How would digitigrade fingers work?

Question

So... I'm fiddling with Yet Another Bipedal Digitigrade Species. There is tons of material on the legs, but seemingly not so much on the hands.

Short Version

So... I'm wondering; is it possible to design a "hand" that can function as both a digitigrade 'foot' and be useful to a civilized tool-user?

Longer Version

As we know, digitigrade forepaws have extended metacarpals, and the range of motion between the metacarpals and proximal phalanges is from roughly in-line to about 90° forward/out. In humans and most other species with grasping fingers, the range is from roughly in-line to about 90° backward/in.

Another challenge is claws, especially the retractile variety, which effectively 'eat up' a useful joint. That said, I'm not sure how much it would really inconvenience most people if the last two digits of their fingers were fused.

An obvious solution is to provide the MC/PP joint with a greater range of motion. Is this plausible? If not, is it plausible that a species with such 'fingers' would be able to grasp objects anyway? (What might the thumb look like?) Or is there any other 'hand' design that could be used?

Notes

• 'Hands' should be able to grasp objects well enough to plausibly be able to survive in any period of development (assume technological progression similar to human).
  • Should be able to grasp a pencil / thin rod. Bonus points if they can use chopsticks.
  • Should be able to grasp a tool handle / thicker rod.
  • Should be able to grasp a ball / apple / etc.
  • Okay if they need to use two hands more often than humans.
• Should be able to 'stand' on forelimbs with weight partially carried by the metacarpals. Should ideally be able to walk like this, but okay if they can't run (on all fours) very well.
• Phalanges are probably longer than otherwise; closer to human proportion.
• There is definitely a thumb, although I'm not entirely sure what it looks like or how it stays out of the way for walking-on-hands. Ideally it is opposable, at least to the tips of the other fingers when said fingers are curled. (Maybe not to each finger joint.)
• Should be able to have "retractable"¹ claws.

(¹ For you pedants out there, yes I know claws are really protractile.)

Answer 1

You may have to be satisfied with less dexterity

Right, so, the thing that makes digitigrade legs work is that their digits, their toes, the things that maintain the creature's balance both while stationary and in motion, are rather large and strong compared to the toes that we as humans have, though this is mainly to allow them to fulfill the same function as what our plantigrade feet do with the shape of their rather robust heels and metatarsals. Point is you need a point of contact that can not only deal with the frictional stresses of locomotion but the compressive stresses on the bones as well, along with strong enough muscles that aren't likely to tear just because you decided to run instead of walk.

Even simply pressing my hand into a theoretically digitigrade posture, allowing only the whole of the underside of my fingers and most of my thumb to touch the table and maybe a little bit of the palm where the fingers connect under the knuckles, hurts, so expect proper digitigrade forelimbs to be rather robust with longer, thicker palms, thicker fingers, and larger, more structurally-sound knuckles. This sadly leads to a sacrifice in dexterity and range of motion, but assuming finger length doesn't shrink relative to the increase in hand size they should still be able to clench the fingers into something like a fist and pick up most things we can, barring extremely tiny/thin things like needles. Obviously they're not going to be doing knife tricks, or stitch-work without an intermediary tool(something tapering and pincer-like that allows their larger digits to work with smaller objects), but they should be fine to do most things.

If you really want claws this limits their dexterity more severely, but with the increase in finger size you'll have larger and more robust fingernails which should be able to deal quite a bit of damage on their own judging from what people can already do to other people's faces with our admittedly frail nails. Clubbing things with their larger forelimbs would also deal considerably more blunt damage than our fists can do as well, so they might not even need weapons but of course with larger and stronger hands comes the ability to wield larger and heavier implements so it's something to consider in their military potential.

Answer 2

Have them knuckle-walk like the great apes do.

Like John mentioned in a comment, having fingers that can flexibly bend in both directions is likely to result in fingers that are either bad at being used for walking, or bad at being used for gripping.

So, you can split the circle by only having them bend in one direction: inwards. They'd then walk around on their knuckles and the backs of their fingers, like a backward-pointing foot, and use the insides of their fingers for gripping things.

This is an inverted posture from the positions you see the "fingers" used in animals like dogs or cats, but which preserves the full functionality of the fingers in this context.

Answer 3

You are asking for opposable thumbs on a digitigrade, and that's a little difficult if you are going digitigrade on both the front and back.

But if you are looking for "paw-like" aesthetics and you aren't totally married to complete digitigrade, let's start with paws that have close to opposable thumb function. Not the complete package, just clawed (non-retractable, I am sorry) with pads.

There are kangaroos, raccoons, koalas, pandas, possums, and opossums. I am leaving out all of the apes because that's just too hand-like, and all of the birds, reptiles, and amphibians because that's too different in format.

Kangaroos, we can eliminate because they rarely get down on all fours, and the design is quite different (mainly because of the hopping). Might be worth a look but, I think it's not a win.

Hie thee to yon google machine and type in two things. First, take that list of animals with paw-like opposable thumb function and type in each animal individually "opossum runs" "raccoon runs" and so on, so you get to see how they move. You may find it fits enough of what you want even though they are not digitigrade. Also, look at the same animals with the search "on hind legs" or "runs on hind legs."

Basically, you can find these animals adopting the stance of digitigrade and plantigrade as it suits them. Structurally, some are plantigrade (bears). If you want real opposable thumbs, plantigrade, at least on this planet's evolutionary track is the way to go. It gives the most versatility.

A cat, which is a digitigrade can stand plantigrade on its back feet if they need to. However, the STRUCTURE is digitigrade, and while there are advantages, one of them isn't opposable thumbs.

You can also find videos of each of these animals listed gripping onto things with surprising dexterity, and if you pushed it a little further, you might have a lot of what you need.

You weren't finding any info on digitigrade hands because, well, digitigrade does not equal hands or anything like that.

On this planet, anyway. You can design what you like. But the examples above are all animals with the kind of versatility you want and at the very least a paw-like structure that isn't something like a bird, reptile, or primate.