3. Seated sit-up machine
Think twice about doing any situp. The tendency to yank your neck forward, round your shoulders, and flex your spine is too dangerous for some people to bother with, especially if they sit at desks for long periods. A study in Clinical Biomechanics found that situps of any kind increased compressive loads on the lower back. The study author concluded that “the issue of using straight legs or bent knees [to perform a situp] is probably not as important as the issue of whether or not to prescribe situps at all.”
To blast your core safely, use exercises that train it to resist flexion and extension and stay stable.
Use these instead:
Plank with arms on a stability ball
Ab rollout Bodysaw with Valslides or towels on a wood floor (shown in the picture above)