Squeeze the ball, crunch it in.
Squeeze the ball the whole time so your legs stay locked around it.
Pinning a medicine ball between your feet and crunching knees and chest together makes both ends of your core close at once, the upper abs curling down while the lower abs drag your legs and the ball inward. The constant squeeze on the ball keeps your legs locked as one unit, removing the leg-swing cheat that ruins most crunch variations. It is a full-range core move that keeps tension from the stretch to the crunch.
The squeeze forces your legs to move as one piece and adds a little resistance to the lower half of the crunch. It also gives you instant feedback, if the ball starts slipping, your focus has drifted and rep quality is dropping.
Shrink the range and slow down. Do not extend your legs all the way out, keep them higher, and focus on curling your hips and shoulders toward each other rather than swinging the legs. The crunch should feel like folding, not kicking.
Light and grippable, roughly 2 to 4 kg for most people. The ball is there for the squeeze and the control, not the load. A heavy ball between your feet mostly punishes your hip flexors and your floor.
