Are age-related changes in perceptual-motor regulation related to an increased falls risk?

Introduction

Decades of research have shown that approximately one in three older adults, aged 65 years or older, falls at least once each year (Campbell et al., 1990; World Health Organization, 2007). This is a problem in our ageing society; as the number of people in this aged cohort continues to increase, leading to an expected increase of falls and falls related medical costs in the coming years (Hendrie, Hall, Arena, & Legge, 2004). The consequences of falling are not only severe in terms of medical costs, for older adults a fall means injuries, decreases in quality of life and could even lead to death (Burns, Stevens, & Lee, 2016).

The current thesis adopts an ecological approach to investigate opportunities for falls prevention. According to an ecological approach, guidance of action is mediated by the perception of ‘opportunities for action’ or ‘affordances’ (Gibson, 1979). These affordances are always action scaled; a person perceives possibilities for action in relation to his or her own action system. For instance, a curb might afford ‘stepping onto’ for me, as I have sufficient leg length and strength to perform a step up, however, it will afford ‘climbing onto’ for a toddler who’s legs might not yet have the length to afford stepping onto the curb. Perception of affordances is therefore influenced by how well a person knows his or her own capabilities or in other words how well one ‘calibrates’ perception and action. To achieve this calibration, one needs to experience of ‘explore’ one’s action system, or, as Gibson (1979) put it: "…we must perceive in order to move, but we must also move in order to perceive" (p. 213). This cyclic nature of perception forms the basis for the perception and action cycle and perceptual-motor coupling.

The perception and action cycle can help to understand successful movement and therefore potentially to explain movement errors (such as the ones leading to falls) as well. If one component of the perception and action cycle is unsuccessful it could affect success in movement. For instance, if a person would not calibrate properly, making one insecure about one’s own action capabilities, it might lead to insecurities in moving around. From this it follows that for successful performance, it is important to successfully couple perception and action.

Aims, Methods and Results per Study

The current study incorporates four inter-related studies with differing methods. The first study (described in Chapter 2) is a systematic review, which sought to investigate the perceptual-motor calibration component of the perception and action cycle. The aim of this study was to assess in what conditions calibration occurs most efficiently, with a sub-aim to assess what is known about age-related changes in calibration. Seven databases were screened to identify literature that combined topics related to ‘perception’, ‘action’ and ‘calibration’ or ‘scaling’. Twenty-three papers satisfied the inclusion criteria.

Results of the first study showed that calibration occurs rapidly if the movements performed to explore the perceptual and action coupling provide relevant information for perception. For instance, when standing height is raised by placing a participant on 10-cm high blocks, calibration occurred rapidly when participants were allowed to walk with the block (allowing much exploration), but not when only allowed standing stationary with no body movement (Mark, Balliett, Craver, Douglas, & Fox, 1990). Furthermore, this study identified a general limitation in the research on calibration; no studies have been identified that have studied calibration to changed action capabilities in an older cohort.

The second, third and fourth study in this thesis (described in Chapter 4, Chapter 5 and Chapter 6) all use the same ‘curb-approach task’ to study the regulation of gait towards a target. For each trial in the curb approach task, participants were positioned at the far end of an 8m long GAITRite pressure sensitive walkway (GAITRite®, CIR Systems, Inc., Franklyn, NJ, USA). At a ‘go’ signal, participants started walking the length of the walkway (placing one footfall on a target that was randomly placed in the first 3 meters of the walk to prevent participants from performing identical walks in each trial), to the end of the walkway where a curb-like platform (L: 2m, W: 1m, H: 0.15m) was positioned. Participants stepped onto the platform and continued to the far end at which a push-button was positioned that signaled the end of the trial. 33 trials were performed per participant.

Outcome measures of the curb-approach task were related to three analyses introduced in previous studies in locomotor pointing (De Rugy, Taga, et al., 2002; Montagne et al., 2000). Firstly, an analysis was introduced that assessed the changes in variability (standard deviation) of the position of foot placements. Secondly, an analysis was introduced that assessed whether the timing of the initiation of adaptations in gait (deviations from a ‘standard’ step) are related to the total amount of adjustments to be made (indicating a perception-action coupling). Finally, an analysis was introduced VII that assessed the strength of perceptual-motor coupling; the degree to which changes in step length depended on the perception of required adjustments.

The second study (as described in Chapter 4) aimed to assess whether successful performance in the curb-approach task required similar perceptual-motor regulation compared to the long-jump run up. Sixteen younger adults were included and ask to perform the curb-approach task. Results confirmed the similarities between the curb-approach and the long jump approach. Regulation seemed to be initiated earlier in the curb-approach compared to the long jump, but a similar pattern was observed in decreasing variability of foot placement and an increasingly stronger perceptual-motor coupling as participants got closer to their target in both tasks. The second study concluded that the curb-approach task would provide an effective paradigm to study perceptual-motor regulation in an older cohort (for whom a long jump would be too demanding).

The third study (Chapter 5) aimed to assess age-related changes in regulation in the curb approach task. In this study, the data collected from the 16 younger participants (study2) was compared to data collected from a cohort of 105 older adults. Results showed that with older age, participants showed less variability in foot placement during their approach. Furthermore, it was shown that with age, participants were more likely to adopt a strategy that involved shortening rather than lengthening of steps. Age-related changes were most prominent in the measures of strength of perceptual-motor regulation. Similar to the younger participants, older participants showed an increased strength of coupling (or in other words; made stronger adjustments) as they got closer to the curb. However, it was also shown that with age the strength of the coupling over all steps increased, indicating that the older participants made stronger gait adaptations.

As the third study identified that age-related changes are most prominent in the measures of perceptual-motor coupling, it was decided to focus the final study (Chapter 6) on the question whether these changes could be related to an increased risk of falls. Ninety-eight participants were included in the analysis for this study, who first performed the curb-approach task and then were entered in a 12 –month follow-up to screen for the occurrence of gait-related falls. Results showed that participants who reported experiencing a gait-related fall showed stronger perceptual-motor coupling (stronger gait adaptations) in stepping onto the curb.

Discussion and Conclusion

The results of this thesis showed that the curb-approach task provides a novel and valid method to measure perceptual-motor regulation of locomotor pointing in in a low demands setting, suitable for the older cohort. Older adults showed stronger gait adaptations compared to their younger counterparts and adaptations in the step onto the curb were stronger still in on older adults prone to experiencing gait-related falls. These results suggest that, in controlling gait, humans are capable of changing the strength of perceptual-motor coupling in accordance with the difficulty of the task. With age, action capabilities decrease and the curb-approach task becomes harder; our results show that this is met with a strengthening of perceptual-motor coupling. Implications of the current thesis are that in falls risk screenings as well as falls prevention, it is important to consider the entire perception and action cycle. An ecologically-grounded functional approach to healthy aging is advocated which considers a person in relation to his/her behavior and environment (Vaz, Silva, Mancini, Carello, & Kinsella-Shaw, 2017).