Does riding hurt horses?

Hi,
I haven't ridden either of my horses in over a year. I read a while ago some studies that have shown that riding hurts horses. Well today a friend told me that those studies are very flawed. That they aren't peer-reviewed, controlled, or published in any scientific journals. I didn't know much about research, so I didn't see these things as important when I first read them.
Is this true that riding does not hurt the horse? My friend showed me a study that says horses can comfortably hold 20% of their weight. But another one said they can't hold any weight on their back (only for 15 minutes in natural collection). I don't know what to believe!
I didn't know anywhere else to ask this question. Please let me know if you'd like me to remove it. I just wanted to ask it here because the community is very kind, knowledgable, and explain things well.

Please help.

I didn't find the right solution from the Internet.

References:
www.artofnaturaldressage.com/forum/viewtopic.php?t=4525
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Thanks