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Comparison of utilized coefficient of friction during different walking tasks in persons with and without a disability
Gait & Posture 22 (2005) 82–88 www.elsevier.com/locate/gaitpost
Comparison of utilized coefficient of friction during different walking tasks in persons with and without a disability Judith M. Burnfielda,b,*, Yi-Ju Tsaib, Christopher M. Powersb a
Pathokinesiology Laboratory, Rancho Los Amigos National Rehabilitation Center, 7601 E. Imperial Highway, 800 Building Room 33, Downey, CA 90242, USA b Department of Biokinesiology and Physical Therapy, University of Southern California, 1540 E. Alcazar St., CHP-155, Los Angeles, CA 90033, USA
Abstract The purpose of this study was to determine if older persons with a disability have greater utilized coefficient of friction requirements than healthy older and younger adults during various walking conditions. Forty-eight community-dwelling adults were divided into five groups based on medical diagnosis and age: CVA (unilateral stroke; mean age 63 years), DM (diabetes mellitus; mean age 70 years), ARTH (lower extremity arthritis; mean age 69 years), SENIOR (healthy; mean age 73 years), and YOUNG (healthy; mean age 29 years). Ground reaction forces (GRF) were recorded as subjects walked across a walkway, ascended and descended stairs, and negotiated a turn. The utilized coefficient of friction (COFU) throughout stance was calculated as the ratio of shear to vertical GRFs, and the peak COFU resulting from a shear force that would contribute to a forward foot slip was identified. Separate one-way ANOVAs were used to identify differences in peak COFU across subject groups for each walking task. The results of this study found that for all conditions evaluated, those with a disability did not demonstrate greater friction requirements then adults without a disability. Friction requirements for the YOUNG group were significantly greater than all disability groups while negotiating a turn, and were greater than the DM group during level walking. These results indicated that the diagnostic groups evaluated in this study are not at any greater risk for slip initiation than the healthy older or younger adults during the tasks evaluated. # 2004 Elsevier B.V. All rights reserved. Keywords: Slip resistance; Gait; Age; Medical condition; Forensic science
1. Introduction Among older persons, slips have been identified as a primary cause of falls in the work and home environments [1,2]. An investigation of occupational injuries to civilian workers over the age of 55 years, reported that slips accounted for more than half (57%) of the falls occurring on level surfaces [2]. Slips contributed to 38% of falls in men and 17% of falls in women in a 1-year study of accidental falls experienced by community dwelling older adults (60– 88 years) [1]. Falls are the leading cause of unintentional injuries resulting in death in persons over the age of 65 years [3] and account for 87% of all fractures occurring in this age group [4]. The serious consequences of falls is underscored * Corresponding author. Tel.: +1 562 401 7177; fax: +1 562 803 5693. E-mail address: [email protected] (J.M. Burnfield). 0966-6362/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2004.07.004
by the inclusion of the reduction of ‘‘deaths from falls’’ as a national priority within the Healthy People 2010 Objectives for Improving Health (Objective 15–27) [5]. During walking, slips result from a loss of traction between the foot and the floor. In the research setting, the traction that an individual requires from the floor surface during walking, or ‘‘utilized’’ coefficient of friction (COFU), can be determined from force plate recordings of ground reaction forces (GRF) [6]. The COFU is defined as the ratio between the horizontal and vertical components of the GRF generated by a person while walking across a dry, noncontaminated surface. The probability of a slip rises when either the friction that an individual utilizes increases or the friction available from the floor surface decreases [7]. As COFU is determined by the ratio of shear to vertical GRFs, it is likely that changes in gait patterns may influence the potential for slip initiation. For example, previous studies
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have shown that peak COFU varies with age [8], gender [8], walking speed [8–10], and the presence of a disability [11]. Those with a disability would appear to be at potentially greater risk owing to the larger changes in gait characteristics and GRFs. To date, only one published study has examined the influence of various medical conditions on peak COFU. Buczek et al. [11] found that persons with a disability had higher peak COFU during level walking when compared to persons without a disability. Only nine persons with disabilities participated in this study and data were averaged across various diagnoses (e.g., amputations, broken leg, osteotomy of the fifth metatarsal). In addition, this study did not systematically examine medical conditions likely to be prevalent in older adults nor did it examine COFU values across a series of walking tasks necessary to function in most work and home environments. The purpose of the current investigation was to compare COFU between persons with and without a disability during selected walking conditions. It was hypothesized that the peak COFU of older persons who have medical conditions that might influence their gait (osteoarthritis, diabetes mellitus, and cerebral vascular accident) would exceed those of younger and older healthy controls during level walking, and while negotiating stairs and turns. Data from this study may be used to guide recommendations related to flooring modifications and environmental design to reduce the risk of injuries related to slips and falls.
2. Methods 2.1. Subjects Community-dwelling adults were divided into five groups based on medical diagnosis and age (Table 1). Thirty-eight subjects, over the age of 55 years, were assigned to one of the following four groups based on medical history: CVA (sustained a unilateral stroke), DM (diagnosis of diabetes mellitus), ARTH (presence of lower extremity arthritis), SENIOR (healthy). A fifth group, YOUNG, consisted of 10 healthy adults between the ages of 20 and 40 years. The three diagnostic groups (CVA, DM,
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and ARTH) were selected as they are among the top five conditions that persons over 70 years of age identified as limiting their ability to perform activities of daily living [12]. Additionally, altered gait and ground reaction force patterns might arise due to the presence of weakness (CVA, ARTH, DM), reduced sensation (CVA, DM), impaired motor control (CVA), and pain (ARTH). Subjects were recruited by word of mouth from the student and faculty population at the University of Southern California (Los Angeles, CA) and from medical facilities around the greater Los Angeles area. Only subjects capable of walking with minimal to no physical assistance across level surfaces and on stairs were included. Subjects with relatively high ambulatory function were selected as they were expected to be the individuals most likely to navigate a variety of situations that might pose a slip risk. Persons in the CVA group were greater than 6 months post a unilateral cerebral vascular accident (diagnosed by a physician) and lacked selective control of the involved lower extremity. Individuals in the DM group had been diagnosed by a physician as having either insulin dependent or noninsulin dependent diabetes mellitus, and demonstrated evidence of peripheral neuropathy (reduced sensation based on a Semmes–Weinstein monofilament testing and bilateral reductions in calf strength as documented by an inability to complete five unilateral heel raises). Subjects in the ARTH group had a physician diagnosis of lower extremity osteoarthritis (hip or knee) which had been previously documented by X-ray or MRI. Subjects with lower limb amputation, joint replacement, or additional neurologic conditions that would influence gait were excluded from participation. In addition, potential subjects were excluded if they had medical conditions that would qualify them for more than one group (e.g., a history of diabetes mellitus and stroke). Subjects in the SENIOR and YOUNG groups had no neurologic or orthopaedic conditions that would alter gait, and denied any history of stroke, diabetes mellitus, or osteoarthritis. Prior to participation, each subject was fully informed of the nature of the study, and signed a human subject’s consent form approved by the Institutional Review Board of the University of Southern California Health Sciences Campus.
Table 1 Subject characteristics (mean (S.D.))
Age (years) Gender Height (cm) Mass (kg) Involved limb Assistive device used Assistance required SBA: stand-by assistance.
SENIOR (n = 10)
CVA (n = 10)
ARTH (n = 8)
DM (n = 10)
YOUNG (n = 10)
72.5 (8.7) 5 females, 5 males 164.0 (11.6) 73.0 (17.1) 10 left None Independent
62.7 (7.2) 2 females, 8 males 174.9 (10.2) 83.7 (18.9) 2 right, 8 left 4 cane SBA on stairs
68.9 (11.8) 7 females, 1 males 167.0 (9.3) 71.7 (18.4) 3 right, 5 left 1 cane Independent
69.7 (5.5) 4 females, 6 males 163.1 (11.6) 71.1 (18.8) 10 left None Independent
28.8 (4.5) 5 females, 5 males 171.8 (7.4) 74.2 (24.2) 10 left None Independent
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2.2. Instrumentation Ground reaction forces (vertical, fore-aft, and mediallateral) were recorded using four AMTI force plates (Model OR6-6-1000; AMTI Corp., Newton, MA) aligned in series. The force plates were covered with smooth vinyl composition tile and were camouflaged within the walkway. Force plate data were sampled at 1200 Hz, and recorded on a Pentium III 1 GHz personal computer. A 64 channel, 12-bit, analog-to-digital card converted analog voltage signals from the force plate (range 10 V) to digital signals. The resolution of the shear force signals was 0.74 N/bit, and the resolution of the vertical force signal was 2.87 N/bit. Light sensitive triggers were used to initiate and terminate data collection as subjects completed each of the walking tasks. The stairs used in this study consisted of four steps. An AMTI force plate (Model OR6-6-1000; AMTI Corp., Newton, MA) mounted on an independent concrete block served as the second step from the lower landing. The rise of each step was 20 cm and the run was 28 cm. 2.3. Procedures All testing was conducted in the Musculoskeletal Biomechanics Research Laboratory at the University of Southern California. To control for the potential influence of footwear on COFU, all subjects were provided with a pair of walking shoes (Rockport World Tour, model M/W WT18; The Rockport Company, LLC, Ronks, PA) for use during testing. Subjects performed four tasks: (1) walking along a level 10 m walkway; (2) walking 5 m, turning 908 and continuing to walk another 5 m; (3) ascending the four-step staircase; and (4) descending the same four-step staircase. The order of tasks was randomized. All subjects were instructed to perform the tasks at a comfortable speed, and two trials were recorded. A trial was considered successful if the subject’s tested foot landed within the force plate.
pass Butterworth filter with a zero-lag compensation [13]. The non-directional resultant shear force was calculated as the vector addition of the anterior–posterior and medial– lateral shear forces. The COFU throughout stance for the more involved limb (CVA; ARTH) or left limb (DM, SENIOR, YOUNG) was calculated as the ratio of the resultant shear to vertical ground reaction forces. During initial loading, the peak COFU resulting from a shear force that would contribute to a forward foot slip was identified (Fig. 1). Data were screened for spuriously high peak COFU values occurring when shear forces were divided by small vertical forces [6]. Typically, non-spurious peak COFU values were observed once the reference limb had been loaded with greater than 65 N of vertical force. Fig. 1 provides representative tracings of the three components of the ground reaction force recorded as a SENIOR subject descended stairs. 2.5. Statistical analysis To determine if peak COFU varied across subject groups, a one-way analysis of variance (ANOVA) was performed. This analysis was repeated for each condition. Statistical analyses were performed using SPSS statistical software (version 10.0; SPSS Inc., Chicago, IL). A significance level of P < 0.05 was used for all statistical comparisons.
3. Results 3.1. Walking velocity and stance duration of turning, stair ascent and descent On average, the YOUNG group walked more rapidly and had a shorter stance duration during turning, stair ascent and stair descent compared to the other groups (Table 2). The CVA and DM groups walked the slowest, and exhibited a longer stance duration while turning and negotiating stairs compared to the remaining groups (Table 2).
2.4. Data analysis 3.2. COFU during level walking Force plate data were analyzed using the DataPac software program (v. 2K2, Run Technologies, Mission Viejo, CA). Digitally acquired anterior–posterior, medial–lateral, and vertical forces were imported from VICON Workstation (Oxford Metrics, Ltd., Oxford, England) into DataPac 2K2 where they were filtered using a fourth order, 45 Hz, low
When averaged across all subjects, the mean peak COFU during level walking was m = 0.23. Values ranged from m = 0.14 for a person in the ARTH group to m = 0.54 for an individual in the CVA group. Peak COFU was found to vary significantly across subject groups (P < 0.001; Fig. 2). Post
Table 2 Mean walking velocity and stance duration for each subject group during each task (mean (S.D.))
Level walking velocity (m/min) Turning stance duration (s) Stair ascent stance duration (s) Stair descent stance duration (s)
SENIOR
CVA
ARTH
DM
YOUNG
86.4 (11.1) 0.76 (0.09) 1.11 (0.15) 0.91 (0.19)
47.0 (13.5) 0.97 (0.15) 2.66 (0.52) 2.23 (0.95)
80.2 (9.9) 0.79 (0.07) 1.12 (0.41) 1.09 (0.43)
70.0 (10.0) 0.89 (0.08) 1.60 (0.91) 1.35 (0.72)
93.7 (12.4) 0.66 (0.12) 0.90 (0.17) 0.79 (0.17)
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Fig. 1. Representative tracings of (a) ground reaction forces and (b) utilized COF during stair descent for a SENIOR subject. Note that the initial spuriously high spike in the utilized COF was due to a relatively low vertical ground reaction force.
hoc analysis revealed that the YOUNG group had a significantly higher peak COFU than the DM group (m = 0.24 versus m = 0.19; P = 0.019). No other differences were identified. 3.3. COFU while negotiating a turn When averaged across all subjects, the mean peak COFU while negotiating turns was m = 0.27. The lowest value was m = 0.15 for a person in the CVA group, while the highest was m = 0.48 for an individual in the YOUNG group. Peak COFU was found to vary significantly across subject groups (P < 0.001; Fig. 3). Post hoc analysis revealed that the YOUNG (m = 0.36) had significantly higher peak COFU than the SENIOR (m = 0.26), CVA (m = 0.21), ARTH (m = 0.27) and DM (m = 0.25) groups (P 0.001). No other differences were identified.
3.4. COFU during stair ascent When averaged across all subjects, the mean peak COFU during stair ascent was m = 0.20. The lowest value was m = 0.08 for a person in the ARTH group, while the highest value was m = 0.46 for an individual in the YOUNG group. The peak COFU while ascending stairs was not found to vary significantly across groups (P = 0.305; Fig. 4). 3.5. COFU during stair descent When averaged across all subjects, the mean peak COFU during stair descent was m = 0.37. The lowest value recorded was m = 0.14 for a person in the YOUNG group, while the highest was m = 0.61 for an individual in the DM group. The peak COFU while descending stairs was not found to vary significantly across groups (P = 0.115; Fig. 5).
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Fig. 2. Comparison of peak utilized COF during level walking for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM). * YOUNG > DM (P < 0.02).
4. Discussion Contrary to our initial hypothesis, the results of this study found that older adults with a disability did not demonstrate greater COFU compared to older and younger adults without a disability. This finding was consistent for each condition evaluated and suggests that those with the specific medical conditions tested in this study are not at greater risk for slip initiation. The results of the current investigation contrast notably with the only previous study that evaluated COFU in persons with a disability. Buczek et al. [11] reported that persons with a disability demonstrated a twofold increase in peak COFU during level walking when compared to persons without a disability. Two important differences between the study by Buczek et al. [11] and the current investigation that likely contributed to the disparate COFU findings are the diagnoses and severity of physical impairment of the subjects assessed. The nine subjects participating in the study by Buczek et al. [11] had a variety of medical conditions including unilateral trans-tibial amputation, transfemoral amputation, unilateral hemiparesis secondary to stroke,
hip fracture, broken leg, and osteotomy of the fifth metatarsal. Two subjects required use of axillary crutches for ambulation, while one used a four-footed walker. In the current study, we selected individuals with relatively high ambulatory function as they were expected to be the persons most likely to negotiate a variety of conditions that might pose a slip risk. Only 4 of 48 subjects required a single point cane. The higher functional status of our subjects could have allowed them to more selectively decelerate the forward momentum of their limb prior to initial contact [14], contributing to lower shear forces during limb loading. Additionally, the reduced reliance of our subjects on assistive devices may have contributed to a more rapid rise in the vertical component of the ground reaction force during weight acceptance. Both these mechanisms could have contributed to the lower COFU values recorded for subjects with a disability in the current investigation compared to those studied by Buczek et al. [11]. Additional reasons for the differences between values recorded in our study and those reported by Buczek et al. [11] likely include differences in footwear, floor characteristics, as well as the limited number of persons with a
Fig. 3. Comparison of peak utilized COF while negotiating turns for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM). * YOUNG > SENIOR, CVA, ARTH, DM (P < 0.001).
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Fig. 4. Comparison of peak utilized COF during stair ascent for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM).
disability tested in the other study. Furthermore, Buczek et al. [11] sampled their ground reaction forces at 500 Hz, did not filter their data, and did not consider COFU values when the vertical ground reaction force was less than 50 N. In contrast, force data from the current study were sampled at 1200 Hz, filtered using a fourth order, 45 Hz, low pass Butterworth filter, and COFU data were visually screened for spurious values. Thus, the methods used by Buczek et al. [11] could have allowed greater shear forces to be divided by smaller vertical forces resulting in higher COFU than those reported in the current study. Buczek and Banks [6] previously suggested that these higher values may be true, but of such short duration that a slip does not occur. Further testing, relating subject-specific COFU values, available slip resistance, and slip outcome should provide insight into the validity of these values. In two of the four conditions tested, subjects in the YOUNG group demonstrated increased peak COFU compared to those with a disability. Friction requirements for the YOUNG group were significantly greater than all disability groups while negotiating a turn, and were higher than the DM group during level walking. The greater COFU observed in the YOUNG during these tasks can be explained by the
fact that these subjects walked more quickly than subjects in the disability groups. Burnfield and Powers [8] investigated the influence of walking velocity on the friction requirements of 60 healthy adults and reported that COFU increased with faster walking speeds. Given the relationship between walking speed and COFU [8], it is not entirely surprising that subjects in the YOUNG group had higher COFU during selected tasks in the current study. Stair descent was the task associated with the highest peak COFU for the SENIOR (m = 0.39), CVA (m = 0.34), ARTH (m = 0.41), and DM (m = 0.41) groups, with values for individual subjects in these groups ranging from m = 0.24 to 0.61. The high peak COFU while descending stairs suggests that there is a greater risk for slip initiation for older adults while performing this task compared to the other tasks studied. In contrast, negotiating a turn was the task associated with the highest peak COFU for the YOUNG group (m = 0.36; range m = 0.28–0.48), suggesting a greater relative risk of slipping for young adults while performing this task. Many state laws and building codes have established that a static COF of m 0.50 represents the minimum slip resistance threshold for safe floor surfaces. Furthermore, the
Fig. 5. Comparison of peak utilized COF during stair descent for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM).
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Americans with Disabilities Act Accessibility Guidelines [15] contain advisory recommendations for static coefficient of friction slip resistance values of m 0.60 for accessible routes (e.g. walkways and elevators) and m 0.80 for ramps. While each group’s average peak COFU fell well below the m 0.50 threshold for the tasks investigated in the current study, wide inter-subject variability was observed. In particular, during stair descent, each group of subjects had at least one individual whose peak COFU exceeded the m 0.50 threshold. This finding would suggest that stair surfaces which meet the minimum m 0.50 threshold may pose a risk for slip onset for some adults.
5. Conclusions The findings of the current study suggest that persons with the select medical conditions are not at any greater risk for slip initiation than healthy older or younger adults during the tasks evaluated. The higher COFU values observed while descending stairs and negotiating a turn suggest an increased risk of slip onset compared to level walking and stair ascent. Recovery from a slip, however, is likely confounded by multiple factors including balance, muscular strength, power, and flexibility. Thus it is possible that once a slip is initiated, persons with a disability may have a more difficult time recovering from the event. Acknowledgements This work was supported, in part, by an award from the California Physical Therapy Fund (#01-08) as well as the American Society of Safety Engineers and National Institute for Occupational Safety and Health. The authors would like to gratefully acknowledge The Rockport Company for their donation of shoes used during subject testing. Additionally, we greatly appreciated the contributions of Richard Souza, MPT and John Popovich, DPT during the data collection and processing phases of this project.
References [1] Berg WP, Alessio HM, Mills EM, Tong C. Circumstances and consequences of falls in independent community-dwelling older adults. Age and Ageing 1997;26:261–8. [2] Layne LA, Landen DD. A descriptive analysis of nonfatal occupational injuries to older workers, using a national probability sample of hospital emergency departments. J Occupation Environ Med 1997;39:855–65. [3] Hoyert DL, Kochanek KD, Murphy SL. Deaths: Final Data for 1997. National Vital Statistics Report, vol. 47, 1999. p. 1–104. [4] Fife D, Barancik JI. Northeastern Ohio trauma study. III. Incidence of fractures. Ann Emergency Med 1985;14:244–8. [5] U.S. Department of Health and Human Services. Healthy People 2010: Understanding and Improving Health. 2nd ed. Washington, DC: U.S. Government Printing Office; 2000. [6] Buczek FL, Banks SA. High-resolution force plate analysis of utilized slip resistance in human walking. J Test Eval 1996;24:353–8. [7] Hanson JP, Redfern MS, Mazumdar M. Predicting slips and falls considering required and available friction. Ergonomics 1999;42:1619–33. [8] Burnfield JM, Powers CM. Influence of age and gender of utilized coefficient of friction during walking at different speeds. In: Marpet MI, Sapienza MA, editors. Metrology of pedestrian locomotion and slip resistance, ASTM STP 1424. West Conshohocken: ASTM International; 2003. p. 3–16. [9] Fendley AE, Medoff HP. Required coefficient of friction versus toppiece/outsole hardness and walking speed: significance of correlations. J For Sci 1996;41:763–9. [10] Skiba R. Sicherheitsgrenzwerte zur vermeidung des ausgleitens auf fussboden. Zeitschrift Fur Arbeitswissenschaft 1988;14:47–51. [11] Buczek FL, Cavanagh PR, Kulakowksi BT, Pradhan P. Slip resistance needs of the mobility disabled during level and grade walking. In: Everett GB, editor. Slips stumbles and falls pedestrian footwear and surfaces, STP 1103. Philadelphia, PA: American Society for Testing and Materials; 1990. p. 39–54. [12] Kramarow E, Lentzner H, Rooks R, Weeks J, Saydah S. Health, United States, 1999 With Health and Aging Chartbook. Hyattsville, MD: National Center for Health Statistics; 1999. [13] Christina KA, Cavanagh PR. Ground reaction forces and frictional demands during stair descent: effects of age and illumination. Gait and Posture 2002;15:153–8. [14] Perry J. Gait Analysis, Normal and Pathological Function. Thorofare, NJ: Charles B. Slack; 1992. [15] Architectural and Transportation Barriers Compliance Board. Americans with disabilities act (ADA) accessibility guidelines for buildings and facilities. Final Guidelines Federal Register, 56. 1991; p. 35408–542.
Comparison of utilized coefficient of friction during different walking tasks in persons with and without a disability Judith M. Burnfielda,b,*, Yi-Ju Tsaib, Christopher M. Powersb a
Pathokinesiology Laboratory, Rancho Los Amigos National Rehabilitation Center, 7601 E. Imperial Highway, 800 Building Room 33, Downey, CA 90242, USA b Department of Biokinesiology and Physical Therapy, University of Southern California, 1540 E. Alcazar St., CHP-155, Los Angeles, CA 90033, USA
Abstract The purpose of this study was to determine if older persons with a disability have greater utilized coefficient of friction requirements than healthy older and younger adults during various walking conditions. Forty-eight community-dwelling adults were divided into five groups based on medical diagnosis and age: CVA (unilateral stroke; mean age 63 years), DM (diabetes mellitus; mean age 70 years), ARTH (lower extremity arthritis; mean age 69 years), SENIOR (healthy; mean age 73 years), and YOUNG (healthy; mean age 29 years). Ground reaction forces (GRF) were recorded as subjects walked across a walkway, ascended and descended stairs, and negotiated a turn. The utilized coefficient of friction (COFU) throughout stance was calculated as the ratio of shear to vertical GRFs, and the peak COFU resulting from a shear force that would contribute to a forward foot slip was identified. Separate one-way ANOVAs were used to identify differences in peak COFU across subject groups for each walking task. The results of this study found that for all conditions evaluated, those with a disability did not demonstrate greater friction requirements then adults without a disability. Friction requirements for the YOUNG group were significantly greater than all disability groups while negotiating a turn, and were greater than the DM group during level walking. These results indicated that the diagnostic groups evaluated in this study are not at any greater risk for slip initiation than the healthy older or younger adults during the tasks evaluated. # 2004 Elsevier B.V. All rights reserved. Keywords: Slip resistance; Gait; Age; Medical condition; Forensic science
1. Introduction Among older persons, slips have been identified as a primary cause of falls in the work and home environments [1,2]. An investigation of occupational injuries to civilian workers over the age of 55 years, reported that slips accounted for more than half (57%) of the falls occurring on level surfaces [2]. Slips contributed to 38% of falls in men and 17% of falls in women in a 1-year study of accidental falls experienced by community dwelling older adults (60– 88 years) [1]. Falls are the leading cause of unintentional injuries resulting in death in persons over the age of 65 years [3] and account for 87% of all fractures occurring in this age group [4]. The serious consequences of falls is underscored * Corresponding author. Tel.: +1 562 401 7177; fax: +1 562 803 5693. E-mail address: [email protected] (J.M. Burnfield). 0966-6362/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2004.07.004
by the inclusion of the reduction of ‘‘deaths from falls’’ as a national priority within the Healthy People 2010 Objectives for Improving Health (Objective 15–27) [5]. During walking, slips result from a loss of traction between the foot and the floor. In the research setting, the traction that an individual requires from the floor surface during walking, or ‘‘utilized’’ coefficient of friction (COFU), can be determined from force plate recordings of ground reaction forces (GRF) [6]. The COFU is defined as the ratio between the horizontal and vertical components of the GRF generated by a person while walking across a dry, noncontaminated surface. The probability of a slip rises when either the friction that an individual utilizes increases or the friction available from the floor surface decreases [7]. As COFU is determined by the ratio of shear to vertical GRFs, it is likely that changes in gait patterns may influence the potential for slip initiation. For example, previous studies
J.M. Burnfield et al. / Gait & Posture 22 (2005) 82–88
have shown that peak COFU varies with age [8], gender [8], walking speed [8–10], and the presence of a disability [11]. Those with a disability would appear to be at potentially greater risk owing to the larger changes in gait characteristics and GRFs. To date, only one published study has examined the influence of various medical conditions on peak COFU. Buczek et al. [11] found that persons with a disability had higher peak COFU during level walking when compared to persons without a disability. Only nine persons with disabilities participated in this study and data were averaged across various diagnoses (e.g., amputations, broken leg, osteotomy of the fifth metatarsal). In addition, this study did not systematically examine medical conditions likely to be prevalent in older adults nor did it examine COFU values across a series of walking tasks necessary to function in most work and home environments. The purpose of the current investigation was to compare COFU between persons with and without a disability during selected walking conditions. It was hypothesized that the peak COFU of older persons who have medical conditions that might influence their gait (osteoarthritis, diabetes mellitus, and cerebral vascular accident) would exceed those of younger and older healthy controls during level walking, and while negotiating stairs and turns. Data from this study may be used to guide recommendations related to flooring modifications and environmental design to reduce the risk of injuries related to slips and falls.
2. Methods 2.1. Subjects Community-dwelling adults were divided into five groups based on medical diagnosis and age (Table 1). Thirty-eight subjects, over the age of 55 years, were assigned to one of the following four groups based on medical history: CVA (sustained a unilateral stroke), DM (diagnosis of diabetes mellitus), ARTH (presence of lower extremity arthritis), SENIOR (healthy). A fifth group, YOUNG, consisted of 10 healthy adults between the ages of 20 and 40 years. The three diagnostic groups (CVA, DM,
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and ARTH) were selected as they are among the top five conditions that persons over 70 years of age identified as limiting their ability to perform activities of daily living [12]. Additionally, altered gait and ground reaction force patterns might arise due to the presence of weakness (CVA, ARTH, DM), reduced sensation (CVA, DM), impaired motor control (CVA), and pain (ARTH). Subjects were recruited by word of mouth from the student and faculty population at the University of Southern California (Los Angeles, CA) and from medical facilities around the greater Los Angeles area. Only subjects capable of walking with minimal to no physical assistance across level surfaces and on stairs were included. Subjects with relatively high ambulatory function were selected as they were expected to be the individuals most likely to navigate a variety of situations that might pose a slip risk. Persons in the CVA group were greater than 6 months post a unilateral cerebral vascular accident (diagnosed by a physician) and lacked selective control of the involved lower extremity. Individuals in the DM group had been diagnosed by a physician as having either insulin dependent or noninsulin dependent diabetes mellitus, and demonstrated evidence of peripheral neuropathy (reduced sensation based on a Semmes–Weinstein monofilament testing and bilateral reductions in calf strength as documented by an inability to complete five unilateral heel raises). Subjects in the ARTH group had a physician diagnosis of lower extremity osteoarthritis (hip or knee) which had been previously documented by X-ray or MRI. Subjects with lower limb amputation, joint replacement, or additional neurologic conditions that would influence gait were excluded from participation. In addition, potential subjects were excluded if they had medical conditions that would qualify them for more than one group (e.g., a history of diabetes mellitus and stroke). Subjects in the SENIOR and YOUNG groups had no neurologic or orthopaedic conditions that would alter gait, and denied any history of stroke, diabetes mellitus, or osteoarthritis. Prior to participation, each subject was fully informed of the nature of the study, and signed a human subject’s consent form approved by the Institutional Review Board of the University of Southern California Health Sciences Campus.
Table 1 Subject characteristics (mean (S.D.))
Age (years) Gender Height (cm) Mass (kg) Involved limb Assistive device used Assistance required SBA: stand-by assistance.
SENIOR (n = 10)
CVA (n = 10)
ARTH (n = 8)
DM (n = 10)
YOUNG (n = 10)
72.5 (8.7) 5 females, 5 males 164.0 (11.6) 73.0 (17.1) 10 left None Independent
62.7 (7.2) 2 females, 8 males 174.9 (10.2) 83.7 (18.9) 2 right, 8 left 4 cane SBA on stairs
68.9 (11.8) 7 females, 1 males 167.0 (9.3) 71.7 (18.4) 3 right, 5 left 1 cane Independent
69.7 (5.5) 4 females, 6 males 163.1 (11.6) 71.1 (18.8) 10 left None Independent
28.8 (4.5) 5 females, 5 males 171.8 (7.4) 74.2 (24.2) 10 left None Independent
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2.2. Instrumentation Ground reaction forces (vertical, fore-aft, and mediallateral) were recorded using four AMTI force plates (Model OR6-6-1000; AMTI Corp., Newton, MA) aligned in series. The force plates were covered with smooth vinyl composition tile and were camouflaged within the walkway. Force plate data were sampled at 1200 Hz, and recorded on a Pentium III 1 GHz personal computer. A 64 channel, 12-bit, analog-to-digital card converted analog voltage signals from the force plate (range 10 V) to digital signals. The resolution of the shear force signals was 0.74 N/bit, and the resolution of the vertical force signal was 2.87 N/bit. Light sensitive triggers were used to initiate and terminate data collection as subjects completed each of the walking tasks. The stairs used in this study consisted of four steps. An AMTI force plate (Model OR6-6-1000; AMTI Corp., Newton, MA) mounted on an independent concrete block served as the second step from the lower landing. The rise of each step was 20 cm and the run was 28 cm. 2.3. Procedures All testing was conducted in the Musculoskeletal Biomechanics Research Laboratory at the University of Southern California. To control for the potential influence of footwear on COFU, all subjects were provided with a pair of walking shoes (Rockport World Tour, model M/W WT18; The Rockport Company, LLC, Ronks, PA) for use during testing. Subjects performed four tasks: (1) walking along a level 10 m walkway; (2) walking 5 m, turning 908 and continuing to walk another 5 m; (3) ascending the four-step staircase; and (4) descending the same four-step staircase. The order of tasks was randomized. All subjects were instructed to perform the tasks at a comfortable speed, and two trials were recorded. A trial was considered successful if the subject’s tested foot landed within the force plate.
pass Butterworth filter with a zero-lag compensation [13]. The non-directional resultant shear force was calculated as the vector addition of the anterior–posterior and medial– lateral shear forces. The COFU throughout stance for the more involved limb (CVA; ARTH) or left limb (DM, SENIOR, YOUNG) was calculated as the ratio of the resultant shear to vertical ground reaction forces. During initial loading, the peak COFU resulting from a shear force that would contribute to a forward foot slip was identified (Fig. 1). Data were screened for spuriously high peak COFU values occurring when shear forces were divided by small vertical forces [6]. Typically, non-spurious peak COFU values were observed once the reference limb had been loaded with greater than 65 N of vertical force. Fig. 1 provides representative tracings of the three components of the ground reaction force recorded as a SENIOR subject descended stairs. 2.5. Statistical analysis To determine if peak COFU varied across subject groups, a one-way analysis of variance (ANOVA) was performed. This analysis was repeated for each condition. Statistical analyses were performed using SPSS statistical software (version 10.0; SPSS Inc., Chicago, IL). A significance level of P < 0.05 was used for all statistical comparisons.
3. Results 3.1. Walking velocity and stance duration of turning, stair ascent and descent On average, the YOUNG group walked more rapidly and had a shorter stance duration during turning, stair ascent and stair descent compared to the other groups (Table 2). The CVA and DM groups walked the slowest, and exhibited a longer stance duration while turning and negotiating stairs compared to the remaining groups (Table 2).
2.4. Data analysis 3.2. COFU during level walking Force plate data were analyzed using the DataPac software program (v. 2K2, Run Technologies, Mission Viejo, CA). Digitally acquired anterior–posterior, medial–lateral, and vertical forces were imported from VICON Workstation (Oxford Metrics, Ltd., Oxford, England) into DataPac 2K2 where they were filtered using a fourth order, 45 Hz, low
When averaged across all subjects, the mean peak COFU during level walking was m = 0.23. Values ranged from m = 0.14 for a person in the ARTH group to m = 0.54 for an individual in the CVA group. Peak COFU was found to vary significantly across subject groups (P < 0.001; Fig. 2). Post
Table 2 Mean walking velocity and stance duration for each subject group during each task (mean (S.D.))
Level walking velocity (m/min) Turning stance duration (s) Stair ascent stance duration (s) Stair descent stance duration (s)
SENIOR
CVA
ARTH
DM
YOUNG
86.4 (11.1) 0.76 (0.09) 1.11 (0.15) 0.91 (0.19)
47.0 (13.5) 0.97 (0.15) 2.66 (0.52) 2.23 (0.95)
80.2 (9.9) 0.79 (0.07) 1.12 (0.41) 1.09 (0.43)
70.0 (10.0) 0.89 (0.08) 1.60 (0.91) 1.35 (0.72)
93.7 (12.4) 0.66 (0.12) 0.90 (0.17) 0.79 (0.17)
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Fig. 1. Representative tracings of (a) ground reaction forces and (b) utilized COF during stair descent for a SENIOR subject. Note that the initial spuriously high spike in the utilized COF was due to a relatively low vertical ground reaction force.
hoc analysis revealed that the YOUNG group had a significantly higher peak COFU than the DM group (m = 0.24 versus m = 0.19; P = 0.019). No other differences were identified. 3.3. COFU while negotiating a turn When averaged across all subjects, the mean peak COFU while negotiating turns was m = 0.27. The lowest value was m = 0.15 for a person in the CVA group, while the highest was m = 0.48 for an individual in the YOUNG group. Peak COFU was found to vary significantly across subject groups (P < 0.001; Fig. 3). Post hoc analysis revealed that the YOUNG (m = 0.36) had significantly higher peak COFU than the SENIOR (m = 0.26), CVA (m = 0.21), ARTH (m = 0.27) and DM (m = 0.25) groups (P 0.001). No other differences were identified.
3.4. COFU during stair ascent When averaged across all subjects, the mean peak COFU during stair ascent was m = 0.20. The lowest value was m = 0.08 for a person in the ARTH group, while the highest value was m = 0.46 for an individual in the YOUNG group. The peak COFU while ascending stairs was not found to vary significantly across groups (P = 0.305; Fig. 4). 3.5. COFU during stair descent When averaged across all subjects, the mean peak COFU during stair descent was m = 0.37. The lowest value recorded was m = 0.14 for a person in the YOUNG group, while the highest was m = 0.61 for an individual in the DM group. The peak COFU while descending stairs was not found to vary significantly across groups (P = 0.115; Fig. 5).
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Fig. 2. Comparison of peak utilized COF during level walking for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM). * YOUNG > DM (P < 0.02).
4. Discussion Contrary to our initial hypothesis, the results of this study found that older adults with a disability did not demonstrate greater COFU compared to older and younger adults without a disability. This finding was consistent for each condition evaluated and suggests that those with the specific medical conditions tested in this study are not at greater risk for slip initiation. The results of the current investigation contrast notably with the only previous study that evaluated COFU in persons with a disability. Buczek et al. [11] reported that persons with a disability demonstrated a twofold increase in peak COFU during level walking when compared to persons without a disability. Two important differences between the study by Buczek et al. [11] and the current investigation that likely contributed to the disparate COFU findings are the diagnoses and severity of physical impairment of the subjects assessed. The nine subjects participating in the study by Buczek et al. [11] had a variety of medical conditions including unilateral trans-tibial amputation, transfemoral amputation, unilateral hemiparesis secondary to stroke,
hip fracture, broken leg, and osteotomy of the fifth metatarsal. Two subjects required use of axillary crutches for ambulation, while one used a four-footed walker. In the current study, we selected individuals with relatively high ambulatory function as they were expected to be the persons most likely to negotiate a variety of conditions that might pose a slip risk. Only 4 of 48 subjects required a single point cane. The higher functional status of our subjects could have allowed them to more selectively decelerate the forward momentum of their limb prior to initial contact [14], contributing to lower shear forces during limb loading. Additionally, the reduced reliance of our subjects on assistive devices may have contributed to a more rapid rise in the vertical component of the ground reaction force during weight acceptance. Both these mechanisms could have contributed to the lower COFU values recorded for subjects with a disability in the current investigation compared to those studied by Buczek et al. [11]. Additional reasons for the differences between values recorded in our study and those reported by Buczek et al. [11] likely include differences in footwear, floor characteristics, as well as the limited number of persons with a
Fig. 3. Comparison of peak utilized COF while negotiating turns for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM). * YOUNG > SENIOR, CVA, ARTH, DM (P < 0.001).
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Fig. 4. Comparison of peak utilized COF during stair ascent for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM).
disability tested in the other study. Furthermore, Buczek et al. [11] sampled their ground reaction forces at 500 Hz, did not filter their data, and did not consider COFU values when the vertical ground reaction force was less than 50 N. In contrast, force data from the current study were sampled at 1200 Hz, filtered using a fourth order, 45 Hz, low pass Butterworth filter, and COFU data were visually screened for spurious values. Thus, the methods used by Buczek et al. [11] could have allowed greater shear forces to be divided by smaller vertical forces resulting in higher COFU than those reported in the current study. Buczek and Banks [6] previously suggested that these higher values may be true, but of such short duration that a slip does not occur. Further testing, relating subject-specific COFU values, available slip resistance, and slip outcome should provide insight into the validity of these values. In two of the four conditions tested, subjects in the YOUNG group demonstrated increased peak COFU compared to those with a disability. Friction requirements for the YOUNG group were significantly greater than all disability groups while negotiating a turn, and were higher than the DM group during level walking. The greater COFU observed in the YOUNG during these tasks can be explained by the
fact that these subjects walked more quickly than subjects in the disability groups. Burnfield and Powers [8] investigated the influence of walking velocity on the friction requirements of 60 healthy adults and reported that COFU increased with faster walking speeds. Given the relationship between walking speed and COFU [8], it is not entirely surprising that subjects in the YOUNG group had higher COFU during selected tasks in the current study. Stair descent was the task associated with the highest peak COFU for the SENIOR (m = 0.39), CVA (m = 0.34), ARTH (m = 0.41), and DM (m = 0.41) groups, with values for individual subjects in these groups ranging from m = 0.24 to 0.61. The high peak COFU while descending stairs suggests that there is a greater risk for slip initiation for older adults while performing this task compared to the other tasks studied. In contrast, negotiating a turn was the task associated with the highest peak COFU for the YOUNG group (m = 0.36; range m = 0.28–0.48), suggesting a greater relative risk of slipping for young adults while performing this task. Many state laws and building codes have established that a static COF of m 0.50 represents the minimum slip resistance threshold for safe floor surfaces. Furthermore, the
Fig. 5. Comparison of peak utilized COF during stair descent for healthy older (SENIOR) and younger (YOUNG) adults and persons with a cerebral vascular accident (CVA), lower extremity arthritis (ARTH), and diabetes mellitus (DM).
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Americans with Disabilities Act Accessibility Guidelines [15] contain advisory recommendations for static coefficient of friction slip resistance values of m 0.60 for accessible routes (e.g. walkways and elevators) and m 0.80 for ramps. While each group’s average peak COFU fell well below the m 0.50 threshold for the tasks investigated in the current study, wide inter-subject variability was observed. In particular, during stair descent, each group of subjects had at least one individual whose peak COFU exceeded the m 0.50 threshold. This finding would suggest that stair surfaces which meet the minimum m 0.50 threshold may pose a risk for slip onset for some adults.
5. Conclusions The findings of the current study suggest that persons with the select medical conditions are not at any greater risk for slip initiation than healthy older or younger adults during the tasks evaluated. The higher COFU values observed while descending stairs and negotiating a turn suggest an increased risk of slip onset compared to level walking and stair ascent. Recovery from a slip, however, is likely confounded by multiple factors including balance, muscular strength, power, and flexibility. Thus it is possible that once a slip is initiated, persons with a disability may have a more difficult time recovering from the event. Acknowledgements This work was supported, in part, by an award from the California Physical Therapy Fund (#01-08) as well as the American Society of Safety Engineers and National Institute for Occupational Safety and Health. The authors would like to gratefully acknowledge The Rockport Company for their donation of shoes used during subject testing. Additionally, we greatly appreciated the contributions of Richard Souza, MPT and John Popovich, DPT during the data collection and processing phases of this project.
References [1] Berg WP, Alessio HM, Mills EM, Tong C. Circumstances and consequences of falls in independent community-dwelling older adults. Age and Ageing 1997;26:261–8. [2] Layne LA, Landen DD. A descriptive analysis of nonfatal occupational injuries to older workers, using a national probability sample of hospital emergency departments. J Occupation Environ Med 1997;39:855–65. [3] Hoyert DL, Kochanek KD, Murphy SL. Deaths: Final Data for 1997. National Vital Statistics Report, vol. 47, 1999. p. 1–104. [4] Fife D, Barancik JI. Northeastern Ohio trauma study. III. Incidence of fractures. Ann Emergency Med 1985;14:244–8. [5] U.S. Department of Health and Human Services. Healthy People 2010: Understanding and Improving Health. 2nd ed. Washington, DC: U.S. Government Printing Office; 2000. [6] Buczek FL, Banks SA. High-resolution force plate analysis of utilized slip resistance in human walking. J Test Eval 1996;24:353–8. [7] Hanson JP, Redfern MS, Mazumdar M. Predicting slips and falls considering required and available friction. Ergonomics 1999;42:1619–33. [8] Burnfield JM, Powers CM. Influence of age and gender of utilized coefficient of friction during walking at different speeds. In: Marpet MI, Sapienza MA, editors. Metrology of pedestrian locomotion and slip resistance, ASTM STP 1424. West Conshohocken: ASTM International; 2003. p. 3–16. [9] Fendley AE, Medoff HP. Required coefficient of friction versus toppiece/outsole hardness and walking speed: significance of correlations. J For Sci 1996;41:763–9. [10] Skiba R. Sicherheitsgrenzwerte zur vermeidung des ausgleitens auf fussboden. Zeitschrift Fur Arbeitswissenschaft 1988;14:47–51. [11] Buczek FL, Cavanagh PR, Kulakowksi BT, Pradhan P. Slip resistance needs of the mobility disabled during level and grade walking. In: Everett GB, editor. Slips stumbles and falls pedestrian footwear and surfaces, STP 1103. Philadelphia, PA: American Society for Testing and Materials; 1990. p. 39–54. [12] Kramarow E, Lentzner H, Rooks R, Weeks J, Saydah S. Health, United States, 1999 With Health and Aging Chartbook. Hyattsville, MD: National Center for Health Statistics; 1999. [13] Christina KA, Cavanagh PR. Ground reaction forces and frictional demands during stair descent: effects of age and illumination. Gait and Posture 2002;15:153–8. [14] Perry J. Gait Analysis, Normal and Pathological Function. Thorofare, NJ: Charles B. Slack; 1992. [15] Architectural and Transportation Barriers Compliance Board. Americans with disabilities act (ADA) accessibility guidelines for buildings and facilities. Final Guidelines Federal Register, 56. 1991; p. 35408–542.