Here is what the Mayo Clinic has to say about Aerobic Exercises.
"During aerobic activity, you repeatedly move large muscles in your arms, legs and hips. You'll notice your body's responses quickly.
You'll breathe faster and more deeply. This maximizes the amount of oxygen in your blood. Your heart will beat faster, which increases blood flow to your muscles and back to your lungs. Your small blood vessels (capillaries) will widen to deliver more oxygen to your muscles and carry away waste products, such as carbon dioxide and lactic acid. Your body will even release endorphins, natural painkillers that promote an increased sense of well-being."
All very well and easy to achieve if you use a manual wheelchair and are prepared to go out for 'walks' to keep fit. For those who are not so fortunate an exercise plan is required.
First and most important, since everyone is different and has different needs, get professional advice from your doctor or physical therapist before you start any program of exercises. You need to get advice on the level of activity that would be good for you and the possible progression towards greater efforts as you build up your ability. There are very many different aerobic exercises. Not all will be possible and not all will be safe for people with certain physical limitations. Make sure that you are comfortable with the plan and that the suggested exercises are ones that you can see yourself persevering with.