Our main finding was that, compared with participants in the comparison group, people with COPD showed marked deficits in postural control (i.e., composite equilibrium scores). More interesting, deficits in postural stability were accompanied by a higher rate of falls (i.e., zero scores) during the SOT trials in the group with COPD. The odds of having frequent falls during the SOT for people with COPD were approximately nine times as high as those for control participants. The sensory analysis did not reveal significant differences between groups (see Table 3). Although lower levels of strength and a modest tendency toward lower physical activity levels were observed in the group with COPD, neither of these two factors was associated with measures of postural control.
The deficits in postural control reported in this study are consistent with previous reports that used functional balance tests to confirm that postural control is impaired in people with COPD. Eisner and colleagues6
found lower scores in the Functional Reach Test (9%; p
<0.0017) when a group with COPD (n
=1,202) with a mean FEV1
of 62% (SD 23%) was compared with a control group (n
=302). Another study3
showed lower scores (p
<0.05) in the Community Balance and Mobility Scale in two groups of people with moderate (FEV1
=45.7% predicted, SD 3.7% predicted) and severe (FEV1
=29.9% predicted, SD 3.7% predicted) COPD compared with a healthy control group. The latter study,3
however, did not reveal significant differences among groups in postural sway measured through posturography. The fact that we found differences between groups could be attributed to the manipulation of sensory information during the SOT, which, compared with conventional posturography, increases the difficulty of the balance task. However, our preliminary analysis showed significant differences (p
=0.008) in the median equilibrium scores for condition 1 between the two groups, suggesting that alterations in postural control in people with COPD are present even in unchallenged situations of relative postural stability. The use of different balance testing protocols may well explain these conflicting findings.
Perhaps the most striking finding of this study was the high rate of zero scores, registered as falls, in the group with COPD during the SOT. In a recent prospective cohort study,28
we found that 101 people with COPD had an annual incidence rate of 1.2 falls per person-year, substantially higher than the rate of 0.24 reported in elderly people.29
The higher rate of falls in this study is thus consistent with these findings and supports recent studies underlining a potential association between postural instability and fall risk in people with COPD.4,7
Despite these results, it is important to reiterate that a fall during the SOT represents a zero score in a specific trial, not an actual fall. In addition, 95% of falls in people with COPD in this study occurred during the most challenging SOT conditions (5 and 6), in which sensory information was dramatically altered. These challenging situations are different from likely conditions during activities of daily living, and, therefore, extrapolation of our findings to real-life situations should be done with extreme caution. Nevertheless, these results reinforce the potential association between fall history and deficits in balance described previously in COPD.7
More longitudinal studies are required to confirm the direct implications of postural control deficits for fall incidence in people with COPD.
Another finding of this study concerns the analysis of sensory ratios. The novelty of this analysis consisted of exploring whether deficits in postural control in people with COPD were catalyzed by specific alterations in the somatosensory, visual, or vestibular domains of postural stability. Sensory ratios did not reveal significant differences between groups, although differences in postural balance between groups seemed to originate mainly from deficits specific to the vestibular control of balance (i.e., conditions 5 and 6) in the COPD group (see Figure 3). It has been postulated that chronic hypoxemia may alter audio-vestibular function; however, the existing literature pertaining to vestibular impairments in COPD is equivocal. For example, El-Kady and colleagues30
investigated audio-vestibular function in people with COPD having hypoxemia (Po2
<75 mm Hg) compared with a control group; although they observed poorer general audio-vestibular function in the group with COPD, the differences were not significant. In a previous study, Nakano and colleagues31
used brain stem auditory evoked potentials to investigate the influence of oxygen deficits on audio-vestibular function in people with chronic hypoxemia (Po2
=58.2 mm Hg); their results indicated that chronic hypoxemia does not alter audio-vestibular function. Hence, more studies are required to investigate potential vestibular deficits and their contribution to postural control and fall risk in people with COPD.
Previous reports have indicated that postural deficits in older adults are associated with muscle weakness, especially during the most demanding balance tasks.11
Thus, a possible explanation for the preferential deficits during SOT conditions 5 and 6 in people with COPD may be a result of the confounding effect of muscle weakness (see Box 1). However, our correlational analysis showed no significant association between knee extensors muscle strength and the vestibular component of balance in the COPD group. More important, when knee extensors strength was correlated with the results of each individual SOT condition for the COPD group, we did not observe any association with conditions 5 or 6 to support a potential confounding effect of muscle weakness in explaining deficits in postural control during such conditions. These findings conflict with those of a previous study19
that used the SOT to investigate the contribution of muscle strength to deficits in postural stability in people with stroke (n
=40) and a control group (n
=40). The researchers found that the strength of some muscle groups (i.e., paretic knee extensors) in the stroke group was correlated with postural sway only in the most challenging SOT conditions (i.e., conditions 5 and 6), supporting the notion that strength plays an important role in situations of maximal instability,11
particularly when the vestibular system is targeted during the SOT. Because the strength values of the stroke group in that study are similar to the values obtained from people with COPD in our study, it is unlikely that the level of muscle strength in the group with COPD was too high for our analysis to detect any significant association with measures of postural control. Disease-specific mechanisms (e.g., spasticity) may partly explain the different associations between muscle strength and postural control observed in the two studies.
On the basis of the existing literature, we expected somatosensory deficits to play a role in explaining the impaired postural control observed in the COPD group. Indeed, proprioception of the lower limbs appears as a primary somatosensory measure associated with postural sway assessed on a firm surface in older adults.11
The fact that postural sway was significantly greater in people with COPD during condition 1 (p
=0.008), therefore, is suggestive of proprioceptive deficits in this group. Previous studies have shown nerve conduction abnormalities32,33
and signs of peripheral neuropathy (i.e., smaller amplitude potentials, increased latency, decreased conduction velocity), especially in the sensory nerves, in people with COPD.34
Peripheral neuropathy, which can be present even in people with moderate COPD,33
may have led to somatosensory deficits,35
alterations in postural balance, and increased fall risk36
in our COPD group. However, none of the participants with COPD had been formally diagnosed with peripheral neuropathy. More important, the sensory analysis did not reveal specific alterations in the somatosensory control of balance (see Table ). In spite of these findings, however, the potential implications of proprioceptive deficits in the regulation of postural control in people with COPD should not be overlooked. The strong association of strength with condition 4 in the healthy control group (r
=0.001), in addition to the lack of correlation in the group with COPD, suggests that differences may exist in the integration of sensory information and in the motor response to postural instability between healthy people and individuals with COPD.
This study has several limitations that need to be considered when results are interpreted. First, although the relatively small convenience sample used in the study was sufficient to detect between-group differences in postural control (i.e., composite equilibrium scores) and the proportion of fallers and frequent fallers during the SOT, a larger sample size would be required to establish more reliable correlations between measures of strength and different aspects of postural control. Second, the fact that people with COPD were moderately older than participants in the control group may have contributed to the deficits in postural control observed in this group. Third, on the basis of previous literature,11
we targeted knee extensors as a muscle group potentially associated with postural sway. However, the assessment of ankle muscles (e.g., gastrocnemius and tibialis) might perhaps be more appropriate to detect correlations between muscle weakness and AP postural sway.37
are important factors contributing to altered postural control and fall risk in older adults; although our exclusion criteria reduced potential confounding factors from other comorbidities that could affect postural control, we cannot rule out the possibility that medications affected postural control in the group with COPD.