It's not just pro cyclists that have scrawny upper bodies...Most endurance athletes including long distance runners, long distance cyclists and triathletes have scrawny and weak upper bodies. It's just the nature of their sport, endurance training isn't conductive to muscle and strength building. The only way for an endurance athlete to increase strength and muscle is to cross-train and supplement their training with weights and resistance training...Cycling is much lower impact than running and better for preserving muscle. The only runners who are muscular are the sprinters because they do a lot of weight training besides their running, majority of marathon runners are scrawny twigs.