Is my alien foot design viable for a compromise between arboreality and cursoriality?

Question

I keep trying to design humanoid and humanesque sapient aliens, but I often run into issues with the feet; In an orthostatic biped, an enlarged first toe (big toe) is probably essential to maintain proper balance, but if this is on a plantigrade foot it will look much too human-like, and digitigrade bipeds, while both plausible and sufficiently inhuman even with a big toe, require special effort to explain why they have human-like hands, which is another thing I would like.

Recently, however, I came up with a new spin on an foot design I've seen in art from time to time, which is essentially a hybrid of a human foot, a chimp foot, and a carnivoran hindpaw. Like a chimp's foot, its first digit is a thumb, but the last four digits are moreso similar to a human's first four toes, and the entire thing is made partially carnivoran-like by the presence of claws and thick pads.

This description may be unhelpful, so I have made a labeled diagram of a footprint from it:

A: heel/talon pad, B: Ball-of-foot pad, C: Heel-of-thumb pad, D: Thumbtip pad, E: Big toe pad, F: 3rd and 4th toe pads, G: 5th/little toe pad, H: Claws

At this point a I feel I should reiterate that this diagram is of the foot print not the foot itself; The foot is not as narrow as shape C, there is simply no padding under much of the arch of the foot.

My idea for this foot design is that it would be a compromise between optimisation for chimp-like arboreal climbing and optimisation for human-like ground-level long-distance running, having a thumb that could be used for climbing without having such long main toes that running would be dangerous and tiring.

My question, then, is this: Is my supposition that this foot would work as a compromise between arboreality and cursoriality valid, or would it only combine the worst of both worlds, creating a foot which is neither good for climbing nor running, more specifically in the context of a vaguely humanoid orthostatic biped?

Answer 1

As a terrestrial, cursorial foot, this is only slightly different to the human foot. When humans are running, it appears to be modern shoes that enforce a plantigrade running habit, whereas humans are capable of a digitigrade running habit, and do so somewhat more often if light or no shoes are worn. A digitigrade running habit allows faster running, however, it reduces endurance without more adaptation that had obviously not occurred here. A plantigrade running habit better allows distance running.

So, if the only difference between a human and this creature was its feet, we could expect this creature to be able to run in a digitigrade fashion relatively easily... but the same doesn't necessarily go for a plantigrade gait. In humans, the point of maximum pressure in plantigrade locomotion runs from the heel, along the outside of the foot. This can be seen in:

https://mass4d.com/blogs/articles/why-examining-foot-posture-is-important-in-sports

In the OP's being's foot, such a path would appear to run A -> C -> B -> E, F, G. However, we don't know from the footprint if the heel-of-thumb pad "C" is bound to the foot as in the lateral web of the human foot, or if it is mobile as in the human thumb. This would appear to be a choice that the OP has yet to make, with each choice coming with its own benefits and drawbacks.

In the case that the heel-of-thumb pad is bound in the position displayed to the side of the foot, the bony and ligament structure of the foot provides a load-bearing surface across which the creature's mass can pass, much as in the image above. However, this also has the disadvantage that the thumb cannot then be spread wide to grasp a tree-branch in arboreal locomotion.

If we consider the differences between a chimpanzee's foot, a human's and the OP's creature, we can see that the chimp foot has long toes that the human and OPs creature lacks, and a thumb that can be spread widely. These allow the chimp to grasp tree branches easily. However, the chimp foot thumb is effectively a very long toe that cannot contribute as greatly to supporting its weight as a human big toe due to its being separate from the body of the foot.

Likewise, if the OP's creature has a separate thumb and a plantigrade gait, its thumb would be less able to support its weight, and attempting to use it to do so would require a large foot-thumb joint that might break down with age more readily given its greater mobility. It would also result in weight effectively being transferred from the heel pad (A) to the ball of the foot pad (B) then the toes, without the thumb taking much weight. There would be no smooth transition from weight on the heel to weight on the ball of the foot and the toes as there is in humans, and that might cause unnecessary mechanical stress in the foot as the creature runs.

The third option, that the creature runs in a digitigrade fashion would reduce the stresses from weight moving from A to B to being entirely on A, E, F & G. However, since this creature has a plantigrade walking habit, a digitigrade running habit would result in a less efficient running gait.

As for arboreal locomotion, this creature has relatively short toes compared to a chimp, more akin to a human's. Without longer toes or claws that do not touch the ground when running like a cat's, these toes and claws would be too short and blunt to provide much grip on a branch. They would be of more use in terrestrial locomotion like a dog's claws. To improve arboreal locomotion, the toes would need to be long and the claws sharp, and the thumb able to spread widely, but this also hinders terrestrial cursorial locomotion.

So, as this creature stands, it is a jack of all trades and a master of none.