Ultrasound Muscle Biomarkers as Predictors of Isometric Quadriceps Muscle Strength and Handgrip in Healthy Adults: an Observational Study

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.