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It seems to be popular to spread abs exercises over the weekly schedule — e.g. one exercise at the end of every training.
OTOH there is an option to do abs all on one day, alone or combined with another muscle group (back?).
Which option is considered more effective?
The best approach is to train them like any other muscle. 2-3 times per week with progressive overload to ensure that the reps or time under strain is relatively consistant over time. This is more difficult with abs, but exercises such as:
All offer the ability to increase the weight over time which will eventually lead to hypertrophy. The best approaches for training abs though will take into account your goals. If its aesthetic, the above should work fine, however if you are training for contact sports anti-flexion and anti-rotation exercises (which aid in the stabilisation of your 'core') will offer the most protective results.
Keep in mind that exercises such as squats and deadlifts will also require core activation and you'll want to incorporate your abdominal training around these.
Lastly, look into the work of Dr. Stuart McGill for good core training techniques. He is probably the foremost expert in the field of sport medicine and lower back and core health