[sirdarling]

I did this when I was younger and noticed the joint stress after working out that way a few times.
What I ended up doing to correct it was forgetting my total body form in the punch and it did not strain my joints:

I would start out the punch correct and then after it reached the apex would allow the weight to throw me off balance forward.
I would end up on one leg leaning forward but allowing the momentum to carry me it eased the strain.

I am not suggesting it, like you said it added a lot of strain... I used 20 lb weights back then, made my punches really hard indeed but I have so many upper body joint/tendonitis problems now. I don't know if this was the cause thought, plenty of other bad decisions to add fuel to the fire, lol.