I used to do triathlons in the 1980s & 90s with a guy who had lost the lower half of his right arm. He is still racing but lives in Melbourne these days. He had a mechanic route both cables to his rear brake lever and had his gear levers on the down tube reversed. He was quite a tidy athlete doing a 9hr Ironman. He rode his bike with his prosthetic arm but swam and ran without it. He was a sub 2hr 50 marathon runner - and claimed it was because he was lighter (by half an arm) than anyone else.