Reverse belly roll: Lie on your back on the floor with your lower back close to the ground, put your hands on your sides, raise your legs at 90 degrees to your upper body, cross your legs and bend your knees slightly. Tighten your abdominal muscles, then exhale, lift your hips slightly, lift your back slightly off the ground for 2 seconds, and then slowly return to the starting position. Boarding in the air: Lie on your back on the floor with your lower back close to the ground. Put your hands on your head and open your arms. Lift your legs and climb on the bike slowly. Exhale, lift your upper body, touch your left knee with your right elbow, hold your posture for 2 seconds, and then restore. Touch the right knee with the left elbow again, hold it for 2 seconds, and then slowly return to the starting position.