In the current study, we examined the influence of floor stiffness (5 conditions) on indices of balance and balance control responses in elderly community-dwelling women following a backwards floor translation perturbation. Our results suggest that appropriately designed novel compliant flooring systems may cause minimal effects on the balance control characteristics of this population. Our first hypothesis was that the MMOS and displacement rates of the COM and COP would not be different for perturbations on the novel compliant floors relative to the control condition, which our results supported in 29 of 36 possible cases (2 floors × 18 dependent variables = 36 possible cases). Regarding our second hypothesis, we observed that excessive reductions in floor stiffness (as created in the two foam conditions) caused substantial negative effects on balance control and balance recovery ability characterized by decreased MMOS in three of four cases (2 floors × 2 dependent variables), and slowed displacement rates in 17 of 32 cases (2 floors × 16 dependent variables). These results indicate that while reducing floor stiffness has the potential to impair balance control responses following a perturbation, the novel compliant floors tested in this study (SmartCell in particular) do not appear to do so.
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 () were always lower for the COP relative to COM (average MMOSCOP
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. COP0–20%
or COP0–2 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
) 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.
Effective intervention strategies are urgently needed to curtail the anticipated rise in incidence of fall-related injuries over the coming decades. Novel compliant flooring systems appear to be a promising approach, capable of providing substantial force attenuative properties with minimal coincident impairments in outcomes from balance and mobility tests (Laing and Robinovitch, 2009
). Our findings provide further support for this intervention approach by suggesting that biomechanical indices of balance and balance control responses are generally not impaired by novel compliant floors such as SmartCell or SofTile following a floor translation perturbation. These results support the introduction of pilot installations to inform the development of clinical trials that test the effectiveness of novel compliant floors at reducing fall-related injuries in older adults.