I'm not entirely sure I understand what you mean.
I mean, if you are relaxed after a few rounds on the heavy bag then you aren't training hard enough. You should be training hard so that you are worn out and tired.
But if you are talking about tension in the body, then you have to train slowly and if you stuff up not fret over it. Just take it easy and just let everything go. You will soon get it.
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jiyasa
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