Abstract:

BACKGROUND: The pathogenesis of patellofemoral pain is complex, and poor movement patterns and incorrect muscle activation patterns have been suggested to increase patellofemoral joint stress and cause injury, but the results of current studies are not uniform. Current biomechanical studies of step-down test in patients with patellofemoral pain have focused on kinematic characteristics, and there is a lack of research on surface electromyographic characteristics. In addition, there are no studies that analyze the differences in biomechanical performance between the healthy and affected sides of patellofemoral pain patients compared with healthy subjects during the downward step test.

OBJECTIVE: To investigate the kinematic and surface electromyographic characteristics of patients with patellofemoral pain during bilateral lower extremity step-downs.
METHODS: Twenty-one men with patellofemoral pain and 21 healthy men were recruited, and both groups of subjects underwent a step-down test. Kinematic, kinetic and surface electromyographic data were synchronously collected using a three-dimensional dynamic capture system, a force platform and a surface electromyographic tester.  
RESULTS AND CONCLUSION: (1) Kinematics: Compared with the healthy control group, the affected and healthy sides of the patellofemoral pain group showed a smaller hip flexion angle (P=0.005, P=0.011), a larger hip internal rotation angle (P=0.033, P=0.039), and a larger knee valgus angle (P=0.001, P=0.001), and the affected side of the patellofemoral pain group exhibited a larger hip internal rotation angle (P=0.013), a smaller knee flexion angle (P=0.043), and a smaller ankle dorsiflexion angle (P=0.002); and compared with the healthy side, the affected side exhibited a smaller ankle dorsiflexion angle (P=0.002). (2) Surface electromyography: Compared with the healthy control group, the patellofemoral pain group showed reduced activation of the vastus medialis oblique (P=0.002), reduced activation of the gluteus medius (P=0.015), and a decreased vastus medialis oblique / vastus lateralis ratio (P=0.010) on the affected side; and compared with the healthy side, there was a reduced activation of the gluteus medius on the affected side (P=0.008). (3) The results indicate that patients with patellofemoral pain have abnormal lower limb biomechanical characteristics during step-down test, which are mainly manifested as changes in kinematic and indexes of the lower limb joints in the sagittal and frontal planes as well as changes in activation levels of the medial femoral and gluteus medius muscles. These changes reflect the fact that patients with patellofemoral pain adopt a stiff landing pattern during step-downs and show pronation of other joints of the lower extremity. In addition, abnormal lower extremity biomechanical characteristics are also present in the healthy limbs of patients with patellofemoral pain, and bilateral lower extremity asymmetry exists in such patients.

Key words: patellofemoral joint pain, step-down, kinematics, surface electromyography, biomechanics, lower extremity, pain