An exercise treadmill includes a tread belt wrapped around front and rear rolls, a motor, a frame, a control panel, a sensor electrically connected to the motor for sensing a rotation of its driving shaft and generating a sense signal if sensing the rotation, and a controller electrically interconnected the sensor and the control panel for receiving the sense signal and generating and sensing a control signal to the motor. The motor may either stop the rotation of the driving shaft for maintaining the tread mill motionless in response to the user's foot impact thereon less than or equal to a predetermined reverse force or oppose a movement of the tread belt in response to the impact larger than the reverse force. Moreover, the tread belt either maintains at its current position or returns to its original position in response to stopping exercise.