It’ll be different for everyone within a certain range. Usain Bolt holds the record at 27.44mph

I would agree with @ARE_you_kidding_me, something just under 28 or 29 mph.

I’ve flown at around 500 miles per hour and my joints were okay with that. I have a feeling that I could fly much faster if given the opportunity.

The upper limit speed to running is based on muscles, not on the joints.

World class competitive downhill skiers fly down slopes aided by skis at speeds above 70 mph. And they have absorb the shocks of bumps and landings at that speed while remaining in control and aerodynamic, so I would say the human joints can handle speeds much great than the speeds generated by running.

Bren Foster has launched a kick recorded at 219 km/h. Think that’s the fastest recorded kick as well.

Not much. In relatively slow speed auto accidents, heads are easily separated from the torso to go through the windshield and continue on, bouncing willy-nilly down the highway.

It’s not a particular digit, each and every has a different length which depends on life-style and daily exercise.