one normally needs to maintain postural control while 
 performing one or more other concurrent tasks such as walking while talking . an individual s 
 attention resources and information processing capacity are presumably limited , and must be 
 shared among all the tasks concurrently being performed1 . therefore , when two tasks are being performed simultaneously , 
 performance of one or both can be impaired , if together they require attention that exceeds 
 an individual s capacity2 . postural control , control of the body s position , orientation and balance , is often 
 regarded as an automatic or reflex - controlled task with minimal attentional demands . performance of the postural 
 control system declines with age due to deterioration of the visual , proprioceptive and 
 vestibular systems , slower nerve conduction velocity and reduced musculature4 . studies have shown that the attentional 
 demands of balance control increase with age in compensation , even as general attention 
 resources decrease5 , 6 . consequently , older adults are more likely to have insufficient 
 attention resources for effective postural control in dual - task situations , which would 
 affect their dual - task performance . reported that older adults have 
 significantly higher values of sway area and center of pressure velocity in older adults 
 during dual - tasks when compared with young adults7 . this suggests why the risk of balance loss , and hence falling , 
 increases among older adults8 . older 
 adults dual - task performance may also be influenced by other factors , such as task 
 difficulty , postural threat , stimulus modalities or response modes . in tasks with increasing 
 difficulty , such as walking backward , an increased cognitive demand occurs and the 
 concurrent execution of a cognitive task may be compromised9 . falls are a common problem for the aged . among community - dwelling adults aged 65 or above impact on the hip is likely in a backward or sideways 
 fall11 , and direct impact on the hip in 
 a fall from standing height can easily be forceful enough to cause a hip fracture in older 
 adults12 . besides physical impact , 
 other possible consequences include psychological problems , a reduction in subsequent 
 activity or even social isolation due to fear of falling13 . many falls occur during transitional 
 movements which require an older person to step backward . backward walking is associated 
 with reduced velocity and stride length , and increased gait variability , and these 
 parameters are associated with a greater fall risk in older adults14 . examples include pulling open a door , backing to sit on a 
 chair , or responding to a forward perturbation . these are common situations that older 
 adults encounter in daily life , but the effects of aging on postural control when stepping 
 backward have not been well - studied . therefore , this study was designed to examine ( i ) 
 whether there are differences between young and older adults in cognitive performance and 
 postural control after stepping back , and ( ii ) to what extent dual - tasking affects cognitive 
 performance and postural control . the hypothesis was that older adults would have poorer 
 cognitive performance and postural stability after stepping back , especially while 
 dual - tasking . a total of fifty - eight subjects , including thirty young adults and twenty - eight older 
 adults , participated in this cross - sectional study . the older subjects , community - dwelling adults aged 65 
 or above , were recruited from elderly centers in hong kong . the subjects were all 
 independent in their daily living activities , able to walk unaided , and native cantonese 
 speakers . candidates were excluded if they had a neurological disorder , myocardial 
 infarction or heart failure , uncontrolled diabetes or hypertension , deformity of the lower 
 limbs , a history of musculoskeletal injury to either lower limb in the previous 12 months , 
 or cognitive or hearing impairment . the validated cantonese version of the mini - mental state 
 examination was used to screen the older subjects for cognitive impairment15 . candidates with scores less than 24 were 
 regarded as impaired and excluded from the study16 . subjects who reported dizziness on the day of the assessment or 
 failed to follow the verbal commands ethics approval was obtained from 
 the ethics committee of the hong kong polytechnic university . written informed consent was 
 obtained from all of the participants prior to their participation . each participant performed three tasks : an auditory stroop test and a stepping - backward 
 task with and without a concurrent auditory stroop test . the classic stroop test requires subjects to name the color in 
 which a word is printed where the word itself is the name of a different color . subjects 
 react more quickly and accurately to congruent stimuli ( when the word corresponds to the 
 color of the printing ) than to incongruent stimuli ( where the word and the print color 
 differ ) . cantonese is a tonal language where meaning depends both on the phoneme and the tone 
 in which it is pronounced . two cantonese words meaning  high and  low were used in this 
 test , and they were pronounced at either a high or low pitch , generating four combinations 
 of pitch and meaning . the subjects were required to respond to the voice as quickly as 
 possible by pressing the right thumb switch when a high pitch voice was heard or pressing 
 the left one for low pitch , disregarding the meaning of the word as pronounced . for the 
 test , each participant had four familiarization 
 trials ( one for each combination ) before sixteen test trials ( four trials per combination in 
 a random order ) . the reaction time ( the time between the sound and the moment when the 
 switch was pressed ) and error rate ( percentage of wrong responses ) were recorded . the stepping - backward task started with the subject standing barefoot on a force plate 
 ( model or6 - 51000 , advanced mechanical technologies inc . , newton , ma , usa ) with the feet 
 shoulder - width apart , looking at a fixed visual target placed two meters from the center of 
 the force plate . the 
 participants were required to make a step backward from one force plate onto another in 
 response to a verbal command . the dominant leg , which was defined as the leg used to kick a 
 ball , was used to step back18 . meanwhile , 
 before landing , the subject was required to press one of the thumb switches ( without an 
 auditory stroop test ) . this served to control for any possible perturbation of postural 
 control arising from the action of pressing a thumb switch . after stepping back the subject was required to maintain a single - leg stance on the dominant leg for 10 seconds and keep 
 looking at the fixed visual target . a prolonged phase of double - leg stance ( with the 
 non - dominant leg stays in the front force plate ) or a shortened single - leg stance of the 
 dominant leg occurs when postural control deteriorates . thus , the duration of the double - leg 
 stance and the sway parameters during the single - leg stance were recorded and compared 
 between the older and young participants . a familiarization trial was given before the four 
 test trials , and rests were allowed if necessary . the center of pressure ( cop ) data 
 collected during the first 5 seconds of the single - leg stance were analyzed to quantify 
 postural control , since the average single - leg stance duration of older adults is normally 
 about 6 to 7 seconds19 . two body - sway 
 measures : ( i ) the total sway path of the cop and ( ii ) the sway area covered by the maximum 
 sway excursion , were used to evaluate the postural control performance after stepping back . 
 the data were recorded at a sampling frequency of 1,000  hz and then re - sampled at 120  hz for 
 analysis using matlab 7.1 software ( the math - works inc . , natick , usa ) . when performing stepping - backward with a concurrent cognitive task , subjects were requested 
 to respond to the auditory stroop test as quickly and accurately as possible while 
 simultaneously performing the stepping - backward task . the auditory stroop test was triggered 
 either when the subjects lifted their dominant leg ( detected by a pressure sensor placed 
 underneath their heels ) , or stepped back onto the force plate . they were instructed that the 
 auditory stroop test could be triggered at any time during stepping back or the single - leg 
 stance . version 20.0 of the commercially available statistical package for the social sciences 
 ( spss ) software ( ibm corp . the average ages , heights and 
 weights of the young and older subjects were compared using the independent t - test . the 
  test was used to compare the gender distribution of the two groups . repeated 
 measures multivariate analysis of variance ( manova ) was used to compare the results of the 
 auditory stroop tests , the stepping - back task and the dual - task performances between the two 
 groups . univariate tests were conducted for each of the measures when an overall 
 statistically significant difference was found . the confidence level for statistical 
 significance the baseline 
 characteristics of the two groups are shown in table 
 1table 1.participants characteristicsvariablesyoung adult group ( n=30)older adult group ( n=28)age ( years)21.3  1.2 72.2  5.7*height ( cm)163.9  7.3154.8  6.4*weight ( kg ) 54.8  8.8 59.1  8.6female , n ( % ) 22 ( 73.3 ) 21 ( 75.0)mean  standard deviation . * significant difference between the young and older adults 
 groups .. the two groups differed significantly in terms of age and height ( both 
 p<0.001 ) , but there was no significant difference in their gender distribution or 
 weights . a significant overall ( group  task ) difference ( f=24.835 ; p<0.001 ) in the auditory 
 stroop test was found by repeated measures manova . univariate analysis demonstrated a 
 significant ( group  task ) difference in reaction times ( p<0.001 ) , but not in error rates 
 ( p=0.064 ) ( table 2table 2.auditory stroop test results during single- and dual - taskingyoung adult group ( n=30)older adult group ( n=28)single taskdual task% change ( % ) single taskdual task% change ( % ) reaction time ( s)0.62  0.100.69  0.15 + 11**1.03  0.32 * 1.81  0.46*+76 * * error rate ( % ) 4.6  5.38.9  12.0 + 9515.2  11.3 * 27.2  21.0*+80 * * significant difference between the young and older adults 
 groups . * * significant difference between single- and dual - tasking . ) . the independent t - test showed there were significant between - group 
 differences in average reaction times and error rates of both the single- and dual - tasking 
 ( p<0.001 ) . the paired t - test showed that , when comparing dual - tasking with 
 single - tasking , both the young and older subjects had significantly increased average 
 reaction times ( p=0.002 and p<0.001 , respectively ) , but that the older adults had 
 significantly higher average error rates ( p=0.001 ) but this was not evident among the young 
 adults ( p=0.1 ) . * significant difference between the young and older adults 
 groups . * * significant difference between single- and dual - tasking . since the average height of the two groups was significantly different , height was treated 
 as a covariate in the statistical analysis . in addition , the double - leg stance durations 
 from the dominant leg landing to the lifting of the non - dominant leg were significantly 
 longer for the older subjects in both single- and dual - tasking ( both p<0.001 ) ( table 3table 3.center of pressure excursions during single- and dual - taskingyoung adult group ( n=30)older adult group ( n=28)single taskdual task% change ( % ) single taskdual task% change ( % ) cop path ( mm)92.3  66.1101.8  71.0 + 10273.1  54.2 * 286.0  60.5*+5cop area ( mm)474.8  343.8679.5  565.5 + 431,081.5  444.9 * 1,180.0  574.2*+9double - leg stance duration ( ms ) 444.5  125.4459.4  108.2 + 3771.1  287.4 * 705.2  201.4*9mean  standard deviation . the younger subjects were simply nimbler and that would be expected to 
 affect subsequent postural performance . therefore , the double - leg stance durations were also 
 treated as covariates in the statistical analysis . repeated measures manova showed no 
 significant group  task difference in either average path length or the area subtended by 
 cop peregrination during the first 5 seconds of the single - leg stance ( p>0.05 ) . the 
 independent t - test revealed there were significant between - group differences in terms of 
 both total sway path and total sway area for both the single- and dual - tasking . but the 
 paired t - test showed that neither the young nor the older adults had a significant 
 difference in total sway path of the cop or the sway area in the comparison of single- 
 versus dual - tasking ( all p>0.05 ) . this study investigated the postural control and cognitive performance of older adults in 
 both single- and dual - tasking . compared with young adults , older adults showed significantly 
 longer reaction times , higher error rates , longer total sway paths and larger total sway 
 areas when stepping backward with and without a concurrent cognitive task . when 
 dual - tasking , both groups had significantly longer reaction times than when single - tasking , 
 but only older adults showed significantly higher error rates . these results suggest that 
 older adults tend to prioritize postural control over cognition when performing a 
 dual - task . for cognitive performance , the results showed that the older adults had a significantly 
 longer average reaction time , and a higher error rate in the auditory stroop test than the 
 young adults . this was as expected because of the well - understood decline with age in the 
 central processing capacity of the brain20 . when an additional postural task was included , both groups 
 demonstrated increased average reaction times . it was hypothesized that if two tasks were 
 performed simultaneously and exceeded the processing capacity , the performance of one or 
 both tasks would be degraded2 . the 
 competition for attention resources generated in this experimental protocol was sufficient 
 that the cognitive performance of both groups was affected in the dual - tasking 
 condition . these results are comparable to those of a study published by our research group in which 
 the participants stepped down from a 19-cm platform with and without an accompanying 
 auditory stroop test18 . while the 
 cognitive task was the same , the postural task was different from the present study . nevertheless , those results showed that older adults had significantly longer reaction times 
 and higher error rates in dual - tasking than single - tasking , as well as when compared with 
 young adults in both conditions . however , the present study found young adults had slower 
 reaction times in dual - tasking than in single - tasking . this was probably due to the task 
 sequence randomization adopted in the present study , which might have minimized the practice 
 effect on the cognitive task results . related to this , lajoie and co - workers examined the 
 effect on auditory reaction time of the maintenance of static posture and gait using eight 
 young and eight older adults . they also found that the reaction times of both groups were 
 fastest during sitting , and concluded that both young and older adults required more 
 cognitive resources for standing and walking than for sitting21 . bergamin and colleagues7 investigated the effects of different secondary tasks , namely a 
 brooks spatial - memory task ( visual construct ) , counting backwards aloud test ( verbal ) , and 
 mental arithmetic task ( cognitive ) on a static postural stability task . they found that the 
 verbal task influenced the performance of the postural task most and they ascribed this 
 finding to either vocalization and/or cognitive demands . the adoption of the auditory stroop 
 test in this study , with the pressing of the thumb switch arbitrarily during stepping 
 backward ( single - task ) , eliminated the influence of a secondary physical task on the 
 postural control task . compared with the younger subjects , the older ones showed significantly longer average sway 
 paths and larger average sway areas in both single- and dual - tasking . this result is in 
 agreement with our previous study on stepping down , which found that older adults have 
 larger body sway than young adults in both single and dual tasks18 . possible reasons for the increased postural sway include 
 the age - related decline in the speed of torque development in the lower extremities22 , delayed onset of postural muscle 
 activation23 , and a diminished capacity 
 to detect and integrate somatosensory information24 . all of these may contribute to a greater sway amplitude and a 
 larger sway area . in contrast , the younger subjects faster ankle torque development and 
 quicker postural muscle responses should have helped them restore balance more quickly , 
 leading to better postural performance25 . the older subjects had similar performance in terms of both postural control measures , but 
 their cognitive performance declined significantly in dual - tasking when compared with 
 single - tasking , showing that they were prioritizing postural control . hypothesis suggested by shumway - cook which proposes that older adults 
 allocate resources to postural control at the expense of cognitive performance in 
 dual - tasking conditions26 . task prioritization tends 
 to be more obvious when the motor task involves a risk of falling , especially for frail 
 older people with balance impairments for whom a fall could have critical survival 
 significance9 . backward stepping may be considered as an alternative to stepping down , which is presumably 
 a more challenging postural task18 , for 
 assessing the balance ability of older adults and/or identifying those at greater risk of 
 falling . the postural task may be more suitable for frail subjects , such as patients with 
 parkinson disease , who often have backward walking deficits which might lead to backward 
 falls . it has also been suggested that assessing backward walking may be illustrative of the 
 degree of basal ganglia impairment27 . however , more balance - related data should be collected before a cut - off point for 
 differentiation can be definitively set . the dual - task paradigm is more demanding of 
 attention , so it might be a more sensitive test than the single - task protocol for testing 
 postural control28 . since this was a cross - sectional study , no causality can be attributed . also , most of the 
 young subjects were university students . the generalizability of the results of the 
 cognitive assessments might be affected due to this limited variety in educational 
 level . only the auditory stroop test was used in this study . further studies are needed to examine the 
 motor and cognitive costs when different cognitive tasks are performed during stepping 
 backward . no electromyographic data was collected , so the muscle recruitment strategies adopted by 
 subjects during stepping back remain to be explored . this could be included in future 
 investigations to search for differences in the strategies employed by different subject 
 groups . future studies might fruitfully compare older fallers with similar non - fallers to 
 find out if there is any difference in postural control performance between them during 
 single- and dual - task activities . it has been suggested that exercise training is beneficial for the static and dynamic 
 balance of older adults with multiple disease conditions performing single and dual 
 tasks29 . however , insufficient evidence 
 has been found to verify these benefits in healthy older adults30 . therefore , one possible direction for future study is to 
 evaluate intervention trials which may increase the performance of postural control and 
 selective attention while walking backward . high quality studies should compare different 
 exercise modalities , and study the optimal dose of exercise needed to improve dual - task 
 performance and reduce the risk of fall . in conclusion , young adults had better postural control and superior cognitive performance 
 in both single- and dual - tasking as they showed significantly shorter reaction times and 
 lower error rates in the auditory stroop test , as well as shorter total sway paths and 
 smaller total sway areas in the postural task . although neither group showed a significant 
 difference in postural control between single- and dual - tasking , they had significantly 
 longer reaction times during dual - tasking , indicating older adults tend to prioritize 
 postural control over cognitive performance when dual - tasking .
[ purpose ] the purpose of the study was to investigate the postural control and cognitive 
 performance of older adults when stepping backward with and without a concurrent cognitive 
 task . 
 [ subjects and methods ] thirty young adults and twenty - eight older adults ( mean 
 age=21.3  1.2 and 72.2  5.7  years , respectively ) were recruited . 
 participants were asked 
 to step backward and then maintain a single - leg stance for 10 seconds with and without a 
 concurrent auditory response task . the reaction time and error rate while performing the 
 cognitive task were recorded . 
 postural stability after stepping back was measured in terms 
 of total sway path and total sway area . 
 [ results ] the older subjects had significantly 
 longer reaction times and higher error rates in both single- and dual - tasking . 
 when 
 dual - tasking , both groups had significantly longer reaction times than when 
 single - tasking . only the older adults showed significantly higher error rates . 
 the older 
 adults also had significantly longer total sway paths and larger total sway areas of 
 single - leg stance after stepping back . 
 neither group showed a significant difference in 
 total sway path and sway area between single- and dual - tasking . 
 [ conclusion ] older adults 
 have poorer cognitive performance and postural stability during both single- and 
 dual - tasking . they tend to prioritize postural control over cognition in dual - tasking .