Abstract
Background Locomotion is an energy costly behaviour, particularly when it entails raising weight against gravity. Minimization of locomotor costs appears a universal default. Avoidance of stair climbing helps humans minimise their energetic costs. In public access settings, demographic subgroups that raise more ‘dead’ weight than their comparison groups when climbing are more likely to avoid stairs by choosing the escalator. Individuals who minimise stair costs at work, however, can accumulate a deficit in energy expenditure in daily life with potential implications for weight gain. This paper tests the generality of avoidance of stairs in pedestrians encumbered by additional weight in three studies.
Methods Pedestrian choices for stairs or the alternative were audited by trained observers who coded weight status, presence of large bags and sex for each pedestrian. Sex-specific silhouettes for BMIs of 25 facilitated coding of weight status. Choices between stairs and a lift to ascend and descend were coded in seven buildings (n = 26,981) and at an outdoor city centre site with the same alternatives (n = 7,433). A further study audited choices to ascend when the alternative to stairs was a sloped ramp in two locations (n = 16,297). Analyses employed bootstrapped logistic regression (1000 samples).
Results At work and the city centre site, the overweight, those carrying a large bag and females avoided both stair climbing and descent more frequently than their comparison groups. The final study revealed greater avoidance of stairs in these demographic subgroups when the alternative means of ascent was a sloped ramp.
Discussion Minimization of the physiological costs of transport-related walking biases behaviour towards avoidance of stair usage when an alternative is available. Weight carried is an encumbrance that can deter stair usage during daily life. This minimization of physical activity costs runs counter to public health initiatives to increase activity to improve population health.
Discussion
As with lifts, avoidance of stairs by choosing a sloped ascent was frequent. The lower rates of avoidance in Chamberlain Square than the station may reflect the greater detour in the square to choose the slope. Avoidance in the square required 76m of walking versus 48.4m for the direct route across the square; at the station, the discrepancy was smaller, 46.1m versus 42.6m. Inevitably, any detour would increase journey times and, typically, pedestrians seek to minimise time and distance [14,15,23,24]. Less frequent avoidance in the square may reflect the greater temporal cost of the indirect route. Nonetheless, stairs are a more energy efficient means of raising weight against gravity than slopes of equivalent angle [29]. Avoiding stairs by choosing a slope will increase both temporal and energetic costs, unlike the reduced costs with mechanised alternatives. This result of avoidance, despite increased cost, may reflect the more gradual force production possible on the slope to achieve the ascent; the actual height of the climb was the same for both alternatives.
General discussion
The studies in this paper reveal consistent effects of demographic grouping on stair avoidance when an alternative is available. In workplaces, and at an outdoor site where the alternative was a lift, overweight pedestrians, those carrying large bags and females were more likely to avoid stairs than their comparison groups. In the final study, this pattern of avoidance occurred where the alternative method of ascent was a sloped ramp. Taken together with a previous summary of avoidance with escalators [9], these studies expand on the original question posed by Brownell and co-workers about the effects of weight status on avoidance of stair use [7,8]. Overweight pedestrians negotiating the built environment are more likely to avoid the physical activity of stair use as part of daily life than healthy weight pedestrians. So are females and those carrying large bags. A bias to minimise the costs of active transport provides a plausible explanation for this generality.
Minimizing energetic cost
During locomotion, humans naturally optimise energetic cost. They adopt a step width, step length and step frequency for walking and choose a step length and frequency for running, all of which minimise the total metabolic cost for completion of the behaviour (see [30]). Humans have an optimal speed for walking and running that minimises the energetic cost per unit distance [31,32], as do other animals [33,34]. Minimisation of transport costs may be a universal default. This minimization requires repeated iterations to optimise the behaviour. Minimisation of transport costs is learnt, linked to the changes in the visual consequences of forward motion [30,35–38]. All of the above studies were for locomotion on the level. Stair climbing, at two and half times the energetic cost of purposeful walking [2], is a metabolically costly barrier encountered during active transport. A consistent bias for pedestrians to minimise the cost incurred by climbing is evident; in shopping malls where journey time is less of an issue than in stations, 92.4% avoid stairs (n = 355,069 [9]). Raising body weight against gravity is energetically costly and appears to be minimised by other animals [39,40]. Energy expenditure serves three main functions, basal metabolic rate, diet-related expenditure on ingested food, and energy for physical activity [41]. Human basal metabolic rate requires 60% of the recommended daily intake and utilizing food a further 6–12% [41]. At least two thirds of recommended intake are required for these recurrent costs of maintaining function that must be met. The only modifiable part of the equation linking intake and expenditure is the remaining third of intake available for movements of the body; it has been estimated that 89% of these movements involve walking [41]. Transport-related walking has deep evolutionary roots. Minimising the proportion of total intake required for transport would be biologically advantageous [30–34].
Nonetheless, the final pair of studies demonstrated that avoidance was not synonymous with minimisation of expenditure. Choosing a sloped ascent increased both temporal and energetic costs. Walking up a slope allows a more gradual force production during the ascent; peak forces at the knee are reduced compared to stairs [28,29]. Similarly, a modified climbing gait in older individuals reduces the forces at the knee and the ankle to a lower proportion of their maximal capacity [42]. Older climbers have reduced resources for climbing and adopt a reduced, and more gradual, increase in force over time for each step [42]. Choosing the slope in the final pair of studies would allow individuals with reduced resources for climbing to maintain output at a lower proportion of total resources, despite increases in energetic and temporal costs. Aggregated avoidance with ramped ascent, 50.1% (95% CI = 49.3, 50.9) exceeded that when the lift was the alternative, 33.5% (95% CI = 33.0, 34.0). Preference for more gradual resource expenditure could increase avoidance when a ramp was available whereas unwillingness to wait for a lift could decrease it.
Perception of stair slope
Recent research on the perception of the slope of stairs provides clues to the mechanisms that may underlie avoidance based on climbing resources. Perception of the angle of hills and stairs is exaggerated in explicit awareness; a 10˚ hill is reported to be about 30˚ and a 23˚ staircase reported to be 45˚ [43–45]. In experimental studies, fatigue from an exhausting run [45–47], carrying extra weight [46] and depleted glucose resources [48], all result in further exaggerations of reported angle. While effects of experimental demand have been proposed as an alternative explanation [49,50], quasi-experimental studies confirmed effects of depleted resources [51] and additional weight carried [43,52] where demand was absent. Travelers waiting for their trains were recruited to complete an interview about the environment [43,51,52]; there was no experiment. Proffitt argued that perception of slope was ‘embodied’ in that resources for climbing influenced explicit perception [44]. Embodied effects of resources facilitate physical activity choices without individuals having to specifically consider resource availability [44].
Echoing the behavioural differences documented here, overweight pedestrians, those wearing heavy bags and females all reported potential climbs as steeper than their comparison groups [43,45–47,51,52]. Estimates of stair steepness scaled by the deadweight of fat mass that would be carried [51]. As noted earlier, females have, on average, a greater percentage of their weight as body fat (25%) than males (12.5%) and hence are encumbered by more deadweight [10]. Consistently, females reported slopes as steeper than males did [43,45,47,49,50]. Further, stairs were reported as steeper by pedestrians who avoided them by choosing the escalator, even when potential effects of demographics were controlled by stratified sampling and statistical adjustment [43]. Perceived steepness appears to be an environmental cue linked to resources that can deter climbing when an alternative is available [43].
At work, the stairs may not be directly visible when a lift is chosen so perception of steepness cannot directly influence choice. Nonetheless, individuals learn about the potential cost of climbing from experience, biasing subsequent choices. As resources change, behaviour, and the associated perception, echo these changes. Body mass is composed of fat free mass and fat mass. Fat free mass that provides resources for climbing was unrelated to perceived steepness [52]. Rather, it was fat mass, i.e. deadweight that must be carried, that was linked to perception [52]. Further, only changes in fat mass, not fat free mass, were related to changes in perception of steepness over a year later [52]. Similar calibration of perception occurred for changes in body size during pregnancy [53] and loss of leg strength with ageing [54]; changes without experience were ineffective [53]. Obese individuals walk less than lean participants, with daily walking distance negatively correlated with body fat [55]. In the only truly experimental study of weight change, increases in body mass, 78% of which was fat, reduced the distance walked equally for healthy and overweight participants [55]; resources changed behaviour. In both longitudinal perception studies, body change over time influenced perception [52,53]. No study has altered perception to change behaviour. The simplest conclusion is that weight carried deters behaviour [55], consistent with the direction of effects in other research [56–58]. Avoidance of stairs seems likely to be a consequence of weight carried. The role of learning in this process, and the potential mediator of perception, is unknown.
Limitations
It is a curiosity that one strength of these data, direct auditing of behaviour, is accompanied by a limitation. Auditing provides matchless accuracy about the actual behaviour performed; accelerometers, for example, cannot identify behaviour. Observational studies of stair use allow a test of the biasing effects of extra weight carried because of the ability to clearly identify the behavioural choice made. Stair climbing is a vigorous member of the family of active transport behaviours. The energetic cost of stair climbing is clear. Work done to raise weight against gravity is relatively independent of the rate of climbing. Height of the climb, not speed, primarily determines cost. Climbing at 60 steps.min-1 required 8.7 METs (Eves & White, unpublished) whereas climbing at almost twice that speed, 110 steps.min-1, cost 9.6 METs [2]. Effects of weight on the lower intensity activity of stair descent here, and on ‘objectively’ measured walking [55], physical activity [57] and sitting time [56], indicate a generalised effect of weight carried on physical activity choices. Nonetheless, auditing will imperfectly measure demographic differences. Sex is generally straightforward but weight status, and the additional weight of a large bag, must be imprecise categories, even when silhouettes optimise coding for weight status [3,21]. The commonality of effects of weight carried on avoidance across different settings, however, does not suggest imprecision in measurement has impeded the research. The fact that demographic differences in avoidance behaviour are linked to perception of an environmental cue that promotes avoidance indicates some triangulation on the question.
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