I was watching Conor McGregor's media workout (2016-08-12) in preparation for UFC 202 and saw that his training includes being hit in the abs:
What's the point in hitting abs during a workout? E.g., does it aim at developing abdominal muscles?
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For Conor McGregor, the point of that particular ab training, is to develop the ability to take a punch, which is as important as being able to deliver.
Collaterally, it will probably also cause some muscle gain in the long run, but probably a negligable amount.
It's important to note that this is not something we should replicate in our own training, as it could cause internal bleeding if done as often as he trains.