I am thinking soft tissue problems rather than worn out joints.
The movement that I am doing is an extended version of the walking movement. (My toes get closer to my shins.)
I can feel a tension in my Achilles tendons which I don't get from normal walking. It is the kind of tension that I have felt when scrambling up and down steep, rocky slopes, however, and which have left my legs sore for a few days afterwards.
I don't think that it is nonsensical to think that doing repeated movements beyond the normal range of movement might cause problems. For example - My niece once worked in a bank call centre and had to use a stupidly designed keypad mounted on a stand next to her desk. That caused her to bend her wrist back slightly further than wrists normally bend when carrying out repetitive tasks. The end result was severe RSI which has left permanent soft-tissue damage in that arm.