Aging is a natural biological process that results in a progressive decline in physical and cognitive functions ^[1][2][1,2]. A significant consequence of aging is its association with physical inactivity or disuse, resulting in a decline in muscle mass, structure, and strength [3]. This leads to changes in the quantity and quality of skeletal muscles, which can worsen muscle weakness and disability in the older population [4]. Accordingly, physical disability affects a substantial proportion of older adults, with 44% of those aged 65 years or older experiencing physical weakness, thereby increasing the risk of impairments in activities of daily living by 54% [5].

As the global population ages, there is a growing focus among healthcare providers to understand and intervene in the factors that increase the risk of health and functional declines in older adults ^[6][7][6,7]. Frailty syndrome, a clinical state characterized by increased vulnerability to stressors, leading to negative health-related outcomes in individuals [8], represents instability and the risk of current or further loss of function [9]. The frequency of frailty syndrome increases with age and is more prevalent among individuals with disabilities, depression, hip fractures, and other comorbidities, such as cardiovascular disease and nervous system disorders [10]. Therefore, maintaining physical health and functional capacity in older adults is a critical public health concern, garnering significant attention in the context of healthy aging. According to the WHO World report on aging and health [11], healthy aging encompasses all the mental and physical capacities that an individual possesses, including cognitive function, sensory function, vitality, motor function, and psychological well-being, with the central goal of preserving and optimizing intrinsic capacity. In particular, the report emphasizes physical activity (PA) as a key strategy to counteract or postpone decreases in intrinsic capacity and conditions like frailty, which is a geriatric syndrome resulting from the declines in multiple physiological systems and thus has become one of the biggest challenges in facilitating healthy aging [12]. PA can serve as a polypill that improves health-related quality of life and functional capabilities while mitigating the physiological changes and comorbidities associated with aging ^{[13][14][15][16]}[13,14,15,16]. It is also a fundamental approach targeting age-related declines in physical function parameters, such as muscle strength, mobility, gait, and balance, which are major concerns in maintaining the intrinsic capacity of frail individuals ^[17][18][17,18]. Over the past few decades, studies exploring the role of PA as a determinant of successful aging in the health and functional status of older individuals have accumulated [19]. Nevertheless, efforts to synthesize findings, identify prominent trends, and identify research gaps in the accumulation are still absent.

2. Effects of Physical Activity on Function in Old Age

By 2050, the number of older adults is expected to almost double to 2.1 billion because of increases in life expectancy ^[20][52]. Nevertheless, an increase in life expectancy may not convert into an increase in lifespan without disability, and these individuals may experience poor general health during their prolonged years [7]. Therefore, maintaining the physical health of older adults is a critical public health concern. Accumulating evidence indicates that PA is a highly effective non-therapeutic approach for promoting healthy aging ^[14][15][14,15]. PA can improve functional capabilities and mitigate physical comorbidities associated with aging [16], directly contributing to quality of life. Therefore, efforts to elucidate the roles of PA in healthy aging have accumulated over the past few decades [19].

A handful of publications between the 1990s and the 2000s have increased to a substantial research field in recent years. This trend reflects the growing interest of institutions and researchers in aging and PA, highlighting their essential roles in human disease, health, and lifespan. These results are meaningful in that this is the first attempt, to our knowledge, to explain and visualize intuitively the research trends in the role of PA in aging, emphasizing that PA is essential for healthy aging. Findings from the author keyword visualization map demonstrate that overall, co-occurring keywords with aging and PA can be categorized into three subject categories: (1) physical function and rehabilitation, (2) lifestyle factors, and (3) cognitive function. In addition, the research trend classified by era indicates that while studies from the early (1990s) to mid-term (2000s) periods mainly investigated the relationship between lifestyle factors, aging, and PA, more recently, studies have been conducted with a focus on specific aging-associated diseases, such as frailty, sarcopenia, and depression. 

“Frailty” is the fastest-growing research keyword in the fields of aging and PA research. This is consistent with recent studies highlighting that frailty affects an estimated 11% of older adults ^[21][53] and is the most common condition influencing older adults in terms of both mortality and morbidity ^[22][54]. Furthermore, the recent generations of older adults tend to have higher frailty levels ^[23][55].

There is a significant reduction in BMI among frail older adults subjected to RT. This suggests a potential avenue for improving frailty by lowering BMI, which is supported by earlier research linking a heightened frailty risk to overweight and obese states based on BMI ^[24][56]. However, it is also notable that BMI does not directly indicate body composition such as adiposity, which becomes more pronounced with age and is associated with progression to sarcopenic obesity ^[25][26][57,58]. Additionally, women may be more susceptible to frailty due to their higher intrinsic adiposity ^[27][59], and older women are more likely to experience obesity-related frailty ^[28][60]. Thus, it is important to acknowledge the limitations of associating BMI with total frailty across sexes.

A reduction in lean body mass with an accompanying increase in fat mass is one of the most striking and consistent changes observed with aging [3]. Low muscle mass is considered an inevitable condition and a key component of physical frailty ^[29][61]. Increasing muscle mass in older adults can be challenging because age-related changes in hormonal profiles and physiological functions can hinder muscle protein synthesis ^[30][62]. However, studies have shown that RT can effectively increase muscle mass in older adults. This beneficial effect of RT on muscle mass has also been observed in individuals with other health conditions and functional limitations ^[31][63]. Furthermore, in older adults, these beneficial changes in body composition characteristics with RT can lower the risk of other common disorders such as metabolic syndrome and diabetes ^[32][64]. Muscular strength, which is directly related to muscle mass, is a crucial component of physical function and is associated with various health outcomes in older adults ^[33][65]. Handgrip and leg strength tests are widely used to measure muscle strength ^[34][66].

Loss of mobility is especially problematic since it has a significant negative influence on quality of life and is strongly linked to poor health outcomes, disability, and loss of independence ^[35][69]. Age-related losses in balance and gait are observed in older adults, who also exhibit increased gait variability and a corresponding rise in fall risk ^[36][70]. These are particularly important given the importance of measures for fall prevention and overall mobility in older adults ^[37][71]. Results demonstrate that RT is an effective way to improve balance and gait speed, which has implications for maintaining independence and quality of life in older adults. The TUG test has been extensively used to assess balance and mobility simultaneously in older adults ^[38][39][40][72,73,74], and previous studies have shown that RT improves TUG test scores in healthy older adults ^[41][75].

A study ^[42](10.3390/healthcare12020197) is the first to analyze research trends in aging and PA studies using a bibliometric analysis with a follow-up meta-analysis with a focus on frailty, which was found to be the most popular co-occurring keyword with aging and PA. The bibliometric analysis revealed that the number of publications on the research topic has increased steadily from the 1990s to the present, indicating a growing interest in understanding the role of PA in aging and its importance in human health. Frailty was found to be the most noteworthy keyword co-occurring with aging and PA. Thus, the scholars further investigated the effects of RT on frail older adults. The meta-analysis found that RT had significant positive effects on physical factors associated with frailty, including handgrip strength, lower limb strength, balance, gait speed, and stair-climbing ability in frail older individuals, with few exceptions such as the TUG or chair stand time tests. These findings indicate that RT is an effective intervention for improving physical function among frail older adults, particularly for tasks that require lower limb muscle strength.