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You've got to work at improving all of your attributes both mental and physical. Knowledge and technique are of the highest importance along with cardio. Then comes strength and flexibility, and they are of equal importance but, you can be greatly lacking in one of these two areas, as long as you have the knowledge, technique and cardio, and still be a great grappler. I personally prefer developing both, because both can give you more options for a wider variety of situations.
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In bjj or sub wrestling i'd definitely say flexibility, but MMA strength becomes a lot more important
