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Biomechanics of human reactions to slip events.

Cham-R; Redfern-MS
NIOSH 2005 Oct; :1-44
Slip and fall accidents have been recognized to be of major importance in occupational health. Epidemiological studies have reported this problem being even more serious in older workers. In light of the aging dynamics of the labor force, the long term goal of this research project is directed at reducing slip-precipitated falls among older adults. The specific aims were (1) to investigate the differences in the biomechanics of slipping between young and older adults and (2) to examine the effect of slippery surfaces warnings on slip-related postural responses. Forty subjects (21 F, 19 M) divided by gender and into two age groups were recruited in this study: "young" between the ages of 20 and 35 years and "older" aged 55 to 70 years old. Each subject was asked to walk on dry and glycerol-contaminated vinyl tile floors, while varying the extent of the a-priori knowledge of the floor's contaminant condition. Specifically, subjects were exposed to the following experimental conditions: (1) baseline "dry floors (subjects knew the floor was dry); (2) unexpected slippery floor (contaminant was applied to floor without subjects' knowledge); (3) alert dry or slippery (subjects were uncertain of the floor's condition), (4) known slippery (subject is aware of the slippery condition). Whole body data were collected at 120 Hz and synchronized with ground reaction forces and electromyographic data sampled at 1080 Hz. A 3D whole body model was developed, validated and used to derive various kinetic and kinematic gait parameters, which were entered as dependent variables in ANOVAs investigating age and warning safety conditions. Also, differences in these biomechanical variables were compared between hazardous and non-hazardous slips. This slip classification was based on a 1.0 m/s PSV threshold. The analysis was divided into 2 parts. The first part related to Aim 1 and considered responses to an unexpected slip (reactive strategies). To date, the results based on the electromyographic (EMG) recordings made from the major muscles of the stance leg (leading/slipping leg) indicated that slip-initiated postural reactive strategies were similar in patterns between young and older adults. However, when experiencing a severe slip, young adults demonstrated a longer, more powerful response compared to older adults. Specifically; the patterns of the reactive strategies consisted of the activation' of the Medial Hamstring and the Vastus Lateralis approximately 175 and 240 ms after HS onto the slippery floor, respectively. Corrective responses were scaled to slip severity with more severe slips generating longer, higher magnitude responses. Delayed Vastus Lateralis latency and Medial Hamstring cessation were associated with an increased slip severity as quantified by PSV. The second part relates to Aim 2, i.e. the impact of slippery floors anticipation on gait biomechanics. The results based on the EMG recordings made from the major muscles of the stance leg, indicated that anticipation of a slippery surface resulted in a general increase in muscle activation magnitude (48% increase) and in ankle/knee co-contraction (30% increase), as well as earlier onsets and longer durations of posterior muscles. Young adults demonstrated earlier onsets and longer muscle activity duration than older adults reducing their slip potential. Finally, we also have results linking slip severity to baseline gait characteristics. This information indicated that adults who normally walk with decreased CAD, greater SLRs, increased FFAs and faster FFASs are at a higher risk of experiencing a hazardous slip. A logistic regression model relating SLR and cadence to slip severity predicted (R2 = 0.45, X2 = 15.30, P < 0.001) that increased SLR and decreased cadence would result in increased probability of hazardous slip. A second logistic regression model relating FFA with slip severity predicted (chi-Square = 16.55; P = 0.005) that increased FFA would result in increased probability of hazardous slip. Additionally, adults with normal gait characterized by greater ankle co-contraction and delayed Tibialis Anterior onset near HS were predisposed to experience less severe slips when encountering an unexpected slippery floor. These findings indicate that older adults' natural gait predisposes them to experience a less hazardous slip. However, both young and older adults experienced hazardous slips at the same rate. These results indicate that baseline gait characteristics, i.e. initial conditions, are not the only contributors to slip severity in older adults. Furthermore, when initial conditions results are combined with the findings of Aim 1, it appears that post-slip reactions in older adults are the main contributors to slip severity. The data analysis of the biomechanical data (e.g. joint moments) is not completed yet. We expect to have more results within the next few months.
Biomechanics; Floors; Accident-prevention; Surface-properties; Injury-prevention; Humans; Age-factors
Human Movement and Balance Laboratory, Department of Bioengineering, University of Pittsburgh, 740 Benedum Hall, 3700 O'Hara Street, Pittsburgh, PA 15261
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Final Grant Report
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National Institute for Occupational Safety and Health
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University of Pittsburgh, Pittsburgh, Pennsylvania