Here are the things that you really, really need to know. The main problem with being in a wheelchair is that your body was designed to walk around. When it sits still, it gets unhappy. This is true for everyone and has been proved in many studies. The study by Paffenbarger [Ref 1] and friends in 1986 is the best known. They showed that, over the years you were going to be healthier and happier and not as likely to die if you exercise. Obvious, really.
Here's how it works.

When you sit still, your heart slows down to your resting heartbeat. When you are frantic and going as hard as you can, it speeds up and goes to maximum. When it is two thirds of the way between resting and maximum, you are now getting a good cardio workout. At that stage, you are starting to pant. You can easily measure this with a Fitbit or something cheaper. Fitbits cost about $200 and give you a readout of how much cardio you get. It is absolutely totally important that you manage this. Lots and lots of research shows that, if you get 22 zone minutes a day, your heart and stuff are feeling good. The American Heart Association says so.
Your body evolved in a world where you might walk 20 km to 30 km in a day and your body is designed to do this. In particular, your blood circulation really needs it. The heart pumps maybe 50 litres of blood per hour into your legs, but, when you are sitting still, how is it going to get back? If you cut a vein, air doesn’t slurp in, so your heart isn’t sucking it up from your legs. In fact, osmotic pressure acts on your flesh to push blood into the tiny little vein pipes. When, enough pressure builds up in the veins the blood trickles back into the heart. This pressure in the veins for hours and hours can lead to swollen veins and legs and the dreaded deep vein thrombosis.
Your body, knowing that humans walked a lot (millions of years ago) developed little one-way valves in the veins and every time your muscles clench and relax, the veins act as a pump. This pumps blood back to the heart and decreases the pressure in the veins. And this gives your veins a rest. Even if your leg muscles aren’t clenching, if you drive your wheelchair vigorously on a suburban sidewalk your legs are being shaken up and down and this does the pumping.
Aside from blood, you have another,  parallel system that pumps lymph around. Lymph is a colourless liquid that contains hunter-killer cells which attack the nasty beasties of infection. Lymph also uses one-way valves and needs clenching or shaking to make it go around. Lymph has to circulate continuously in order to find the bad guys. Bad circulation of lymph equals bad immune system.
So, bottom line – you really need exercise to tone up these systems and make them do their jobs. It’s not that the blood and lymph just need to go around, it’s what they do while they’re going around. [Ref 2,9]

If you don’t exercise, you aren’t burning calories and you tend to put on weight. It is almost impossible to diet just by being strong. Look at the chubby people around you. It doesn’t work. Wheelchair users are 40% more likely to get diabetes than the able-bodied. But there is good news. Those who exercise have the same incidence of diabetes as the able-bodied. This happens because the body becomes slimmer and healthier. [Ref 3]

Muscles, joints and bones need to be used in order to be happy. Unused muscles waste, really quite quickly. You have a lot of them in your torso and, unless you use them, they become steadily weaker. Your posture goes downhill and you become less flexible. Really bad idea. Even if you don’t have core strength, you still get a more flexible torso from exercise.
Joints also need the right kind of use. They need to be pushed one way and then the other way. All your joints are designed to push and pull. When you do that, the liquid in them swishes one way and then the other, lubricating the surfaces. When you push your wheelchair’s pushrim, you only push and you don’t pull so, sooner or later, you get wrist, elbow and shoulder pain. 65% of people who have used a wheelchair for more than ten years have this problem.
Strangely enough, bones are also fussy. They adjust to the load they are carrying. Little old ladies have thin old bones. Little old ladies who do resistance weight training don’t. (Don’t mess with them.) The more you load bones, the stronger they get. The less load the weaker. If they are only strong enough to hold you up while you sit, they are going to be overmatched when you fall. And they will break. Old people’s hips break regularly for this reason. You have to transfer regularly, and you’d like them to be up to the challenge if you fall. So, you need to do stuff to make them strong. [Ref 2]

If you maintain pressure on flesh, the blood squeezes out of the space between the cells and the little capillaries. Then the flesh doesn’t get oxygen and it gets unhappy. If you do vigorous exercise (into the cardiac zone) you are inevitably squeezing and then releasing this pressure and the blood is seeping merrily around. Bed sores are a problem of long term pressure. If you exercise regularly, you only do it for a short while but it still helps. Overall, when you have more muscles available you are inclined to move more. And that’s all you need.

This is a biggie. The actual cause of death for many wheelchair users is that they can’t breathe enough. If you never pant, the muscles in your chest that are in charge of breathing, waste away until you can just breathe enough to sit still. As you live a sedentary life, the connective tissue of the chest becomes less supple and the first step along the way is that you can’t cough properly. When the condition is well established, and you get a fever, then, when your respiration rate wants to go up, it can’t. Consequently, you can’t cool yourself and you can’t keep your acid balance right. You really, really don’t want to go down this path. [Ref 4]

Several studies show that exercise reduces urinary tract infections by about 70% - to the level of the able-bodied. Exercise increases the flow of blood and urine. The kidneys need the increased blood flow to clean the whole system out from time to time.
Millions of years ago, when your bowels were getting used to upright posture, you spent a lot of time walking. So they were designed in a jiggling tummy and they could take advantage of the jiggle and the increased heart rate. You spent time every day jiggling the bowels while you walked. The bowels use this jiggling to help their operation. They are not just long pipes running from here to there. They do stuff. In particular, there is a good blood supply to the walls and fluids are transferred continuously into and out of them. Also, the jiggling can be seen as stirring the pot. So, you need to bump your heartrate into the cardiac zone and jiggle your body.[Ref 2]

This is the big one. The elephant in the room is the disaster that put you in the wheelchair in the first place. You are not leaping across mountain tops, emitting small stars. But the human mind basically averages your happiness level and when it’s worse than usual, chemical steps are taken to bring you back to level. Same if you get too cheerful - a depression is stalking you.
It has been said that happiness is a state of mental health, and I think that is right. So, if you are in a frame of mind that says, “What the hell, I don’t want to strive to improve my life,” then you are actually not healthy, mind-wise. Many studies have shown that mental health needs exercise. People who sit still are going to get sad. Why would you tolerate that for the rest of your life?
The simple act of revving up your system will improve your mood, but you also need other stuff. You need social interactions, sunshine, trees, rivers, passing dogs who want to give you a friendly sniff. This need is real. You can live alone and inert, but you can’t be happy and healthy when you do.[Ref 5,6,7]

Note that, for multiple sclerosis, exercise improves your prognosis considerably.[9]

1. Before you do anything, get a Fitbit or a cheap knockoff. It’s the only way to know whether you are getting enough exercise. Near enough is not good enough. You need a clear rule about whether you are doing your duty – or not! Once you’ve got one, you can be sure you’re getting exercise right. Otherwise not.
2. You can go on excursions in your wheelchair. This seems to be limited pretty much to the young and strong because pushing along suburban sidewalks is just plain hard. Also, this is a bad idea because it will probably lead to arm and shoulder damage. Even with E-assist wheels you have the same arm and shoulder problem. The motion is identical; you just goa bit faster.
3. Swimming is excellent, though you’re not going to meet many dogs. It’s a really fine way to get your heart going boompety-boomp and make all the little connections in your torso do their job. Fitbits are waterproof. Unfortunately, it’s not very convenient so few people do it.
4. Then there are rowing machines, which are quite cheap and give you quite good exercise. You can buy a dog to watch you, but there aren’t many trees and sunshine and people. Here’s the gold standard – about$2,700:  Concept2 RowErg Rowing Machine – Flex Fitness Equipment New Zealand It may need some adaptation.
5. Same with home weight systems. It’s not really fun so few people will do it for decade after decade. But a dog and a TV help. You can get a budget version or a better system here for $500 -$1500: https://www.gymjunkie.co.nz These systems offer cables, so you don’t have to handle free weights.
6. Fancy exercise wheelchair. This has been designed specifically for fun outdoor exercise. It’s a standard manual wheelchair but it has a fancy push-pull stroke (no levers or pedals or batteries) and is getting towards the performance of a bicycle. It’s easy to do 5 km in 30 minutes. Excellent, fun exercise – you’ll do it for the rest of your (longer) life. Wall to wall sunshine and trees and dogs and people. (And rain) Available in New Zealand for about $20,000 at: https://OctalWheels.com

1. Paffenbarger, R. S., Hyde, R. T., Wing, A. L., & Hsieh, C. C. (1986). Physical activity, all-cause mortality, and longevity of college alumni. New England Journal of Medicine, 314(10), 605–613.
   https://doi.org/10.1056/NEJM198603063141003
2. Marieb, E. N., & Hoehn, K. (2018). Human Anatomy & Physiology (11th ed.). Pearson.
3. Sherri L. LaVela, Frances M. Weaver, Barry Goldstein, Ke Chen, ScottMiskevics, Suparna Rajan, David R. Gater, Jr. Diabetes Mellitus in Individuals With Spinal Cord Injury or Disorder.   J Spinal Cord Med. 2006;29:387–395
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5. Matteo Ponzano , Robert Buren , Nathan T Adams , Jane Jun , Arif Jetha, Diane E Mack , Kathleen A Martin Ginis. Effect of Exercise on Mental Health and Health-related Quality of Life in Adults With Spinal Cord Injury: A Systematic Review and Meta-analysis. Arch Phys Med Rehabil 2024 Dec;105(12):2350-2361. doi:10.1016/j.apmr.2024.02.737. Epub 2024 Mar 29.
6. Frank R Kruisdijk, Ingrid JM Hendriksen, Erwin CPM Tak, Aartjan TF Beekman and Marijke Hopman-Rock.Effect of running therapy on depression
   (EFFORT-D). Design of a randomised controlled trial in adult patients.BMC Public Health,2012 12-50.
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8. Joseph Peters , Libak Abou, Laura A. Rice, Kady Dandeneau, Aditya Alluri , Amadeo Felix Salvador and Ian Rice. The effectiveness of vigorous training on cardiorespiratory fitness in persons with spinal cord injury: a systematic review and meta-analysis. Spinal Cord (2021) 59:1035–1044; https://doi.org/10.1038/s41393-021-00669-7
9. Shelley S. Selph,  Andrea C. Skelly, Ngoc Wasson, Joseph R. Dettori,  Erika D. Brodt, Erik Ensrud,Diane Elliot, Kristin M. Dissinger, Marian McDonagh. Physical Activity and the Health of Wheelchair Users: A Systematic Review in Multiple Sclerosis,Cerebral Palsy, and Spinal Cord Injury.Archives of Physical Medicine and Rehabilitation 2021;102;2464-81.
10. Halil Akkurt, Hale U. Karapolat, Yesim Kirazli, Timur Kose. The effects of upper extremity aerobic exercise in patients with spinal cord injury: a randomized controlled study. European Journal of physical and rehabilitation Medicine 2017 april;53(2):219-27 doi: 10.23736/s1973 9087.16.03804-1