You probably know crunches are old-school, but you may not know why they’re not very effective. What’s their weakness? Most women initiate crunches with their hip flexors without engaging much of their core. This may get the surface muscles in your abs, but it ignores the ones underneath, which are also essential to a flat stomach.
Also, Crunches mimic the sitting posture we use for much of the day. We don’t want to exacerbate this ‘hips flexed/shoulders hunched’ position. The point of training is to fix the gaps and do something different. Crunches repeat a similar movement pattern.
A better bet for flat abs? Focus on moves like the plank and side plank and side plank that work often neglected areas of your core. And don’t forget your butt, too. Weak glutes push your stomach out and give you a belly even if you don’t have one.