The trunk, which includes the back and abdominal muscles, is a weak link for most cyclists. Because they don t have the torso support to resist the tremendous forces which their powerful leg muscles can generate.
Any force directed into the pedals also goes up into the torso.
Look at tired riders. In fact, a rider will never get stronger by pushing pedals as long as his torso absorbs the forces he creates, because he is negating the resistance of the pedals. Most riders give away significant pedal power because they do not possess adequate torso strength. Back muscles are not adequate in themselves to supply the needed torso rigidity. Evolution has left us with musculature designed for quadripedal animals, and the muscles which could support a hanging, horizontal spine don t stabilize a vertical one subjected to all the impacts and forces that upright posture dictates.