A sensor includes a sheath that is elongated along a longitudinal axis; a spacer positioned within the sheath and defining first and second channels having lengths that extend along the longitudinal axis; a first elongated member positioned within the first channel; and a second elongated member positioned within the second channel. The insole corrects the shape of the arch of the sole and thus improves blood circulation. The means for restricting the axial displacement of a clutch retainer with respect to a metal inner ring is achieved by engagement between the concave engaging sections and the convex engaging sections when the clutch retainer is assembled on the outer-diameter side of the cam surface.