1. Pulling on your neck
It might feel natural to pull on your head as you crunch away, but frankly your hands are supposed to be just supporting your head. The hyper-flexion of the cervical spine can stress the discs between the vertebrae. Also, the actual focus of the work takes place in your midsection, not your neck. Try unlocking your fingers behind your head, or remove your hands completely.
2. Doing just bodyweight moves
While there’s a bit of controversy surrounding whether you should train with low reps (with added weight) or high reps (using just bodyweight) for abs, one thing is for certain: If you don’t overload the muscles of the midsection, they simply won’t grow stronger. If you’re using only bodyweight moves (and your bodyweight doesn’t fluctuate wildly), then the only way you can get stronger is by doing more reps (and with bodyweight you always need to reach failure to be effective) or reducing your between-sets rest periods. If you’re doing moves like cable crunches, you can physically increase the resistance from one workout to the next, and that’s where you can build up those abs. That’s why they call it progressive overload, folks.
3. Arching your back
You’ve heard it so many times that it’s practically beaten into your brain: Maintain a flat or slightly arched back when you train with weights. And while that’s good advice to protect the spine, a flat back means you aren’t contracting your abs actively — though they may be isometrically contracting to keep your core tight. You often see this when guys are doing cable crunches or decline crunches with an arched back. A better way to get the abdominals actively engaged is to round your lower back, which allows the rectus abdominis to shorten. Just watch out for explosive movements when rounding your back.
4. Not holding the peak contraction
Because the abdominals are postural and always at work throughout the day, it makes it that much harder to fatigue them via training. They’re naturally fatigue resistant, primarily made up of slow-twitch muscle fibers. That’s a good thing because we need them to have a ton of endurance to support us throughout the day, as well as in the gym. Because of that, you need to take advantage of each rep’s peak contraction, squeezing and holding for a full count rather than going through the motions doing speed reps. The better able you are at contracting the six-pack at the right time, the better they’ll respond to whatever stimulus (high rep/low weight or low rep/high weight) you apply to them.
5. Pulling through your quads
We’ve all done sit-ups and have heard they’re no good, but that’s only a half-truth. The problem with moves in which your feet are anchored, whether under a sofa or on a decline bench, is the tendency to pull through your hip flexors, a group of muscles that act to flex the femur (thigh bone) and pull the knee upward. To focus on your abs, you want to minimize the effect of the hip flexors. One way to do that is to consciously avoid contracting your quads when doing ab movements in which your feet are anchored.