Physical activity is defined as any bodily movement that leads to energy expenditure through skeletal muscle [1], and is primarily categorized into Leisure-Time Physical Activity (LTPA) and occupational physical activity [2]. Participation in LTPA has increased, driven by factors such as enhancements in physical and physiological fitness [3] and the pursuit of enjoyment [4-6]. It has been demonstrated to offer a range of health benefits, including improvement on mental health [7], and reductions in the risk of chronic disease [8], cardiovascular disease [9], and stroke [10]. In contrast, although occupational physical activity involves comparable levels of energy expenditure, it has been associated with an increased risk of chronic diseases [7] and cardiovascular disease [11], and appears to have limited effects on physical fitness [12]. These observed differences in health outcomes may be attributed to variations in the context, intensity, recovery capacity, and level of autonomy associated with the type of physical activity. In this regard, the consistent health advantages observed with LTPA, as opposed to occupational physical activity, underscore its critical role in the broader relationship between physical activity and health.

Meanwhile, Health-Related Quality of Life (HQoL) is a comprehensive concept that serves as an indicator of an individual’s overall well-being by evaluating physical function, psychological state, and social interactions. It is influenced by various factors, comprising physical health, personal beliefs, mental well-being, social relationships, and environmental conditions [13]. This concept is widely applied across diverse fields, including physical therapy, geriatric health evaluations, and clinical research, as it enables a holistic understanding of an individual’s health status by monitoring health conditions, predicting mortality, and evaluating treatment effectiveness [14-16].

A previous longitudinal study by Wendel-Vos et al. [17] examining the relationship between LTPA and HQoL found that LTPA was associated with the mental components of HQoL. In addition, the study by Vuillemin et al. [18] demonstrated that individuals who met the public health recommendations for LTPA exhibited notably higher scores in the vitality dimension of HQoL. Similarly, research by Tessier et al. [19] indicated that changes in LTPA were associated with mental health and vitality, key components of the mental aspect of HQoL, regardless of sex. Therefore, these findings suggest that LTPA has a positive influence on the psychological components of HQoL.

Recent social and environmental changes have continuously influenced physical activity patterns, which could significantly impact HQoL. In particular, advancements in digital technology, changes in work environments, and lifestyle shifts following the COVID-19 pandemic have become major contributors to the decline in physical activity levels [20]. The extensive integration of telecommuting and virtual education has compelled numerous individuals to engage predominantly in a sedentary lifestyle throughout the day, thereby resulting in a substantial decline in levels of physical activity and negatively impacting HQoL. Most previous studies have focused primarily on scrutinizing the relationship between LTPA and HQoL, with a shortage of investigations exploring the trend of the relationship between those modifications.

Overall, our study addressed three main aims to identify trend of the relationship between LTPA and HQoL on U.S. adults. Specifically, the first aim was to examine temporal trends in HQoL across years. The second aim was to ascertain whether the association between LTPA and HQoL differs across time. The final aim was to examine whether sex moderates the time-varying association. The present study utilized Time-Varying Effect Modeling (TVEM), which represents an extension of linear regression. The TVEM is a statistical approach that allows for the estimation of regression coefficients as continuous functions of time, facilitating the examination of how these coefficients change over time [21]. This model is beneficial for detecting temporal variations in the levels of outcomes and the associations between variables of interest [22].

This study examined temporal trends in self-reported unhealthy days in U.S. adults and investigated the trend of the relationship between LTPA and HQoL over time. Among U.S. adults, the mean number of unhealthy days increased between 2018 and 2019, peaking at 8.83 days. By 2020, this number declined to 7.45 days, potentially reflecting a short-term improvement in perceived health. However, it subsequently rose again, reaching a peak of 9.06 days at the end of 2023, indicating a lower overall HQoL in the post-pandemic period.

The association between LTPA and HQoL among U.S. adults from 2018 to 2023 did not appear to change over time, and when analyzed after controlling for time-invariant covariates, we found a statistically significant association between LTPA and HQoL. Furthermore, when the association between LTPA and HQoL from 2018 to 2023 was analyzed by sex, the strongest associations were observed in 2018 for both men and women. However, no statistically significant sex-based moderation was found.

The trend in self-reported unhealthy days among U.S. adults showed an increase from 2018-2019, followed by decline around 2020, and then a gradual rise again, peaking at the end of 2023. Notably, the U.S. Department of Health and Human Services (HHS) declared a public health emergency due to COVID-19, and by April 2023, over 140 million U.S. COVID-19 cases have been reported by CDC. The temporary decline in unhealthy days observed around 2020 may reflect changes in health perception or reporting behavior during the early stages of the pandemic. In contrast, the sustained increase through 2023, despite the official end of the public health emergency in 2023, may reflect long-term negative impacts of the COVID-19 pandemic. This trend aligns with previous findings. Hay et al. [30] reported lockdowns, social distancing, and fear of infection were associated with declines in HQoL.

In the results of the association between LTPA and HQoL over time, we showed that the association did not change over time and that there was a statistically significant association between LTPA and HQoL after controlling for time-invariant covariates. According to the previous studies, Vuillemin et al. [18] found that participants who met Public Health Recommendations (PHRs) for LTPA had better HQoL than those who did not, and Tessier et al. [19] found that participants who increased their LTPA over three years had higher HQoL. Furthermore, White et al. [31] also found that physical activity affects self-efficacy, which in turn affects mental health, a component of HQoL. These previous studies support our findings that the association between LTPA and HQoL is statistically significant.

The association between LTPA and HQoL by sex from 2018 to 2023 was analyzed. While the estimated odds ratios were highest in 2018 for both males and females, the overall trends between sexes were largely parallel, and the 95% CI overlapped throughout the study period. This indicates that no statistically significant sex-based moderation was observed, suggesting the association is known to exist regardless of sex. This is consistent with the findings of Brown, Carroll, Workman, Carlson & Brown [32], who also reported that the association between physical activity and HQoL did not vary significantly by sex.

This study has the following limitations. First, the BRFSS is a telephone interview survey, so there may be under- or over-reporting of LTPA and HQoL measures. Second, the data does not track individual longitudinal data; therefore, independent samples must be compared yearly, limiting the ability to explain causal relationships over time. Lastly, we were unable to separately analyze Hispanic or Latino individuals due to the absence of a consistent ethnicity variable across all study years. This may have led to the misclassification of Hispanic participants within other racial categories, thereby restricting the generalizability of our findings to this important population subgroup. Despite these limitations, this study is significant in utilizing responses from a large-scale health survey targeting hundreds of thousands of people across the United States and using continuous data for six years.

This study investigated the trend of HQoL in U.S. adults across time and the temporal relationship between LTPA and HQoL. The mean number of unhealthy days increased from 2018 to 2019, declined around 2020, and then rose again, reaching its highest level in late 2023. This pattern partially overlaps with the COVID-19 pandemic period, suggesting that large-scale societal events may coincide with fluctuations in perceived health status.

Across the years, participation in LTPA was consistently associated with higher odds of reporting better HQoL, and this association remained stable throughout the pandemic period, with no evidence of moderation by sex. While causal inferences cannot be drawn from this observational data, the robustness of this association underscores the potential value of maintaining regular LTPA as part of public health strategies aimed at supporting perceived well-being during societal challenges.