Neuromuscular electrical stimulation (NMES) is widely used as a method to increase muscle strength and improve physical function even at a low-moderate exercise intensity (7). However, NMES effect may prove to be insufficient because the exercise intensity is determined by the electrical stimulation endurance level of the user (7). On the other hand, transcutaneous electrical stimulation, so-called transcutaneous electrical nerve stimulation (TENS), is effective for pain relief (8). Knee pain independently reduced quadriceps strength and activation (9). TENS restores inhibited quadriceps motor function (central and muscle activation) through pain relief (10). Moreover the combined application of electrical stimulation (ES) and volitional contractions (VC) is said to be more effective than ES or VC alone (11). Therefore, a hybrid training system (HTS) that resists the motion of a volitionally contracting agonist muscle using the force generated by its electrically stimulated antagonist (NMES) was developed as a way to combine the application of electrical stimulation and voluntary contraction (12). HTS is a method that eliminates the disadvantages of both volitional exercise and NMES (13). It has been reported that HTS is a new training technique that can increase both muscular strength and muscle mass (12, 13, 14, 15). Recently, HTS is showing promise as a countermeasure for the musculoskeletal disuse of astronauts because the HTS technique can generate exercise resistance within the body even if there is no gravity (1). In addition, HTS can be utilized during many different types of exercise (e.g. knee extension exercise, squat and hip flexion, walking exercise, and cycling exercise) (17, 18). It seems to be more effective for improvement of muscle strength and physical function to combine HTS with easy exercise (e.g. knee extension, walking, and squats) for KOA patients.