Originally Posted by
kenji666
Having strong thumbs helps.
Most experienced mechanic use the base of the thumb adjacent to the middle of the palm to lever tight tires over. When I first started in the bike Biz, back in the Bronze Age, The two most experienced mechanic had large callouses there, big enough to make sort of an edge which could catch and lift the tire much the same way a tire jack does.
If you use your thumbs to mount tires all day, the supporting muscles get tired and it's easy to dislocate or sprain it. That's an injury that's painful and slow to heal since you keep reinjuring it during the work day.