How does support grip strength and crush grip strength differ so much?

Question

Apparently I can hold on to heavier dumbbells than others, but they can crush my hand much harder in a handshake. This tells me that support grip and crush grip can be very much apart. I believe there's not really a carryover from gripping strength to bar holding strength. I can only close the Trainer gripper, about 100 lbs. for a few reps, and my hands and forearms are dead.

I can, however, hold on to a 200 lb. dumbbell for a few seconds (one-handed).

I can also hang from a bar with almost my bodyweight added (200 lbs. + 200 lbs. = 400 lbs. of hanging support).

However, I know a guy who can't do either of the prior feats, but his grip in a handshake is quite impressive (overpowers mine). He also can't close much more than me in a gripper, but has a strong crushing grip.

My main question is, how does crushing grip differ from support in a muscular sense? Aren't they both the same recruitment of muscles used to close the hand?

How is it supposed to make sense that I can barely squeeze together a 100 lb. gripper spring, but can hold on to a 200 lb. dumbbell handle? Or support 200% of my weight hanging freely from a bar, but fail to crush an apple with my dominant hand?

I always hear people telling me I'm not too strong with my hands in handshakes for some reason (my grip strength is about average for an adult male in crush PSI), but average men can't hold a 200 lb. dumbbell in one hand. Anyone here could explain how the hand strength varies so much in different lifts (e.g. crush 200 lb. gripper, can't hold on to a 200 lb. bar, or hang with 200 lbs. added to bodyweight, but can't open a jar of pickles)?

Answer 1

For support grip and crush grip you use two different muscles (both are used, but in either movement, there is a different main muscle).

For the support grip, you flex the distant part of your phalanges (fingers) by the action of the flexor digitorum profundus.

For the crush grip, you flex the proximal part of your phalanges by the action of the flexor digitorum superficialis.

There are several explanations why the support grip is stronger. The simplest one would be that flex. dig. profundus is stronger. However, since they are of roughly equal size, the more likely explanation is a biomechanical one.

Because of the position of the muscles' tendons, with one (profundus) going under and through the other (superficialis), there is a better leverage for the profundus muscle. Also, the greater thickness allows for greater force production. This makes sense in evolutionary terms since we have done more "hanging" than "crushing" with our hands.

Here is a visual of the position of the tendons as they insert onto the phalanges.

Answer 2

Generally one has a greater mechanical advantage holding onto something, such as a barbell, via a "pure" support grip than via a "pure" crushing grip.

Same reason it's easier to hold onto a barbell weighing "X" pounds than closing a hand gripper rated at the same "X" lbs.

Usually people don't use a "pure" support grip to deadlift or pullup -- they usually "augment" it with a little bit of their crushing grip to hold onto the barbell more securely, making the resulting grip look like something more in between a pure support and a pure crushing grip. In most gym exercises, the latter is only used to reinforce the former anyway, hence it's not surprising why it would often trail behind. Likewise, I would not expect a lot of carryover into support grip strength by training up the crushing grip.