Development of high slip-resistant footwear outsole using rubber surface filled with activated carbon/sodium chloride.

High slip-resistant footwear outsoles can reduce the risk of slip and fall on wet and icy surfaces. Falls on wet and icy surfaces can cause serious life-threatening injuries, especially for older adults. Here we show that footwear outsoles using the rubbers filled with activated carbon or sodium chloride produce higher friction force and reduce the slip rate in walking. We have identified that small depressions were formed on outsole materials filled with activated carbon or sodium chloride during friction between the rubber and surface leading to some air ingress into the interface. While there are air bubbles between the rubber and surface, real contacts are surrounded by water with negative pressure (Laplace pressure). It is considered that the negative pressure promotes real contact formation, which causes high friction. We consider that the outsole materials filled with activated carbon or sodium chloride can reduce the risk of slip-and-fall accidents.

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.

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 (RCOFML) for the older adults. Reduced RCOFML 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 RCOFML, 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.

Required coefficient of friction in the anteroposterior and mediolateral direction during turning at different walking speeds.

This study investigated the required coefficient of friction (RCOF) and the tangent of center of mass (COM)-center of pressure (COP) angle in the mediolateral (ML) and anteroposterior (AP) directions during turning at different walking speeds. Sixteen healthy young adults (8 males and 8 females) participated in this study. The participants were instructed to conduct trials of straight walking and 90° step and spin turns to the right at each of three self-selected speeds (slow, normal, and fast). The ML and AP directions during turning gait were defined using the orientation of the pelvis to construct a body-fixed reference frame. The RCOF values and COM-COP angle tangent in the ML direction during turning at weight acceptance phase were higher than those during straight walking, and those values increased with increasing walking speed. The ML component of the RCOF and COM-COP tangent values during weight acceptance for step turns were higher than those for spin turns. The mean centripetal force during turning tended to increase with an increase in walking speed and had a strong positive correlation with the RCOF values in the ML direction (R = 0.97 during the weight acceptance phase; R = 0.95 during the push-off phase). Therefore, turning, particularly step turn, is likely to cause lateral slip at weight acceptance because of the increased centripetal force compared with straight walking. Future work should test at-risk population and compare with the present results.

Misalignment of the Desired and Measured Center of Pressure Describes Falls Caused by Slip during Turning.

In this study, desired center of pressure (dCOP) was introduced to evaluate dynamic postural stability. The dCOP is defined as a virtual point on the ground, where the moment around the body center of mass (COM) becomes zero when dCOP and the measured COP (mCOP) coincide. We hypothesized that, when the misalignment of the dCOP and mCOP (dCOP-mCOP) increases up to a certain value due to a large perturbation during walking, it becomes difficult to make a compensatory step and to recover balance of COM and to continue gait. Here we tested this hypothesis in slipping during turning. The study involved twelve healthy young adult males with an average age of 21.5±1.9 yrs. The subjects were asked to (1) walk straight and turn 60 degrees to the right with the right foot (spin turn) on a dry floor surface, and (2) walk straight and 60 degrees spin turn to the right on a slippery lubricated surface. The dCOP-mCOP during turning in the slip trial with fall were significantly larger, particularly in x-direction (i.e., the medial-lateral direction during straight walk), than that in no-slip trial and slip trial without fall. The receiver operating characteristic (ROC) analysis indicated that the dCOP-mCOP in x-direction is good indicator of falling (area under the curve (AUC) = 0.93) and the threshold in the dCOP-mCOP in x-direction to distinguish for fall or no-fall was 0.55 m. These results support our hypothesis in slipping during turning.

Development of a high slip-resistant footwear outsole using a hybrid rubber surface pattern.

The present study examined whether a new footwear outsole with tread blocks and a hybrid rubber surface pattern, composed of rough and smooth surfaces, could increase slip resistance and reduce the risk of fall while walking on a wet floor surface. A drag test was performed to measure static and dynamic coefficient of friction (SCOF and DCOF, respectively) values for the footwear with the hybrid rubber surface pattern outsole and two types of commercially available boots that are conventionally used in food factories and restaurant kitchens with respect to a stainless steel floor covered with glycerol solution. Gait trials were conducted with 14 participants who wore the footwear on the wet stainless steel floor. The drag test results indicated that the hybrid rubber surface pattern sole exhibited higher SCOF (≥0.44) and DCOF (≥0.39) values than the soles of the comparative footwear (p<0.001). Because of such high SCOF and DCOF values, the slip frequency (p<0.01), slip distance (p<0.001), and slip velocity (p<0.001) for the footwear with the hybrid rubber surface pattern outsole were significantly lower than those for the comparative footwear, which resulted in no falls during trials.

Kinematics of center of mass and center of pressure predict friction requirement at shoe-floor interface during walking.

We aimed to determine whether inclination angles of the line connecting the whole body center of mass (COM) to the center of pressure (COP) (COM-COP angle) help predict the required coefficient of friction (RCOF) in young adult males during the weight acceptance and push-off phases in transient movements such as turning, gait termination and initiation, and steady-state movements such as straight walking. Seventeen healthy young adult males were asked to (1) walk in a straight line, (2) turn 60° with either foot (step and spin turns), and (3) initiate and terminate walking on a dry level floor. Peak absolute values of the ratio between resultant horizontal and vertical ground reaction forces during the weight acceptance and push-off phases (RCOFh and RCOFt, respectively) were calculated. COM-COP angles θh and θt at the instant of RCOFh and RCOFt, respectively, were also calculated. Bivariate regression analysis demonstrated that the |θh| and |θt| tangents were significant predictors of RCOFh (R = 0.878; R(2) = 0.770; p<0.001) and RCOFt (R = 0.918; R(2) = 0.843; p<0.001), respectively. The results suggest that COM and COP kinematics (i.e., the COM-COP angle) serve as a predictor of friction requirement during the weight acceptance and push-off phases in steady-state movements such as straight walking and transient movements such as turning as well as gait termination and initiation.

Effect of turning angle on falls caused by induced slips during turning.

The present study examined whether turning increases the risk of falling because of an induced slip. Fifteen healthy adult male participants were instructed to walk in a straight line and turn 30°, 45°, and 60° to the right with the left and right feet on a stainless steel floor covered with 70 wt% glycerol solution. The frequency of trials in which falls occurred was examined. The maximum distance (D(max)), magnitude of relative velocity (v(r_max)) between the whole body center of mass (COM) and COM of the foot (sliding foot) segment, and the parallel and perpendicular components of the relative distance/velocity (D(parallel_max), D(perpendicular_max)/v(r_parallel_max), and v(r_perpendicular_max)) during the slipping period were calculated using kinematic data collected from a three-dimensional motion capture system. Mean frequency of fall trials increased significantly as turning angle increased (p<0.001). Depending on the turning angle and pivoting foot, the difference between the frequency of fall trials for 60° turning (55.6%) with the right foot and that for straight walking (13.3%) was 42.3%. Distance and relative velocity between the whole body COM and COM of sliding foot segment (corresponding approximately to center of pressure) perpendicular to the progressing direction increased with turning angle. Bivariate regression analysis demonstrated that D(perpendicular_max) value (R(2)=0.834; p<0.01) and v(r_perpendicular_max)value (R(2)=0.594; p<0.05) were significantly correlated with the frequency of falls.

"Walking-mode maps" based on slip/non-slip criteria.

"Walking-Mode Maps", based on Slip/Non-Slip criteria, are proposed. Slip/Non-Slip criteria are expressed as follows: for a slip to occur: |F(h)/F(n)|(h) > or = mu(s) or |F(h)/F(n)|(t) > or = mu(s); for no slip to occur: |F(h)/F(n)|(h) < mu(s) or |F(h)/F(n)|(t) < mu(s). |F(h)/F(n)|(h) and |F(h)/F(n)|(t) are the maximum peak value of the traction coefficient at heel-strike and toe-off respectively, and mu(s) is the static friction coefficient between shoe sole and walkway. The "Walking-Mode Map" for the level-surface shows the possible regime of each walking mode as Slip, Slip possible or No slip. Our results, which are based upon a small population and strictly-specified test conditions, indicate that a static friction coefficient higher than 0.47 will prevent both forward and backward slips under our test conditions and under a range of step lengths of 0.55 to 0.95 m and walking speeds of 1.0 to 1.9 m/s. We also found that shorter steps reduce the chance of a slip. A "Walking-Mode Map" for inclined-surface ambulation has also been developed. The inclined-surface results show the friction and inclination-angle-related regime of each walking mode: Slip, Forward slip at heel-strike phase in descending, Backward slip at toe-off phase in ascending, or No slip. Again, extreme caution must be taken in generalizing these small-sample results (n=1, for the inclined tests) to the general population.