Great answers. I suspect, as is suggested, that lack of adequate load against the leg muscles is the prime culprit, not lack of protein, in the decrease in lower body strength. ~1.5 g/protein/kg body wt/day is the general recommendation. It’s easy to get, most get more than that. With physician’s clearance, I’d suggest stationary cycling (or bicycling), eliptical, treadmill walking, especially at grade, as effective ways to increase lower body strength.