Exercises to strengthen the core include abdominal crunches, bicycle crunches and leg raises. Abdominal crunches primarily exercise the upper abdominal muscles, bicycle crunches focus on the obliques, and leg raises exercise the lower abdominal muscles and hip flexors.
To perform an abdominal crunch, lie on your back with your knees bent. Lift your shoulders off the ground, curling them toward the pelvis to contract the abdominal muscles, then lie back down.
For bicycle crunches, lie on your back with your shoulders off the ground and hands behind your head. With your legs off the ground, do a bicycling motion with your legs while touching one elbow to the opposite knee as it rises towards your chest.
To perform leg raises, lie on your back with your legs straight. Raise your legs to a 90-degree angle, then lower them back to the ground.