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CONTEXT It is very important to empirically determine the optimal ankle position for the quadriceps femoris (QF) strengthening during isometric exercises. OBJECTIVE To examine the effect of different ankle positions on torque and electromyography (EMG) activity of QF during maximal isometric contraction. STUDY DESIGN Within-subject repeated measures. SETTING University laboratory. PARTICIPANTS Thirty-six healthy volunteers (15 males and 21 females). MAIN OUTCOME MEASURES The isometric strength of the QF was measured at 3 different ankle positions: active dorsiflexion (AD), active plantar flexion (AP), and neutral position (NP). Simultaneously, 3 different ankle positions were assessed for EMG activity of the vastus medialis, vastus lateralis, and rectus femoris muscles during maximal voluntary isometric contraction. RESULTS The peak torque per body weight and average peak torque were significantly higher in AD than in AP and NP (P < .01). The vastus medialis and rectus femoris maximal voluntary isometric contraction EMG activity were significantly higher in AD than in AP and NP (P < .01). The vastus lateralis maximal voluntary isometric contraction EMG activity was significantly higher in AD than in AP and NP (P < .01), and was significantly higher in AP than in NP (P < .05). CONCLUSIONS These results indicate that the 3 different ankle positions affect the QF torque and EMG activity. In particular, AD position may be more efficient for improving QF strength than AP and NP position. Future studies should prove whether long-term duration QF isometric exercise effects muscle strength and functional performance in different ankle positions.