Introduction: Lunge exercises were commonly used in injury rehabilitation and physical training to strengthen quadriceps and hamstrings. Common lunge exercises are forward, side and reverse lunge. Lunge exercises were known to facilitate the muscle co-activation during motion. The co-activation of the quadriceps and hamstrings could affect movement strategy and provide stabilization of the knee joint, joint moment could be used to assess the joint stability. Lunges could train the unilateral lower limb muscle group, adjust the posture or muscle strength compensation, thus, to identify lunges that facilitate balanced activation of the quadriceps and hamstrings might be beneficial in knee joint injury prevention and stabilization of movement. Therefore the purpose of this study was to examine the co-activation of the quadriceps and hamstrings during multiple planes of lunges. Methods: There were twelve healthy collegiate individuals volunteered to participate in this study (age 20.7 ± 1.4 yrs, height 174.6 ± 3.9 cm, weight 68.8 ± 8.5 kg). Surface EMG signals were collected by five Delsys Trigno sensors at 1000 Hz. Sensors were placed on rectus femoris, vastus lateralis, vastus medialis, biceps femoris, semitendinosus of the dominant leg. The muscle co-activation ratios (Q:H) were calculated as mean quadriceps activation divided by mean hamstring activations during movements. Results: The quadriceps activation and knee extensor moment of side lunge was greater than other two lunge exercises, the hamstrings activation of reverse lunge during descending phase and ascending was greater than other two lunge, The Q:H ratios of reverse lunge was smaller than other two lunge exercises. Conclusion: A resulting Q:H ratio equal or close to 1.0 indicates a more balanced muscle activation. The smaller Q:H ratio (close to 1.0) during descending phase of reverse lunge could be peculated that reverse lunge was a more hamstring dominant exercise than other two lunge exercises. A significant greater Q:H ratio of side lunges could indicate that a stronger quadriceps muscle activation was existed and tended to lead to greater stresses on the knee joints during descending phase.