A support frame for an exercise apparatus includes a base, and a smoothly arched moving arm member slidably coupled to the base. The moving arm member defines a first end connected to a wheel pedal unit and an opposing second end connected to a saddle unit, such that a distance difference between the first and second ends in the vertical direction, as well as in the horizontal direction, changes subject to changing the relative position between the moving arm member and the base. Thus, by means of moving the moving arm member relative to the base, the distance difference between the first and second ends is relatively changed, causing change of the relative position between the wheel pedal unit and the saddle unit. This allows the user to select a normal upright sitting exercise mode or a recumbent sitting exercise mode.