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

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.