MATERIALS AND METHODS Subjects The research program was approved by the Human Subject Committee in Taipei Physical Education College. Knee joint laxity was surveyed by a KT-2000 test (MED Metric Co., San Diego, USA) in the Department of Physical Education and Health at Taipei Physical Education College. The anterior drawer laxity of the knee joint in this study was defined as anterior displacement over 3 mm discrepancy between bilateral knee joints by a KT-2000 test under 20-pound force. According to this criterion, Dale M. et al. demonstrated that 85% (33/39) of this population have complete ACL tears and 15% (6/39) have partial tear. One hundred and seventy-three students were tested and 30 met the criterion, including 22 males and 8 females, although no obvious trauma history was available in some of them. Two males were excluded due to other causes that might affect muscle strength or knee stability such as lower limbs fracture and arthritis. Besides, eight female students were eliminated to avoid possible gender difference. Twenty male students were included in the experimental group. Another 8 male students with stable knees without any medical history of knee injuries were included in the control group. The participants in both experimental and control groups lived a sedentary lifestyle. Stable knees were defined as a discrepancy less than 1.5mm between bilateral knees by a KT-2000 test under 20-pound force.