1/ Cable crunch, make sure to
not keep your lower back straight and instead slowly wrap the spine to engage abs muscles
2/ Knee raises, either doing it on the floor or using abs straps to a pull-up bar. Same path, wrap the spine until your knees are close to the sternum. Never keep your lower back staright or it will engage the hip flexors
3/ Janda sit-ups, popularized by Gareth Stapstead, instead of locking your feet in the front, you're locking your feet in the back, who will engage the hamstrings but partially disengage the hip flexors, and then increasing abs recruitment
Keep regular crunches for your warmup and/or for a back decompression protocol if you have already developped upper abs. If not you can consider also weighted crunches
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