What it strengthens:
Abs, hip flexors
This is a gentle, effective exercise that targets your deep core muscles. Exercises like this help to activate your core and train you to use it more effectively.
Lie on your back with your knees bent and feet flat on the floor, arms by your sides. Tighten your abs to pull your hips back towards you and flatten your lower back against the floor. Lift one leg at a time so your knees are above your hips and your ankles are level with your knees (90 degree angle in knees). Keep your abs pulled in like this the entire time so your back does not lift up. Maintain the angle in your knee and lower one foot toward the ground without moving anything else and return. Lower the other leg in the same manner to complete one repetition.
To make harder:
If you want more of a challenge, try lifting your head and shoulders off the ground with your head supported by your hands (without tucking your chin to your chest) to engage your abs from top to bottom.