This observational study aims to determine whether ultrasound-derived muscle biomarkers can predict isometric quadriceps muscle strength and handgrip in healthy adults aged 18-89 years.

The main questions it aims to answer are:

• Which ultrasound muscle characteristics are associated with isometric quadriceps muscle strength and handgrip?
• Can ultrasound-derived biomarkers be used to develop predictive models of muscle strength in healthy adults?

Participants will undergo:

• Ultrasound assessment of the quadriceps and forearm flexor muscles;
• Measurement of isometric quadriceps muscle strength and handgrip using standardized dynamometry protocols;
• Assessment of functional capacity, mobility and balance;
• Completion of questionnaires related to health status, physical activity, nutrition, fatigue and health-related quality of life.

The study will also assess the reliability of ultrasound measurements and explore the relationship between ultrasound variables, muscle strength, physical function and health-related factors.

Peripheral muscle strength is an important determinant of physical function, independence and health outcomes. Reduced quadriceps muscle strength and handgrip have been associated with functional limitations, frailty, hospitalization and mortality. Although muscle ultrasound is increasingly used to assess muscle morphology and quality, its ability to predict muscle strength remains incompletely understood.

The MUSCLE-US study is a monocentre observational study designed to investigate the relationship between ultrasound-derived muscle biomarkers and isometric quadriceps muscle strength and handgrip in healthy adults. Participants aged 18-89 years will be recruited from community settings and stratified by age and sex.

Ultrasound assessments will be performed on the quadriceps and forearm flexor muscles using standardized acquisition procedures. First-order variables (e.g., muscle thickness, cross-sectional area, pennation angle, fascicle length), second-order texture features and higher-order ultrasound biomarkers will be extracted and analysed.

Isometric quadriceps muscle strength and handgrip will be measured using standardized dynamometry protocols. Additional assessments will include functional capacity tests, mobility and balance measures, physical activity, nutritional status, fatigue, dyspnoea, health-related quality of life, lifestyle factors and comorbidity profiles.

The primary objective is to identify ultrasound biomarkers associated with quadriceps muscle strength and handgrip, and to develop predictive models of muscle strength. Secondary objectives include assessment of ultrasound measurement reliability and exploration of relationships between ultrasound biomarkers, muscle strength, physical function and health-related characteristics.

Recruitment, assessor training, ultrasound standardisation procedures and reliability analyses will be conducted during the initial phase of the project, followed by the main recruitment and data collection phase and subsequent statistical modelling and dissemination activities.

Inclusion Criteria:

• Healthy adults aged 18-89 years.
• Resident in Spain for at least 5 years.
• Living independently in the community and able to complete all study assessments.
• Able to understand and follow verbal instructions in Spanish.
• Able and willing to provide written informed consent prior to study participation.

Exclusion Criteria:

• Neurological, musculoskeletal, or systemic disorders affecting muscle strength or mobility.
• Recent surgery or acute injury affecting the upper or lower limbs.
• Recent use of medications affecting the musculoskeletal system (e.g., systemic corticosteroids for >1 week during the previous 6 months or muscle relaxants during the previous week).
• Cognitive or psychiatric conditions limiting participation.
• Pregnancy.
• Recent postpartum period (≤3 months after delivery).
• Presence of clinically relevant pain, fatigue, physical deconditioning, or functional limitations that may interfere with study procedures or measurement validity.
• Acute illness at the time of assessment.
• Uncontrolled hypertension (≥140/90 mmHg).
• Resting peripheral oxygen saturation ≤90%.
• Moderate-to-severe pain (Numeric Rating Scale ≥4/10).
• Any medical condition considered by the investigator to contraindicate study participation.

Community-dwelling healthy adults aged 18-89 years residing in Spain. Participants will be recruited through community advertisements, institutional dissemination channels, and collaborating centres. The study aims to investigate the relationship between ultrasound-derived muscle biomarkers and isometric quadriceps muscle strength and handgrip in a healthy adult population.

Individual participant data (IPD) that underlie the results reported in publications, after de-identification, will be made available. Shared data may include demographic characteristics, anthropometric variables, isometric quadriceps muscle strength, handgrip, ultrasound-derived muscle variables, functional capacity measures, and patient-reported outcomes/questionnaire data collected as part of the study. Direct identifiers will be removed before data sharing. Data sharing will comply with applicable ethical approvals, informed consent provisions, and data protection regulations.

De-identified individual participant data and supporting documents will become available beginning 12 months after publication of the primary study results and will remain available for at least 5 years thereafter.

Researchers who provide a methodologically sound proposal for secondary analyses or validation studies may request access to the de-identified dataset. Requests will be reviewed by the study investigators. Data will be provided following approval of the proposal and execution of a data-sharing agreement, when required, in accordance with ethical and legal requirements. Supporting documents, including the study protocol, statistical analysis plan, informed consent form template, and analytic code, will be made available upon reasonable request.