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Older people often have increased difficulty maintaining balance, and are vulnerable to falls. As noted in section 1.3.3, falling is a more common cause of accidents for older pedestrians than collisions with vehicles. In section 4.3 below, data on these accidents are discussed in more detail. Problems with stability affect the way older people walk, and their ability to combine walking with cognitive tasks. In this section we look in detail at changes in older people’s balance and stability.

Many aspects of the way older people walk point to more conservative patterns of movement. Older people walk more slowly than younger people, and take smaller steps (Ketcham and Stelmach, 2001). Older people with a history of falling tend to show larger reductions (Wolfson et al., 1996). Older people also tend to place their feet further apart when stepping, and tend to have both feet on the ground for longer during each stride (Shumway-Cook and Woollacott, 2001). Laboratory studies suggest that they need to notice obstacles earlier to avoid them (Patla et al., 1992), and that their walking is more affected by a concurrent task (Chen et al., 1996). Chen et al. (1991) found that older adults stepped over obstacles more

conservatively than younger adults, moving more slowly and taking a smaller step over the obstacle.

Falls are the seventh leading cause of death in the USA among people aged over 75, and about half of the over 75s who have a fall causing injury become afraid of further falls, many to the extent that they avoid certain kinds of situation (Ochs et al. 1985; cited in Shumway-Cook and Woollacott, 2001). Older women are more likely

to fall than older men. Although the liability to fall is affected by environmental variables, such as the stability of the walking surface, as well as the constitution of the person, changes in muscle strength, joint mobility, and strategies that recruit different muscle groups to maintain stability all affect the capacity of older people to maintain balance (Shumway-Cook and Woollacott, 2001). Three sensory systems, each of which performs less well in older people, contribute to the maintenance of balance (Woollacott, 2000). Somatosensory systems detect mechanical events occurring in contact with the body and the relative position of the limbs; the vestibular system provides information about position within a frame of reference defined by acceleration (i.e. gravity); and vision locates the viewer in relation to other objects. In certain situations, some created artificially in the laboratory, these systems can provide conflicting information, and older people appear to change the relative weight they give to them, perhaps becoming more dependent on visual information in particular (Maylor and Wing, 1996).

Tests of balance can be directly linked to dynamic mobility. Tang et al. (1998) found that their older participants’ capacity to cope with a sequence of walking tasks in a laboratory, which they called the Sensory-oriented Mobility Assessment Instrument (SOMAI), correlated with static tests of balance in conditions in which visual, vestibular, and somatosensory information was reliable. That is, those who were good at the SOMAI tended to be the best static balance performers when they had all three sources of information. However, in general, there was not a correlation between SOMAI performance and static balance with restricted sensory input. The exception was that those who did poorly on static balance tests without reliable somatosensory information tended to do less well on the SOMAI when peripheral vision was obscured. The SOMAI task thus shows the importance of all three balance senses to dynamic mobility, and that studies of balance have wide relevance to the physical mobility of older pedestrians.

When an incident such as a trip occurs, older people have greater difficulty avoiding a fall, and may use different strategies to younger adults. Laboratory studies of recovery from loss of balance are summarised by Shumway-Cook and Woollacott (2001). Older adults, particularly those with a history of falls, are more likely to use hip movements than ankle movements to recover balance than younger adults (e.g. Sundermier et al., 1996). A strategy based on hip movement recruits larger muscle groups and could reflect a loss of confidence in ankle strength. Another strategy is to take steps. Maki et al., (2000) subjected participants to a lateral movement of the platform they were standing on. They found that healthy older adults (average age 69 years) were more likely than younger adults (average age 24 years) to make extra arm movements or to knock the standing leg with the other one as they tried to recover balance. Maki et al. hypothesised that older adults would be more likely to use a stepping strategy. In fact the difference was small, and a majority of adults of all ages used a stepping strategy. However, older people tended to take more steps when stepping to recover balance, whether they had been simply standing in place until perturbation or had been walking on the spot.

There has been a lot of interest in the role of attention in recovering balance and posture, and a number of studies have looked at responses in dual task situations. Typically, participants are asked to perform a mental task such as counting while standing on a platform that is moved backwards. In dual task situations both younger and older adults are adversely affected, but the effect tends to be greater for older adults. For example, Brown et al. (1999) found that the time to perform an

arithmetic subtraction increased more following platform movement in older adults (68–89 years) than for younger adults (21–36 years). Stepping responses were far more common for older people, who in this study only rarely used hip strategies, and strategy selection was similar in dual and single-task conditions. However, when there was a second task, adults of all ages initiated stepping further from the limit at which balance would have been lost. That is, they kept a greater safety margin in the dual-task condition.

The way older people respond to a sudden perturbation is affected by the level of their concurrent cognitive processing. If the cognitive load is greater, they respond more conservatively. One practical reason why studies of balance recovery in dual- task conditions are relevant is that the pedestrian task normally is a dual task. The pedestrian has to control their movement while scanning and evaluating the

environment, looking in shop windows, negotiating other pedestrians, or monitoring traffic while crossing the road. Pedestrians may also be talking to people they are walking with, or thinking about the goals of their journey.

One hypothesis is that older people respond more conservatively to loss of balance because they have less strength. Chen (1993; cited in Shumway-Woollacott, 2001) argued, on the basis of a model, that recovery from a trip relies on the ability to build up force quickly, rather than muscle strength as such, something older adults do less effectively (Thelen et al., 1996). A similar study to the one described in the previous paragraph focused on muscle activity on trials in which an ankle strategy was used to recover balance (Rankin et al., 2000). Muscle activity in the lower leg began slightly later in the older group (about 12ms later for the agonist muscle) in both single and dual-task settings, with no dual-task decrement for either group. The amplitude of muscle response was reduced for both age groups in the dual-task condition, but there was a larger reduction in the amplitude of agonist response for older than for younger people. Rankin et al. argued that the agonist muscle response is more important for retaining balance in this situation, and that increased

frequency of stepping strategies in dual-task situations is a consequence of this reduced muscle response. However, Brauer et al. (2001), using a slightly different method, did not find a decrease in muscle response in dual-task conditions for healthy or balance-impaired older people. The detailed relationship between muscle activity and balance recovery therefore remains an open question for future

research.

Older people with balance difficulties are less able to cope when the sensory information used to maintain balance is restricted. Shumway-Cook and Woollacott

(2000) used a choice reaction time auditory task to compare the dual-task performance of healthy older adults to older people with balance or falling difficulties. In different conditions they restricted different sensory cues. For example, ‘‘sway-referencing’’ (tilting the platform in a manner that tracked body sway) was used to reduce somatosensory input. For healthy older adults, dual-task performance was worse only when both visual and somatosensory cues were removed. For those with balance difficulties a reduction of visual or somatosensory information was sufficient to produce a decrement in the dual-task condition. When both vision and somatosensory input were compromised, people in this group all fell in both single and dual-task conditions. As Shumway-Cook and Woollacott carefully noted, the impaired group were also older, more likely to be living in an institution, taking more medication, and typically had poorer health. Although it is, therefore, not possible to say that the differences are directly linked to balance status, it is clear that more frail older people will have particular problems with mobility when

environmental conditions are sub-optimal.

Other studies have analysed the way older people respond to balance perturbation in simple situations. Tang and Woollacot (1998) induced slips in older (60–84 years old) and younger (21–29) adults in a laboratory, and found that older adults returned the swinging foot to the ground more quickly, which they took to reflect a

conservative approach to balance. However, the older people also made larger compensatory arm movements and were more likely to trip over the swinging foot as it returned to the walking surface.

Pavol et al. (2001) subjected a sample of people aged over 65 years to a trip. The participants were supported in a harnesss and walked along a track ‘‘at a self- selected, ‘normal’ speed’’ (p. M429). None had a history of repeated falls. They knew they would be tripped, but not exactly where on the track or on which trial. An obstacle was elevated about 5cm from the track in front of the swinging leg to impede its path. The apparatus recorded the amount of support required from the harness following the trip, and ‘‘falls’’ were recorded when full support was required. Kinematic analysis characterised three patterns of response to trips (for details, see Pavol et al.) and found, among other things, that for two of these, people who were walking faster were more likely to fall following a trip. This implies that slower or more conservative walking would be a rational defence against the potentially serious consequences of falling.

Cao et al. (1998) examined responses to a signal to stop walking or suddenly turn to investigate the process of avoiding collisions. The signal to stop was given as participants walked down a track at comfortable walking speed, reported to be within 10% of 1.3m/s for older (65–85, mean 73.8 years) and younger (18–30, mean 21.8 years) adults, male and female. They found that older adults took longer to begin to decrease forward velocity, although all groups reduced forward

acceleration within 250ms. Older men then decelerated more quickly, and older women more slowly, than younger adults. Thus older men partly compensated for

the initial delay. The magnitude of differences was of the order of tens of

milliseconds. It is possibly surprising that the groups could be matched so closely for comfortable walking speed given the data on walking speed discussed above, and it is conceivable that differences on the task were an artifact of differences in the percentage of maximum effort being made to achieve that rate of progress. Nevertheless, as Cao et al. noted, the pattern of results is consistent with other findings that older adults’ walking is affected by diminished muscle strength in the lower limbs.

Older people walk using more conservative patterns of movement, but are vulnerable to loss of balance. Sensory decline and reduced strength appear to be important factors in this higher rate of falling. Older people tend to respond to loss of balance differently to younger adults, and are less able to recover balance. Older people are less able to cope with a secondary task while walking. Some older people have particular problems with balance, and are at risk of repeated falls. The

consequence of falling can be serious for older people.