Bosu Ball leg raise exercise – bent knee, to the side, alternating This exercise is great for the beginner who is working on core strength or an improvement in balance. Start by standing on the top of the Bosu with your feet shoulder width apart. As you contract the core, put all your body weight on the left leg. Slowly lift the right leg to the side as high as possible by bending the knee. Lead with your knee and keep your foot flat. Once your right leg is lifted, slowly lower the leg to the Bosu. Repeat the movement with the left leg, by placing all your body weight on the right leg which is planted on the Bosu. Keep the core contracted, back flat and eyes facing forward. This exercise should not be completed quickly.