What is the best wa6 to restrengthen a knee that has undergine ACL surgery?

ACL exercises. If the rest of the knee is healthy then squats are the single best exercise to rebuild knee strength after an acl.
Acl rehab. A graduated program of weight training and aerobic exercise like a bicycle or elliptical can be beneficial for strengthening the knee after acl reconstruction. There are rebab protocols online or check with your surgeon.