When you are back to doing what you do best. Or, when you are back home from a long trip.
Timmy had been on the mend from a broken foot, but now that he's all better and playing ball again, he's <back in the saddle>.
Joey had been on a long business trip but now he's <back in the saddle>.
1.) To begin doing what you stopped doing before, and often after recovering from an illness or a set-back.
2.) Riding a horse
1.) The doctor said that I have fully recovered, so I am ready and will be back in the saddle
2.) His teenage son got a new position in the polo club, and he is now back in the saddle