Fitness Ball Crunches
Lie flat on the exercise ball, feet flat on the ground, hands on the side of the head, arms open. Tuck your chin slightly toward your chest, exhale, contract your abdominal muscles and lift your upper body about 45 degrees, hold it for 2 seconds, and then slowly return to the starting position. In order to maintain balance, your feet can be separated more. If you increase the difficulty, you can do it with your feet together.

Leg curl
Lie on your back on the floor with your lower back close to the ground. Put your hands on the side of your head and open your arms. Raise your legs up to 90 degrees to your upper body, cross your legs, and bend your knees slightly. Exhale, contract your abdominal muscles, raise your upper body, keep your lower back from the ground, hold for 2 seconds, and then slowly return to the starting position. Pay attention to keeping the chin slightly closed to the chest.
Weight-bearing abdominal crunch
First adjust the pull-down weight. The fulcrum of the gantry is adjusted to the highest position. Kneel on the side of the plane of the gantry with your knees 60-100cm away from the base of the gantry, subject to the head not hitting the gantry. Hold the rope pull handle with both hands behind your head, keep your arms and legs fixed, hold your chest, exhale while contracting your abdominal muscles and pull down. Hold for 2 seconds to lift your upper body, then slowly return to the starting position and repeat.

Reverse curl
Lie on your back on the floor, with your lower back close to the ground, your hands on both sides of your body, raise your legs 90 degrees to your upper body, cross your legs, and bend your knees slightly. Tighten the abdominal muscles, then exhale and lift up the buttocks slightly, keep the lower back slightly off the ground, hold for 2 seconds, and then slowly return to the starting position.
Traditional Crunches
Lie on your back on the floor with your lower back close to the ground. Put your hands on the side of your head and open your arms. Lay your legs flat on the ground and bend your knees. Tuck your chin slightly toward your chest, contract your abdominal muscles, exhale and lift your upper body, keeping your lower back from the ground, hold for 2 seconds, and then slowly return to the starting position.