, to predict human slips

Measuring shoe-floor friction is critical for assessing the safety of footwear products. Portable devices for measuring coefficient of friction (COF) are needed. This study introduces such a device and evaluates its ability to predict human slip events across shoe designs. A portable device (18 kg) was utilized to measure 66 unique shoe-floor-fluid coefficients of friction (COF). Consistent with the shoes, flooring, and fluid contaminants from the COF tests, participants (n = 66) were unexpectedly exposed to the fluid while walking. Slip predictions were made based on a separate training data set. Slip predictions were made prospectively and using logistic regression analyses. The slip predictions were valid (p < 0.001), 91% sensitive, and 64% specific. The logistic regression fit also revealed that the COF values predicted slip outcomes (p = 0.006). This device is expected to expand the capacity of researchers, product developers, forensic engineers, and safety professionals to prevent slips and enhance human safety.

Quantification of Gait Stability During Incline and Decline Walking: The Responses of Required Coefficient of Friction and Dynamic Postural Index

This study aims to investigate the gait stability response during incline and decline walking for various surface inclination angles in terms of the required coefficient of friction (RCOF), postural stability index (PSI), and center of pressure (COP)-center of mass (COM) distance. A customized platform with different surface inclinations (0°, 5°, 7.5°, and 10°) was designed. Twenty-three male volunteers participated by walking on an inclined platform for each inclination. The process was then repeated for declined platform as well. Qualysis motion capture system was used to capture and collect the trajectories motion of ten reflective markers that attached to the subjects before being exported to a visual three-dimensional (3D) software and executed in Matlab to obtain the RCOF, PSI, as well as dynamic PSI (DPSI) and COP-COM distance parameters. According to the result for incline walking, during initial contact, the RCOF was not affected to inclination. However, it was affected during peak ground reaction force (GRF) starting at 7.5° towards 10° for both walking conditions. The most affected PSI was found at anterior-posterior PSI (APSI) even as low as 5° inclination during both incline and decline walking. On the other hand, DPSI was not affected during both walking conditions. Furthermore, COP-COM distance was most affected during decline walking in anterior-posterior direction. The findings of this research indicate that in order to decrease the risk of falling and manage the inclination demand, a suitable walking strategy and improved safety measures should be applied during slope walking, particularly for decline and anterior-posterior orientations. This study also provides additional understanding on the best incline walking technique for secure and practical incline locomotion.

Shoe Tread Wear Occurs Primarily during Early Stance and Precedes the Peak Required Coefficient of Friction

Worn shoes contribute to injuries caused by slip-and-fall accidents. The peak required coefficient of friction (RCOF) has been associated with tread wear rate. However, the temporal relationship between RCOF and shoe wear is unknown. The purpose of this study was to determine whether the contact region at the time of peak RCOF is consistent with the region of shoe wear. The shoe contact region at peak RCOF was imaged by frustrated total internal reflection. Images of worn tread after months of use were captured. The worn tread region was more posterior than the contact region at RCOF and did not correlate with the contact region at the time of RCOF. The contact regions observed during earlier stance (within 83 ms of heel contact) were more consistent with the worn region, suggesting that RCOF may not directly cause tread wear. These results serve to motivate future studies to identify early-stance gait parameters associated with tread wear development.

Gait Biomechanics for Fall Prevention among Older Adults

In our currently ageing society, fall prevention is important for better healthy life expectancy and sustainable healthcare systems. While active outdoor walking is recommended as adequate exercise for the senior population, falls due to tripping and slipping exist as the primary causes of severe injuries. Minimum foot clearance (MFC) is the lowest vertical height of the foot during the mid-swing phase and indicates the risk of tripping. In contrast, coefficient of friction (COF) factors determine the occurrence of falls from slipping. Optimisation of the MFC and the COF for every step cycle prevents tripping and slipping, respectively. Even after the initiation of hazardous balance loss (i.e., tripping and slipping), falls can still be prevented as long as the requirements for balance are restored. Biomechanically, dynamic balance is defined by the bodily centre of mass and by the base of support: spatially—margin of stability and temporally—available response time. Fall prevention strategies should, therefore, target controlling the MFC, the COF and dynamic balance. Practical intervention strategies include footwear modification (i.e., shoe-insole geometry and slip-resistant outsoles), exercise (i.e., ankle dorsiflexors and core stabilisers) and technological rehabilitation (i.e., electrical stimulators and active exoskeletons). Biomechanical concepts can be practically applied to various everyday settings for fall prevention among the older population.

Effects of anterior load carriage on gait parameters: A systematic review with meta-analysis

Anterior load carriage is common in occupational work and daily activities. Our primary purpose was to systematically review previous work concerning the biomechanics of walking with anterior load carriage. A secondary goal was to conduct a meta-analysis on common gait parameters relevant to front load carriage. An electronic database search yielded eight qualified articles. Meta-analyses were performed for four gait variables: stride length, heel contact velocity, required coefficient of friction, double support time. When possible, subgroup analyses by age were conducted. Results suggest that walking with front load carriage may shorten the stride length, particularly among young adults, but has small effects on the other three variables. Findings should be interpreted with caution given the limited number of studies included and small sample size per study. Future work investigating these four variables and others is needed to further our understanding of the impact of front load carriage on gait.

Relationship between Friction Coefficient and Surface Roughness of Stone and Ceramic Floors

Slips and falls are common occupational incidents worldwide. The friction on a floor surface is one of the critical environmental factors affecting the risk of a slip. In this research, we conducted friction measurements on stone and ceramic floor tiles under dry, wet, and water–detergent (WD) solution covered conditions using a horizontal pull slip meter (HPS). Our purposes were to quantify the slip resistance of commonly used stone and ceramic floors under different surface conditions and to validate the curvilinear relationship between the coefficient of friction (COF) and surface roughness of the floors proposed in the literature. The COF data were analyzed together with a surface profile parameter (Ra) of the floor samples. The results showed that the COFs of the stone floors were significantly (p < 0.0001) higher than those of the ceramic floors. All the floors under the dry conditions were slip resistant when adopting the ANSI 1264.2 criterion. Two and five ceramic floors were not slip resistant under the wet and WD solution covered conditions, respectively. Three polynomial regression equations were established to describe the relationship between the COF and Ra. The curvilinear functions of these models indicate that the three-zone (initial growth, steady-growth, and plateau) concept concerning the COF–Ra relationship in the literature was valid when static COF values measured using an HPS were adopted. In addition, the three-zone concept was valid not only on WD solution covered surfaces but also on dry and wet surfaces.

Unconstrained slip mechanics and stepping reactions depend on slip onset timing

Slips can occur at any time during stance. Accordingly, time-dependent tangential ground reaction forces likely produce a diverse range of slipping foot mechanics when traction is lost, thus requiring flexible recovery strategies to prevent falls. However, previous research has focused on slip onset in early stance, often with experimental anteroposterior constraints on the slipping foot, despite the diversity of environmental slips and falls. This study aimed to determine the effects of slip onset time on slip direction, severity (distance and velocity), and compensatory stepping responses. Ten young adults received slipping perturbations at different times during the stance phase of walking via a wearable device that reduces available friction while allowing the slipping foot to slide freely within the horizontal plane. Slip direction, distance, and peak velocity, compensatory step direction and distance, and upper body angular momentum magnitude and plane of rotation were derived from kinematic data. All outcome measurements significantly correlated with the time of slip onset. Slip direction and the plane of rotation of angular momentum deviated widely from the sagittal plane, exhibiting laterally-directed components exceeding those in the anteroposterior direction. As slip onset occurred later in stance, slip severity decreased while compensatory steps became longer and progressed from a posterior to anterior placement. These results provide insight into critical times within stance when slips are most severe, and into the diversity of slipping mechanics caused by changes in slip onset time.

Effect of foot-floor friction on the external moment about the body center of mass during shuffling gait: a pilot study

Herein, we investigated the effect of friction between foot sole and floor on the external forward moment about the body center of mass (COM) in normal and shuffling gaits. Five young male adults walked with normal and shuffling gaits, under low- and high-friction surface conditions. The maximum external forward moment about the COM (MEFM-COM) in a normal gait appeared approximately at initial foot contact and was unaffected by floor condition. However, MEFM-COM in a shuffling gait under high-friction conditions exceeded that under low-friction conditions (p < 0.001). Therein, MEFM-COM increased with an increasing utilized coefficient of friction at initial foot contact; this effect was weaker during a normal gait. These findings indicate that increased friction between foot sole and floor might increase tripping risk during a shuffling gait, even in the absence of discrete physical obstacles.

Gait kinetics impact shoe tread wear rate

BACKGROUND

Adequate footwear is an important factor for reducing the risk of slipping; as shoe outsoles wear down, friction decreases, and slip and fall risk increases. Wear theory suggests that gait kinetics may influence rate of tread wear.

RESEARCH QUESTION

Do the kinetics of walking (i.e., the shoe-floor force interactions) affect wear rate?

METHODS

Fourteen participants completed dry walking trials during which ground reaction forces were recorded across different types of shoes. The peak normal force, shear force, and required coefficient of friction (RCOF) were calculated. Participants then wore alternating pairs of shoes in the workplace each month for up to 24 months. A pedometer was used to track the distance each pair of shoes was worn and tread loss was measured. The wear rate was calculated as the volumetric tread loss divided by the distance walked in the shoes. Three, mixed linear regression models were used to assess the impact of peak normal force, shear force, and RCOF on wear rate.

RESULTS

Wear rate was positively associated with peak RCOF and with peak shear force, but was not significantly related to peak normal forces.

SIGNIFICANCE

The finding that shear forces and particularly the peak RCOF are related to wear suggests that a person's gait characteristics can influence wear. Therefore, individual gait kinetics may be used to predict wear rate based on the fatigue failure shoe wear mechanism.

Effects of Manual Material Handling Workload on Measures of Fall Risk

OCCUPATIONAL APPLICATIONS We found, contrary to expectations, that performing a fatiguing simulated heavy manual material handling (MMH) task did not adversely affect the risk of trip-induced falls when compared to a less-fatiguing light MMH task. However, when considering these MMH tasks together rather than in comparison, our results provide evidence for adverse effects of fatigue on both gait and the ability to recover balance after tripping. The current results provide additional evidence that physical fatigue increases fall risk, start to clarify the mechanisms by which this increase occurs, and can help in developing and evaluating fall prevention strategies targeting these mechanisms.

Perturbation-based balance training targeting both slip- and trip-induced falls among older adults: a randomized controlled trial

BACKGROUND

Falls are the leading cause of injuries among older adults. Perturbation-based balance training (PBT) is an innovative approach to fall prevention that aims to improve the reactive balance response following perturbations such as slipping and tripping. Many of these PBT studies have targeted reactive balance after slipping or tripping, despite both contributing to a large proportion of older adult falls. The goal of this randomized controlled trial was to evaluate the effects of PBT targeting slipping and tripping on laboratory-induced slips and trips. To build upon prior work, the present study included: 1) a control group; 2) separate training and assessment sessions; 3) PBT methods potentially more amenable for use outside the lab compared to methods employed elsewhere, and 4) individualized training for older adult participants.

METHODS

Thirty-four community-dwelling, healthy older adults (61-75 years) were assigned to PBT or a control intervention using minimization. Using a parallel design, reactive balance (primary outcome) and fall incidence were assessed before and after four sessions of BRT or a control intervention involving general balance exercises. Assessments involved exposing participants to an unexpected laboratory-induced slip or trip. Reactive balance and fall incidence were compared between three mutually-exclusive groups: 1) baseline participants who experienced a slip (or trip) before either intervention, 2) post-control participants who experienced a slip (or trip) after the control intervention, and 3) post-PBT participants who experienced a slip (or trip) after PBT. Neither the participants nor investigators were blinded to group assignment.

RESULTS

All 34 participants completed all four sessions of their assigned intervention, and all 34 participants were analyzed. Regarding slips, several measures of reactive balance were improved among post-PBT participants when compared to baseline participants or post-control participants, and fall incidence among post-PBT participants (18%) was lower than among baseline participants (80%). Regarding trips, neither reactive balance nor fall incidence differed between groups.

CONCLUSIONS

PBT targeting slipping and tripping improved reactive balance and fall incidence after laboratory-induced slips. Improvements were not observed after laboratory-induced trips. The disparity in efficacy between slips and trip may have resulted from differences in dosage and specificity between slip and trip training.

TRIAL REGISTRATION

Name of Clinical Trial Registry: clinicaltrials.gov Trial Registration number: NCT04308239. Date of Registration: March 13, 2020 (retrospectively registered).

Worn region size of shoe outsole impacts human slips: Testing a mechanistic model

Shoe outsole tread wear has been shown to increase slip risk by reducing the tread's ability to channel fluid away from the shoe-floor interface. This study establishes a connection between geometric features of the worn region size and slipping. A mechanistic pathway that describes the relationship between the worn region size and slip risk is assessed. Specifically, it is hypothesized that an increased worn region size leads to an increase in under-shoe fluid pressure, which reduces friction, and subsequently increases slipping. The worn region size, fluid pressure, and slip outcome were recorded for 57 participants, who were exposed to an unexpected slip condition. Shoes were collected from each participant and the available coefficient of friction (ACOF) was measured using a tribometer. A greater shoe worn region size was associated with increased slip occurrence. Specifically, a 1 mm increase in the characteristic length of the worn region (geometric mean of its width and length) was associated with an increase in slip risk of ~10%. Fluid pressure and ACOF results supported the mechanistic model: an increase in worn region size correlated with an increase in peak fluid pressure; peak fluid pressures negatively correlated with ACOF; and increased ACOF correlated with decreased slip risk. This finding supports the use of worn region size as a metric to assess the risk of slipping.

Predicting Hydrodynamic Conditions under Worn Shoes using the Tapered-Wedge Solution of Reynolds Equation

Slips and falls are a leading cause of injuries in the workplace. The risk of slipping increases as shoe tread wears. Knowledge of the mechanics relating shoe wear to slip risk is needed to develop fall-prevention strategies. This research applies a rectangular, tapered-wedge bearing solution to worn shoes and compares the results to experimentally measured under-shoe fluid pressure results. Changes in the size of the shoe outsole worn region and fluid dispersion capabilities were recorded for four, slip-resistant shoes which were systematically abraded. The film thickness predicted by the solution correlated well with the measured force supported by the fluid. The results provide support that the tapered-wedge solution can be used to assess slip risk in worn shoes.

Angular momentum regulation may dictate the slip severity in young adults

Falls cause negative impacts on society and the economy. Slipping is a common initiating event for falling. Yet, individuals differ in their ability to recover from slips. Persons experiencing mild slips can accommodate the perturbation without falling, whereas severe slipping is associated with inadequate or slow pre- or post-slip control that make these individuals more prone to fall. Knowing the discrepancies between mild and severe slippers in kinematic and kinetic variables improves understanding of adverse control responsible for severe slipping. This study examined differences across these participants with respect to center of mass (COM) height, sagittal angular momentum (H), upper body kinematics, and the duration of single/double phase. Possible causality of such relationships was also studied by observing the time-lead of the deviations. Twenty healthy young adults performed walking trials in dry and slippery conditions. They were classified into mild and severe slippers based on their heel slipping speed. No inter-group differences were observed in the upper extremity kinematics. It was found that mild and severe slippers do not differ in the studied variables during normal gait; however, they do show significant differences through slipping. Compared to mild slippers, sever slippers lowered their COM height following a slip, presented higher H, and shortened their single support phase (p-value<0.05 for all). Based on the time-lead observed in H over all other variables suggests that failure to control angular momentum may influence slip severity.

Human-Like Gait Adaptation to Slippery Surfaces for the NAO Robot Wearing Instrumented Shoes

Gait development for bipedal/humanoid robots has been a field of study with a lot of attention for several years and is becoming increasingly important as robots slowly become part of our daily lives. Therefore, it is expectable that robots should adopt human-like behaviors in order to make their interactions with humans more natural and studies have been made involving robots that have a natural, human-like gait. However, very few focus on scenarios with slippery floors. In this paper, the humanoid robot NAO is used and the effects of a human-based walking pattern on the robot’s balance when walking on floors with different slipperiness degrees were analyzed. The simulations are done having the robot equipped with specially developed shoes that enable the measurement of the friction coefficient. From that analysis, an algorithm that automatically adapts the gait parameters to the floor’s slipperiness was developed, in order to prevent the robot from suffering unexpected disturbances and possibly falling over. This paper focusses on preventing balance disturbances, instead of correcting them.

Characterizing the shoe-rung friction requirements during ladder climbing

Ladder slip and falls cause negative financial and health impacts due to their severity and frequency. However, frictional requirements of climbing, which presumably influence slip risk, are unknown. The purpose of this study was to quantify frictional requirements during climbing at different ladder angles. The required coefficient of friction (RCOF) was calculated during ladder climbing and assessed for three ladder angles (75.5°, 82.8°, and 90°). Data was collected from 10 participants at each angle. Kinetic data and kinematic data of the climber's shoe was collected and used to map friction and normal forces to the shoe. These forces were then used to calculate the RCOF. The RCOF and friction force at 90° was higher than at lower angles (75.5° and 82.8°). RCOF was correlated with shoe angle and body angle supporting both the relevance of shoe orientation and body position to slip risk. This study suggests that frictional requirements of ladder climbing are dependent on ladder angle, shoe orientation, and body angle.

Influence of averaging time-interval on shoe-floor-contaminant available coefficient of friction measurements

Available coefficient of friction (ACOF) is a common metric of footwear traction performance. ACOF is the ratio of friction to normal force, often averaged over a time-interval. The time-interval needed to achieve repeatable and valid ACOF is unknown. A post-hoc analysis was performed on nine shoe-floor-contaminant combinations to assess the repeatability and bias of data averaged across 4 time-intervals (2 ms, 50 ms, 100 ms, 200 ms) after the target normal force was reached. The ability to predict human slips was assessed for ACOF across these intervals. Differences in repeatability and validity across the four intervals were small. However, statistically significant differences were observed for the shortest compared with the longest interval (lower repeatability yet modestly improved predictive ability). Given the limited impact of time-interval on the results, a shorter interval of 50 ms is recommended to enable testing of smaller floor samples.

Prediction of coefficient of friction based on footwear outsole features

Traction testing of footwear is expensive, which may create barriers for certain users to assess footwear. This study aimed to develop a statistical model that predicts available coefficient of friction (ACOF) under boundary lubrication conditions based on inexpensive measurements of footwear outsole features. Geometric and material hardness parameters were measured from fifty-eight footwear designs labeled as slip-resistant. A robotic friction measurement device was used to quantify ACOF with canola oil as the contaminant. Stepwise regression methods were used to develop models based on the outsole parameters and floor type to predict ACOF. The predictive ability of the regression models was tested using the k-fold cross-validation method. Results indicated that 87% of ACOF variation was explained by three shoe outsole parameters (tread surface area, heel shape, hardness) and floor type. This approach may provide an assessment tool for safety practitioners to assess footwear traction and improve workers' safety.

Effect of heel base area and walking speed on the utilized coefficient of friction during high-heeled walking

BACKGROUND

The utilized coefficient of friction (uCOF) and the risk of slipping are known to increase as the heel height of shoes increases. The heel base area of shoes can also affect the uCOF.

OBJECTIVE

The purpose of this study is to investigate the effect of the heel base area of high heels and walking speed on the uCOF during walking and their interaction effect.

METHODS

The walking experiment was conducted at the speed of 1.0 m/s and 1.25 m/s using four 9-cm high heels having different heel areas (narrow, moderate, wide, and wedge heels).

RESULTS

The peak uCOF was significantly lower when wearing the wide heels than when wearing the other heels. Wearing the narrow and moderate heels reduced the vertical ground reaction force (GRF) owing to the early timing of the peak anterior-posterior GRF and increased the peak uCOF. As the walking speed became faster, the peak uCOF became greater with more increases by the interaction effect when wearing the narrow and moderate heels than when wearing the wide and wedge heels.

CONCLUSIONS

These results imply that wearing narrow high heels should be considered carefully, as the potential for a slip could be high owing to the increase in the peak uCOF. If it is inevitable to wear narrow high heels, it is critical to walk at a slower speed than usual. It is better to wear high heels with a wide heel area, e.g., 3 cm*3 cm, rather than narrow high heels or even wedge heels to reduce the possibility of slipping.

Analysis of required coefficient of friction in running and walking

The popularity of running has increased over the past few years. However, just a few studies in running have focused on the friction between surface and shoe/foot. Changes in friction can affect aspects of human motion, such as safety, motion pattern and efficiency among others. The aim was to investigate the effects of cadence (walk, self-selected running and imposed-running), stance sub-phases (absorption and propulsion) and footwear (barefoot and shod) on the required coefficient of friction (RCOF) of regular runners. Twenty healthy runners (12 males, 8 females, 29.4 ± 4.9 years, 70.4 ± 9.6 kg) participated in this study. Two force plates were used to measure the ground reaction forces (GRF) in order to calculate the RCOF for each condition and the stance phase was divided in sub-phases. In walk, the RCOF was smaller in the absorption than in propulsion phase (p < 0.001). Results evidenced effects of the cadence (p < 0.001), stance sub-phases (p < 0.001) and footwear (p < 0.001) on the RCOF. There was interaction effect in cadence with stance sub-phases (p < 0.001) and footwear with stance sub-phases (p < 0.001). Our results show RCOF is influenced by cadence and footwear condition in the absorption phase.

Predicting slips based on the STM 603 whole-footwear tribometer under different coefficient of friction testing conditions

Assessing footwear slip-resistance is critical to preventing slip and fall accidents. The STM 603 (SATRA Technology) is commonly used to assess footwear friction but its ability to predict human slips while walking is unclear. This study assessed this apparatus' ability to predict slips across footwear designs and to determine if modifying the test parameters alters predictions. The available coefficient of friction (ACOF) was measured with the device for nine different footwear designs using 12 testing conditions with varying vertical force, speed and shoe angle. The occurrence of slipping and the required coefficient of friction was quantified from human gait data including 124 exposures to liquid contaminants. ACOF values varied across the test conditions leading to different slip prediction models. Generally, a steeper shoe angle (13°) and higher vertical forces (400 or 500 N) modestly improved predictions of slipping. This study can potentially guide improvements in predictive test conditions for this device. Practitioner Summary: Frictional measures by the STM603 (SATRA Technology) were able to predict human slips under liquid contaminant conditions. Test parameters did have an influence on the measurements. An increased shoe-floor testing angle resulted in better slip predictions than test methods specified in the ASTM F2913 standard.

Effects of age-related changes in step length and step width on the required coefficient of friction during straight walking

BACKGROUND

Slipping is one of the leading causes of falls among older adults. Older adults are considered to walk with a small anteroposterior (AP) component and a large mediolateral (ML) component of the required coefficient of friction (RCOF) owing to a short step length and a wide step width, respectively. However, limited information is available.

RESEARCH QUESTION

What are the effects of aging on the resultant RCOF (RCOF_res) and its ML (RCOF_ML) and AP (RCOF_AP) components during straight walking?

METHODS

We used the kinetic and kinematic data of 188 participants aged 20-77 years from a publicly available database (National Institute of Advanced Industrial Science and Technology Gait Database 2015). The participants were divided into the following three groups: young group (n = 56; age range, 20-34 years), middle-aged group (n = 50; age range, 35-64 years), and old group (n = 82; age range, 65-77 years).

RESULTS

The RCOF_res and RCOF_AP were lower in the old group than in the other groups, indicating a lower slip risk in this group. However, the RCOF_ML was higher and the step width was greater in the old group than in the other groups. The higher RCOF_ML and lower RCOF_AP in the old group might be associated with slips in a more lateral direction.

SIGNIFICANCE

Our findings suggest that older adults have a high risk of slipping in a more lateral direction. Shoes with high-slip resistance in the lateral direction are recommended to prevent hazardous lateral slips among older adults.

Generalizability of Footwear Traction Performance across Flooring and Contaminant Conditions

Background

To prevent slip and fall events at the workplace, mechanical slip testing is conducted on shoes. Such experiments may involve redundant testing across floorings and contaminant conditions, causing wasted time and effort.

Purpose

Quantify the correlations between shoe traction across different contaminant-flooring conditions to reduce redundant slip testing efforts.

Methods

The available coefficient-of-friction (ACOF) was quantified for 17 shoes across five floorings and three contaminant conditions. Redundant testing conditions were identified when the shoe ACOF values for one floor-contaminant condition were highly correlated with a second floor-contaminant condition.

Results

High correlations were observed among quarry floorings across different contaminants and among vinyl (composite tile) floorings with the same contaminant. However, vinyl floorings exhibited low correlations with quarry floorings. Low correlations were also observed across contaminants within vinyl tiles.

Conclusions

This study was able to determine the generalizability of traction performance of shoes across vinyl and quarry floorings. This information is anticipated to reduce redundant traction testing of shoes across vinyl and quarry floorings.

Two Novel Slip Training Methods Improve the Likelihood of Recovering Balance After a Laboratory-Induced Slip

Task-specific balance training is an approach to fall prevention that has the potential to reduce the number of slip-induced falls. However, a limitation of current task-specific training methods is that they require non-trivial financial and/or equipment resources. This pilot study evaluated the efficacy of two low-cost, low-tech methods for slip recovery training in improving balance recovery ability. The two methods were: 1) repeated unexpected slip training (UST), which involved repeated unexpected slips while walking (similar to current methods of task-specific slip recovery training); and 2) volitional slip-recovery training (VST), which involved practicing a slip recovery response after volitionally stepping to induce a slip-like perturbation. Thirty-six young adults completed one training session (UST, VST, or control), followed by one unexpected, laboratory-induced slip while walking on the following day. Compared to controls, UST and VST resulted in a higher proportion of successful balance recoveries from the laboratory-induced slips. UST improved both proactive control and the reactive stepping response after slipping, while VST improved the ability to arrest the motion of the slipping foot. Based on these preliminary results, UST and VST may provide practical, cost-effective methods for slip recovery training.

Coefficient of friction testing parameters influence the prediction of human slips

Measuring the available coefficient of friction (ACOF) of a shoe-floor interface is influenced by the choice of normal force, shoe-floor angle and sliding speed. The purpose of this study was to quantify the quality of slip prediction models based on ACOF values measured across different testing conditions. A dynamic ACOF measurement device that tests entire footwear specimens (Portable Slip Simulator) was used. The ACOF was measured for nine different footwear-contaminant combinations with two levels of normal force, sliding speed and shoe-floor angle. These footwear-contaminant combinations were also used in human gait studies to quantify the required coefficient of friction (RCOF) and slip outcomes. The results showed that test conditions significantly influenced ACOF. The condition that best predicted slip risk during the gait studies was 250 N normal force, 17° shoe-floor angle, 0.5 m/s sliding speed. These findings can inform footwear slip-resistance measurement methods to improve design and prevent slips.

Decrease in required coefficient of friction due to smaller lean angle during turning in older adults

We investigated age-related differences in the required coefficient of friction (RCOF) during 90° turning, the difference of RCOF during step and spin turn, and how affects observed differences. Sixteen healthy young and healthy older adults (eight men and eight women in each group) participated. Participants performed 90° step and spin turns to the right at a self-selected normal speed. Older adults turned with lower RCOF than the young adults during both step and spin turns. This was associated with reduced mediolateral (ML) RCOF component (RCOF_ML) for the older adults. Reduced RCOF_ML in older adults was associated with reductions in the ML component of the lean angle of the body during turning. This age-related gait changes during turning can be compensatory mechanisms that allowed older adults to turn while reducing the risk of slipping. Spin turns exhibited lower RCOF, resulting from significantly lower RCOF_ML, than step turns in young and older adults; thus, spin turning is a safer turning strategy for preventing lateral slips. This may suggest that, in older adults, slip prevention may take precedence over balance recovery after slips sustained during turning. These results illustrate a turning gait mechanism that helps prevent slips and falls, and how age affects this mechanism.

Kinematics and kinetics of the shoe during human slips

This paper quantified the heel kinematics and kinetics during human slips with the goal of guiding available coefficient of friction (ACOF) testing methods for footwear and flooring. These values were then compared to the testing parameters recommended for measuring shoe-floor ACOF. Kinematic and kinetic data of thirty-nine subjects who experienced a slip incident were pooled from four similar human slipping studies for this secondary analysis. Vertical ground reaction force (VGRF), center of pressure (COP), shoe-floor angle, side-slip angle, sliding speed and contact time were quantified at slip start (SS) and at the time of peak sliding speed (PSS). Statistical comparisons were used to test if any discrepancies exist between the state of slipping foot and current ACOF testing parameters. The main findings were that the VGRF (26.7 %BW, 179.4 N), shoe-floor angle (22.1°) and contact time (0.02 s) at SS were significantly different from the recommended ACOF testing parameters. Instead, the testing parameters are mostly consistent with the state of the shoe at PSS. We argue that changing the footwear testing parameters to conditions at SS is more appropriate for relating ACOF to conditions of actual slips, including lower vertical forces, larger shoe-floor angles and shorter contact duration.

Performance testing of work shoes labeled as slip resistant

The variability in friction and slip propensity across slip resistant (SR) shoes is poorly understood. This study aimed to quantify the impact of shoe design features on the available coefficient of friction (ACOF) across shoes labeled as SR. Differences in ACOF and the slipping rate across SR shoes were also quantified. Twelve shoes were tested across five types of flooring and three contaminant conditions using a whole shoe mechanical slip tester. Geometric and hardness parameters were measured to determine the effect of heel outsole design on ACOF. The rate of slipping was evaluated for three of the shoes on vinyl tile with canola oil using human subjects. Differences in ACOF were significant across shoe outsole designs (p < .001). ACOF was correlated with geometrical and hardness parameters. Rate of slipping was lower for the highest ACOF shoe (p < .001). This information can be used to guide SR shoe selection and design.

Shoe-Floor Interactions in Human Walking With Slips: Modeling and Experiments

Shoe-floor interactions play a crucial role in determining the possibility of potential slip and fall during human walking. Biomechanical and tribological parameters influence the friction characteristics between the shoe sole and the floor and the existing work mainly focus on experimental studies. In this paper, we present modeling, analysis, and experiments to understand slip and force distributions between the shoe sole and floor surface during human walking. We present results for both soft and hard sole material. The computational approaches for slip and friction force distributions are presented using a spring-beam networks model. The model predictions match the experimentally observed sole deformations with large soft sole deformation at the beginning and the end stages of the stance, which indicates the increased risk for slip. The experiments confirm that both the previously reported required coefficient of friction (RCOF) and the deformation measurements in this study can be used to predict slip occurrence. Moreover, the deformation and force distribution results reported in this study provide further understanding and knowledge of slip initiation and termination under various biomechanical conditions.

Predictive multiscale computational model of shoe-floor coefficient of friction

Understanding the frictional interactions between the shoe and floor during walking is critical to prevention of slips and falls, particularly when contaminants are present. A multiscale finite element model of shoe-floor-contaminant friction was developed that takes into account the surface and material characteristics of the shoe and flooring in microscopic and macroscopic scales. The model calculates shoe-floor coefficient of friction (COF) in boundary lubrication regime where effects of adhesion friction and hydrodynamic pressures are negligible. The validity of model outputs was assessed by comparing model predictions to the experimental results from mechanical COF testing. The multiscale model estimates were linearly related to the experimental results (p < 0.0001). The model predicted 73% of variability in experimentally-measured shoe-floor-contaminant COF. The results demonstrate the potential of multiscale finite element modeling in aiding slip-resistant shoe and flooring design and reducing slip and fall injuries.