We used a floor translation paradigm to induce a balance perturbation to thirteen elderly community-dwelling women. Outcome measures included the displacement rates and margins of safety for both the underfoot centre-of-pressure and whole-body centre-of-mass across two novel compliant floors (Smart-Cell, SofTile), two basic foam surfaces (Firm-Foam, Soft-Foam) and a standard ‘Rigid’ floor as a control condition.

The centre-of-mass and centre-of-pressure margins of safety, and all centre-of-mass displacement rates, were not significantly lower for the two novel compliant flooring systems compared to the control floor. The centre-of-pressure displacement rates were similar to the control floor for the SmartCell floor condition. The majority of the margin of safety and displacement rate variables for the foam floors were significantly lower than the control condition.

A promising intervention approach that is particularly relevant for high-risk settings is to decrease the stiffness of the floor to attenuate impact forces in the event of a fall (Casalena et al., 1998; Maki and Fernie, 1990; Laing et al., 2006; Laing and Robinovitch, 2009; Laforest et al., 2000; Nevitt and Cummings, 1993; Simpson et al., 2004; Healey, 1994). While the clinical effectiveness of intervention strategies such as exercise, pharmacological agents, and wearable hip protectors depend on active user compliance and adherence, novel compliant floors are a passive intervention approach that may protect against multiple types of fall-related injuries. However, evidence exists that some low stiffness surfaces could impair balance maintenance and balance recovery abilities, thereby increasing the risk of falls. Simple compliant foam surfaces have been postulated to impair balance by decreasing the quality of sensory inputs provided by the proprioceptive and pressure receptors in the ankle and foot (Betker et al., 2005, Lord and Menz, 2000; Ring et al., 1989), and by altering gait mechanics including decreased trunk stability and reduced toe clearance (Marigold and Patla, 2005; Maclellan and Patla, 2006). Thus, the challenge for this intervention approach is the design of floors that substantially attenuate impact force while minimizing potential impairments in balance and mobility.

Novel compliant flooring systems are generally designed to provide a dual-stiffness response characterized by minimal deflection during locomotion, and a transition to increased compliance at the higher loads associated with fall-related impacts. Such products have been shown to attenuate the peak force applied to the proximal femur by 25–50% during simulated sideways falls, in addition to having minimal effects on successful completion of static (quiet stance) and dynamic balance and mobility tasks (Timed Up and Go test, backwards floor translation) in older community-dwelling women (Laing and Robinovitch, 2009). However, no information currently exists on the influence of novel compliant floors on biomechanical variables associated with balance control responses following externally induced perturbations (i.e. the biomechanical variables to control balance recovery), despite the fact that evaluating balance recovery abilities is a critical component of a comprehensive balance assessment (Chiu et al., 2003).

From a biomechanical perspective, to maintain an upright posture during quiet stance the vertical projection of the whole-body centre-of-mass (COM) must fall within the limits of the base of support (BOS) (Winter, 2009). To accomplish this, models related to ankle stiffness (Winter et al., 1998; Winter et al., 2001) and reactive muscle strategies (Morasso and Schieppati, 1999; Morasso and Sanguineti, 2002) predict that adjustments in the location of the underfoot centre-of-pressure (COP) are used to guide or ‘shepherd’ the trajectory of the COM towards an equilibrium position. In the event of a balance perturbation that causes the COM to shift anteriorly (such as being nudged from behind), recovering balance without changing the BOS via stepping responses requires the COP to be rapidly shifted anteriorly towards the toes to decelerate the COM. If the COM trajectory cannot be altered quickly enough by the anterior COP displacement and consequently reaches the BOS boundaries, the individual must increase the BOS by taking a step in order to prevent a forward fall (Winter, 2009). Thus, the proximity of the COM to the BOS boundaries after perturbation (margin of safety (MOS)) provides an indication of the degree of instability following the perturbation. For elderly persons, a large MOS may be of increased importance in recovering balance due to age-related declines in strength, reaction time, and other sensori-motor capabilities. Underfoot compliance (as might be associated with a low stiffness floor) has the potential to impair balance recovery responses by decreasing effective ankle stiffness and the rate of COP displacement in response to changes in ankle torque. Consequently, changes in support surface stiffness may influence COP dynamics, which in turn could influence COM trajectories (Marigold and Patla, 2005; Maclellan and Patla, 2006). However, the interplay of COM and COP movements has never been examined in response to balance perturbations (i.e. during balance recovery responses) on novel compliant flooring systems.

Our results are in accordance with a series of proposed balance control mechanisms used to maintain feet-in-place balance (Winter et al., 1998; Winter et al., 2001; Morasso and Schieppati, 1999; Morasso and Sanguineti, 2002). In all floor conditions, the COM moved anteriorly within the BOS immediately following the perturbation. In response, we observed a rapid shift of the COP towards the toes to decelerate the COM and shift it back towards its initial position. As a consequence, the MMOS (Table 2) were always lower for the COP relative to COM (average MMOS_COP to MMOS_COM ratios were 0.45, 0.48, 0.41, 0.63, and 0.43 for the Rigid, Smart-Cell, SofTile, Firm-Foam, and Soft-Foam conditions, respectively). In addition to these amplitude differences, we observed displacement rates to be consistently larger for COP compared to COM, with ratios for the largest COP to largest COM rates averaging 2.12, 1.96, 2.07, 2.30, and 2.22 for the Rigid, SmartCell, SofTile, Firm-Foam, and Soft-Foam conditions, respectively. Although these general trends existed in all trials, the differential decreases in MMOS and displacement rates observed across floors (predominantly in the foam conditions) indicates that surface compliance has the potential to impair elements of balance and balance control responses.

During the initial phase of COP movement (i.e. COP_0–20% or COP_{0–2 cm}), we unexpectedly found that the displacement rates were often greater for the compliant floors than those on the control floor. One potential explanation for this observation may relate to baseline pre-perturbation ankle stiffness. It is possible that, in response to the lower-stiffness surfaces, participants increased co-contraction of the muscles spanning the ankle to increase the effective stiffness of the ankle joint and the resulting angular displacement in response to applied loads. If this initial co-contraction was strong enough it may have also contributed to faster COP displacements by increasing the gain of the stretch reflex in the plantar flexors immediately post-perturbation (Nielsen et al., 1994). However, the decision was made a priori to avoid controlling baseline ankle stiffness to allow for more realistic balance control responses across floors, rendering this explanation mainly speculative. Nevertheless, these potential benefits are likely outweighed by the decreased MMOS and displacement rates typically observed later in the response for the foam, and to a lesser extent, the SofTile conditions.

These results add to existing literature related to the effects of compliant flooring on balance and postural stability. Postural sway is commonly observed to increase in both the anterior–posterior and medial–lateral directions during quiet stance on compliant foam surfaces compared to rigid surfaces (Lord et al., 1991; Lord and Menz, 2000; Gill et al., 2001). In addition, walking on foam compared to rigid floors has been associated with lowered COM trajectory and toe clearance (Marigold and Patla, 2005), and increases in step length, step width, and step width variability (Maclellan and Patla, 2006). There is less clarity regarding the influence of common flooring products such as carpet. Redfern et al. (1997) report that thick carpet can lead to increases in anterior–posterior sway for older adults when exposed to a moving visual environment, while Dickinson et al. (2001, 2002) found no effect of carpet on sway in older adults. Encouragingly, and with particular respect to novel compliant flooring systems, Laing and Robinovitch (2009) report that the amplitude and velocity of quiet stance sway in the medial–lateral direction were not different between a rigid surface and the SmartCell floor examined in the current study. Furthermore, the times required to complete the Timed Up and Go test (a predictor of fall risk (Podsiadlo and Richardson, 1991; Lundin-Olsson et al., 1998; Chiu et al., 2003) were not different for SmartCell, SofTile and a rigid floor condition. Finally, when exposed to a floor translation task, participants successfully regained balance equally well on the SmartCell, SofTile and rigid floors. Our results extend from their findings, and demonstrate that the displacement profiles of the COM (a balance indicator) are not affected by the novel compliant flooring systems, and that the balance control variable is minimally affected by these surface conditions.

It is important to consider the current results in concert with reports of the force attenuative properties of various flooring materials. For a sideways fall with an impact velocity of 4 m/s, SmartCell and SofTile have been reported to reduce the peak force applied to the proximal femur (compared to a rigid floor) by 33.7% and 51.2% respectively (Laing and Robinovitch, 2009). In contrast, force attenuation values for common compliant floors average 7% for wooden floors, 15% for carpets without underpadding, and 24% for carpets with underpadding (Gardner et al., 1998; Simpson et al., 2004; Maki and Fernie, 1990). That the novel compliant floors tested in this study can more than double the force attenuative capacity of traditional products, without concomitant impairments in balance and stability, supports their value as a promising intervention strategy.

A host of additional factors must be considered when assessing the clinical feasibility of novel compliant floors. The relatively low profile of the SmartCell flooring (25 mm thick) makes it suitable for installation in newly built facilities or via retrofitting. Retrofitting issues that have been successfully addressed include installation of ramps or transition markers between traditional and compliant flooring zones, ensuring sufficient clearance for doors, and maintaining standard heights for infrastructure including toilets and sinks. In contrast, the SofTile system tested (100 mm thick) is likely more appropriate for outdoor applications including patios, gardens, and walkways. However, lower profile models (50 mm thick) do exist that may be adaptable for certain indoor applications. In addition, overlays such as carpet or vinyl may be required for certain settings, which may affect the force attenuation and balance results reported in the literature. Finally, an important consideration is that flooring systems should minimally influence the work demands of facility staff (e.g. rolling wheelchairs or lift-assists). Additional research is warranted to assess the potential influence of novel compliant flooring systems on these issues.

There were several limitations associated with this study. First, we restricted our perturbation to the anterior–posterior direction, and our participants to feet-in-place responses based on recent video footage that demonstrates that 32% of falls occurred as a result of inappropriate transfer skills during primarily feet-in-place activities (e.g. rising from a chair, putting on a jacket while standing) (Robinovitch et al., 2009). However, as change-in-support stepping reactions are also prevalent control responses (Maki and Mcilroy, 2005), additional studies are required to assess the influence of compliant flooring during such balance control strategies in both AP and ML directions. Second, this study population was drawn from community-dwelling elderly women as they are at greater risk of hip fracture compared to both younger adults and age-matched males (Cumming et al., 1997; Jacobsen et al., 1990; Cummings and Melton, 2002). However, as compliant flooring may be most suitable for installation in high-risk environments such as retirement homes, future studies should investigate the potential effects of these floors on the balance control characteristics of the residents in these settings. Third, as this study sought to isolate the influence of flooring type (and not footwear) on balance recovery ability, our participants were barefoot throughout the experimental protocol. Accordingly, our findings may not directly extend to common situations where footwear is worn in which case foot-floor contact areas would likely be higher, and consequently, local floor deformations lower. However, in addition to isolating the influence of flooring, the current results provide insights into how such flooring systems might influence balance during worst-case scenarios where seniors are not wearing shoes during activities of daily living e.g. transferring from the bedroom to the bathroom in the middle of the night. Fourth, although the current study investigated classic variables used to assess stability during feet-in-place responses (including COM and COP) (Winter et al., 1998; Winter et al., 2001; Morasso and Schieppati, 1999; Morasso and Sanguineti, 2002), the use of electromyography in future studies could provide further insights into potential differences in the timing and amplitude of control responses across flooring conditions. Finally, not all commercially available compliant flooring systems were tested in the current study. As the design principles and materials employed across products differ substantially, so too may their influences on force attenuation, balance, and mobility. Consequently, additional tests with a wider range of products are required to provide consumers with an evidence-base to guide purchase and implementation decisions.