The purpose of this cross-sectional, observational study is to evaluate the relationship between quadriceps muscle quality-specifically muscle echogenicity and thickness change during isometric contraction-and clinical outcomes such as pain, physical function, quality of life, and Patient Global Assessment (PtGA) in female patients with knee osteoarthritis. Participants will undergo a non-invasive ultrasound evaluation of their thigh muscles and complete standard clinical questionnaires to help determine how muscle structural and functional properties impact the daily lives of patients with knee osteoarthritis.

Knee osteoarthritis (OA) is a highly prevalent degenerative joint disease characterized by pain, loss of function, and reduced quality of life. A critical factor related to the clinical course of the disease is quadriceps muscle weakness and deterioration in muscle quality, including intra-muscular fatty infiltration and fibrotic changes.

This single-center, cross-sectional observational study aims to comprehensively assess quadriceps muscle quality using ultrasonography (USG) and investigate its relationship with core clinical outcomes in patients with knee OA. Specifically, the study will measure muscle echogenicity (a surrogate marker of fat infiltration and fibrosis) and the rate of muscle thickness change during maximum voluntary isometric contraction (MVIC), which reflects the dynamic functional response of the muscle.

Female patients aged 40 to 70 years with radiographically confirmed Kellgren-Lawrence grade 2-3 knee OA will be recruited from the physical medicine and rehabilitation clinic. Following informed consent, participants will undergo clinical assessments using standardized patient-reported outcome measures. These include the Visual Analog Scale (VAS) for pain intensity, the Patient Global Assessment (PtGA), and the Knee Injury and Osteoarthritis Outcome Score (KOOS) subscales for physical function (KOOS-PS) and quality of life (KOOS-QoL).

Ultrasonographic evaluations will be performed by a single evaluator using a linear high-resolution probe (5-10 MHz). Rectus femoris (RF) and vastus intermedius (VI) muscle thicknesses will be recorded at rest and during a 5-second MVIC to calculate the thickness change ratio. Furthermore, muscle echogenicity (grayscale median) will be quantified via image analysis software and mathematically corrected for subcutaneous fat thickness to ensure accuracy.

By evaluating both the structural and functional ultrasonographic features of the quadriceps muscle simultaneously, this study seeks to provide a holistic understanding of how muscle quality impacts the core outcome domains recommended by OMERACT-OARSI for knee osteoarthritis, ultimately contributing to better prognostication and more individualized rehabilitation strategies.

Inclusion Criteria:

• Female sex
• Aged between 40 and 70 years
• Diagnosed with clinical and radiographic primary knee osteoarthritis (Kellgren-Lawrence Grade 2 or 3)
• Ability to perform isometric contraction at 90° of knee flexion
• Voluntary participation and provision of written informed consent

Exclusion Criteria:

• History of knee surgery within the last 6 months
• Intra-articular injection (e.g., corticosteroids, hyaluronic acid) within the last 3 months
• Presence of inflammatory rheumatic diseases (e.g., Rheumatoid Arthritis, Spondyloarthropathy)
• Known neuromuscular disorders affecting the lower extremities
• Severe cardiopulmonary comorbidities that prevent participation in clinical assessments
• Severe patellofemoral instability

Female patients aged 40-70 years diagnosed with knee osteoarthritis (OA) who apply to the Physical Medicine and Rehabilitation Clinic of Izmir City Hospital will be screened according to the study criteria. Individuals diagnosed clinically and radiographically with Kellgren-Lawrence Grade 2-3 knee osteoarthritis, who are capable of performing isometric quadriceps contractions, and who volunteer to participate in the study will be included after signing the informed consent form.

No final decision has yet been made regarding the future availability of the data.