Is this due mainly from muscle atrophy and otherwise you could perform these movements?

Yes. I am physically unable to do them due to muscle weakness /lack of muscle. Pain is also an issue with these. Altho Dr says once i start to build muscle i should slowly add those routines in, as tolerable.
I tried a leg curl machine a few days ago and could not even move it with zero resistance (only using one leg).
Right now for legs im mostly doing ( extention/abduction /adduction/calf machine / i can step up on box, but cannot step down with weight on R leg)

Also thanks for the reply!

First off, I'm no physical therapist but I take from your question that you need to move things along more quickly than physical therapy.

My first suggestion is to work the good leg intensely. There are studies that show working an uninjured limb strengthens the injured limb...even when the injured limb is in a cast. I know the fear is that you will only make the imbalance worse but your weaker limb will eventually start to close the gap.

When I say intensely, I mean heavy low rep powerful movements. These movements will have the highest neural effort and therefore, have the highest sympathetic response in the other limb.

Since you can step up on a box, when you complete an intense set with your good leg, immediately do step ups with the weak leg superset style.

Since you have trouble lowering yourself with your weak limb, you can build up by attempting to lower yourself on the first rep of each step up rep. Even if all you do is drop and have to catch yourself with your strong leg, you will still be firing the motor units and re-learning the pattern.

I'm sure that these surgeries have made you intimate with the anatomy of your leg. The quadriceps not only extend the knee they also close the hip. Work that angle also. Using some ankle weights or a dumbbell resting across your ankle, lift your leg as if you were stepping over an obstacle.

It is also very important for knee health to keep knee and hip dominant movements in balance. You can not ignore the hamstrings and glutes.

You will probably be doing unilateral leg work for a while. I suggest looking up articles written by Michael Boyle (or his student Ben Bruno). He is a strength and conditioning coach who has had success lowering the incidence of knee injuries in athletes using unilateral exercises.

Feel free to PM me.

Can you do any type of running/jogging? I have had two knee surgeries myself (ACL and meniscus) and I know the thing that really helped me regain the movement in my legs was forcing my knees to bend. The natural way to do this is from walking/jogging/running. Be careful about this though as running on a treadmill (which it sounds like you can't do) can put added pressure that you don't want on the knee. I also try to stay away from machine type exercises (i.e. leg curl machine). The weight bearing on your limbs is unnatural and may cause you added stress on your joints. I'm in agreement with the previous poster and would only add that the most natural movements are the best for you at this point in time.

Have you thought about swimming exercises? What about pilates/yoga? I know they aren't weight training, but if you are unable to do weight training at this time anyway...

Hey bud,
I understand what your surgeon is saying. Right now, your ligaments in your now must be weak. You would probably need to strengthen them some how. But, if you are able to do, running in the water to have resistance against that area may help strengthen your legs. Give that a shot. And testosterone injections, do you have low testosterone? Because yes, that will help you gain muscle but if you are in your late 40s to 50s since when you are in your early to mid 30s. That is when your testosterone starts to drop. So, that would make total sense. The side effects though could be detrimental though such as rage if it is too much. I would speak to your doctor about that though instead of a regular nurse. I would try running in the water before anything since it is resistance exercise that can help you gain strength and muscle. Try that and see what happens.