If you are new to boxing, work your footwork, proper punching technique and power on it. Don't just come out swinging for like 2 mins then go sit down. Work in rounds(maybe 2 min and 30 sec break). Move around the bag. Learn how to keep your proper stance and fire your jab, one-two, and one-two-threes. For the first while, you will be simply pushing the bag with your punches, and that is when it hurts to punch. Try snapping your punches and work on aim.
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