For those who have lost the use of their legs and/or feet there is an alternative. I am talking about hand cranking for propulsion. There are just a few manufacturers producing hand crank trikes. Here is one designed for off road riding.
From their website …
The Explorer II Off-Road Handcycle is a unique handcycle designed to tackle extreme off-road conditions and provide unparalleled opportunities to get up close and personal with places that until now have been inaccessible. Places like mountains, trails and the beach are no longer out of bounds, the Explorer Off-Road Handcycle will take you there and to the top. This unique off-road handcycle has already taken many users to the highest peaks of Europe, including the Alps and the Tatra mountains. The Explorer also took part in a successful climbing expedition to Mount Kilimanjaro – the highest peak of Africa. The Explorer ll is the most advanced off-road handcycle of it’s kind with features such as full independent suspension, hydraulic disc brakes all round, Schlumph Mountain Drive and a choice of drivetrain ranging from 16-22 gears. One of the great features of the Explorer is the chest steering used to keep the bike in a straight line or negotiating turns when pedaling. The handlebars are there for flying downhill.
This YouTube video is accompanied by the following description:
The Explorer II off-road handcycle , is a hand powered trike for people with disabilities, however it can bring a lot of fun also for able bodied. We created this handcycle so that break the boundary of mountain biking and allow the disabled persons to reach what until now was impossible. Now we at SPORT-ON believe that we have succeeded!
HERE is a link to their website.