Seated pullups are an excellent regression for standard pullups. Sit on the floor or a bench directly under a bar or rings. Grasp the bar with an overhand grip. Keeping your feet on the ground to assist the movement, pull your chest toward the bar by driving your elbows down and back. Focus on engaging your back muscles rather than just pulling with your arms. Control the descent back to the starting seated position.
The exercise is best performed with gymnastic rings