Boxing - heavy bag question

In doing the heavy bag activity, I found that it was giving me credit for multiple hits every time I swung. I would swing my arm and see my avatar on the screen hit the bag two or three times fast, which of course got my allotted hits done faster and gave me less of a workout. It doesn't do this with hitting targets or any other activity so far. Any suggestions on what I might be doing wrong?|||Sorry I have not seen this happen before.