I used to do biathlon... Personally, my arms were more painful. Still, the rifle and the thrusts on it. You run the last kilometers just on your legs, because your arms are useless. Feet can rest somewhere, from the descent, for example. My arms work for 90 percent of the race. And your back gets very tired, but it depends on the age and build of the athlete. - It's all very subjective. с: