I have used the ball on my core training . Itīs really good , since w it you can reach the deep muscles not touched w regular sit ups or chrunches. to accomplish that you must do several things w your core muscles ( srt of tighthening ) and KEEP that during every rep . I think that same applies for keeping the stability while sitting on it. The less full the ball is the harder it is to use.
However I have NOT used the ball for the way you mentioned....
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want explode your head? smoke dynamite
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