Our vulnerability is really located in the Achilles tendon. Even if standing upright, if they lacked an Achilles tendon like in chimps and gorillas, early humans would have been slower than a snail race. “Reverse-engineered” computer models point that in this case, our ancestors would have had difficulties in running. Lacking the spring impulse in their step would have halved the human top speed at a double energy waste. Even so, the upright walk would have been much more effective than the knuckle walking of the chimps and gorillas.

“Our research supports the belief that the earliest
humans used efficient bipedal walking rather than chimp-like ‘Groucho’ walking,” said lead researcher Bill Sellers, a computational primatologist at the University of Manchester in England.
Efficient sprinting was unknown for the first humans, but walking could have been the norm.