Abstract: A treadmill exercising apparatus comprising a generally planar frame equipped with a planar slider bed and head and tail rollers journalled respectively at the head and tail ends of the frame, over which an endless nylon belt is trained. The treadmill frame at its head end is equipped with a cross member that pivotally mounts a motor assembly including an electric drive motor and shaft driven by same that is keyed to a stepping pulley aligned with a stepping pulley keyed to the head roller, and between which a drive pulley belt is tensioned by a tensioning device that subjects the motor assembly to uniform spring biasing action about the pivotal mounting of the motor assembly for uniformly tensioning the pulley belt into drive transmitting relation with the respective pulleys, and with the motor assembly including a hand crank arrangement for manually relieving the pulley belt tension for changing of the pulley belt with respect to the stepping pulleys for convenient speed changing of the treadmill belt.