When I grasped thin objects like a zipper tab my thumb and index finger used to bend ferociously. Trapping the zipper tab against my thumbnail made the zipper tab slide out of my hand. After analyzing the muscle imbalance in my hand, my OT gave me exercises. Caution: Bad practice can strengthen muscles that are already too strong. The photos below illustrate the kind of individualized exercise program an OT can create for you.
The 1st exercise is touching my thumb to my index finger. The vibrator stimulates the weak muscles at the base of my thumb that were not working (see black arrow).
The 2nd exercise is straightening my fingers against a rubber band. Initially my middle finger straightened more than my index finger (see arrow on left).