Captain’s chair. Sitting in a chair, press your back against the chair’s back. Stabilize yourself by holding the chair’s handrests, then slowly lift your knees toward your chest, then return them to the starting position, keeping the motion controlled and deliberate through the whole exercise.
Back extensions. Back extensions can be done on the floor or with a Roman chair at the gym. To do the floor version, lie facedown on the floor, arms straight out in front of you, palm down, legs behind you. Slowly lift your right arm and left leg off the floor, hold them for several seconds an inch or two off the ground, then lower and repeat with the opposite arm and leg.
Crunches on an exercise ball. Sit on an exercise ball, feet flat on the floor. Let the ball roll back until you’re lying on it, thighs and torso parallel to the floor. Cross your arms over your chest and tuck your chin. Contract your abdominals raising your torso to no more than 45 degrees. For better balance, spread your feet wider. Want to challenge the obliques (the muscles either side of your abs)? Make the exercise less stable by moving your feet closer together. Exhale as you contract; inhale as you return to the starting position.
Vertical leg crunches. Lie on the floor, lower back pressed to the ground, hands behind your head. Extend your legs straight up, crossed at the ankles, slightly bent at the knee. Contract your abdominal muscles by lifting your torso toward your knees. Make sure to keep your chin off your chest with each contraction, and exhale as you contract upward, inhale as you return to the starting position.
Reverse crunches. Lie on the floor, lower back pressed to the ground. Put your hands beside your head or extend them out flat to your sides — whatever feels most comfortable. Crossing your feet at the ankles, lift your feet off the ground, knees bent, until your calves are parallel to the floor. Once in this position, press your lower back on the floor as you contract your abdominal muscles. Your hips will slightly rotate and your legs will reach toward the ceiling with each contraction. Exhale as you contract; inhale as you return to the starting position.