Last week: Wear & tear model is outdated. Your cartilage, just like other bodily tissues [muscles, bones, tendons], respond positively to progressive loading. Do 100 bicep curls every day, your body will adapt. Do 100 squats a day, your body (including that knee cartilage) will adapt. It's a simplification, but you get the point.
This Week: How much is too much. Astronauts & Marathoners.
How much is too much: the optimal load likely exists on a U-shaped Curve (see award-winning photo -->>).
Too little load or too much load & your risk of injury is increased.
Let's play Astronauts & Marathoners: We need gravity. That constant mechanical force of gravity ensures we've got the bones, tendons, etc, strong enough to live on this earth.
Astronauts don't have gravity, and when they return to earth they experience detriments to ALL bodily tissues: bones, tendons, muscles - even their eyes shrink due to lack of loading.
Marathoners have the opposite issue. Normal amounts of gravity, but subject themselves to high reps of high load when we're talking 100+ MPW.
The thing in common? They're both at an increased risk of injury. Gotta stay in dat sweet spot.
Closing: Often, those with osteoarthritis are less physically active than those without. Load your body, train smart (avoid excessive overloading), strength is protective, enjoy the process.
Source: Bryan Heidersheit PT, PhD Running Medicine Podcast with guest Ross Miller, PhD