Three months after ACL reconstruction, you can run, but do not run more than 200 meters each time, and you can only run as much as you want comfortably after 4.5 months after surgery. The running speed should also be slow in the three months after surgery, and only fast after four months. Vigorous exercise should be done only if all three of the following are met: ① More than six months after surgery. ② The muscle strength of the operated leg is more than eighty-five percent of the good leg. ③ The strength of the posterior thigh muscles (posterior thigh group muscles or called N cord muscles)/the strength of the anterior thigh muscles (quadriceps) is > 70% (i.e., H/Q ratio > 70%).