A portable exercise apparatus assisting one person to do sit-up exercise is provided, which includes a footrest, a harness, a resilient retraction means connected between the footrest and harness, and an anchoring means for anchoring the footrest to a stationary support. In use, a person wears the harness connected with the resilient retraction means which connects to the footrest, and seats on the floor with his feet against the footrest. With the assistance of the apparatus, the exerciser alternates between backward prone and sitting upright positions to do sit-up exercise. Another portable exercise apparatus assisting two persons to do sit-up exercise is also provided, which includes a footrest and a hand-pulling device. In use, each person is seated on opposite sides of the footrest, with their feet against the footrest. Each person grips one handle of the hand-pulling device and alternates between sitting upright and backward prone positions.