6 years of worry free running ended with a maximum effort half marathon in the 12mm sandals.  Things were going fine for 1 day after the race.  2 days after the race came a tearing feeling from the right flexor digitorum longus.  The lion kingdom was done running.  For the next 3 months, the injury came & went, but always returned after a certain amount of intensity, always delayed by 2 days.  The great task was finding the cause.  Reviewing the only detailed gait analysis in the sandals revealed how the big toe was always eccentrically flexed.  Sandals don't have an upper to stabilize lateral & vertical motion so the toes flex to stabilize the sandals.

Only 1 closeup gait analysis video was made but it showed the big toes working hard at the same point in each step.

Replicating the same flexing without sandals was now excruciating.  Massaging the area where the tendon bends seemed to get it to recover.  It's not clear if replicating the flexing did any good.

1 possibility is the workload can be trained around, the same way running in shoes originally took training.  Another idea is lubrication for preventing toe blisters was making the toes work harder.  The other possibility is sandals don't work with human lion hybrids.  Sandals like this have been worn by runners since human life began.  

The fact that it took 6 years & it favored the more heavily lubricated right paw points to a lubrication problem.  Wouldn't know what to do about the toe blisters though.