I have trained many client with different problems. i think for endurance as LaRue mentioned, an upper ergometer should be used. Depending on where the amputee occured. i would use any muscle that is left. I have made light sand bags out of socks to train many people with problems such as no hands or feet. It may not be conventional but, these people need help. so sometimes we must be creative in doing so. Upper body should not present much of a problem if done right.