Sarcopenia in older adults is associated not only with loss of muscle mass but also with deterioration of muscle quality, particularly intramuscular fat infiltration. While muscle mass is commonly assessed, muscle quality remains insufficiently explored in routine clinical practice.

The FATUS-OLD study aims to evaluate the clinical relevance of a novel ultrasound-based multiparametric approach to assess intramuscular fat infiltration and muscle volume in hospitalized older adults undergoing rehabilitation. The main hypothesis is that higher intramuscular fat infiltration at baseline is associated with poorer recovery of physical performance at 6 months, independently of muscle volume.

This non-invasive, rapid, and radiation-free imaging approach could improve sarcopenia phenotyping and help identify new prognostic biomarkers for clinical follow-up and future interventional trials.

FATUS-OLD is a prospective, monocentric, open-label, non-randomized observational study conducted at Nantes University Hospital. The study includes older adults aged 75 years and older hospitalized in a rehabilitation day-hospital program for gait or balance disorders.

Participants undergo ultrasound assessments of skeletal muscle using a CE-marked ultrasound device capable of measuring muscle volume and intramuscular fat infiltration through matrix-based analysis. Measurements are performed at baseline and during follow-up.

The primary objective is to assess the prognostic value of baseline intramuscular fat infiltration on physical performance recovery at 6 months, evaluated using the Short Physical Performance Battery (SPPB). Secondary objectives include the relationship between muscle volume, muscle quality, muscle strength, appendicular lean mass measured by bioelectrical impedance analysis, and clinically significant events such as falls, loss of independence, rehospitalization, and mortality.

The study is exploratory and aims to validate innovative ultrasound-derived biomarkers of muscle quality in a real-life geriatric population.

Inclusion Criteria:

• Age ≥ 75 years
• Hospitalized in rehabilitation day-hospital program
• Written informed consent

Exclusion Criteria:

• Moderate to severe neurocognitive disorders
• Inability to comply with study procedures

All participants undergo standardized clinical assessments and ultrasound-based muscle evaluations as part of routine rehabilitation follow-up.