I think there's something to be said about practicing until the muscle fatigues, in which case it will start to burn. But at that point, that's where you STOP. That burning is essentially the lactic acid build-up in the tissues, and once you stop and the tissue gets re-oxygenated, the burning should subside quickly. So I don't have an issue with someone practicing until the muscle gets fatigued--as long as people understand what's going on, and to stop before damage is done...as Matt stated.
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