Kurt E. Beschorner

Impact of joint torques on heel acceleration at heel contact, a contributor to slips and falls

Slips/falls are a health burden in the workplace. Previous research has implied a relationship between foot dynamics at heel contact and slips/falls; however, heel acceleration has received little attention. Heel acceleration as the heel contacts the ground is the result of the combined effort of the leg joint torques to control motion of the foot. This study aims to examine the association of heel acceleration with fall risk, and explore the main joint torque determinant of heel acceleration at contact. Sixteen young and eleven older adults walked on known dry floors and in slippery environments expected to be dry. Heel acceleration at heel contact in the direction of motion, i.e. anterior/posterior, was compared between slip-recovery and slip-fall outcomes. Results showed that subjects that recovered contacted the floor with a greater heel deceleration (p < 0.05) than fall subjects. Knee torque alone explained 76% of the heel acceleration variability (p < 0.01). These data suggest that walking with reduced knee flexion torque at heel contact results in a reduced heel deceleration, a potential risk factor for slip-initiated falls.

Modeling Mixed-Lubrication of a Shoe-Floor Interface Applied to a Pin-on-Disk Apparatus

While slip-and-fall accidents are a serious health concern, few attempts have been made to tribologically model the shoe-floor-contaminant interface. To this end, modeling techniques are introduced here for shoe and floor materials operating in mixed lubrication. The proposed analytical model results are compared with experimental data in order to assess the validity of the developed model. Coefficient of friction (COF) values are generated using a pin-on-disk apparatus across a range of sliding speeds with two different shoe materials operating in the mixed-lubrication regime. The model solves for the contact condition using Hertzian contact mechanics theory and the hydrodynamic pressure condition using the Reynolds equation. The amount of contact deformation is adjusted iteratively such that the summed force from the fluid and contacting asperities is equivalent to the total normal force. The model predicts friction values based on the proportion of the load supported by the fluid versus the proportion of the load supported by contacting asperities. The model-generated COF-velocity plots showed close agreement with experimental values for both shoe materials studied. In addition, the model predicts that as the speed between the surfaces increases, the hydrodynamic lift increases. This in turn decreases the contact area and the load borne by the contacting surfaces. Hence, the model presented serves as an initial step towards developing shoe-floor-contaminant friction models.

Effects of foot placement, hand positioning, age and climbing biodynamics on ladder slip outcomes

Ladder falls frequently cause severe injuries; yet the factors that influence ladder slips/falls are not well understood. This study aimed to quantify (1) the effects of restricted foot placement, hand positioning, climbing direction and age on slip outcomes, and (2) differences in climbing styles leading to slips versus styles leading to non-slips. Thirty-two occupational ladder users from three age groups (18–24, 25–44 and 45–64 years) were unexpectedly slipped climbing a vertical ladder, while being assigned to different foot placement conditions (unrestricted vs. restricted toe clearance) and different hand positions (rails vs. rungs). Constraining foot placement increased the climbers likelihood of slipping (p < 0.01), while younger and older participants slipped more than the middle-aged group (p < 0.01). Longer double stance time, dissimilar and more variable foot and body positioning were found in styles leading to a slip. Maintaining sufficient toe clearance and targeting ladder safety training to younger and older workers may reduce ladder falls. Practitioner Summary: Ladder falls frequently cause severe occupational fall injuries. This study aims to identify safer ladder climbing techniques and individuals at risk of falling. The results suggest that ladders with unrestricted toe clearance and ladder climbing training programmes, particularly for younger and older workers, may reduce ladder slipping risk.

Shared and task-specific muscle synergies during normal walking and slipping

Falling accidents are costly due to their prevalence in the workplace. Slipping has been known to be the main cause of falling. Understanding the motor response used to regain balance after slipping is crucial to developing intervention strategies for effective recovery. Interestingly, studies on spinalized animals and studies on animals subjected to electrical microstimulation have provided major evidence that the Central Nervous System (CNS) uses motor primitives, such as muscle synergies, to control motor tasks. Muscle synergies are thought to be a critical mechanism used by the CNS to control complex motor tasks by reducing the dimensional complexity of the system. Even though synergies have demonstrated potential for indicating how the body responds to balance perturbations by accounting for majority of the data sets variability, this concept has not been applied to slipping. To address this gap, data from 11 healthy young adults were collected and analyzed during both unperturbed walking and slipping. Applying an iterative non-negative matrix decomposition technique, four muscle synergies and the corresponding time-series activation coefficients were extracted. The synergies and the activation coefficients were then compared between baseline walking and slipping to determine shared vs. task-specific synergies. Correlation analyses found that among four synergies, two synergies were shared between normal walking and slipping. However, the other two synergies were task-specific. Both limbs were contributing to each of the four synergies, suggesting substantial inter-limb coordination during gait and slip. These findings stay consistent with previous unilateral studies that reported similar synergies between unperturbed and perturbed walking. Activation coefficients corresponding to the two shared synergies were similar between normal walking and slipping for the first 200 ms after heel contact and differed later in stance, suggesting the activation of muscle synergies in response to a slip. A muscle synergy approach would reveal the used sub-tasks during slipping, facilitating identification of impaired sub-tasks, and potentially leading to a purposeful rehabilitation based on damaged sub-functions.

Solution of Reynolds Equation in Polar Coordinates Applicable to Nonsymmetric Entrainment Velocities

Reynolds equation in polar cylindrical (polar) coordinates is used for numerous tribological applications that feature thin fluid films in sliding contacts, such as chemical mechanical polishing and pin-on-disk testing. Although unstated, tribology textbooks and literary resources that present Reynolds equation in polar coordinates often make assumptions that the radial and tangential entrainment velocities are independent of the radial and tangential directions, respectively. The form of polar Reynolds equation is thus typically presented, while neglecting additional terms crucial to obtaining accurate solutions when these assumptions are not met. In the present investigation, the polar Reynolds equation is derived from the cylindrical Navier―Stokes equations without the aforementioned assumptions, and the resulting form is compared with results obtained from more traditionally used forms of the polar Reynolds equation. The polar form of Reynolds equation derived in this manuscript yields results that agree with the commonly used Cartesian form of Reynolds equation but are drastically different from the typically published form of the polar Reynolds equation. It is therefore suggested that the polar form of Reynolds equation proposed in this technical note be utilized when entrainment velocities are known to vary with either radial or angular position.

Required coefficient of friction during level walking is predictive of slipping

The required coefficient of friction (RCOF) is frequently reported in the literature as an indicator of slip propensity. This study aimed to further develop slip prediction models based on RCOF by examining slips under moderately slippery conditions where the RCOF was approximately equal to the available coefficient of friction. Baseline RCOFs were found for normal walking trials and then an unexpected slip was introduced with a moderately slippery boot-floor contaminant combination for thirty-one subjects. Slip outcomes (i.e., whether a subject experienced a slip) were assessed based on the displacement of a marker placed on the heel. A logistic regression analysis was used to model the impact of RCOF on slipping. Results showed that subjects who walked with a greater RCOF were found to have a higher probability of slipping. The predicted probability of a slip across the RCOF ranged from 3% to 95% and an increase of 0.01 in RCOF was associated with a slipping odds ratio of 1.7. Thus, modest differences in RCOF can have a dramatic impact on slip propensity. This study shows that RCOF can be a sensitive and valid predictor of slipping in realistic frictional environments.

Biomechanical response to ladder slipping events: Effects of hand placement

Ladder falling accidents are a significant, growing and severe occupational hazard. The factors that contribute to falls from ladders and specifically those that influence the motor response from ladder falls are not well understood. The aims of this research were to determine the effects of hand placement (rung versus rail) on muscle activation onset and peak activity timing in response to slipping on a ladder and to sequence the timing of events following slip initiation. Fifteen unexpected slips from 11 experienced ladder climbers were induced with a freely spinning rung under the foot, while subjects were randomly assigned to a rung versus rail hand grasping strategy. EMG onset time and peak activity time from five bilateral muscles (semitendinosis, vastus lateralis, triceps, biceps and anterior deltoid) were analyzed. Results indicated that significantly slower muscle activation onset and peak response times occurred during rail hand placement, suggesting that grasping ladder rungs may be preferable for improving the speed of the motor response. The triceps muscle activated and reached peak activity earlier in the slip indicating that subjects may initially extend their arms prior to generating hand forces. The study also revealed that slips tended to occur around the time that a foot and hand were in motion and there were just two points of contact (one hand and the slipping foot).

A Microscopic Finite Element Model of Shoe-Floor Hysteresis and Adhesion Friction

Abstract Few efforts have attempted to model the tribological interaction of shoe–floor contacting surfaces despite high prevalence of slipping accidents. Hysteresis and adhesion are the two main contributing mechanisms in shoe–floor friction at the microscopic asperity level. This study developed a three-dimensional microscopic finite element model of shoe–floor surfaces to quantify the effect of surface topography, shoe material properties and sliding speed on hysteresis and adhesion friction. The validity of the model was assessed by comparing model predictions to pin-on-disk experimental data. The model predicts that hysteresis friction increases for harder shoe materials, rougher shoe surfaces and rougher floor surfaces, while adhesion increases for smoother shoe surfaces, smoother floor surfaces and decreasing sliding speed. The effects of shoe material and floor roughness on the predicted hysteresis friction values were consistent with the experimental data. The effects of sliding speed on adhesion friction were moderately consistent with the experimental data. In addition, the predicted hysteresis magnitudes were consistent with experimental data. This model is a significant step toward development of a comprehensive shoe–floor friction model.

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

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.

Derivation of Reynolds Equation in Cylindrical Coordinates Applicable to Pin-on-Disk and CMP

Traditional tribology references typically provide the cylindrical (or polar) Reynolds equation, which may not be applicable when entrainment velocities vary with radius and/or angle. However, entrainment velocities are known to vary with angle for some cases of pin-on-disk contact and chemical mechanical polishing (CMP). A form of Reynolds equation is derived in this manuscript from the Navier-Stokes equations without entrainment velocity assumptions. Two case studies, related to pin-on-disk and CMP, are presented and results from the derived form of Reynolds equation are compared with results from the traditionally used form. Pressure distributions obtained from the two forms of Reynolds equation varied greatly in magnitude and in pressure shape. Therefore, a new form of the cylindrical Reynolds equation derived in this manuscript is used when entrainment velocities are known to vary with radius or angle.Copyright