Loaded overhead, full sit-up, full burn.
Keep your arms locked overhead, let your abs do the lifting not your shoulders.
Holding a medicine ball locked overhead through a full sit up stretches the lever your abs have to move, every rep starting with the load behind your head and finishing with it stacked toward the ceiling. Your core does all the hoisting while your arms just hold their position, which is what separates it from a throw or a press. It turns the basic sit up into a loaded movement you can actually progress.
Distance. Load held long overhead multiplies the demand on your abs compared with the same weight hugged to your chest. It also forces a controlled, honest rep, momentum with a ball overhead simply does not work.
Yes, bent arms sneak the ball toward your chest and shrink the lever, quietly making every rep easier. When the arms start folding, the set is effectively over. Drop to a lighter ball or fewer reps and keep the arms long.
Unanchored is stricter, your core has to do everything. If you cannot complete a rep without your feet lifting, anchor them lightly to build strength, then wean off the anchor as your core catches up.
