Patellofemoral pain is a common disease, and it often couples with patellar malalignment. Previously, imbalance of the different quadriceps muscle heads was believed to be a causative factor of patellar malignment. However, findings from electromyography studies on quadriceps activities and patellar alignment in the last two decades were inconsistent. In order to better understand the relationship between quadriceps muscle imbalance and patellar malalignment, shear wave elastography (SWE), which can measure the passive and active muscle stiffness, may shed some new lights to this area. This study aims to examine whether the quadriceps muscle stiffness can predict the patellar position. The healthy young adult will be recruited in this study. Active and passive quadriceps muscle stiffness will be measured with the SWE in different conditions and the result will be correlated with the patellar position measured with magnetic resonance imaging (MRI). Also, electromyography (EMG) activity will be measured during the active stiffness measurement so as to establish the level of agreement between the EMG and SWE data. Linear regression will be used to calculate the relationship between different factors. The total number of subjects needed will be 55 by setting an effect size of 0.15 with two predictors, 80% power, and α at 0.05 in the linear regression.

This project will be conducted at PolyU campus and an MRI clinic in Hong Kong. Hopefully, it could be started in July 2021 and will last for 1 year.

Inclusion Criteria:

• Healthy and in the range between 18 and 45 years.

Exclusion Criteria:

• Knee pain in the last 3 months;
• Previous history of knee surgery;
• People with contraindications to having MRI scanning.

Healthy subject without patellofemoral joint problem