The present invention includes two rotatable sleeves, two springs, two pins, two plastic sleeves and a Z-shaped crank. The two springs are respectively disposed in the two rotatable sleeves. The two rotatable sleeves are respectively accommodated around corresponding left end and right end of the crank. One end of each of the two springs is abuttingly connected on an inner wall of one of the two rotatable sleeves and one other end abuttingly connected on an end wall of the crank. The two pins are respectively penetrated through the two rotatable sleeves and are insertingly connected on the crank. The two plastic sleeves are respectively accommodated around left and right ends of the crank to closely cover the two pins. Thus, the present invention not only provides a smooth and pleasant appearance, but also allows a pedal to a position parallel to a bicycle without affecting a boxing effect.