Objective To determine key muscles during muscle strength training of patients with knee osteoarthritis (KOA) by studying the correlation of KOA symptoms and disease stages with knee extensor/flexor strength. Methods Ninety-nine community volunteers were recruited and stratified into healthy group, single KOA group, and bilateral KOA group. The relationship between isokinetic extensor/flexor strength and KOA symptom, full-body aerobic exercise capacity, joint function were analyzed by knee isokinetic muscle strength test, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), 6-minute walk test, and CS-30 test. ResultsKnee isokinetic extensor peak torque of the symptomatic leg was significantly lower than that of the asymptomatic leg in single KOA group. There were significant differences in knee isokinetic extensor/flexor peak torque of both the severe and mild symptomatic leg in bilateral KOA group. Knee isokinetic extensor peak torque in bilateral KOA group was significantly lower than that in healthy group. Knee isokinetic extensor/flexor peak torque of KOA patients was positively related to 6-minute walk test, CS-30 test, while negatively related to age. Knee isokinetic extensor peak torque was negatively related to pain score and physical function score of WOMAC. Conclusions Both knee extensor and flexor strength are related to KOA. Therefore, during KOA rehabilitation therapies, muscle training for knee extensors (particularly quadriceps femoris muscles) and knee flexors (hamstring muscles, gastrocnemius muscles) need to be valued.