Squeezing and holding muscles is indeed efficient to hold your muscle tone.
I do this during hydrotherapy and with my physiotherapist.
Since 2 weeks now I have been practising in the pool with a half kilo ankle weight on my injured leg.
I also cycle daily, up to 15 minutes.
No change yet in my mobility. I still can’t bend my knee sufficiently, so I limp.
And as a result, I still suffer from pain in R knee, R hip and lower back.
Once the hardware is removed, I should be able to walk normally.
Will keep you informed.