Knee osteoarthritis (OA) is a common degenerative joint disease that leads to pain, reduced mobility, and functional limitation. Quadriceps femoris weakness plays a significant role in disease progression and disability. Isotonic strengthening using the N-K Table allows adjustment of load angles to optimize muscle activation; however, the effect of different load angles on quadriceps activation in patients with knee osteoarthritis remains unclear.

This study aims to compare quadriceps femoris activation at 0° and 50° N-K Table load angles in patients with knee osteoarthritis using surface electromyography (sEMG). A randomized cross-over experimental design is applied to determine whether adjusting the load angle improves maximum voluntary contraction (MVC) during isotonic knee extension exercise.

Knee osteoarthritis (OA) is the most prevalent joint disease and a leading cause of disability among older adults. The progressive degeneration of articular cartilage and structural remodeling of the joint result in chronic pain, stiffness, and limited movement. Among the factors contributing to disability, quadriceps femoris weakness plays a central role by impairing knee stability and shock absorption during gait.

Isotonic strengthening exercises using the N-K Table provide a controlled method to restore quadriceps strength while minimizing excessive joint loading. The N-K Table allows adjustment of the load arm angle, which may better align external resistance with the physiological torque profile of the knee extensors, including the vastus medialis, vastus lateralis, and rectus femoris. Although previous studies in healthy adults suggest that higher load angles may optimize torque production, the specific effect in patients with knee osteoarthritis has not been clearly established.

This study employed a randomized cross-over experimental design. A total of 43 female participants aged 50-65 years with unilateral or bilateral knee osteoarthritis (Kellgren-Lawrence grade 2-3) were enrolled. Each participant performed isotonic knee extension exercises at 0° and 50° N-K Table load angles in random order. Exercise intensity was set at 75% of the individual ten-repetition maximum (10RM). Surface electromyography (sEMG) was used to measure maximum voluntary contraction (MVC) of the vastus medialis, vastus lateralis, and rectus femoris. The primary outcome was the difference in MVC between the two load angles. Secondary variables included age, body mass index, and physical activity level.

The study was conducted at the Department of Physical Medicine and Rehabilitation, Dr. Cipto Mangunkusumo National General Hospital, Jakarta, Indonesia, between January and June 2024. Ethical approval was obtained from the institutional ethics committee, and all participants provided written informed consent prior to participation.

Inclusion Criteria:

• Diagnosed with unilateral or bilateral knee osteoarthritis (OA), grade 2 or 3, in the subacute or chronic phase based on the criteria of the American College of Rheumatology (ACR) and Kellgren-Lawrence classification.
• Female, aged 50-65 years, based on ACR diagnostic criteria.
• Knee flexion-extension range of motion of at least 0-120 degrees.
• Normal cognitive function as assessed using the Montreal Cognitive Assessment Indonesia Version (MoCA-INA) (score ≥26-30)

Exclusion Criteria:

• Knee joint instability due to ligament laxity in the cruciate or collateral ligaments obtained from physical examination (valgus/varus test and anterior/posterior drawer test) or previous supporting examination.
• Presence of deformity in the form of genu varus, genu valgus, or genu recurvatum (Q angle 12-20°).
• History of knee injury (fracture, ligament rupture, or meniscus) or having undergone knee surgery.
• Pain or instability of the ankle joint obtained from physical examination.
• Stroke or disorders of the central and peripheral nervous system.
• Patients with metabolic diseases such as diabetes mellitus and heart disease or other autoimmune diseases based on medical records in unstable condition with pre-prandial capillary blood glucose ≠ 80-130 mg/dL or 1-2 hours post-prandial capillary blood glucose ≠ <180 mg/dL, systolic blood pressure ≠ 90-140 mmHg and diastolic blood pressure ≠ 60-90 mmHg.
• Pain durin loaded knee extension exercise greatetr than 3 on the Visual Analogue Scale (VAS)

There is no plan to share individual participant data.

This is an open-label study. Due to the nature of the exercise intervention, participants and investigators are aware of the load angle applied during testing.