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.

The RESTart study (Reference equations and normative data for isometric quadriceps and handgrip STrength in Southern European populations) is a multicentre observational cross-sectional study conducted in Spain, Portugal and Italy.

Peripheral muscle dysfunction, particularly reduced quadriceps muscle strength and handgrip, is common in people with chronic respiratory diseases and is associated with impaired physical function, hospitalisation and mortality. Accurate interpretation of muscle strength measurements requires population-specific reference equations and normative values. However, currently available reference data may not adequately represent Southern European populations.

The primary objective of the study is to develop reference equations and normative values for isometric quadriceps muscle strength and handgrip in healthy adults aged 18-89 years. A secondary objective is to validate/assess clinical application of these reference data in people with chronic respiratory diseases, including chronic obstructive pulmonary disease, interstitial lung disease, asthma, bronchiectasis and cystic fibrosis.

Healthy participants will be recruited from community settings using proportional stratification according to age, sex and country. Participants with chronic respiratory diseases will be recruited from collaborating clinical centres.

Primary outcomes are isometric quadriceps muscle strength and handgrip assessed using standardised dynamometry protocols. Secondary outcomes include functional capacity, mobility and balance, symptoms, physical activity, health-related quality of life, and anthropometric variables; lung function and laboratory parameters only when available from medical records.

The study is expected to provide robust reference equations and normative values for Southern European populations, supporting improved identification and monitoring of peripheral muscle dysfunction in both clinical practice and research.

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.

Community-dwelling healthy adults and adults with chronic respiratory diseases recruited from participating centres in Spain, Portugal and Italy. Healthy participants will be recruited from the general population to develop reference equations and normative values for isometric quadriceps and handgrip strength. Participants with chronic respiratory diseases will be recruited from clinical settings to externally validate the developed reference equations and assess their clinical applicability.

De-identified individual participant data underlying the results reported in publications arising from this study, including demographic characteristics, anthropometric measurements, peripheral muscle strength assessments, functional capacity outcomes, patient-reported outcomes/questionnaire scores, and other variables collected according to the study protocol, upon reasonable request to the principal investigator.

Beginning 12 months following publication of the primary study results and ending 5 years after publication.

Researchers who provide a methodologically sound proposal for secondary analyses consistent with the aims of the study (e.g., individual participant data meta-analyses, validation studies, methodological research, and other scientifically justified secondary analyses).