Knee osteoarthritis (OA) is a degenerative joint disease characterized by pain, muscle weakness, and functional limitations that significantly affect lower limb muscle performance. Structural and physiological alterations in the quadriceps muscle, particularly in the vastus medialis obliquus (VMO), have been reported in individuals with knee OA, including reductions in muscle cross-sectional area, intramuscular fat infiltration, vascular changes, and impaired neuromuscular activation.

Muscle oxygenation (SmO₂) is an objective indicator of the metabolic status and circulatory capacity of skeletal muscle. However, the oxygenation dynamics of the quadriceps muscle before and after exercise in individuals with knee OA have not been sufficiently investigated.

The aim of this study is to evaluate VMO muscle oxygenation at rest, during exercise, and during the recovery period in individuals with knee OA and to compare these parameters with age- and sex-matched healthy individuals. In addition, the study will examine the relationship between muscle oxygenation parameters, muscle strength, functional performance, and pain levels.

Understanding these physiological responses may provide insight into the metabolic mechanisms underlying muscle dysfunction in knee OA and may contribute to the development of more individualized rehabilitation strategies.

Knee osteoarthritis (OA) is a degenerative joint disorder associated with pain, muscle weakness, and functional limitations that negatively affect lower extremity muscle performance. Previous studies have reported structural and physiological alterations in the quadriceps muscle in individuals with knee OA, particularly in the vastus medialis obliquus (VMO). These alterations include reduced muscle cross-sectional area, increased intramuscular fat infiltration, vascular changes, and impaired neuromuscular activation, which may contribute to decreased knee joint stability and progression of the disease.

Muscle oxygenation (SmO₂) is an important physiological parameter reflecting the balance between oxygen delivery and utilization in skeletal muscle and provides objective information about muscle metabolic and circulatory function. Despite the potential relevance of SmO₂ in understanding quadriceps dysfunction in knee OA, studies investigating exercise-induced oxygenation responses and recovery dynamics in this population remain limited.

This observational, comparative cross-sectional study aims to evaluate VMO SmO₂ dynamics in individuals with knee OA and compare these parameters with healthy controls matched for age and sex. The study will also investigate the associations between SmO₂ parameters, muscle strength, functional performance, and pain intensity.

Individuals diagnosed with knee OA by a physical medicine and rehabilitation physician and healthy volunteers meeting the eligibility criteria will be invited to participate. Written informed consent will be obtained from all participants prior to participation. Participants who meet the inclusion criteria will be allocated into two groups: a knee OA group and a healthy control group. All participants will undergo the same assessment procedures during a single evaluation session.

In accordance with the recommendations of Osteoarthritis Research Society International (OARSI), in participants with bilateral knee OA, the target knee for analysis will be determined using a predefined strategy: (1) the knee with greater pain at rest or during activity will be selected; (2) if pain levels are equal, the knee with greater radiographic severity (Kellgren-Lawrence grade) will be selected; (3) if both pain and radiographic severity are equal, the dominant leg (the extremity preferred for kicking a ball) will be designated as the target knee. The target knee will serve as the reference knee for follow-up measurements, including SmO₂ and knee-specific clinical assessments.

To our knowledge, no previous study has directly compared SmO₂ parameters between individuals with knee OA and healthy controls. Therefore, a study investigating SmO₂ changes in a similar muscle group and using the same measurement technique in individuals who underwent anterior cruciate ligament reconstruction was used as a reference. Sample size estimation was performed using G*Power software (version 3.1) based on a two-tailed independent samples t-test model for comparing the mean difference between two groups. The effect size reported for VMO SmO₂ in the reference study (Cohen's d = 1.02) was used for the calculation. With an alpha level of 0.05, statistical power of 0.80, and a group allocation ratio of 1:1, the required sample size was calculated as 17 participants per group, resulting in a total of 34 participants.

Before the measurements, participants will rest in a seated position for 10 minutes. Pain intensity will be assessed using the Visual Analog Scale (VAS). Following the rest period, SmO₂ measurements will be performed using a near-infrared spectroscopy device (Moxy Monitor) placed over the VMO muscle. Baseline SmO₂ values will be recorded during a 60-second measurement period.

After the baseline measurement, participants will perform a standardized mini-squat exercise task. The mini-squat was selected as a safe and functional closed-chain exercise commonly used in individuals with knee osteoarthritis to activate lower-extremity musculature, and it was combined with isometric hip adduction using a ball placed between the knees to enhance vastus medialis obliquus activation and medial knee stabilization. Immediately following exercise, VMO muscle oxygenation will be monitored, and the recovery process will be recorded. Recovery time will be defined as the duration required for the SmO₂ value to return to its baseline level while the participant rests in a seated position.

After completion of the recovery measurements, participants will rest for 10 minutes to minimize the effects of fatigue. Subsequently, muscle strength will be assessed using an isokinetic dynamometer through maximal voluntary isometric knee extension contractions performed at 15° of knee flexion. This joint angle was selected because the vastus medialis obliquus demonstrates greater activation near terminal knee extension. Additionally, testing at 15° places the quadriceps in a biomechanically advantageous position for torque production while minimizing joint stress. Participants will perform three maximal contractions, each lasting approximately 6 seconds, with standardized verbal encouragement and 60-second rest intervals; the mean peak torque value will be used for analysis. Functional performance will then be assessed using the 30-Second Sit-to-Stand Test. All measurements will be performed during a single session, and participants will not be followed longitudinally.

Statistical analyses will be performed using SPSS. Data distribution will be assessed using normality tests. Descriptive statistics will be presented as mean ± standard deviation or median as appropriate. Group differences between individuals with knee OA and healthy controls will be analyzed using independent samples t-test or Mann-Whitney U test. Associations between muscle oxygenation parameters, muscle strength, functional performance, and pain will be examined using Pearson or Spearman correlation analyses. A p-value < 0.05 will be considered statistically significant.

Inclusion Criteria:

• Age between 45 and 65 years
• Body mass index (BMI) below 30 kg/m²
• Willingness to participate in the study

Additional criteria for the knee osteoarthritis group:

• Diagnosis of knee osteoarthritis confirmed by a physician according to the American College of Rheumatology diagnostic criteria
• Bilateral knee osteoarthritis classified as Kellgren-Lawrence grade 2 or 3

Exclusion Criteria:

• Presence of another orthopedic or rheumatologic condition affecting the lower extremity
• Uncontrolled hypertension
• Presence of lower extremity arterial or venous disorders
• Presence of a neurological disease
• Cognitive, learning, or perception impairments
• Knee extension deficit
• Physical activity level exceeding 600 MET-min/week according to the International Physical Activity Questionnaire Short Form (IPAQ-SF)
• Participation in a physiotherapy and rehabilitation program for knee osteoarthritis within the last 6 months
• Intra-articular knee injection within the past 6 months

The study population will consist of adults aged 45-65 years, including individuals diagnosed with knee osteoarthritis (Kellgren-Lawrence grade 2-3) according to the American College of Rheumatology criteria and age- and sex-matched healthy controls. All participants will meet the predefined eligibility criteria and provide written informed consent prior to participation.