Duration or Timing? What is the Optimal Core for Older Adults to Engage in Moderate-to-vigorous Physical Activity to Prevent Disability?

Background The positive association between the total duration of physical activity and performances of physical function may vary at different times of the day as circadian rhythm regulates individuals in response to external stimulations. We aimed to examine the association of timing-specic and overall moderate-to-vigorous physical activity (MVPA) with performances of physical function in older adults. Methods A cross-sectional analysis was conducted among 118 older adults (mean age = 70.0 ± 5.0 years). We assessed and identied timing-specic (morning: 06:01-12:00; afternoon: 12:01-18:00; evening: 18:01-24:00) and overall MVPA using a triaxial accelerometer. Different measures of physical function were evaluated including handgrip strength (by grip dynamometer), gait speed (ve-meter walk test), basic functional mobility (timed up and go test), and lower limb strength (ve times sit-to-stand test). Multivariate linear regression models adjusting for covariates were used to investigate the associations. Results Participants spent 0.4 hours in MVPA per day on average, half the time spent during the morning (47.7%), followed by during the afternoon (29.9%) and evening (21.6%). The time spent on overall MVPA was generally associated with better physical function performances. There was statistical evidence for the percentages of MVPA engagement during the morning ( B = 0.214, 95% condence interval [CI] 0.001 to 0.428) and afternoon ( B = -0.273, 95% CI -0.518 to -0.027) associated with basic functional mobility but with contrary directions; the percentage of MVPA engagement during the evening was associated with less time spent in gait speed performance ( B = -0.237, 95% CI -0.468 to -0.006). Conclusions Our ndings inform implications that the overall MVPA engagement was more important than timing-specic MVPA to older adults’ physical function performances. Strategies for accumulating time of MVPA is more practical and effective than encouraging to engage MVPA in specic timing for the enhancement of functional ability and therefore prevent disability among older adults.


Background
Disability, a physical or mental condition that impairs or limits an individual's movements in daily life, is one of the commonest symptoms in older populations; the prevalence was estimated to be 25% in the population aged over 65 years in 2008 in the United States [1]. It places not only a heavy burden of physical strain and emotional stress on the family caregiver [2] but also an increase in medical care expenditure [3]. Several studies have shown that physical function is an important predictor of disability in older adults [4,5]. Previous research addressing older adults' disability has shown that engaging su cient levels of moderate-to-vigorous physical activity (MVPA) contributes to better performances of physical function (e.g., handgrip strength, gait speed, basic functional mobility, and lower limb strength) [6][7][8].
There is a growing interest in establishing a link between the effects of circadian rhythms and su cient physical activity in older adults [9]. An individual's circadian rhythm is dynamic over their lifespan and deteriorates in the later years of life [9]; however, it still regulates individuals' physiology and metabolism in response to external stimulations [10,11]. Previous studies have indicated that older adults who were more active, indicated by the amount of physical activity or walking time, during the morning had lower risks of obesity [12] and mortality [13]. Despite this, the relationship between timing-speci c physical activity and physical function remains to be investigated.
There may be an optimal time of the day for MVPA engagement to modify human circadian rhythms and maximize physical function in the aged populations.
There are several measurements to assess physical activity at different times of the day and the accelerometer is one of the ideal measurements. The accelerometer tracks the orientation and movement in individuals objectively [14], records the changes in small time increments, and therefore accurately identi es different timings of the day for older adults' MVPA. Measurement by accelerometer can address the limitations of reporting and recall biases [15].
We hypothesized that the MVPA engagement during the morning was associated with older adults' physical function after controlling for its overall duration and therefore investigated the association of timing-speci c and overall MVPA with a range of measures of physical function among community-dwelling older adults.

Participants
Older adults aged over 60 years who could walk independently were recruited using local advertisements and announcements from 28 different residential neighborhoods in Taipei City, Taiwan. Among these participants, 118 out of 130 older adults wearing the accelerometer for a consecutive seven days completed the on-site examinations. There were no differences in sex and age in the included and excluded participants (data not shown). Detailed methods and procedures have been reported elsewhere [16].

Outcome variables
Several measures of physical function, which have been validated for the evaluation of physical function in older adutls [17], were assessed by on-site examinations: 1. Handgrip strength: We measured the handgrip strength of both of the participant's hands using the hydraulic hand dynamometer (Jamar Plus + Digital Hand Dynamometer, Lafayette Instrument Company, USA). A higher measure of handgrip strength indicated better performance, and the maximum strength measurement was selected from three attempts with a one-minute gap between attempts. 2. Gait speed: Each participant had only one attempt to walk 11 meters as fast as possible. The time spent walking in the central ve meters was calculated. A shorter time spent on the 5-meter walk test indicated better performance of gait speed.
3. Basic functional mobility: Participants were instructed to rise from a standard chair, walk three meters forward, return to the chair, and sit down as soon as possible. Each participant repeated the same attempt twice and the attempt with the shorter time, indicating better performance, was used in the analyses.

Lower limb strength:
We assessed lower limb strength using the ve times sit-to-stand test. Participants were requested to sit on a standard chair and repeat stand up and sit down ve times, as fast as they could. Each participant had two attempts and the attempt with the shorter time, indicating better performance, was selected.

Exposure variables
Time spent in MVPA was measured by a triaxial accelerometer (ActiGraph GT3X+, Pensacola, FL, USA). We followed the information on data collection and processing criteria suggested by a systematic review of standard protocols [18]. The different time intervals were identi ed in which the MVPA (≥ 2,020 counts/minute) occurred [19]: a) morning: 06:01-12:00; b) afternoon: 12:01-18:00; and c) evening: 18:01-24:00 based on previous studies [13,19,20]. We calculated the percentage of the three time intervals of MVPA engagement for each participant. The ActiLife software version 6.0 was used to process the accelerometer data.

Covariates
The covariates including i) sex, ii) age, iii) body mass index (BMI; calculated by weight in kilograms divided by height in meters squared), iv) marital status, v) employment, vi) living status, vii) educational level, viii) self-rated health, ix) depression, x) diabetes, xi) hypertension, xii) hyperlipidemia and xiii) monitor wear time were assessed by an interviewer-administered questionnaire (i-xii) and an accelerometer (xiii).

Statistical analyses
We analyzed data from 118 older adults who completed all the study variables. We selected the covariates for different measures of the physical function using univariate linear regression models (Appendix Table 1

Results
The characteristics of 118 older adults (70.3% female; mean age = 70.0 years) were presented in Table 1. Participants spent 0.4 hours in MVPA per day on average, almost half the time was during the morning (47.7%), followed by the time spent during the afternoon (29.9%) and evening (21.6%) ( Table 1). Table 2

Discussion
Our data showed that the engagement in longer MVPA was generally associated with better performances of physical function and some statistical evidence for associations between speci c-timing MVPA and physical function in older adults. To our knowledge, this is the rst study examining the association between the time spent in MVPA at different times of the day and physical function in the context of older adults. Furthermore, we used an accelerometer to objectively measure MVPA and provide such robust data. There were some limitations to this study. First, the association of the percentages or duration of timing-speci c and overall MVPA with physical function should be interpreted with caution, as the size of our study was small. Second, although we regarded the same period of the day for each participant as sleep duration (i.e., 00:00-06:00) so those older adults would not engage in physical activity at that period, there may be some inconsistency of lifestyle between participants and this may cause bias. However, our data showed that the sleep periods in more than 90% of the participants highly overlapped. Third, as we did not use a standardized procedure to screen the cognitive ability of participants before the on-site examinations, the older adults' understanding and reactions to the examinations may be confounded by their levels of cognition. Finally, the association of the percentages and duration of timing-speci c and overall MVPA with physical function cannot be extrapolated as a causal relationship because of the cross-sectional design.
Our data showed positive associations of the overall duration of MVPA with performances of physical function. This result was consistent with previous studies, suggesting the engagement of MVPA could retain muscle density and function in older adults [6,7,21]. By contrast, our results did not show the bene ts of physical function from higher levels of MVPA during the moring, inconsistent with previous ndings [12,13]. Interestingly, there were diverse associations of the MVPA engaging in the morning and afternoon with the performance of basic functional mobility. Some studies suggested that the increased levels of serum vitamin D, which could be synthesized by sun exposure [22], could directly strengthen older adults' bones and muscles [23] as well as indirectly improve their muscle strength [24] through promoting sleep quality of older population [24,25]. However, our result regarding the MVPA engagement during the morning and the performance of basic functional mobility did not fully support the hypothesis. It implied that sun exposure may not be the only reason to impact the associations between MVPA engagement and performances of physical function. There may be some other factors, especially work in the morning, that may in uence this association. For example, a pilot showed preliminary evidence that exercising in the fasted or fed state during the early morning would have different effects on facilitating adaptations in muscle [26] and this may in uence the association between the MVPA during the morning and physical function observed. Future research should investigate the pathway toward better physical function among older adults with comprehensive factors. There was a signi cant association between the MVPA engagement during the evening and the performance of gait speed. The association between evening MVPA engagement and gait speed may be attributable to the lower crime-related safety. Older adults who perceive themselves living in safer neighborhoods indicating by lower levels of crime and more comprehensive crossings were more likely to be associated with more MVPA [27] and lead to better physical function [28].
Despite some associations showed between the timing-speci c MVPA and physical function, it seemed that the duration of MVPA engagement across the time of the day was more important to older adults' performances of physical function. Our ndings may imply that older adults accumulated more duration of MVPA, even through interrupted sections, would have better performances of physical function, irrespective of the timing of the day for MVPA engagement.

Conclusion
This study suggests that engaging in more MVPA, regardless of the speci c time of the day, is associated with better physical function performances among older adults. Designing strategies/programs with increasing the duration of MVPA engagement across the time of the day could bene t older adults' physically functional ability and may, therefore, prevent disability.

Declarations
Ethics approval and consent to participate