Effects of Isometric, Isotonic, and Eccentric VMO Exercises in Patellofemoral Pain Syndrome

Patellofemoral pain syndrome (PFPS) is a prevalent condition affecting physically active individuals and is associated with anterior knee pain during functional activities. Alterations in patellofemoral joint biomechanics, particularly related to quadriceps muscle imbalance and insufficient activation of the vastus medialis oblique (VMO), are considered key contributing factors.

Although VMO strengthening is widely used in clinical practice, there is limited evidence comparing the effects of different contraction types-specifically isometric, isotonic, and eccentric exercises-applied within the terminal knee extension range (0-30°), where patellofemoral joint stress and stabilization demands are highest.

This study is designed as a three-arm, parallel-group, randomized controlled trial to investigate the comparative effects of these exercise modalities on pain, functional outcomes, and muscle architecture. The intervention period will last 8 weeks, with sessions conducted three times per week.

A total of 90 participants aged 18-40 years with clinically diagnosed PFPS will be recruited and randomly allocated into three groups: isometric, isotonic, and eccentric exercise groups. All participants will receive a standardized physiotherapy protocol including superficial heat application, transcutaneous electrical nerve stimulation (TENS), and therapeutic ultrasound prior to exercise.

Primary outcomes will include pain intensity assessed by the Visual Analog Scale (VAS) and functional status evaluated using the WOMAC index, 30-second sit-to-stand test, and stair ascent/descent test. Secondary outcomes will include ultrasonographic assessment of muscle architecture, including muscle thickness, fascicle length, pennation angle, and physiological cross-sectional area of the VMO muscle. Additionally, selected hip muscles (adductor group and tensor fascia lata) will be evaluated.

Assessments will be performed at baseline (T0) and after the 8-week intervention (T1) by a blinded assessor. Randomization will be conducted using a computer-generated sequence with allocation concealment.

Statistical analyses will include repeated measures analysis to evaluate within-group and between-group differences, with significance set at p < 0.05.

The findings of this study are expected to contribute to evidence-based rehabilitation by identifying the most effective type of VMO strengthening exercise for improving clinical outcomes and muscle structure in individuals with PFPS.

Inclusion criteria

• Aged between 18 and 40 years
• Clinically diagnosed with patellofemoral pain syndrome
• Presence of anterior knee pain for at least 3 months
• Pain during activities such as stair climbing, squatting, running, or prolonged sitting
• Pain intensity of at least 3/10 on the Visual Analog Scale (VAS)
• Willingness to participate and provide written informed consent

Exclusion criteria

• History of knee trauma or surgery
• Meniscal injury, ligament injury, or knee instability
• Advanced osteoarthritis
• Neurological disorders
• Rheumatologic diseases
• Participation in a structured knee rehabilitation program within the last 6 months
• Pregnancy