What’s different between a repeated action building muscle and a repeated action damaging muscle/things?

Question

First part’s obvious, you do exercises frequently and it builds muscle but, some examples of the second, I remember Burnie of Rooster Teeth talking about going to the doctor for bad shoulder pain and finding out it was because he was frequently reaching back to his kids in the back seat while driving or, back when I had a warehouse job, I sprained both my knees after pulling big heavy cages around for a day or two. Why did Burnie’s repeated reaching back cause damage and why did my repeated pulling heavy cages cause damage when other repeated actions just build muscle?

In: 1

Answer 1

Overdoing it, or doing it in an angle so wrong that it damages other things than muscles.

Muscles get stronger by repairing small microtears that happen during regular or exercise use. Ligament, cartilage, etc. does not.