What is knee extension on ball (core)?

Knee extension on ball (core)

1. Sit up straight on an exercise ball with your feet flat on the floor, hip-width apart. As you do this exercise, avoid slumping your shoulders and arching your back.
2. Rest your hands on the ball near your hip or a steady object next to you. (If you feel very stable on the ball, rest your hands in your lap.)
3. Slowly straighten one leg at the knee. Slowly lower it back down. Repeat with the other leg.
4. Repeat 8 to 12 times with each leg.