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Leigh J. Allin, Maury A. Nussbaum and Michael L. Madigan

Falls are the leading cause of nonfatal injury in the United States. 1 A large percentage of falls are due to slipping. For example, slip-induced falls are responsible for 40% to 50% of all occupational fall-related injuries, 2 and slipping causes a greater proportion of fall-related injuries

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Anna Lee, Tanvi Bhatt, Xuan Liu, Yiru Wang, Shuaijie Wang and Yi-Chung (Clive) Pai

individuals’ dynamic stability. Dynamic stability is simultaneously affected by the center of mass (COM) position and its velocity relative to the base of support (BOS) (ie, COM state) to prevent environmental-induced falls. 9 Specifically, slip-perturbation training is induced by a sudden forward

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Sarah A. Wyszomierski, April J. Chambers and Rakié Cham

Slips and falls are a serious public health concern in older populations. Reduced muscle strength is associated with increased age and fall incidence. Understanding the relationships between specific joint muscle strength characteristics and propensity to slip is important to identify biomechanical factors responsible for slip-initiated falls and to improve slip/fall prevention programs. Knee corrective moments generated during slipping assist in balance recovery. Therefore, the study goal was to investigate the relationship between knee flexion/extension strength and slip severity. Isometric knee flexion/extension peak torque and rate of torque development (RTD) of the slipping leg were measured in 29 young and 28 older healthy subjects. Motion data were collected for an unexpected slip during self-paced walking. Peak slip velocity (PSV) of the slipping heel served as a slip severity measure. Within-sex and age group regressions relating gait speed-controlled PSV to strength of the slipping leg revealed significant inverse PSV-knee extension peak torque and PSV-knee flexion/extension RTD relationships in young males only. Differences in PSV-strength relationships between sex and age groups may be caused by greater ranges of strength capabilities in young males. In conclusion, the ability to generate higher, more rapid knee flexion/extension muscle moments (greater peak torque/RTD) may assist in recovery from severe slips.

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Dennis E. Anderson, Christopher T. Franck and Michael L. Madigan

The effects of gait speed and step length on the required coefficient of friction (COF) confound the investigation of age-related differences in required COF. The goals of this study were to investigate whether age differences in required COF during self-selected gait persist when experimentally-controlling speed and step length, and to determine the independent effects of speed and step length on required COF. Ten young and 10 older healthy adults performed gait trials under five gait conditions: self-selected, slow and fast speeds without controlling step length, and slow and fast speeds while controlling step length. During self-selected gait, older adults walked with shorter step lengths and exhibited a lower required COF. Older adults also exhibited a lower required COF when walking at a controlled speed without controlling step length. When both age groups walked with the same speed and step length, no age difference in required COF was found. Thus, speed and step length can have a large influence on studies investigating age-related differences in required COF. It was also found that speed and step length have independent and opposite effects on required COF, with step length having a strong positive effect on required COF, and speed having a weaker negative effect.

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Shuaijie Wang, Yiru Wang, Yi-Chung (Clive) Pai, Edward Wang and Tanvi Bhatt

Falls can cause serious, devastating consequences for older adults, 1 , 2 and slipping alone accounts for ∼25% of falls among older people. 3 , 4 Although ∼30% of slip-related falls are injurious, 5 not all slips lead to falls, and quite a few of them result in harmless transient postural

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Beatriz H. Thames and Stacey L. Gorniak

When humans adjust their grip to prevent the slipping of objects grasped by the fingertips, the component of their grip force (the force normal to the object surface) generated at the fingertips must be sufficient to generate a tangential frictional force that can prevent slip. 1 This sufficient

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Adriana V. Savescu, Mark L. Latash and Vladimir M. Zatsiorsky

This article proposes a technique to calculate the coefficient of friction for the fingertip– object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the imposed displacement method), a 3-D force sensor was moved horizontally while the subjects applied a specified normal force (4 N, 8 N, 12 N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk).The normal force and the tangential force (i.e., the force due to the sensor motion) were recorded. The coefficient of frictiond) was calculated as the ratio between the tangential force and the normal force. In the second experiment (the beginning slip method), a small instrumented object was gripped between the index finger and the thumb, held stationary in the air, and then allowed to drop. The weight (200 g, 500 g, and 1,000 g) and the surface (sandpaper, cotton, rayon, polyester, and silk) in contact with the digits varied across trials. The same sensor as in the first experiment was used to record the normal force (in a horizontal direction) and the tangential force (in the vertical direction). The slip force (i.e., the minimal normal force or grip force necessary to prevent slipping) was estimated as the force at the moment when the object just began to slip. The coefficient of friction was calculated as the ratio between the tangential force and the slip force. The results show that (1) the imposed displacement method is reliable; (2) except sandpaper, for all other materials the coefficient of friction did not depend on the normal force; (3) the skin–sandpaper coefficient of friction was the highest µd = 0.96 ± 0.09 (for 4-N normal force) and the skin–rayon rayon coefficient of friction was the smallest µd = 0.36 ± 0.10; (4) no significant difference between the coefficients of friction determined with the imposed displacement method and the beginning slip method was observed. We view the imposed displacement technique as having an advantage as compared with the beginning slip method, which is more cumbersome (e.g., dropped object should be protected from impacts) and prone to subjective errors owing to the uncertainty in determining the instance of the slip initiation (i.e., impeding sliding).

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Lucinda E. Bouillon, Michael Hofener, Andrew O’Donnel, Amanda Milligan and Craig Obrock

gluteus maximus and gastrocnemius when squatting with an unstable shoe. 13 Despite numerous studies assessing muscle activity with unstable surfaces, the findings are unclear which creates confusion for clinicians when designing exercise programs. Recently, an unstable platform using a slip

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Bart Van Gheluwe, David Kerwin, Philip Roosen and Rudi Tielemans

Prior research into eversional control in running has reported the occurrence of calcaneal slipping, defined as the difference between calcaneal and heel counter eversion within the shoe. Inadequate heel fit has been proposed to explain the observed slippage. Therefore, the purpose of the current study was to investigate whether eliminating excess space around the calcaneus by narrowing the heel counter or by introducing heel inserts could minimize eversional slipping of the calcaneus within the shoe and, in so doing, improve eversion control of the calcaneus. Three-dimensional rearfoot kinematics of 20 subjects running on a treadmill at 3.8 m · s−1 were recorded using a three-camera Vicon movement analysis system. ANOVA analyses of the angular displacement data revealed a significant (p < .05) reduction in maximal eversion of the calcaneus in shoes with a narrower heel fit though the heel counter of the same shoes displayed more eversion than their wider counterparts. Calcaneal eversion in the shoes with a wider heel counter, however, could be reduced significantly (p < .05) using heel inserts. Maximal relative eversion (referenced to the value at heel strike) and eversional velocity were only affected by the heel inserts, the latter, however, not significantly. All these results show that heel fit affects the eversional behavior of the calcaneus during ground contact and that running shoes equipped with a rigid heel counter will improve rearfoot control, thus only reducing the risk for overload injuries if a snug heel fit used.

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Bart Van Gheluwe, Rudi Tielemans and Philip Roosen

The purpose of the current study was to examine rearfoot motion using three-dimensional recording techniques and shoes with varying heel counter rigidity. The results showed that only the more rigid shoes tended to create significant differences between calcaneal and shoe heel eversion during ground contact, except close to heel-strike. The rigid shoe thus resisted eversional forces. The calcaneus, however, was not prevented from slipping within the shoe and from everting as much or nearly as fast as in a shoe with a more flexible heel counter. In order to simulate two-dimensional recording techniques commonly used in rearfoot studies, a two-dimensional reduction of the original three-dimensional angular data was performed. However, the results produced were similar if torsional movement between the foot and shoe was ignored.