Are planks effective for building nice abs?
I've tried a few basic positions and the idea of holding a position for as long as I can, and having a workout for my abs and core areas, is pleasing, compared to dreadful sit-ups / crunches that make my neck so tired.
Can planks be as effective for building flat abs / 6-pack, like crunches and sit-ups are?
If so, I rather do a couple minutes of planks everyday ...
But I'm guessing there are diminishing returns with planks ...
The main function of the rectus abdominus is to curl the torso forward. Contrary to popular belief and method, bending at the hips works hip/leg flexors, with minimal involvement to the abdominal muscle.
For growth, you need to stress the muscle as you would any other, with progressive overload in the hypertrophy range for sets/reps. To start, you can do bodyweight exercises such as traditional crunches and hanging leg curls. (For the curls, the initial part will work the hip flexors, then the final curl up works the abs.)
You need to maintain a fairly neutral spine, and avoid curling your neck or putting your hands behind your neck/head to "pull" yourself up. These will lead to the neck pains you are describing.
Also, do not neglect the other core muscles. Obliques are the muscles responsible for rotation of the torso, and the lower back muscles are worked by extension type exercises. All of these together need to be worked, or you risk imbalance that causes back pain as other muscles try to compensate.
Since building a sixpack will require some muscle growth, I'd argue that neither are effective. You'll be at a point where you can do dozens of sit-ups very quickly, which might be fine for the endurance of your abs, but won't help growth very much. Same for holding 3 mins of planck. Fine, but not very helpful on your way towards a sixpack.
While not having a sixpack myself, I'd think that you'll have to implement some kind progressive overload for a sixpack, while staying in the range of 8-12 reps (hypertrophy). If you're in a gym, you might look at cable crunches, which is a loaded version of crunches, which you'll be able to adapt to your abilities. If you have access to weight plates, you could also do weighted sit-ups.
To some extent, standing barbell presses might have an effect, too, since they require a strong core to stabilize.
