Strength of the grip and strength of the fingers are highly correlated. The contribution of the index finger is about 25% of the total force, the middle finger 35%, the ring finger 25% and 14% of the force is contributed by the pinky. Looking at these values you can see that the radial side or thumb side of your hand contributes about 60% of the hands gripping power and the ulnar side 40%.
Body size and finger length are moderately correlated with strength. Much of the closing power of the hands comes from the extrinsic muscles that make up the forearm, as fingers are moved by tendons which are pulled by these muscles.  It has been shown in studies that as little as a 2 cm change in circumference of the forearm can affect gripping power.
The absolute best way to increase overall hand strength is training, whereby the musculature is forcibly lengthened, commonly referred to as eccentric exercise or negative only training. Tyler Hobson the designer of Pendulum built the Pendulum Gripper not only to exercise by closing the hand, but developed a tool for lengthening contractions.
Using the foot pedal on the Pendulum Gripper to assist in negative only training
To maximize hand strength load the gripper with a weight that can be squeezed tight for a full second and then lowered slowly for an 8 count. Quickly the athlete uses their foot to close the gripper and once again hold the weight with no movement for a full second before lowering the weight in 8 seconds.  The hand  must be halfway open on the count of 4. Every inch of the exercise is important, being half way down in four seconds means that the athlete is 1/4 of the way down in 2 seconds and 1/8 of the way down in 1 second.
Each rep is counted from the squeezed position until the index finger becomes almost completely extended as shown
Perfect repetitions is what the coach and athlete are after, the exercise is completed when the lifter can no longer control the decent of the weight at an 8 second pace.  When 8 perfect reps are achieved in exactly 8 seconds, 2 1/2 lbs. can be added.
Pendulum Grippers ready to be shipped.….waiting to Get the hands Strong