Double knee op - ouch ! Are you still in rehab post-op ?
Muscle only grows when provided a stimulus plus the protein your body needs to build it, and adequate rest to repair.
Some form of resistance training like kettlebell squats (or just body weight callisthenics) can work wonders for fast twitch fibres, as can sustained cardio based exercise for slow twitch fibres such as indoor rowing, mountain biking, swimming amongst many others.
The obvious other factor is time, it won’t regrow overnight.
Good luck with rebuilding your leg strength and mobility/flexibility, if you put the work in it will happen.
Op was October 23. Picked the bike up march 24.
Knees are great but the deep nerve damage in the thighs are the issue. Apparently they don’t normally tourniquet but my legs were BIG so they did.
Can take a few years so they say so I’ll just plug on and see where I end up.
Next step weight loss