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.