Reference Equations and Normative Data for Isometric Quadriceps and Handgrip Muscles STrength in Southern European Populations - RESTart: a Multicentre Study

This observational study aims to learn about peripheral muscle strength in healthy adults and people with chronic respiratory diseases. The study aims to develop reference data for quadriceps muscle strength and handgrip in adults living in Southern Europe and to assess whether these reference data can be used in people with chronic respiratory diseases. The main questions it aims to answer are: * What are the normal values for quadriceps and handgrip muscle strength in healthy adults aged 18-89 years living in Spain, Portugal and Italy? * Can the reference values developed in healthy adults be applied to people with chronic respiratory diseases? Participants will attend one study visit. During this visit, researchers will: * Measure quadriceps muscle strength and handgrip. * Collect information about age, sex, height, weight and other health characteristics. * Perform simple functional tests related to mobility and physical performance. * Ask participants to complete questionnaires about symptoms, physical activity and health-related quality of life.

Healthy adults cohort Inclusion Criteria: * Age between 18-89 years. * Spanish, Portuguese or Italian, or permanently resident in one of those countries for at least five years. * Be community-dwelling (i.e., not hospitalised or institutionalised). * Be functionally independent and capable of performing study assessments without assistance or limiting pain. * Be able to follow verbal instructions in the local language (Spanish, Portuguese or Italian, as appropriate). * Provide written informed consent. Exclusion Criteria: * Acute or chronic musculoskeletal conditions affecting strength: recent fractures, severe osteoarthrosis, ligament injury, acute tendinitis in hands/knee areas, among others. * Recent surgery (e.g., within the past six months) in upper or lower limbs. * Neuromuscular disorders: Parkinson, multiple sclerosis, stroke, peripheral neuropathies, among others. * Non-communicable diseases affecting strength or functional status: active cancer, any unstable heart diseases; any CRD; active rheumatological diseases, obesity (BMI≥30kg/m2), uncontrolled metabolic diseases, among others (not included: controlled dyslipidaemia or arterial hypertension). * Cognitive impairments (e.g., dementia) or psychiatric disorders impairing following instructions. * Aid devices for walking or compromised functional mobility. * Moderate-to-severe pain during the assessment (VAS ≥4). * Systemic corticosteroids use for over one week in the previous six months (i.e., risk of muscle atrophy in type II fibres; or steroid myopathy). * Muscle relaxants use during the previous week (i.e., risk of not full muscle activation). * Pregnancy and postpartum (i.e., potential physiological changes which may alter muscle strength). * Current smoking. Chronic respiratory disease cohort Inclusion criteria: * Aged 18-89 years. * Clinical diagnosis of a chronic respiratory disease (e.g., COPD, interstitial lung disease, asthma, bronchiectasis, cystic fibrosis), confirmed by spirometry or clinical records. * Clinically stable (no exacerbation within the past four weeks). * Capacity to provide informed consent and complete study procedures. Exclusion criteria: * Coexisting conditions which meet the exclusion criteria defined for healthy participants, apart from chronic respiratory diseases. * Current requirement for hospitalisation or acute medical management. * Severe functional limitation preventing completion of assessments.