Australas Phys Eng Sci Med (2012) 35:187–191 DOI 10.1007/s13246-012-0144-2
SCIENTIFIC PAPER
Clinical assessment of dynamic coefficient of friction effects in shoe-sole trituration of patients with drop foot Jamshidi Nima • Salami Firooz
Received: 23 August 2011 / Accepted: 27 May 2012 / Published online: 12 June 2012 Ó Australasian College of Physical Scientists and Engineers in Medicine 2012
Abstract The aim of this study was examining the effect of human factors such as plantar friction, contact period time, and impulse on shoe-sole trituration of drop foot patients. Twenty-five patients with drop foot and twenty normal subjects were recruited in the study. The force plate and its related software’s recorded human factor (coefficient of friction, ground reaction force, time of stance phase) as time dependent parameters. Dynamic coefficient of friction patterns were categorized based on their magnitude versus time when the longitudinal axis of the sole was plotted as the Y-axis and the transverse axis of the sole as X-axis during stance phase. The result of this research indicated that the average coefficient of friction among drop foot patients is 77.53 % (p value \0.05) lower than the normal subjects. Also the time of stance phase among drop foot patients is 7.56 % (p value \0.05) greater than normal subjects. There is no difference in the peaks, of vertical ground reaction force between normal and control group. The findings of this research revealed that the time of stance phase has a key role in shoe-sole trituration of patients with drop foot. Keywords
Steppage gait Force plate Shoe design
J. Nima (&) Faculty of Engineering, Department of Biomedical Engineering, University of Isfahan, HezarJerib.st, P.O. Box 81746-73441, Isfahan, Iran e-mail:
[email protected] S. Firooz Faculty of Biomedical Engineering, Department of Biomechanics, Politecnico di Milano, Milan, Italy
Introduction There is a wide range of topics within the broad scope of footwear science, including influence of kinematics and kinetics data on human movement and human performance in the area of clinical biomechanics and rehabilitation engineering. Also footwear has a great role in the prevention and treatment of disorder of lower extremity such as drop foot. Shoe properties and human factors are key factors for the function of footwear [1, 2]. Understanding ground reaction force spatial relationship between normal and steppage gait is intuitively helpful in designing better footwear. Valuable information of ground reaction force obtained from a force plate, could give us a better understanding of acting force during gait [3, 4]. In neurologic gait disorder ground reaction force data would be very useful in modeling, analyzing and designing better orthosis and prosthesis [5, 6]. Drop foot is symptom of gait problem. Patients with drop foot usually lose their ability to pull the toes up while the leg is moving forward, and a simple routine like walking becomes a challenge for people with dropped foot. The shoe sole of patients with drop foot usually triturates very fast. Friction forces in human movement are the cause of shoe trituration. Friction in human movement depends on human and environmental factors. In this research human factors which may play roles on shoe trituration including coefficient of friction, contact period time and impulse among patients with drop foot have been investigated. Human walking requires adequate friction between the sole of the foot and the floor. Causes of shoe-sole trituration are complex involving environmental and human factors. Environmental factors include characteristics of floor design (floor material and micro-structure), shoe design (sole material [7–11], surface micro-structure and tread
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patterns and style [12, 13], contaminants, environment, elevations, steepness of an incline, lighting and even floor compliance [14]. ‘‘Human factors include sensory capabilities, biomechanics, neuromuscular control and information processing’’ [15]. These factors affect on friction and make a complicated, multi variable problem. The shoe sole triturate rapidly among the patients with drop foot in comparison with normal subjects. Dynamic coefficient of friction (COF) is a very important index that represents the human factors during gait. In the previous researches the primary goal of friction measurement was to reduce injuries due to slipping, several human centered methods have been developed to evaluate shoes and floors14. Furthermore the quantified COF was used as a German standard index for determining slip resistance of a floor [16, 17]. Static COF is the most common measurement for rating slip potential; there has been much debate over whether it is the most realistic measure [18–22]. ‘‘Results from laboratory experiments involving human subjects indicated that most foot slips occur under dynamic foot movement conditions and thus a dynamic COF may be a more appropriate measure of the frictional capabilities of a shoe/floor condition’’ [18]. Change in gait by footwear was investigated by several researchers using force plate that measures the three orthogonal components of ground reaction force as well as plantar friction as the frictional coefficient between the flooring material and the sole. ‘‘Gait pattern was demonstrably changed by footwear and was influenced by the kinematics constraint of the ankle joint and stiffness of the sole of the shoe’’ [23]. There have been limited researches utilizing this technique under the dynamic conditions among those with drop foot. The complex interactions between the shoe-trituration factors, however, make the results of these studies difficult to understand and apply, thus the environmental factors have been excluded. Excluding the shoe and floor surfaces and shoe material and focusing on biomechanical factors will provide an opportunity for clinical researchers to better understand the contributions from biomechanical factors that affect on shoe titrations between normal and neuropathic gait. The main goal of this research was to determine which biomechanical factor of patients with steppage gait, plays a key role on shoe-sole trituration. The reason that contact period time was considered as one of the important factors is long time interval of excreting friction that leads to more trituration. On the other hand it seems that the vertical impulses affect on trituration because it is a representation of normal load. Our purpose is to distinguish the role of the medio-lateral (x), antero-posterior (y) and absolute coefficient of friction on shoe sole trituration. Better understanding of the cause of shoe sole trituration during walking will facilitate the design of better sole and
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improve the rehabilitation of patients with drop foot. This study would help researchers to assess the role of dynamic COF measurements on shoe sole trituration. In previous researches COF was often used as a threshold to determine whether a condition is likely to lead to a fall and the probability of a slip. This research may expand the application of force plate data for designing more comfortable shoes for drop foot patients by highlighting the human factors such as dynamic COF.
Materials and methods The effective parameters derived from force plate have been categorized by statistical methods and the significant levels have been defined. The effects of these factors were clarified through statistical analysis of experimental data derived from force plate. The coefficient of friction, impulse, and the contact period time have been derived from Kistler force plate (400 9 600 9 35 mm, model 9286aa) by its associated software, BioWare [24]. Equation 1 states the friction in solid and rigid objects on rigid surface in normal mode and is known as Amonton’s law. Ffriction ðtÞ ¼ lðtÞ NðtÞ
ð1Þ
N (t) represents the normal load and l(t) represents the friction coefficient during time. Friction forces in human gait do not obey Eq. 1. The Eq. 1 shows that the friction is produced by a sliding contact between two bear surfaces [25]. Human gait friction as a result of the rolling and sliding contact of the foot sole dose not satisfy the Eq. 1. These terms are time dependent. Impulses are the areas under the force curve in medio-lateral (x), antero-posterior (y), and vertical direction of force component (z). The Fh represents the vector summation of the medio-lateral (x), antero-posterior (y) forces during human gait. Fv represents the vertical component of ground reaction forces in z direction. The amount of friction required to prevent slipping during the gait is shown by COF (l(t)), and defined as the amount of shear force utilized per normal force. The COF represents two parameters: COFx and, COFy. COFx and, COFy could be captured from the mediolateral (x), and antero-posterior (y) components of ground reaction force. Tsetup represents contact period time (s). COFx, COFy, and COFxy respectively represent the coefficient of friction in medio-lateral (x), antero-posterior (y) direction, and the absolute component. The absolute coefficient of friction is calculated through Eq. 2. Walking direction is positive Y-axis. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi COFxy ¼ ðCOFx2 þ COFy2 Þ ð2Þ Patient enrollment participants were recruited from departments of orthotic prosthetic at Isfahan Red
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Crescent Society-Rehabilitation Center. The patients were recommended by physicians to use the drop foot orthosis. The normal subjects were selected from Isfahan University of Medical Sciences. Patients were excluded if they were mentally ill, were newly diagnosed, planned to discontinue receiving services from the clinic within the 12-month follow up period, or did not have a touch-tone. Each subject was asked to wear a nylon socks without shoe to exclude the environmental factors such as shoes parameters. The complex interactions between the shoetrituration factors, however, make the results of these studies difficult to understand and apply, thus the environmental factors have been excluded. The subjects was instructed to start walking at self-selected speed which determined by themselves, while looking straight ahead horizontally after hearing a beep sound. For removing the effects of initial acceleration of gait, the initial three steps have been ignored, the fourth step was the only one examined in the present experiment [26]. Each subject was then given time to become acquainted with the laboratory ambiance and performed some trials before the data collection. A valid trial consisted of the participants striking their heel on the force platform without altering their normal gait. A subset of the database was used which contained 240 gait cycles of 20 normal male subjects (mean ± S.D. age: 27.55 ± 10.6 years, mass: 67.72 ± 13.19 kg, height: 173.5 ± 5.89 cm) and 300 gait cycles of 25 male patients (mean ± S.D. age: 30.24 ± 11.87 years, mass: 62.28 ± 14.42 kg, height: 168.48 ± 15.84 cm) with drop foot. There were no differences in the age, weight, height, and body mass index between the patients with drop foot and normal subjects (p [ 0.05). The statistical trends of the derived parameters from force plate measurements were examined by SPSS version 17. The parameters were expressed as mean ± standard deviation. Differences in the derived parameters between normal and drop foot patients were classified by two-tailed Student’s t test. Differences with a significance level (p) lower than 0.05 were categorized significant.
Results The normalized impulses in medio-lateral (x), antero-posterior (y), and vertical direction of force component (z) in control and normal groups have been shown in Table 1. Tsetup, COFx, COFy, and COFxy are respectively shown in Table 1. As shown in Table 1, the COFx, COFy, and COFxy parameters in control group are greater than the patients group (COFx_control group [ COFx_patients group, COFy_control group [ , COFy_patients group and COFxy_control group [ COFxy_patients group), but the
189 Table 1 List of parameters derived from force plate measurements Control group (n = 20) Normalized impulse Z (s) (Sec) Tsetup (s) COFx
0.6536 ± 0.1762
Patients (n = 25) 0.7566 ± 0.188
p value 0.104
0.883 ± 0.1232
0.955 ± 0.0994
0.005
0.3075 ± .49579
0.0006 ± .07391
0.000
COFy
0.0239 ± .03015
-0.0041 ± .0047
0.000
COFxy
0.3026 ± .2845
0.0683 ± .0648
0.000
Mean ± SD (standard deviation) %BW means normalized force by dividing on body weight of each subject
Fig. 1 The COF patterns during a normal and drop foot stride in medio-lateral (x) and antero-posterior (y) direction and the absolute component (H and P respectively represent the normal subjects and drop foot patients)
duration of stance phase in patients is longer than the control group (Tsetup_control group \ Tsetup_patients group) and the Normalized Impulse Z did not show any considerable difference. All the mentioned factors in Table 1 have effect on trituration. Since the duration of stance phase in patients is longer and the coefficient of friction is less than control groups therefore the main factor of shoe sole trituration among patients is duration of stance phase. Analysis on duration of stance phase data revealed that the contact of patients’ shoe sole with the ground is 7.54 % higher than the control group. The differences between COF patterns of normal subjects and drop foot patients in X and Y directions have been illustrated in Fig. 1.
Discussion Measurement of kinetics friction is a challenging task. In this research the effective environmental factors in floor-
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shoe interface such as drainage capacity between shoe and floor, viscoelastic mechanical properties of the shoe sole, floor surface roughness [9, 27–32], floor surface waviness [31, 32], and contaminant condition [27–31] which considered to be important by other researchers have been ignored using bare foot. But in this study, the goal was not to quantify the COF in drop foot patients in different conditions; instead the aim was to assess the effect of human factors on shoe-trituration among drop foot patients. For comparing two groups the environmental factors assume to be similar during walking. Due to rolling and sliding friction during the human gait the laws of dry friction equation (Amontons’ First Law) is not true. Human gait friction as a result of the rolling and sliding contact of the foot sole dose not satisfy the Amontons’ First Law. Three main stages in a single stance phase are heel strike, foot flat and toe off. The critical phases of gait that high friction is required for preventing falls are heel strike and toe off. In heel strike and toe off the friction force is mostly generated by sliding. In foot flat the friction is mostly generated by rolling. This time history of plantar friction behavior can be explained by a sliding/rolling motion between flooring material and sole [23]. Foot forces normally generated during gait require friction to counteract the shear forces and prevent slip [18].
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shoe sole of these patients. Also it is possible to inject air between sole layers during manufacturing process using plastic air blowing shoes injection molding machine. Air injection between sole layers leads us to produce lighter and softer shoe sole. Softer shoe sole will increase the friction between outer shoe sole and the ground. On the other hand, increasing the coefficient of friction during both phases of heel strike and toe off causes pain relief, ulcer prevention and improved mobility. The result of this research may improve therapeutic footwear. Footwear has the potential to hide or to emphasize gait disorder. The importance of footwear is improving the steppage gait disorder by decreasing the instability thereby the selfesteem of patients, quality of life, well-being, and depression will be improved. In this study a method was proposed for evaluation of relationship between measured data force plate and biomechanical parameters which are effective on shoe-sole trituration. The vertical, medio-lateral and antero-posterior forces have been calculated using force plate data. For safety the coefficient of friction must be greater than the ratio of horizontal and vertical ground reaction force. Acknowledgments The authors gratefully acknowledge the Center of Excellence in Biomedical Engineering of Iran, and the Isfahan Sport Administration, Sport Championship Center for their help in conducting this project.
Conclusion Findings revealed that the vertical impulses do not play important roles on shoe sole trituration; but the contact period time among drop foot patients is greater than normal subjects. Thereby, contact period time play an important role on shoe sole trituration. Further research is required to confirm our findings. The finding of this research indicated that the instability of steppage gait is connected to low coefficient of friction. Further researches may discover the cause of instability, slips and fall [33–35] when the shoefloor-contaminant factor is ignored. Also it is possible to increase the amount of available friction and more stable gait through increasing the size of tread width [12] and tread depth [13]. Knowledge about the trituration of shoe sole in neuropathic gait may have therapeutic effects on patients. In neuropathic gait due to low level of friction on shoe sole, patients feel instability consequently they prefer to not walk. Lake of walking leads to weakness of neuro-muscloskeletal system. Findings of this research may improve the gait of these patients through increasing the coefficient of friction between shoe sole and ground. Previous studies indicated that harder shoe materials are associated with a smaller friction coefficient [14]; therefore it is suggested to use soft and anti-trituration material for
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