I'd say an upright rider can put way more force on a chain than a bent rider can. In fact, I've broken chains on uprights but never on a bent. Those of you who claim they can do 400-600 pounds (or more) in a leg press, can you do it with ONE LEG (one pedal at a time?) I can't. Standing and pulling on the bars can exert not only a huge force, but it can instantaneously be much higher than someone spinning while seated.