Characterization of the protective capacity of flooring systems using force-deflection profiling

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Abstract

‘Safety floors’ aim to decrease the risk of fall-related injuries by absorbing impact energy during falls. Ironically, excessive floor deflection during walking or standing may increase fall risk. In this study we used a materials testing system to characterize the ability of a range of floors to absorb energy during simulated head and hip impacts while resisting deflection during simulated single-leg stance. We found that energy absorption for all safety floors (mean (SD) = 14.8 (4.9) J) and bedside mats (25.1 (9.3) J) was 3.2- to 5.4-fold greater than the control condition (commercial carpet). While footfall deflections were not significantly different between safety floors (1.8 (0.7) mm) and the control carpet (3.7 (0.6) mm), they were significantly higher for two bedside mats. Finally, all of the safety floors, and two bedside mats, displayed 3–10 times the energy-absorption-to-deflection ratios observed for the baseline carpet. Overall, these results suggest that the safety floors we tested effectively addressed two competing demands required to reduce fall-related injury risk; namely the ability to absorb substantial impact energy without increasing footfall deflections. This study contributes to the literature suggesting that safety floors are a promising intervention for reducing fall-related injury risk in older adults.

Introduction

Fall-related injuries represent a major health concern for the elderly population. Nearly 54,000 Canadians over the age of 65 are hospitalized for a fall every year [1]. The estimated economic burden of fall-related injuries in Canada is approximately $2 billion in annual treatment costs [2] and is expected to rise to about $4.4 billion by 2031 [1], when a projected 24% of Canadians will be 65 years of age or older [3]. Thirty-eight percent of fall-related hospitalizations in the elderly are caused by hip fracture [1]. Over 25% of hip fracture patients die within a year of the injury, and more than 50% suffer a major decline in mobility and functional independence [4], [5]. Fall-related head injuries are an equally alarming public health issue. Sadigh et al. [6] report that up to 11% of falls sustained by nursing home residents resulted in head or neck injury. Traumatic brain injury accounts for 50% of all unintentional fall-related deaths in the elderly [7]. The incidence of fall-related injuries is likely to grow as the elderly population increases, emphasizing the need for development and implementation of practical interventions not only to reduce the risk of falling in the elderly, but also to minimize the severity of injury when fall-prevention fails.

Safety floors (also referred to in the literature as ‘low stiffness’, or ‘compliant’ floors) are a promising approach for reducing the incidence and severity of fall-related injuries in older adults. As individuals in nursing-care facilities, hospitals and rehabilitation settings are at a significantly higher risk of falling compared to home-dwelling seniors [8], [9], flooring systems with the potential to decrease injury risk would be particularly relevant in these environments. Evidence of the relative protective capacity across flooring materials can be derived from studies that use mechanical test systems. For traditional floors, estimates of force attenuation values during sideways falls on the hip range between 7% for wooden floors, 15% for carpets, and 24% for carpets with common underpadding when compared to rigid flooring [10], [11], [12]. Differences in carpet unlayments alone have been shown to significantly influence peak loads during simulated falls on the hip [13]. ‘Safety floors’ are reported to provide substantially greater force attenuation, with values ranging from 15.2% [14] to 50% [15] during simulated falls on the hip. These benefits translate to risk for head injury with attenuation of peak headform accelerations of up to 58% for some models of safety floors [16]. Benefits have also been observed in studies with human volunteers with a 4.5 cm thick layer of foam rubber reported to reduce peak force applied to the proximal femur by 15% and buttocks by 24% during sideways and backwards falls, respectively [17], [18]. Benefits in terms of applied pressure may be even more pronounced than force reduction. For example, when compared to an unpadded impact, a 1.6 cm layer of foam incorporated into a wearable hip protector decreased peak pressure applied over the greater trochanter by 73% compared to a more modest peak force reduction of 19% [19]. As injury risk is usually directly related to applied loads, the force attenuation properties of safety flooring systems are suggestive of their potential as an intervention to reduce the risk of fall-related injuries.

However, a potential limitation to the clinical effectiveness of safety floors is that they could paradoxically increase fall incidence rates by impairing balance maintenance and recovery abilities. Control of balance is dependent on the central nervous system continually integrating visual, vestibular, and somatic sensory information about the surrounding environment, orientation of body parts, and forces acting on the body [20]. Soft foam surfaces detrimentally affect somatic sensory inputs from proprioceptive muscle spindles and cutaneous receptors in the foot and ankle, leading to a delayed or misguided balance control response [21], [22]. Thus, a crucial design element for effective safety floors is the ability to provide substantial force attenuation without allowing deflections that may impair balance control during gait and stance activities. Previous studies involving quiet stance and backwards perturbations in elderly individuals revealed clinically insignificant differences in postural sway between some safety floors and traditional floors [15], [23]. In order to examine a more dynamic task scenario, the flat-foot stage of single-leg stance during a natural cadence gait-cycle was simulated in this study to contribute to existing literature on locomotor adaptations, footfall deflections and toe-floor clearance distances while walking on compliant surfaces [22], [24], [25]. In addition, this approach corresponds to recommendations that “future research into the role of floor compliance in balance in older adults should include studies of walking” [33].

The newest generation of safety floors are designed with the competing demands of force attenuation and fall risk in mind [14], [15], [22], [23]. Several floors of this category use buckling column designs to absorb energy under high energy loading scenarios. Increased floor displacement during the impact phase of a fall should theoretically increase the system's energy absorption capacity [26] with concomitant decreases in the average force applied to musculoskeletal tissues. However, it is unknown whether force-deflection properties differ across a range of traditional and novel flooring systems, and whether these properties are influenced by the surface geometry of the impacting body part.

Accordingly, the focus of this study was to characterize a range of commercially available floors based on their ability to absorb energy during simulated head and hip impacts while resisting displacement when subjected to ground reaction forces representing single-leg stance. Our goals were to test the hypotheses that: (1a) flooring type and (1b) indenter shape (i.e. hip vs head) would significantly influence energy absorption during simulated head and hip impacts; (2) peak deflection during simulated single-leg stance would differ significantly across floors; and (3) safety floors would display a larger energy absorption-to-foot deflection ratio compared to traditional carpeting and bedside mats.

Section snippets

Floor conditions

We investigated three general categories of floors (12 floors in total), each with different subtypes, to provide a wide range of flooring conditions (details in Fig. 1, Table 1). The ‘carpet’ category included four models. The control condition was a ‘commercial’ grade carpet (0.25 in. thick) used typically in stores and office settings. The ‘residential’ carpet was a loop design (0.25 in. thick) targeted for homes and retirement communities. The ‘Berber’ carpet (0.40 in. thick) was a looped pile

Head and hip impact simulation

The force-deflection profiles for each floor-indenter combination are displayed in Fig. 3. ANOVA results demonstrated a significant main effect of indenter type on energy absorption across all floors with ehip (mean (SD) = 15.7 (10.9) J), being 9.6% higher than ehead (14.2 (9.6) J) (F11,24 = 106.8, p < 0.001). Furthermore, a significant interaction effect existed whereby energy absorption was higher with the head indenter compared to hip indenter for the safety floors, but lower for the bedside mats (F

Discussion

In the current study, we used a materials testing system to characterize the ability of a range of floors to absorb energy during simulated head and hip impacts while resisting deflection during simulated single-leg stance. Our first hypothesis was supported in that energy absorption for all safety floors (mean (SD) = 14.8 (4.9) J) and bedside mats (25.1 (9.3) J) was 3.2- and 5.4-fold greater than the control condition (commercial carpet). Furthermore, indenter shape also had a significant

Conflict of interest statement

None of the authors have any conflicts of interest related to the materials or details presented in this document. No persons other than the authors had input into any aspect of the study and subsequent reporting of data.

Acknowledgements

This research was funded in part by an operating grant obtained from the Natural Science and Engineering Research Council of Canada (NSERC; grant # 386533-2010). MG was supported by the RBC – Your Future by Design Retirement Research Undergraduate Fellowship; JC was supported in part by the Canada Research Chair Program.

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