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How fast can a digitigrade bipedal ape possibly run?

I've raked through any question I could find on here talking about speedy bipeds or digitigrade bipeds (that aren't robotic/synthetic), but none that pinpoint a (rough) believable speed estimate. I don't expect to see these outrunning cheetahs, but perhaps they could beat ostriches (70 kph) and ideally, even catch up to animals like wildebeest (80 kph) for a few milliseconds.

In short, I'm looking to estimate the top speed of some digitigrade bipeds, assuming they have the anatomy to favor it. These creatures are apes, but ecological circumstances over time forced their ancestor on the ground. They have ape-like features in their forelimbs which allow for tool use and grasping etc, and they're social with human-level language and intelligence. In terms of recommending changes to their anatomy, go for it- the only guidelines I have around these creatures is that they evolved from apes and therefore have no tails, plus they must have hands with thumbs.

These carnivorous apes are fast-sprinting ambush predators that thrive in wide open plains, deserts, savannahs, and tundra with big game. There are barely any trees for them to climb anymore (they can only climb with tools and willpower); their ancestors diverged millions of years before the apes that would eventually lead to humans diverged in this world, so their ancestors have had a lot of time on the ground to sort things out.

Unlike humans, they are digitigrade and rely on speed instead of endurance. They have many fast twitch muscle fibers, efficient robust cardiovascular systems (for a mammal), and long legs. To run fast, they would have to sacrifice some stability and lean forward, but they could have very long strides, perhaps just under 5 meters (basing this on an ostrich).

Sure, their legs bear some similarities to ostriches like ligaments and reduced toes, but the angle between their body and legs is different from a bird or dinosaur, as they have to stand straight and run leaning forward- at what angle, I'm not sure. Maybe 45 degrees? These are apes and therefore tailless, so the only counterweight they could have are their arms and maybe a spear. It would probably be fine if they just fell over or leapt after sprinting full throttle.

Of course, it's not like they had the time birds have to get bipedal digitigrade running together. Birds are going to be better at leg injury avoidance than these creatures ever will. Perhaps, taking another note from ostriches, they rely heavily on ligaments in their lower extremities and because of the haste (relative to ostriches) with which they developed this adaptation, they're prone to ligament injuries.

Speed is going to be everything to these creatures- finding dinner, being accepted by social circles, and meeting partners. I expect the most intense speeds at which they can run to take a toll on them over time, most likely with athletes. Regular exercising in a cushy, modern life would be harmless, of course.

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    $\begingroup$ Ostrich are a digitigrade biped. Since you're asking about a digitigrade biped that has the performance of an ostrich, you've answered your own question. What are you expecting from us? $\endgroup$
    – sphennings
    Commented May 26, 2022 at 21:52
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    $\begingroup$ "they're social with human-level language and intelligence" - in that case they would probably not need to evolve to be super fast. One great advantage of high human intelligence was that it makes persistence hunting easier, where even though you're slower than your prey you can track them until they're too exhausted to keep running. Or you can use your intelligence to develop tools like javelins and arrows to compensate for lack of speed. If you already have human intelligence you don't need to be very fast and vice versa. $\endgroup$
    – GiantSpaceHamster
    Commented May 27, 2022 at 15:15
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    $\begingroup$ Humans adopt a digitigrade posture when accelerating while running for speed (as opposed to running for endurance). (Example showing Usain Bolt.) A top level human athelete reaches about 36 km/h. (And humans can easily outrun a cheetah over even medium distances, and will also outrun a wildebeest over longer distances. Cheetahs have ridiculously low endurance, and even wildbeest cannot compete with us over more than maybe a few miles. In fact, the ostrich is only runner always better than humans.) $\endgroup$
    – AlexP
    Commented May 28, 2022 at 12:26
  • $\begingroup$ Honestly it would make a lot more sense if you said they weren't human level intelligence, as that makes their speed mostly redundant for hunting. Though you could maybe keep it plausible by saying that sexual selection started driving things at one point. $\endgroup$
    – Vakus Drake
    Commented Jun 3, 2022 at 18:23
  • $\begingroup$ I would look at insects and contrive it in terms of speed divided by either body mass or body length. Also, many organisms are constrained by metabolism and breathing. Also terrain is a definitive attribute for survival, so being able to leap into trees, or go where the runner can't makes the runner go hungry. Co-evolution sets prey to have enough speed to not go extinct while staying in the predators habitat. $\endgroup$
    – EngrStudent
    Commented Jun 8, 2022 at 2:21

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There's no definitive answer to this question, as it largely depends on the specific anatomy of the creature in question. However, we can make some educated guesses based on what we know about other bipedal animals.

For example, ostriches can reach speeds of up to 70 kph, and wildebeest can reach speeds of up to 80 kph. If we assume that these creatures have a similar anatomy to ostriches (long legs, ligaments for shock absorption, etc.), then it's reasonable to believe that they could reach similar top speeds.

However, if we assume that these creatures have a more human-like anatomy ( shorter legs, less efficient cardiovascular system, etc.), then their top speed would likely be lower. Based on this, we can estimate that the maximum speed of a digitigrade bipedal ape would fall somewhere between 70-80 kph.

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  • $\begingroup$ Cheetahs can do 120 km/h and they don't have the dinosaurian/avian respiratory system. $\endgroup$
    – Keith Morrison
    Commented Jun 19, 2022 at 3:34
  • $\begingroup$ A bear can run as fast as a galloping horse, 40mph (64kph). Seems like a decent lower bound, given a bear's body plan and limb length; I would definitely expect faster from a more streamlined creature. $\endgroup$
    – Tom
    Commented Aug 6, 2022 at 3:57

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