On Bob's Board (See link at bottom of Index page), there sometimes are spirited debates over the leg extension issue. Basically, your OS & PT are generally the ones you follow for guidance. Any guidance you start to follow by yourself may work out fine but then you are sort of on independent ops - So evaluate if you really want/need to detour from the guidance your professionals are providing. Here's what I know from what I have read in journal articles, past Board discussions and recall from my own experiences:
PTs and OS straddle the fence on whether leg extensions are okay to do. The thought is that there are other exercises out there that can be performed to make significant progress on your quads without having to do leg extensions. The group that is against ACL patients doing leg extensions are concerned because the exercise can put a shear (cross) stress on the ACL as you work the leg against the support arm. Also, for patella autograft people, their existing patella tendon is firing on only 2/3 of a tendon (until that area recovers 90% of it's lost matl - takes 2 yrs) and what that means is that during the recovery period, they are somewhat more prone to developing patella tendonitis. This happens from working resistances too aggressively.
This exercise is considered an "Open Chain" exercise meaning that the foot is not supported by a platform. As a result, when resistance is worked against the support arm, the lower leg acts like a lever arm and there is movement in the knee joint area potentially resulting in the shear stress.
A journal article I had read indicated that the way they performed ACL reconstructions years ago was much more of a concern in regards to doing leg extensions but I must have lost the brain cell that stored memory on this because i cannot recall why it was more of a problem then and not now. The article went on to say that the leg extensions were not necessarily more dangerous than some fitness ctr exercises but that actually a number of them can be harmful if not given some guidance (initially) by an experienced PT.
My PT gave me the go-ahead to work leg extensions at 3 1/2 mos when we were reviewing all the fitness ctr exercises I was then allowed to do. I, however, was reading a debate on the leg extensions on Bob's Board and decided to put that exercise on the backburner at that time.
As it turned out, I was at 85% full muscle strength doing everything else at the fitness ctr by my 6 mos post-op checkup. My OS released me for all sports but wanted me back at 7 1/2 mos to see if I was at full strength. Don't you just hate those perfectionists ! I told him about not doing the leg extensions. He told me point blank to start them and that at 6 mos post-op there was nothing that I should be concerned about. So...............I started doing them.
I must tell you that I started with low resistances doing the fitness ctr exercises and increased the resistances very gradually to minimize any chance of setbacks. I did not do the "No pain, no gain" philosophy. I did many reps at low resistances. I did all the exercises that could be done as single leg exercises (Leg Extensions, Leg Curls & Leg Presses). My philosophy was that I would tire out the leg rather than strain it. I did between 50-100 reps on the Leg Extension machine and about 50 reps on the Leg Press machine, 30 reps on the Leg Curl machine, etc, etc on the rest of the exercise machines. In many cases, I found it much easier to do the first 50 reps on the Leg Extension machine and then after doing a bunch of other exercises (including some upper body exercises) , I would then go back and knock out the last 50 reps on the Leg Extension machine.
At 7 1/2 mos post-op, I was at 100% full muscle strength and my leg muscle tone circumference readings were 3/8" larger than my good leg ! OS was impressed.
There have been plenty that have started Leg Extensions in the 3-6 mos window. But there have been at least 3 that have EMailed me (from Bob's Board postings) with Patella Tendonitis concerns developing and that their PTs had them stop doing the Leg Extensions and basically put them in the corner for being bad ! Patella Tendonitis can be temporary for a month or can be the more longer debilitating type that can last.....well, a real long time.
Chances are that those that decide to start Leg Extensions at 6 mos post-op, work gradually with many reps and low resistances, gradually increase the resistances, listen to their body/leg ....then they will be fine. But then there is the OS & PT protocol/opinions..........so..what to do ??