Abstract: A power transmission system particularly useful for a foot-operated bicycle comprises a pair of axle sections coaxially-mounted with respect to each other, each fixed to one of the crank arms and each rotatable independently of the other; and coupling mechanism between the pair of axle sections and the chain wheel effective to cause the angular velocity of one crank arm to decrease relative to the other for a portion of a rotary cycle, and then to increase relative to the other for the remaining portion of a rotary cycle, such that as the one crank arm approaches the lower dead-center position after completing a forceful downward stroke, the other crank arm has passed the upper dead-center position preparatory to starting another forceful downward stroke.