Abstract:[Objective] To observe the effects of different duration of exercise on the isokinetic strength of knee extensor and flexor.[Methods] A total of 54 college students were included into this study and divided into 5 groups by random number method. Of them, 11 per-sons were in the 10-second group, 11 in the 15-second group, 11 in the 20-second group, 10 in the 25-second group and 11 in the 30-sec-ond group, and corresponding duration of knee exercises conducted for 12 weeks. The Biodex S4 isokinetic test system was used to measurepeak torque (PT), relative peak torque (peak torque / body weight, PT/BW), average power (AP) and hamstring / quadriceps (H/Q) at angularspeeds of 60°/s and 180°/s before and 12 weeks after exercise. [Results] In term of extensor, the PT and PT/BW were significantly increasedin all the 5 groups (P<0.05), and AP were significantly increased in the 10-second group, 15-second group, 20-second group and 30 secondgroup at 60°/s angular velocity 12 weeks after exercise compared with those before exercise (P<0.05). Furthermore, the PT, PT/BW and APwere significantly increased in the 10-second group, 20-second group and 30-second group at 180°/s angular velocity (P<0.05), with themost significant changes in the 20-second group and 30-second groups. There was no significant difference in the above indexes among the5 groups at any corresponding time point (P>0.05). In term of flexor, the PT and PT/BW were significantly increased in the 10-second group,15-scond group, 20-second group and 30 second groups (P<0.05), and AP was significantly increased in the 15-second group, 20-secondgroup and 30-scond groups at 60°/s angular velocity (P<0.05), while the PT, PT/BW and AP in 10-second, 20 second and 30-second groupswere significantly increased at 180°/s angular velocity (P<0.05). However, there were no significant changes in H/Q before and after exercise in all the 5 groups (P>0.05), and there were no significant differences in H/Q among the five groups at any corresponding time points (P>0.05). [Conclusion] Traditional exercises does improve the maximum muscle strength, explosive power and muscle endurance of knee exten-sor and flexor. Among them, exercises lasting 20 seconds and 30 seconds have the most significant improvement in knee extensor and flexorstrength.