Time to stand up faster: underutilization of real-world sit-to-stand transition velocity in aging research

Introduction. Frailty reflects a decrease in physiological reserve that increases the vulnerability to adverse events following minor illness insults to health. [1] Higher levels of frailty correspond to age-related health deficits that incur a greater likelihood of physical immobility, cognitive disorders, and premature mortality. [2] One of the most effective strategies to prevent and manage frailty is physical exercise. Specifically, exercise had a more consistent positive effect on frailty than diet, geriatric assessments, and optimal medication control. [3] However, due to the development of more detailed movement guidelines [4] and low population rates of engaging in exercise, there is greater interest in targeting postures that comprise most of our waking hours and specifically breaking up sedentary behaviours with upright activities.Studying innovative methods to monitor and then intervene on to promote healthier movements patterns among our frail population are needed.Advances in wearable monitors has equipped researchers within relatively precise tools to measure peoples' activity and postures to the fraction of a second (see Figure 1). Monitors worn on the thigh have equipped researchers with the ability to compute both the number of sitto-stand (STS) transitions and their velocity. Over the past decade, sedentary breaks, or the number of STS transitions has garnered major attention, showing that more frequent STS breaks are positively associated with several health markers. [5] For example, more frequent STS breaks were associated with better waist circumference [5], triglycerides [5], and blood pressure regulation [6] independent of overall sedentary activity. But the velocity by which these STS transition occur is not often considered. With aging, the ability to stand up independently is paramount. Adverse physiological changes in the musculoskeletal system, cardiovascular system, and neurological systems occur with sedentary aging that make slow the STS transition. This opinion article outlines the potential utility of STS-velocity (STSv) in aging research with the objective of outlining its potential utility and to promote more discourse on this metric and inclusion in researcher's repertoire of physical behaviour outcomes for studies of older adults.-FIGURE 1 -Discussion: Intensity Complementing Frequency. Older adults tend to break up their sedentary time less frequently than younger adults. [6] The frequency of STS transitions is highly variable, and examining how often they occur may be missing an important characteristic of this behaviour. Using aerobic activity as an example, the frequency of STS may be analogous to step count frequency, whereas STSv may be conceptualized as minutes of moderate-vigorous aerobic physical activity. Herein, the intensity by which stepping occurs is at minimum, equally important to characterizing movement and the evidence base for physical activity intensity is substantial. For example, movement guidelines for physical activity rely on intensity-related physical activity rather than frequency (e.g., step counts). [4] Using this logic, the velocity by which each STS occurs may provide valuable information that might be more impactful than STS frequency alone. Accordingly, a faster STS transition provokes a greater cardiovascular and cerebrovascular response as well as require greater muscular power. [7] Of note, pronounced decreases in arterial pressure, decreases in prefrontal cortex oxygenation, and increases in heart rate are observed for 20-30 seconds following a single STS transition. [7] Thresholds for what constitutes slow/fast or thresholds similar to activity (light, moderate, and vigorous velocities) are an important area for establishing this understudied metric.The decision to stand up is inherently influenced by several factors, and often not a reflective or well-thought-out choice unless the person has mobility issues. It is plausible that between group studies (e.g., frail vs non-frail) or within-group interventions (e.g., exercise training study in older adults) may conclude no differences/changes in postural measures based on the frequency of STS transitions. It is positioned that STSv is particularly relevant for an older adult population, and that the transition from chair rising being an automatic task to one that is a conscious decision is especially important for frail, older adults. If such transitions were faster, but STSv not quantified, in one-group or post-intervention, the null hypothesis would be accepted and a type two error concluded. The consideration of STSv may improve researchers' toolbox in understanding clinically relevant aging and frailty-related research questions.Discussion: Degree of Lower-Limb Functional Capacity. Based on laboratory testing in highly controlled conditions, it is well-established that faster STSv (e.g., via a 5-time STS test) is associated with better mobility [8] and a reduction in fall risk [9], or vice versa. This test reflects their maximal level repeatedly doing the same action in a very short period of time, providing clinically relevant information. But how well this reflects their usual STSv when not under the direction supervision of an evaluator are less clear. While there is some evidence to indicate that maximal STSv and habitual STSv are positively correlated, [10,11] habitual STS is markedly lower, plausibly more variable, and more accurately conveys the true physical behaviours of the person. Studies of small samples are have documented that older adults and those with higher physical frailty levels (via the phenotype) exhibit slower STSv. [10,12] Arising faster, more often, likely would attribute to a faster (or attenuated decline over time) in maximal STSv.Related, the degree of detailed information for each transition, incurs detailed analysis on the STS patterns over the day. For example, most individuals take between 10-90 STS transitions per day (or ~70-630 per week) [6,13], studying free-living STSv permits insights in the changes throughout the day, day-to-day, and lend itself to the potential of lifestyle interventions. In relation to aging and frailty, STSv may closely reflect musculoskeletal function and identifying strategies to ensure STSv is sufficiently high could be an important measure of overall mobility and reducing the risk of disability for geriatricians and other members of the healthcare team. It is positioned that STSv, or the ability to stand up quickly, may prove to be a useful clinically relevant endpoint in studies examining understanding the cardiovascular, neural, and muscular function alterations with age and frailty developments.Discussion: Quantifying Free-Living STSv. Advancements in thigh-worn accelerometry hardware and software have equipped postural researchers with tools to measure habitual standing and sedentary time. With the inclusion of sedentary time into movement guidelines, [4] more researchers studying physical behaviours are opting for monitors worn on the thigh compared to the other locations that cannot, or more indirectly assess sedentary postures (i.e., waist or wrist). Importantly, these monitors are well-tolerated by older adults and able to be worn 24-hrs/day for a full week with transparent dressing even on older, clinical populations with fragile skin. [14] Whether there are a sufficient number of transitions needed to reliably assess STSv or periods of wear longer than one week needed to reliably assess STSv is less clear.Methodological work assessing the practical and technical aspects of STSv is needed.Computationally, commercially available software for thigh-worn monitors provides the raw acceleration data. Currently available commercial software put forth by accelerometer companies do not quantify STSv, but it would be unsurprising for this to change with an increased inclusion of this metric among researchers. Rather, several groups have developed their own analysis programs and engaged in open-sciences practices to enable others with the ability to measure STSv. [15][16][17] Eventually, it is expected that a harmonized analytical approach will be adopted (e.g., similar filtering, sampling rate requirements, strategies to denote the beginning and end of an STS transition, etc.). Currently, few studies have been conducted examining free-living STSv [15], but such information can be readily determined from thigh-accelerometers and would inherently complement both STS frequency and overall sedentary time. Whether there are methodological considerations that would challenge the validity of STSv measures are unclear. These could include for example chronic conditions or health issues that create unilateral imbalances (e.g., unilateral knee osteoarthritis). Similar there may be utility in studying STSv beyond simply a median or peak velocity and the acceleration profile or STSv phenotype could hypothetically provide useful information. Details on the technical aspects on measuring STSv and the heterogeneity of the field are beyond the scope of this paper and are discussed elsewhere. [15] Regardless, this potentially important metric can be determined prospectively and retrospectively. Devices positioned on the thigh for free-living conditions (e.g.., activPAL, Axivity, Fibion SENS, etc.) that provide the raw accelerations could be used to determine STSv at home or outpatient settings. Current analysis pipelines require data to be downloaded and analyzed using custom software programs. Advancements in this analytical strategy could enable practitioners (e.g., physiotherapists, physicians, kinesiologists, etc.) to rapidly determine a person's STSv in-person or virtually could help guide care treatment planning or movement recommendations. Subsequent guidelines or recommendations to help providers interpret such data would be practically useful. As devices become more advanced and progressively easier to implement in clinical settings, the utility of this metric could provide unique information on patients' mobility. Once a greater awareness of STSv is achieved, sufficient understanding of its validity/reliability, and measurement characteristics are optimized, there is great potential for answering research questions on the aging process and testing models to improve physical function and reduce fall risk among geriatric populations.The ability to stand up quickly is a fundamental aspect of human movement requiring a coordinated physiological effort. Preliminary work indicates that the velocity of the STS transition may be impaired with aging and higher frailty levels. It is argued that the intensity by which STS transition occur may be as important, if not more than the frequency of transitions.A greater integration of STSv and probing the clinical and methodological components of STSv is needed to inform clinical practice and assist researchers advance their scientific inquiry.Conflicts of interest: MWO is the Chair of the Sedentary Behaviour Research Network (SBRN). The work does not reflect SBRN and SBRN was not involved in this article.