Non-Strength Post-OP Initial & Mid-Recovery Exercises
It is best to consult with your OS before doing any new exercise. Your OS's protocol may not recommend what's described below or may have a timetable for when to phase this in. In general, it is best to start flex and extension exercises immediately after the operation. However, those that had more than the classic ACL reconstruction and/or cartilage trim/cutout (ie. MCL/PCL ligament repair) may have to follow a different protocol that requires a little later start on flex exercises.
It is best to recover extension right away or...never lose it from day one out of the hospital. In order to maintain extension, the OS may recommend the use of the Immobilizer or a recovery brace locked at 0 degrees for those periods when you are not frequently doing flex exercises.
When a person is resting on a couch or bed and not doing flex exercises, then it is best to rest with the leg completely straight, making sure that the leg is not resting with a slight bend in it.
In the Initial Strength Recovery Exercise Section, there is a Quad exercise that you try to push your knee into the floor. That exercise is also helpful for maintaining the extension.

Those that do not have 0 degree extension, may also try prone hangs. Prone hangs can be done while resting on a couch with the leg resting straight out but requires that the foot be suspended (supported) and the underside of the knee is not supported. Resting in that position sometimes is helpful since gravity works on lowering the leg into a straight leg position.

Flex exercises for me started 2 hours after the operation. A hospital technician was hooking a hospital version CPM unit onto my bed and I was flexing at 50 degrees to start. When I left the hospital, 23 1/2 hours after the operation (my max period for outpatient service), I was already up to 90 degrees flex. At home, I had a small portable CPM unit which was dropped off the day before my operation and I was to use that for 8 hrs a day until I reached the max flex for the machine (which was 125 degrees). I broke up the 8 hrs flexing into 2 hr segments throughout the day. That enabled the leg to get flex at both the beginning and at the end of the day and minimize the time during the night when the leg could stiffen up.

At 10 days, I reached 125 degrees and I then returned the machine. I then did flex exercises at home manually and at therapy to get the rest of the way to full flex.

The following are some basic flex exercises:

These are done frequently. If those with CPMs are using them for 8 hrs a day. Then that may give you an idea that flex exercises can be done frequently !

1. Initially Flex Exercise - Sitting position on bed, floor or couch. Loop towel around foot and gently pull towel toward you as your knee bends as far as is comfortable with perhaps slight discomfort but not pain. Keep repeating.

2. Heel Slides - This is usually started at therapy during the first weeks. Lay on your back with legs up in the air resting on the wall. Perhaps place a towel under the injured leg's foot to facilitate sliding motion. Since the leg muscles are extremely weak in the first few weeks of recovery, it is recommended that the good leg's foot/ankle is placed under the injured leg's foot/ankle to support the injured leg while doing this exercise. (It's best to do this first with PT guidance.)
The exercise is started by easing up on the good leg's support of the injured leg and allowing the injured leg to bend while the heel is sliding down the wall. Bend leg as much as you can and then use the good leg's foot/ankle to push the injured leg's foot/ankle back up to the starting position. Keep repeating the  exercise. Eventually, the injured leg should bend more and more.

3. Stationary lifecycle exercise is a good flex motion but this again should be done at therapy especially in the beginning. There is danger of losing your balannce or straining your vegetable leg during the early recovery period. Pedalling a full circle usually can be attempted once you can flex the leg around 120-125 degrees. Pedal ver slowly the first time to ensure that you can flex the leg comfortably without hurting it as you complete the pedal circle.

4. Flex Leg Pulldowns - If your leg is strong enough that you can hold it up in the air by itself and can also bend and straighten it without a hand, etc for assistance, then this exercise can be tried to get to full flex later in recovery (ie. 5-12 weeks post-op).
Find a good TV program to watch for about an hour. That will take your mind off the exercise. Lay on your back and raise the injured leg straight up into the air. With both hands, pull down the leg with moderate pressure. You should feel some discomfort but no pain. Hold that lowest bent position as long as you can (ie. a few minutes). Then push the leg up to relieve any tension and then pull it back down again. keep repeating but mostly hold it down rahther than do repetitions.

Measuring progress rather crudely:

While doing the above exercise and with the leg bent and the foot/heel in the lowest position possible, place your hand sideways between your heel and your butt. Hopefully, your entire hand will not fit in the space. If so, see how many fingers width the heel and butt are and then the next time you finish the above exercise, you can remeasure and see if the width has reduced to less fingers in-between. That's the crude method.

What the PT uses to measure your flex angle accurately while you bend your knee while on one of the therapy tables is called a goneometer. It's basically a protractor that facilitates leg angle measurements.

Scar Tissue Massaging
Scar tissue massaging usually kicks in when the stitches are removed from the pencil diameter sized arthroscopy holes. Your OS should give you information at that time about scar tissue massaging. If not, then ask !

Some people are more prone to scar tissue than others. There will be those that did nothing to minimize scar tissue and they come out fine and then there will be others that did every single thing to minimize scar tissue formation and they wind up with some scar tissue concerns - so go figure !

In general, it is best to do something about it, rather than to do nothing.

I performed the following to help minimize scar tissue formation.

First, The flex exercises from day one after the operation, helps to keep scar tissue from adhering.

Second, I would use moderate rotating thumb pressure and try to rub out each of the arthroscopy holes. I would do this twice a day and I would do it on each hole for about 30 seconds.

Third, With my leg resting straight out, I would grasp my kneecap with both hands and gently with moderate pressure, move it sideways. I would do this for about a minute and then move the kneecap up and down for another minute. I did this twice a day.

Later, in my 5th/6th week post-op, my PT indicated that my patella (the 2/3 remaining after the graft was removed) was very tight. (This really concerns only those with patella autografts - so skip this if you ahd a different reconstruction method. )

My PT suspected there was scar tissue clinging onto it. This is the area right below the kneecap. It feels like a bone but it's actually the patella tendon and it can actually be moved sideways, if you have enough strength to move it.

My PT used both her hands and tried moving the patella sideways. My PT was really working it while I was sitting on my duff watching this great feat of strength ! LOL
All of a sudden we heard this twang noise and I felt something. It was the patella breaking free of some scar tissue. We both looked at each other and started to laugh. It just seemed funny !

Mobility, scar tissue massaging & flexing all help to minimize the formation of scar tissue.