BOSU Ballast Ball Twisting SitUps
The BOSU ballast ball makes these twisting sit-ups more challenging than
most other forms of resistance such as dumbbells or medicine balls. The
ballast material (sand) inside the ball shifts as you rotate towards one
side which creates extra momentum which your muscles will have to stop.
While any form of situps is going to work the hip flexors as well as the
abdominals, this form of sit-ups is great for the obliques as well.