I had quite a few times that I couldn’t stand up over the years. Go to get out of bed and my legs buckle and I end up on my knees. Seemed like a back problem to me, but once I figured out that I could ease the spasm with the kneeling hip flexor stretch (which was convenient since I was already stuck kneeling), I could get standing again.
During covid, I hadn’t had any of these hip flexor issues since I’d figured out how to stretch to avoid them, but I knew I had some imbalance issues, probably from all those years I didn’t get any exercise and at that point had only been getting exercise in one plane of motion from being back on the bike for a few years. So, I went to see a PT. It was brutal, first test was to stand/balance on one leg for 10 seconds. Couldn’t do it. Spent lots of time on a bosu ball and doing 1-legged squats and sit-to-stand type exercises, as well as lots of lateral motion work. And I do have some kind of compression issue in my low back, but it only seems to come into play when I’m at the limits of back bends. I use a balance board every couple days now to keep my balance and all the supporting muscles sharp as I age (have arthritis in my feet and broke my tib/fib looping out riding a wheelie 15 years ago).