A serial mediation model of physical exercise and loneliness: the role of frailty and depression

Abstract

Background

Frailty, depression, and loneliness are significant risk factors that hinder successful aging. Physical exercise has been widely recognized as an effective intervention to improve both the physical and mental health of older adults. Guided by the integral conceptual model of frailty, this study aimed to explore the relationships among physical exercise, frailty, depression, and loneliness, providing theoretical support for designing targeted exercise interventions to alleviate loneliness in older adults.

Methods

This study employed a structural equation model (SEM) and bootstrap method to examine a serial mediation model, investigating the roles of frailty and depression in the relationship between physical exercise and loneliness. A descriptive and cross-sectional design was adopted, and data were collected from 505 older adults aged 60 and above in China between February and July 2023. The data were collected using the Physical Activity Rating Scale (PARS-3), the UCLA Loneliness Scale (ULS-8), the Tilburg Frailty Indicator (TFI) and the Patient Health Questionnaire-9 (PHQ-9).

Results

The findings revealed a significant direct negative relationship between physical exercise and loneliness (Effect = -0.063, 95% CI: -0.085 to -0.040). Furthermore, frailty and depression were found to mediate this relationship both independently and serially. The independent mediation effect of frailty was − 0.072 (95% CI: -0.090 to -0.055), while the independent mediation effect of depression was − 0.010 (95% CI: -0.019 to -0.003). The serial mediation effect of frailty and depression was − 0.007 (95% CI: -0.011 to -0.002). Collectively, the total indirect effect of the three mediation pathways accounted for 58.55% of the observed relationship.

Conclusions

This study demonstrated a negative correlation between physical exercise and loneliness among older adults, with frailty and depression serving as significant mediators in this relationship. The findings suggest that physical exercise may alleviate loneliness in older adults by enhancing physiological function and fostering psychosocial empowermen.

Background

Loneliness in older adults, defined as a distressing emotional state stemming from ‌perceived social isolation and lack of meaningful connections [1]. Loneliness is increasingly recognized as a critical public health issue. Substantial evidence links chronic loneliness to ‌multifaceted health risks‌, including reduced quality of life, higher prevalence of mental disorders (e.g., depression), physical comorbidities (e.g., hypertension), and elevated mortality/suicide rates [2,3,4]. Clinical studies suggest that declining adaptive capacity and compromised bodily resilience in older adults can exacerbate feelings of isolation, creating a bidirectional cycle between loneliness and deteriorating health [5].

Scholarly definitions of loneliness vary. Weiss conceptualized it as emotional distress from unmet desires for social connection [1], whereas Sermat and Peplau emphasized discrepancies between expected and actual relationship quality [6, 7]. Despite conceptual divergence, loneliness universally reflects ‌(1)‌ dissatisfaction with interpersonal relationships, ‌(2)‌ subjective self-perception rather than objective isolation, and ‌(3)‌ an inherently unpleasant emotional state.

Weiss conceptualizes loneliness as two distinct dimensions: ‌social loneliness and ‌emotional loneliness. These dimensions may coexist or manifest independently [1]. Specifically, social loneliness is closely tied to ‌social isolation‌, which results from a lack of connection and integration into society [8]. Empirical studies indicate that functional limitations in ‌activities of daily living (ADL)‌ among older adults exacerbate social isolation by restricting participation in social activities and reducing interpersonal interactions, thereby amplifying perceptions of loneliness [9,10,11].

Frailty, recognized as a ‌multidimensional syndrome of diminished physiological reserves and resilience‌, heightens vulnerability to stressors and disease susceptibility in older adults [12]. Emerging evidence delineates its bidirectional relationship with ‌activities of daily living (ADL)‌: frailty exacerbates functional decline through mechanisms such as impaired physiological function, chronic disease burden, cognitive-psychological deterioration, and a self-reinforcing frailty-ADL vicious cycle [13, 14]. Notably, frailty exhibits ‌early-stage reversibility‌, where timely interventions (e.g., resilience-building programs) can mitigate disability progression and improve health outcomes [15]. The World Health Organization prioritizes frailty management as a cornerstone of healthy aging, asserting that it - more than chronological age - serves as a critical biomarker of functional capacity and quality of life.

While existing research has predominantly examined frailty through a biomedical lens, emerging evidence underscores its psychosocial dimensions. Longitudinal data reveal that frail older adults exhibit 1.5-fold higher loneliness prevalence than their non-frail counterparts, with loneliness independently predicting a 40% increased frailty risk [12]. The integral frailty model conceptualizes physical, social, and psychological domains (Fig. 1), with synergistic interactions exacerbating disability and mental health risks. Importantly, multimodal interventions (e.g., exercise, nutritional support, cognitive-behavioral strategies) demonstrate frailty reversibility, advocating integrated care frameworks [17,18,19].

Fig. 1

Physical exercise and loneliness in the integral conceptual model of frailty

Among these interventions, physical exercise is considered the preferred strategy for preventing and treating frailty in older adults. Research has shown that a combination of resistance training, balance exercises, and other forms of physical activity can significantly improve lower limb strength and balance in frail older adults [20]. Concurrently, aerobic modalities mitigate psychological frailty by alleviating negative emotions and enhancing self-efficacy [21]. Social engagement inherent in group-based exercise programs further attenuates social frailty through strengthened interpersonal connectivity, synergistically addressing physical, psychological, and social frailty domains [22, 23].

Prior studies have examined frailty in relation to physical exercise and loneliness separately, yet no study has systemically examined their triadic interactions. Guided by the Integral Conceptual Model of Frailty, we posit that: 1) High physical frailty may engender negative self-schemas and social isolation risks through excessive preoccupation with physical vulnerability; 2) Social frailty manifests as diminished social networks and inadequate support systems; 3) Psychological frailty is marked by heightened susceptibility to negative affective states. These intersecting risk domains, whether operating independently or synergistically, potentiate adverse psychosocial outcomes such as depression and loneliness.

As a multidimensional health intervention, physical exercise augments physiological reserve capacity while attenuating physical and psychological frailty through enhanced systemic resilience. Concurrently, its social engagement component reduces social frailty by mitigating isolation mechanisms inherent in limited interpersonal connectivity [24, 25]. We therefore posit H1: Frailty mediates the association between physical exercise and loneliness in older adults.

Depression and loneliness often co-occur in older adults, with correlation coefficients ranging from 0.4 to 0.6 [26, 27]. Evidence suggests bidirectional associations: depression mediates loneliness through social withdrawal and negative self-appraisals that impair social engagement, while loneliness exacerbates depressive symptoms via chronic stress responses, dysregulated cortisol secretion, and hippocampal dysfunction [28, 29]. Depression and loneliness in older adults are not isolated phenomena but rather a complex interplay involving physiological, psychological, and social factors.

Empirical evidence confirms physical exercise’s antidepressant efficacy via multimodal mechanisms [30, 31]. Physiological pathways involve neuroendocrine adaptations (dopaminergic/serotonergic regulation), anti-inflammatory modulation (IL-6/TNF-α reduction), and neuroplastic enhancement (hippocampal neurogenesis) [32, 33]. Psychosocial processes include negative affect regulation and social reinforcement through self-efficacy cultivation [34]. Given depression’s established role as both exercise-modifiable risk factor and loneliness precursor in aging populations, we posit H2: Depression mediates the association between physical exercise and loneliness in older adults.

Frailty serves as a critical prodromal marker for late-life depression, which frequently manifests as somatic symptomatology (e.g., fatigue, psychomotor retardation) rather than overt emotional distress - a phenotype termed “masked depression” [35]. This clinical overlap with frailty arises from shared pathophysiological substrates, particularly chronic neuroinflammation marked by elevated IL-6, CRP, and TNF-α [36, 37]. Vascular depression variants further exemplify this nexus through motoric slowing and muscular weakness that presage frailty development [38, 39].

From a biological perspective, chronic inflammation may impair hippocampal neurogenesis and prefrontal cortex function through the “brain-immune axis,” while simultaneously accelerating muscle loss, thereby creating a vicious cycle of “inflammation-frailty-depression” [40]. Loneliness, as an extension of this cycle, may be further exacerbated by frailty and depression, leading to a negative feedback loop of “self-isolation and heightened loneliness.”

This study proposes physical exercise may serve as a potential therapeutic strategy to disrupt the frailty-depression-loneliness triad. The possible mechanisms could include anti-inflammatory effects, physiological improvements, enhanced neuroplasticity, and psychosocial empowerment. These multidimensional changes might ultimately contribute to alleviating loneliness among older adults. Given the cross-sectional nature of this study, Hypothesis 3 is proposed as a speculative framework: Frailty and depression play a serial mediating role in the relationship between physical exercise and loneliness in older adults.

Methods

Recruitment and participants

This study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of Zhejiang Normal University (ZSRT20240125).

A purposive criteria-based sampling approach was employed to enhance the representativeness of the sample relative to the target population. Five cities in Zhejiang Province were selected for investigation based on population size, level of economic development, and regional spatial distribution. Specifically, Hangzhou was chosen as the most populous city in the province, Ningbo as the city with the highest per capita GDP, and Jinhua, Quzhou, and Wenzhou as cities representing distinct geographic regions within Zhejiang.

Before the surveys commenced, all participating researchers received training to ensure uniformity in the procedures and requirements across the surveys.

Prior to data collection, investigators coordinated with local sub-district offices and community neighborhood committees to recruit older adults who met the inclusion criteria. The inclusion criteria were as follows: (a) aged 60 years or older; (b) no language communication barriers or severe cognitive impairment, ensuring the ability to complete the questionnaire; and (c) provision of informed consent and voluntary participation. Exclusion criteria included: (a) serious mental or physical illnesses; (b) inability to read or understand the questionnaire; and (c) unwillingness to cooperate with the survey.

From February 25 to July 1, 2023, ten researchers were divided into two groups conducted a questionnaire survey among 600 older adults (aged ≥ 60 years) across the five selected cities. After excluding 95 questionnaires due to insincere responses, defined as those with identical answers for all questions, incomplete responses, or unclear answers, a total of 505 questionnaires were retained for final analysis.

Instruments

The questionnaire used in this study was developed in Chinese and consisted of three sections. First, participants were informed that the survey adhered to the principles of voluntariness and anonymity. They were assured that their responses would be accessible only to the research team and would not be used for commercial or non-research purposes. The second section collected demographic information about the older adults, including age, gender, education level, and other relevant background details. The third section included the standardized scales required for this study, as described below:

Physical activity rating scale (PARS-3)

The questionnaire was developed by Komio Hashimoto, translated in Chinese and revised by Liang Deqing et al. [41]. The questionnaire measured the level of physical activity in terms of intensity, time, and frequency. The PARS-3 Chinese version of the scale is evaluated using the 5-point scoring method and the scale consists of three questions. The total physical activity level was calculated by multiplying intensity by (time-1) by frequency, with 100 as the highest score and 0 as the lowest score. According to the theory of physical activity level, scores ≤ 19 are recorded as low exercise level, 20–42 are recorded as moderate exercise level, and ≥ 42 are recorded as high exercise level.

UCLA loneliness scale-8 (ULS-8)

The scale was adapted by Hays and DiMatteo based on the UCLA-20 scale (University of California Los Angeles Loneliness Scale), consisting of 8 items [42]. This study translated and revised it. The translated version has acceptable reliability (Cronbach’s α coefficient of 0.757) and validity (The fit indices: χ²/df = 4.577, CFI = 0.921, GFI = 0.959, RMSEA = 0.082, SRMR = 0.064) when used in the Chinese context. Among them, 6 items are positively stated as “loneliness” and 2 items are phrased negatively as “non-loneliness.” Each item is rated using a Likert 4-point scoring method, with scores of 1–4 indicating never, rarely, sometimes, and always. The non-loneliness items are reverse-scored. The score on this scale ranges from 8 to 32, where higher scores indicate higher levels of loneliness.

Tilburg frailty index (TFI)

TFI was developed by Gobbens et al. [16], translated in Chinese and revised by Si et al. [43]. The scale was developed on the basis of the integral conceptual model of frailty, and measured the frailty status of the older adults from three dimensions: physical, psychological and social frailty, with a total of 15 items. The scale items adopt the two-classification scoring method, and the scoring range is 0 ~ 15 points, and 5 points and above points are frailty. The higher the score, the heavier frailty. The scale demonstrates good reliability with a Cronbach’s α coefficient of 0.787.

Patient health questionnaire (PHQ-9)

PHQ-9 based on DSM-IV is used to evaluate depression symptoms, comprising nine items. This questionnaire was translated and revised by Chen and has been widely used in the Chinese context [44]. The scoring criteria for PHQ-9 scale were proposed by Kroenke [45]. Each item is scored from 0 to 3, with 0 indicating not at all, 1 indicating several days, 2 indicating more than half the days, and 3 indicating nearly every day. The total score ranges from 0 to 27, with cutoff points at 5, 10, 15, and 20, categorizing the severity of depression as follows: no depression (0–4), mild (5–9), moderate (10–14), moderately severe (15–19), and severe (20–27). Clinically, a score of ≥ 5 indicates a potential association with significant depressive symptoms, with higher scores indicate more severe depression. The scale shows good reliability with a Cronbach’s α coefficient of 0.865.

Data collection

Informed consent was obtained from all participants prior to their involvement in the study. The purpose of the research, along with assurances of confidentiality and anonymity, was clearly explained to ensure participants were fully aware of the procedures and any potential risks. Participants were informed of their right to withdraw from the study at any time without consequence.

The study was conducted as an anonymous survey, and all data collected were used solely for scientific research purposes. To minimize external interference and ensure accurate responses, researchers, accompanied by community staff, visited the homes of older adults to administer the questionnaires. Researchers guided participants throughout the questionnaire completion process, assisting them in understanding the meaning of the items. For participants who had difficulty reading or comprehending the questions, researchers read the items aloud and provided additional explanations as needed to facilitate accurate responses.

On average, participants took approximately 20 min to complete the questionnaire. All questionnaires were collected immediately after completion to ensure data integrity.

Data analysis

Data processing and statistical analysis were conducted using SPSS 26.0 (IBM, Armonk, NY, USA) and AMOS 27.0 (IBM, Armonk, NY, USA). Independent sample t-tests and one-way ANOVA were performed to compare group differences. To address potential common method bias, Harman’s single-factor test was applied. Pearson correlation analysis was used to examine relationships between variables. Regression analysis and mediation effects were tested using the bias-corrected percentile bootstrap method.

Mediation effects were tested using AMOS. AMOS is a software designed for SEM. It enables the simultaneous estimation of relationships among multiple variables, including direct, indirect, and total effects. For confirmatory factor analysis, AMOS can intuitively construct factor models and provide rich fitting indices to help us evaluate the fitting degree of the model, which is crucial for our research. The Bootstrap method was employed. The parameter settings as follows: the number of bootstrap samples was set to 5,000 to ensure the stability and accuracy of the estimates, the confidence level was set to 95% for calculating the confidence intervals of the mediation effects. In the output, the primary statistical values of interest included the estimated mediation effect, standard error, t-value, and 95% confidence interval. These values are reported in the Results.

A significance level of p < 0.05 was set for all statistical tests.

Reliability and validity

Confirmatory Factor Analysis (CFA) was employed to assess the internal validity of each construct. For the ULS-8, the fit indices were as follows: χ²/df = 4.577, CFI = 0.921, GFI = 0.959, RMSEA = 0.082, SRMR = 0.064. While the chi-square value can be influenced by sample size, value less than 3 are considered ideal, and values less than 5 are relatively acceptable. For the TFI, the fit indices were χ²/df = 1.340, CFI = 0.970, GFI = 0.968, RMSEA = 0.026, SRMR = 0.038, and the PHQ-9 were χ²/df = 1.103, CFI = 0.999, GFI = 0.989, RMSEA = 0.014, SRMR = 0.018. All meet the validity score standards for scales (χ²/df < 3, CFI > 0.9, GFI > 0.9, RMSEA < 0.08, SRMR < 0.08), indicating a good fit for these scales. In terms of reliability, the Cronbach’s α coefficients for the ULS-8, the TFI, and the PHQ-9 were 0.757, 0.787, and 0.865 respectively. Both Cronbach’s α coefficients and composite reliability (CR) exceeded the threshold of 0.7, confirming good reliability for the mentioned scales.

Common method bias test

Since all structural data were collected through self-reported surveys, common method bias (CMB) is a potential concern. To mitigate the impact of CMB on the validity of the results, several procedural measures were implemented during the survey process, including balanced item ordering, anonymous questionnaire administration, and standardized measurement protocols. Additionally, Harman’s single-factor test was conducted to assess the extent of CMB. The results indicated that the first principal component accounted for 24.47% of the total variance, which is below the recommended threshold of 40%, suggesting that common method bias was not a significant issue in this study.

Results

Descriptive analysis

From February to July 2023, a total of 600 questionnaires were distributed. After excluding questionnaires with excessively short response times and highly consistent answers, 505 valid responses were collected, with a total valid rate of 84.16%.

Table 1 Descriptive information

Overall status and variance analysis of physical exercise in the older adults

Overall status of physical exercise in the older adults refer to Table 2. Differences in loneliness, frailty, and depression scores of older adults people with different exercise levels refer to Table 3. The results suggest that with an increase in exercise levels, loneliness, frailty, and depression scores of older individuals gradually decrease, and there are significant differences in each score.

Table 2 Physical exercise status of the older adults

Table 3 Comparison of loneliness, frailty and depression in different physical exercise levels

Correlational analysis

Table 4 shows the average value (M) and standard deviation (SD) of physical exercise, loneliness, frailty, and depression. Spearman’s analysis method was used to test the correlation coefficient between physical exercise and loneliness, frailty, and depression. The results demonstrated that all variables were significantly correlated, with correlation coefficients ranging from 0.41 to 0.67, supporting the measurement model and the construction of the mediation effect model.

Table 4 Mean, standard deviations, and correlation

Path analysis of mediation model

Before testing the path model, confirmatory factor analysis was conducted on physical exercise, loneliness, frailty, and depression. According to Kline (1998) [46], the recommended χ2/df ratio is less than 3; CFI and TLI values should be greater than 0.90; RMSEA values should be less than 0.08, indicating an acceptable model. The measurement model in this study fits the data well (χ2/df = 1.618, CFI = 0.933, TLI = 0.928, RMSEA = 0.035, SRMR = 0.047).

In the mediation effects model (Fig. 2; Table 5), all path coefficients are significant (P < 0.001), it can be concluded that the mediation effect of frailty and depression between physical exercise and loneliness is significant. Using Bootstrap testing to verify the model, with 5,000 repetitions and calculating a 95% confidence interval. The final model results show that the confidence interval of the mediation effect from physical exercise to frailty -depression to loneliness is [-0.011- -0.002], which does not include 0, confirming a significant mediation effect.

From Table 5, it is evident that the direct effect of physical exercise on loneliness is -0.063, with an indirect effect (mediation effect) of -0.089, accounting for 58.55% of the total effect. Frailty and depression play a partial mediating role in the relationship between physical exercise and loneliness, confirming H3.

Fig. 2

Frailty and depression play a serial mediating role in the relationship between physical exercise and loneliness

Table 5 The mediation effect analysis of frailty and depression in the influence of physical exercise on loneliness

Discussion

Based on the integral conceptual model of frailty, the high correlation between depression and loneliness, the shared pathological mechanisms of frailty and depression, and the established effects of physical exercise on frailty, depression, and loneliness, this study examined the roles of frailty and depression in the relationship between physical exercise and loneliness. The findings suggest that physical exercise may alleviate loneliness in older adults by enhancing physiological function and fostering psychosocial empowerment, providing theoretical support for the design of exercise-based interventions targeting loneliness in this population. It is important to note that, as a cross-sectional study, this research can only describe the associations between these factors and cannot infer causality.

Direct effect of physical exercise on loneliness

Scientific evidence indicates that one of the most significant benefits of regular physical activity may be the improvement of quality of life related to physical fitness, particularly through the maintenance or enhancement of age-related functions in key physiological systems, including the endocrine, respiratory, cardiovascular, and musculoskeletal systems [47]. Long-term and regular physical exercise not only effectively enhances physiological reserves and resilience in older adults, reducing the risk of social isolation due to increased bodily vulnerability or disease, but also modulates the levels of certain neurotransmitters, such as monoamines (e.g., dopamine and serotonin) and endorphins [48]. These neurotransmitters can stimulate brain function at a broader level, fostering more positive self-perceptions and thereby reducing a range of negative emotions and cognitive patterns. This process is similar to the effect of physical exercise on subjective well-being, both of which are achieved through the sense of vitality and pleasure derived from physical activity itself.

In addition, long-term longitudinal studies have shown that older adults who consistently engage in physical exercise have a social network size 40% larger than those who are sedentary [49] In daily life, many older adults view participation in physical activities as a primary means of social engagement, which alters their social environment. This environmental change can influence their attitudes and beliefs toward life, thereby shaping their perceptions of their own circumstances and enhancing their perceived social support. High levels of social support have been shown to effectively reduce feelings of loneliness among older adults [3].

In summary, this study suggests that physical exercise may be associated with a reduction in loneliness among older adults. This association may manifest in two ways: on the one hand, physical exercise may help alleviate emotional loneliness by directly enhancing physiological resilience and providing a sense of vitality and pleasure derived from the activity itself; on the other hand, through indirect psychosocial mechanisms such as expanding social networks and strengthening social support, physical exercise may positively influence the reduction of social loneliness.

The mediating role of frailty between physical exercise and loneliness

According to the integral conceptual model of frailty, the combined effects of physical, psychological, and social frailty are significant contributors to adverse outcomes such as diseases and psychological disorders. These frailty factors can be mitigated through health promotion interventions, including physical exercise, nutrition, cognitive training, psychological support, and socioeconomic assistance. Among these, physical exercise is the preferred strategy for preventing and treating frailty in older adults [12]. Studies have shown that effective exercise interventions can improve the condition of approximately 10–20% of individuals with physical frailty or pre-frailty during follow-up periods [47], which has positive implications for alleviating social isolation caused by limited mobility.

The stress-buffering model suggests that functional decline in older adults is a significant stressor [50]. Resilient older adults may adopt proactive and problem-focused coping strategies, gaining growth from effective coping. Research has confirmed that physical exercise can promote the development of psychological resilience in older adults [51]. The author posits that those who improve their resilience through exercise demonstrate higher self-control, resilience, and positive attitudes when facing adversity, which helps alleviate internal dysregulation, maintain mental health stability, and reduce psychological frailty and associated loneliness.

As individuals age, older adults tend to prioritize maintaining their current health and abilities over improving them. For example, leisure exercise activities involving social interactions are not only enjoyable and low-risk but also fulfill the need for love and belonging. This study found that older adults prefer participating in group-based physical activities, such as walking with family or friends. This finding is understandable because China is a country with a collectivist culture, and group-based physical exercise may provide an ideal social environment for collective-oriented individuals to satisfy their desire for collectives. The integral conceptual model of frailty indicates that a broader social network significantly reduces social frailty levels, and older adults with lower social frailty tend to experience less loneliness. Therefore, group-based physical exercise creates more opportunities for social interaction, offering significant value in reducing loneliness caused by social isolation or social frailty.

The mediating role of depression between physical exercise and loneliness

As individuals age, changes in brain biochemistry and structure often occur, accompanied by shifts in personality traits such as social withdrawal, passivity, dependency, and stubbornness. These changes reduce older adults’ resilience to stress, thereby increasing the risk of depression and other psychological issues. Additionally, environmental stressors and challenges (e.g., retirement, empty nest syndrome, loss of loved ones, disability, and illness) significantly impact mental health, leading to reduced social connections and emotional isolation [51].

Depression not only impairs an individual’s ability to maintain relationships but also negatively affects interpersonal interactions, increasing the risk of loneliness. Research indicates that depression and loneliness are related yet distinct constructs: loneliness may unpredictably predict subsequent depression, while depression has a stable prospective effect on loneliness [52]. Therefore, simultaneously assessing depression and loneliness is crucial for breaking the vicious cycle between the two.

This study found that depression mediates the relationship between physical exercise and loneliness in older adults. The neurobiological hypothesis suggests that physical exercise can enhance positive emotions and reduce depressive symptoms by modulating the HPA axis’s response to stress, promoting the secretion of endorphins and monoamine neurotransmitters, and increasing the release of neurotrophic factors [50]. Even when the level of social support in physical activities is low, these physiological mechanisms may still provide some emotional improvements, as exercise meets the physiological needs embedded in human evolutionary mechanisms, thereby enhancing resilience to negative emotions such as loneliness.

Furthermore, the distraction hypothesis posits that physical exercise can alleviate depression and anxiety by diverting attention from negative emotions or promoting positive behaviors [53]. At the same time, social interactions during exercise provide opportunities for relaxation and social support, further reducing depression and loneliness. In summary, physical exercise mitigates depression in older adults through biological, psychological, and sociological pathways. Given the close link between depression and loneliness, the experience of loneliness may also decrease as a result.

The serial mediating role of frailty and depression between physical exercise and loneliness

Research indicates that a vicious cycle exists between frailty and depression in older adults. Frailty increases the risk of depression by impairing brain function through chronic inflammation, oxidative stress, and neuroendocrine dysregulation (e.g., HPA axis dysfunction) [54]. Conversely, depression exacerbates frailty as individuals often adopt unhealthy behaviors (e.g., reduced physical activity, poor nutrition), accelerating muscle loss and functional decline [55]. This bidirectional relationship places individuals experiencing both frailty and depression at higher health risks. The author posits that loneliness reflects the social isolation resulting from limited mobility or intensified negative emotional experiences. Therefore, comprehensive interventions are needed to address this complex interplay.

Studies have demonstrated the significant efficacy of comprehensive interventions in alleviating frailty, depression, and loneliness. For example, cognitive behavioral therapy (CBT) combined with exercise interventions can increase the remission rate of depression by 40% in frail older adults [56]. Additionally, multicomponent interventions integrating exercise, nutrition, and psychosocial support can improve frailty by 20% and reduce depressive symptoms by 35% [48]. These findings underscore the importance of multidimensional interventions in breaking the “frailty-depression” vicious cycle.

Based on the evidence, the author suggests that exercise interventions for loneliness in older adults should adopt a stratified approach tailored to frailty levels, depressive symptoms, and available social resources to enhance adherence and effectiveness. Specific strategies include:

For Social Frailty: Community or group-based exercise programs (e.g., Tai Chi, square dancing) may enhance social interactions and alleviate depressive symptoms, social isolation and loneliness.

For High and Physical Frailty: Resistance training (e.g., elastic band exercises, dumbbell training) may improve muscle function, physical resilience, and ADL.

For Psychological Frailty: Personalized exercise plans based on physical condition and personal interests, starting with low-intensity activities (e.g., walking, yoga) and gradually increasing intensity and duration, may enhance psychological resilience and foster positive experiences.

Future studies should further explore the underlying mechanisms, focusing on shared molecular pathways (e.g., inflammatory pathways, neuroplasticity) and longitudinal relationships among frailty, depression, and loneliness. This will facilitate the development of personalized exercise interventions to optimize outcomes.

Conclusions

This study demonstrated a negative correlation between physical exercise and loneliness among older adults, with frailty and depression serving as significant mediators in this relationship. The findings suggest that physical exercise may alleviate loneliness in older adults by enhancing physiological function and fostering psychosocial empowermen.

Limitations and prospects

This study has several limitations that should be acknowledged:

Limited Generalizability to Disabled Older Adults: The findings of this study can not reflect the situation of disabled older individuals, as they were not specifically included in the analysis.

Regional and Spatial Bias: The respondents were primarily from Zhejiang, China, which may introduce regional and spatial biases. Future studies could expand the sample to include diverse geographical areas to enhance the generalizability of the conclusions.

Multidimensional Nature of Frailty: While frailty is a multidimensional construct, this study was unable to precisely predict the effects of each dimension (e.g., physical, psychological, social) on loneliness. Further research is needed to explore these relationships in greater detail.

Sample Heterogeneity: The study did not account for potential influences of sample differences, such as gender, family structure, urban–rural disparities, or educational and cultural backgrounds. Future studies could stratify samples based on these factors to provide more nuanced insights.

Post-Pandemic Context: The potential impact of changes in living habits and health awareness among older adults following the COVID-19 pandemic could not be estimated in this study. Future research should consider these evolving dynamics to better understand their influence on the results.