in the saddle—
1. riding a horse:
- She was hardly in the saddle before she called out “race you to the bridge.”
2. in a position of leadership:
- It is his bad luck to be in the saddle when his country has to decide which road it is now going to follow.
on the saddle—on the seat of a bicycle or motorcycle:
- I was just sitting on the saddle pushing the bike along with my foot on the curb.