<p>A crank assembly (10) including a mechanism capable of connecting the crank arm (12) to a shaft (30) to drive the shaft in rotation in either a unidirectional drive mode or a bidirectionally driving mode, the mechanism being quickly adjustable between modes. A coupler (50) in the mechanism may be slidable between a first position engaging the crank arm (12) with a hub portion (36) fixedly mounted on the shaft (30) and a second position in which a unidirectional clutch (34) can operate to allow rotation of the crank arm freely about the shaft in one direction but cause rotation of the shaft in the opposition direction. The coupler (50) may be arranged to be coupled only when the shaft is in a selected angular position.</p>
