Part of the issue might be in using shadow boxing to practice speed and power -- instead of using it primarily for perfecting your form, movement, and chaining together smooth combinations.

if you shoot you hands out at top speed and power with nothing to stop them then you are likely going to be putting a lot of stress on your joints (esp. elbow and perhaps shoulder), or you are going to be training in a lot of counter-tension and/or bad form by stopping those punches short using muscle power.

Either you practice perfect form or you are practicing to do it imperfectly. What sense does it make to be practicing to do something incorrectly?

Practice speed and power (in the best form you can manage) against the bag. Practice perfecting your form and combinations by shadow boxing while relaxing.

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HerbM