Reallocation of time spent on sedentary behavior by time spent on physical activity reduces dynapenia in older adults: a prospective cohort study

ABSTRACT BACKGROUND: Dynapenia is characterized by mobility limitations in the older population when combined with aggravating behavioral factors that can increase the risk of morbidity and mortality. OBJECTIVE: To investigate the hypothetical effects of reallocation of time spent on sedentary behavior (SB), moderate-to-vigorous physical activity (MVPA), and sleep on dynapenia in older adults. DESIGN AND SETTING: A prospective cohort study using exploratory surveys in Alcobaça City, Bahia State, Brazil. METHODS: In total, 176 older adults (≥ 60 years) of both sexes participated in this study. Dynapenia was assessed using the handgrip strength test with cutoff points of < 27 kg for men and < 16 kg for women. MVPA and SB were assessed using the International Physical Activity Questionnaire, and sleep was assessed using the Pittsburgh Sleep Quality Index. RESULTS: Effects on reallocation were found for the shortest times, such as 10 minutes (odds ratio (OR) 0.92; 95% confidence interval (CI): 0.85–0.99); substituting MVPA with SB increased the chances of dynapenia by 58.0% (95% CI: 1.01–2.49). Analyzing the substitution of 60 minutes/day of SB with 60 minutes/day of MVPA revealed a protective effect, with a lower OR for dynapenia of 37.0% (OR 0.63; 95% CI: 0.40–0.99). The reallocation of sleep time did not significantly reduce dynapenia. CONCLUSIONS: Substituting the time spent sitting with the same amount of time spent on MVPA can reduce dynapenia, and a longer reallocation time confers greater health benefits in older adults.


INTRODUCTION
Aging is commonly accompanied by a significant reduction in muscle performance, since skeletal muscle mass and strength are affected by this process. 1 The age-related decline in muscle strength is termed dynapenia. This condition exposes older adults to a greater risk of mobility limitations. 2 It is directly influenced by behavioral factors such as the level of physical activity (PA), exposure to sedentary behavior (SB), and quality and duration of sleep. 3 Moderate-to-vigorous physical activity (MVPA) is an established component of healthy aging and can improve the health and longevity of the population. 4 Insufficient levels of physical activity are prevalent worldwide; in older adults, this prevalence reportedly ranges from 4.9% (Sweden) 5 and 29.0% (Portugal) 5 to 33% in Brazil. 6 PA levels among older adults remain below the minimum 150 to 300 minutes per week recommended by the World Health Organization. 7 These low levels induce several deleterious muscle adaptations, including reductions in muscle volume, power, and strength, which are aggravating factors for older adults. 8 Concomitantly, advancing age has been associated with high SB, 3 with an estimated sedentary time of older adults of 9.4 hours per day, ranging from 8.5 to 10.7 hours per day, according to a systematic review of 22 studies. 9 Consequently, SB is independently associated with reduced muscle strength, which contributes to reducing the functionality and autonomy of older adults. 10 Therefore, exposure to dynapenia may play a role in the relationship between PA, MVPA, and SB. Establishing and quantifying the associations between such variables is thus a priority for informing potential lifestyle guidelines and interventions, ultimately mitigating poor health outcomes. Regarding sleep, its relationship with aging and strength and its close association with the development of adverse health conditions have been described. 12 A study found that low handgrip strength was independently associated with poor sleep quality in middle-aged and older adults. 13 Although the association between SB, PA, and sleep has been investigated in the literature, 14,15 studies examining the relationship between dynapenia and SB, PA, and sleep, especially their effects when assessing the reallocation of the exposure time of older individuals to these activities, are lacking. Therefore, investigating sleep hour time, MVPA, and SB in relation to dynapenia is relevant; an isotemporal substitution modeling shows the ability not only to control the effect between activities but also the effect of substitutions of time spent, reducing the heterogeneity of associations, thus facilitating public health recommendations. 16 We hypothesized that the hypothetical reallocation of time in MVPA by SB would increase the odds of dynapenia.

OBJECTIVE
To investigate the hypothetical effects of the reallocation of time spent on SB, MVPA, and sleep on dynapenia in older adults.

Study design
This was a prospective and observational cohort study, part of the In February 2020, 249 participants were excluded due to a lack of information (59 due to death, 36 due to relocation to another city, 18 due to refusal to participate, 25 due to not meeting the inclusion criteria, and 105 due to not being locatable); 48 were excluded because they already had dynapenia at the beginning of the study, and 6 were excluded due to a lack of information on handgrip strength, resulting in a final study population of 176 individuals (Figure 1).

Ethical consideration
This study complied with the procedures and protocols of the

Sleep
The time spent on nocturnal sleep was measured by the question, "During the past month, how many hours did you sleep at night?, " from the Pittsburgh Sleep Quality Index, 26

Data analysis
Epidata software, version 3.1b, was used to prepare the database, and the analyses were performed using SPSS software (version Descriptive statistics were used to identify the sample, including the distribution of absolute and relative frequencies, medians, means, standard deviations (SDs), and interquartile ranges.
The difference between groups with and without dynapenia was measured using the Mann-Whitney U test. For the association between the covariables and dynapenia, inferential statistics were used (Pearson's chi-square test).
To determine the hypothetical effects of the reallocation of time spent on sleep, SB, and PA on dynapenia, the isotemporal substitution approach was used. 30 Isotemporal substitution analyses were performed using logistic regression, with an estimate of odds ratio (OR) and 95% confidence interval (CI

RESULTS
This study included 176 older adults of both sexes, with a median age of 66.0 years. The incidence of dynapenia during the followup period was 17% (n = 30).  In the isotemporal substitution analyses ( Table 2 Despite its health benefits, PA levels among older adults remain below the recommended 150 minutes/week. 33 It has been shown that even at low levels, small changes in the inactive profile can improve and maintain the health of older adults. 3  This factor has been assessed by sedentary activity; the study by Hammer and Stamatakis 10 addressed the daily time spent on TV and internet use and its inverse association with muscle strength, highlighting that older adults who watched TV for ≥ 6 hours per day had less handgrip strength than older individuals who watched TV for < 2 hours per day. In older adults, sleep and muscle strength vary according to the aging process. As modifiable parameters, they can interact and influence each other. 36 The results of the current study did not show significant changes in the reallocations of sleep time by SB or MVPA, which can be explained by the mean sleep rate of the population, which was of the recommended regular amount (~7 hours).
However, recent investigations have identified strong evidence between the quality and amount of sleep and muscle strength. 36,37 Pourmotabbed et al. showed that both short (< 6 hours) and long (> 8 hours) periods of sleep could lead to an increase in the risk of sarcopenia (decline in muscle mass, strength, and performance). 37 In the isotemporal substitution model, no studies reported on dynapenia as an outcome; however, with sarcopenia and its that an increase in a behavior will be accompanied by a decrease in the equal duration of the others while the total time in all behaviors is kept constant. 40 These findings may be important in preparing specific recommendations for PA and SB in older adults.
This can be useful for primary health and health professionals on how to use discretionary time in a way that is beneficial to health in daily practice.
With the need for future studies that complement our results, monitoring the high exposure to SB already present in the population can influence the development of dynapenia, even if PA levels remain above the recommended parameters.
Among the limitations of this study is the isotemporal substitution method, which is hypothetically applied, and the lack of estimating the change in behavior via a direct assessment.
Moreover, we implemented an instrument that indirectly assesses PA and SB, which does not estimate mild intensity, which is considered important for the composition of the day in 24 hours.
Nevertheless, the strengths of the study should be highlighted, such as its representative sample, the follow-up having been performed by the same assessors throughout the study period, in addition to the measurement of muscle strength with the hydraulic dynamometer, considered the gold standard for large populations, its originality, and its configuration in a longitudinal design that no other studies have utilized.

CONCLUSION
Substitution of the time spent on MVPA with the same amount in SB is associated with an increased risk of dynapenia. The opposite also occurs; longer time spent on MVPA correlates with greater benefits, drastically reducing the risk of developing dynapenia, thus directly reflecting on the reduction of the limiting impacts of the decline in muscle strength.