Objective:To observe and analyze the surface electromyography activity of quadriceps femoris of knee osteoar-thritis(KOA)patients. Method:The surface electromyography of vastus lateralis,rectus femoris,and vastus medialis were recorded and analyzed in 30 KOA patients and 30 normal people during isokinetic knee flexion or extension at veloci-ties of 60 °/s,90 °/s or 180 °/s.We also record clinical indicators(muscle thickness and degree of knee pain)of the KOA group and evaluate the correlation between electromyographic data and clinical indicators. Result:Compared with the healthy control group,the KOA group had significantly lower knee extensor mo-ment during isokinetic knee flexion or extension at velocities of 60°/s(P＜0.001),90°/s(P＜0.01),or 180°/s(P＜0.01).Statistics showed that there were significant differences in the median frequency difference rate of vastus lateralis(P＜0.05)and root mean square value of rectus femoris(P＜0.01)between the healthy control group and the KOA group under 60°/s angular velocities.There was a significant difference in the root mean square value of rectus femoris(P＜0.05)between the two groups at velocities of 90°/s.Statistics revealed that the median frequency difference rate(P＜0.05)and root mean square value(P＜0.05)of rectus femoris had changed significantly at the velocities of 180°/s.The results of both multiple regression and random forest algo-rithm showed that knee extensor moment was the strongest predictor between electromyographic characteristics and clinical indicators. Conclusion:The strength of quadriceps femoris in KOA patients was decreased.Thus,the patient may need more muscles to complete the same action,which was easy to cause muscle fatigue.The knee extensor moment was closely related to the quadriceps femoris thickness and the degree of knee pain.It offers important advantag-es for the diagnosis and assessing the severity,which may provide the way for the future study of KOA.