For decades I've been puzzled that there are no pedal-powered wheelchairs: a substantial number of the people who use wheelchairs can walk a little, many of the people who use walkers look as though they'd like to move a little faster once in a while, and renting a pedal-powered wheelchair sounds like a lot more fun than being driven somewhere to pedal a stationary bike. I suppose many of the last category require close supervision, but at least one patient was referred to a regular gym.
When I twisted my knee last winter, I discovered that pedal-powered wheelchairs have been available for some time, and are quite cheap: mine was well under a thousand dollars even though I bought a lot of accessories. It doesn't fit into my car and is strictly for outdoor use, so shopping malls are still off limits, but it's been about fifteen years since I drove to a shopping mall anyway.
The outdoor-only wheelchair is so new that they haven't settled on a name for it yet; it's variously called "step-through bike", "crank- forward bike", "semi-recumbent", "flat-footer", and "comfort bike", and when I DuckDuckGoed the model I bought, I found that the maker calls it a "lowstep",
What it is, is a bicycle with two down tubes instead of a down tube and a top tube ("step through"), with the cranks on the forward down tube ("crank-forward"). The rear down tube meets the seat tube, like the down tube on a diamond-frame bike. This semi-recumbent construction allows the seat to be so low that one can put both feet on the ground while firmly seated (flat-footer). The handlebars are so high that one sits bolt upright, which requires the saddle to be replaced with a tractor-like seat, which may account for calling it a "comfort bike" even though one *needs* a cushy seat to compensate for the inability to transfer some of one's weight to the pedals when one sees a bump coming.
Viewed as a bike, it's nothing much: the inefficient posture makes it so hard to pedal that riding a mile on my flat-footer takes as much effort as riding ten miles on my diamond-frame bike — but it's a whole bunch easier than walking, and being able to exercise without putting any weight on my bad leg allowed me to improve so fast that I stopped using a cane before I'd found one that would fold up to fit into the basket.
And it lets me go three times as fast as I could walk when I had two working knees, so I frequently pedal to a store I'd walked to only three or four times, and last time out I went half a mile beyond it, which I'd never done on foot.
When I was researching the model I bought, I found dozens of reviews that enthusiastically stated that it's a marvelous toy, and it did indeed give me the thrill that I got when I first learned to balance. (Partly because one has to learn how to steer and balance all over again!)
Fun is a major factor -- one day I got out of the car stiff and sore from having sat for two hours. In former days I'd have been cripping around for the rest of the day, but after a while I put safety pins into my pant legs, hopped onto my pedal powered wheelchair, took a lap around the neighborhood, and came back feeling much better.
It's not all perfect. One major downside is that if you are over twelve, you can't operate it on a sidewalk, so you have to learn how to ride a bike, and instructors who can teach you how to ride in traffic are much scarcer than "instructors" who think that wearing a helmet is all there is to it and don't even tell you how to put it on. I've run into "safety experts" who don't know which side of the road to use!
A minor disadvantage is that it doesn't climb worth a nickle; it requires first gear on slopes that I had never before detected. Perhaps this isn't all that minor in neighborhoods hillier than mine, but I find that the lack of a top tube and the extremely high handlebars make the bike into a tolerable substitute for a cane when I have to walk up a slope.
One would think that a less-efficient bike would be better at burning off fat, but an upright bike is inefficient because it forbids the rider to use his largest muscles — the ones that burn up calories. Riding a flatfoot burns up less energy than walking for the same length of time. But it burns up more calories than sitting on the sofa because your knee hurts!
The bike is also difficult to start. On a diamond frame, you start by straddling the bike, putting your foot on one pedal, then lifting yourself into the saddle by pushing down on the pedal. This imparts enough momentum that you have plenty of time to get the other foot on the other pedal.
The gentle nudge that one can give a pedal while seated on a flatfoot won't get you going unless you are on a slight downslope, or smooth pavement that's dead level. It's neccessary to drasine -- to push on the pavement with both feet in alternation -- until some speed is built up. This is probably easier to learn if one has no experience with riding a bicycle; I keep attempting the quick start in unsuitable conditions and end up pushing with only one foot, which is inefficient, awkward, and distracting.
But it didn't take long to get over being terrified at being so slow off the mark at intersections; after all, I cross streets on foot all the time, so I know how to judge whether a hole is big enough. Aunt Granny of Warsaw