Abstract: A crank arm (200) includes a mechanical ring amplifier (232), having a cavity and a connecting bridge (140), to amplify the radial strain exerted on the crank arm by a rider. Strain sensors (234, 235) are positioned to detect the amplified radial strain. The crank arm (200) may include further strain sensors (228, 229) adapted to detect tangential strain on the crank arm, allowing both the tangential and radial forces exerted by a bicycle rider to be measured. The crank arm (200) may also comprise means for adjusting the position of a pedal axle relative to the crank arm, via the loosening, rotation and retightening of a carrier disc held by the crank arm, where the carrier disc has an eccentrically mounted aperture for holding the pedal axle.