## 2 Materials and Methods
### Variables  
#### 2.4.1 Characterization Variables  
The demographic profile included age (in years), sex (male/female), height (in centimeters), and weight (in kilograms). Educational level was classified according to the International Standard Classification of Education (ISCED-97) [26], distinguishing the following categories: no formal education, preschool education, primary education, lower secondary, upper secondary, post-secondary non-tertiary, first-cycle tertiary education, and currently enrolled in education. Body Mass Index (BMI) was calculated using the standard formula (kg/m\({}^{2}\)) and categorized as underweight (<18.5), normal weight (\(\geq\)18.5 to <25), overweight (\(\geq\)25 to <30), and obese (\(\geq\)30). Regarding chronic health conditions, participants were provided with a definition of long-term disease as a health problem of extended duration. They were then asked whether they suffered from any such illness, disease, disability, or condition. The response format was dichotomous: \"Yes\" or \"No\". Physical condition was evaluated through three domains: handgrip strength (HGS), motor difficulties, and physical inactivity. HGS was measured using a Smedley portable dynamometer (TTM, Tokyo, Japan; capacity: 100 kg). Participants were instructed to  
Figure 1: Sample selection flowchart. exert maximum force while grasping the device. Two measurements were taken per hand, and values were considered valid if the intra-hand difference did not exceed 20 kg [27; 28]. The highest value recorded across all trials was retained and subsequently normalized by body weight (expressed as kg/weight in kg; HGS ratio). To assess mobility and upper limb/fine motor difficulties, participants were asked whether they experienced limitations in a range of 10 functional tasks, including walking 100 m, remaining seated for two hours, rising from a chair, climbing one or multiple flights of stairs, bending, kneeling, squatting, reaching overhead, pulling or pushing heavy objects, lifting over 5 kg, and picking up a coin from a table. Responses were dichotomous (\"Yes\" = 1, \"No\" = 0). Based on this information, a composite variable was created to reflect +3 mobility and upper limb/fine motor limitations, categorizing individuals with more than three reported mobility impairments and additional difficulties in upper extremity or fine motor tasks. Lastly, physical inactivity was determined by self-reported frequency of moderate (e.g., gardening, car washing, walking, etc.) and vigorous (e.g., sports, heavy housework, physically demanding jobs, etc.) activities. Participants who reported never engaging in either were classified as physically inactive. Mental health was evaluated through two key indicators: depression and perceived loneliness. Depression was assessed using the EURO-D scale [29], developed by a European consortium to evaluate depressive symptomatology in older populations. This instrument comprises 12 dichotomous items that capture a range of emotional and cognitive symptoms, including sadness, pessimism, suicidal ideation, guilt, sleep disturbance, anhedonia, irritability, appetite changes, fatigue, concentration difficulties, lack of enjoyment, and tearfulness. The total score ranges from 0 to 12, with higher scores indicating more severe depressive symptoms. The EURO-D scale has demonstrated good convergent validity with other established mental health measures and robust reliability across multiple studies [30; 31; 32]. For analysis purposes, scores were dichotomized: values from 0 to 3 were classified as an absence of depression, while scores \(\geq\)4 indicated the presence of depressive symptoms [33]. Perceived loneliness was measured using the Three-Item Loneliness Scale [34], which assesses perceived social isolation. Participants rated how frequently they felt unaccompanied, left out, or isolated from others, using a 3-point Likert scale (1 = almost never, 2 = sometimes, 3 = often). The total score ranged from 3 to 9, with higher scores reflecting greater perceived loneliness.