A treadmill includes an external frame, an internal frame connected to the external frame, a belt in the form of a loop supported on the internal frame and a buffering apparatus for buffering the internal frame. The buffering apparatus includes a crossbar and two buffer sets. The crossbar is rotationally provided on the external frame between various positions. Each of the buffer sets includes a plurality of buffering blocks made with different elastic coefficients so that a selected one of the buffering blocks is brought into contact with the internal frame when the crossbar is in a related one of the positions.