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

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.