Sprinting is the only time most people find out their actual top speed, and the number is usually lower than expected : and holdable for a far shorter time than expected.
The short answer
An untrained but healthy adult sprints at roughly 24 to 30 km/h (15 to 19 mph) at peak. Trained amateur athletes reach 30–35 km/h. Elite sprinters exceed 40 km/h.
How that compares
- Untrained adult : 24–30 km/h peak
- Recreational athlete : 28–33 km/h
- Trained sprinter : 35–40 km/h
- Usain Bolt, 2009 world record : about 44.7 km/h peak, 37.6 km/h averaged over 100 m
The interesting part is how narrow that range is. The fastest human ever recorded is under twice the speed of an ordinary adult. Compare that with strength, where trained lifters move several times the load of an untrained person, and sprinting looks strongly constrained by biology rather than training.
Why you cannot hold it
Top speed lasts a few seconds because of how the muscles are fuelled. Maximal sprinting runs almost entirely on stored ATP and phosphocreatine, which is depleted within about 6–10 seconds. After that, output falls whether you like it or not.
This is visible in elite races: sprinters do not accelerate through the line, they decelerate least. Bolt hit peak speed around the 60–70 m mark and was slowing over the last 30 m, as was everyone beside him.
What sets top speed
Research points to ground force rather than leg turnover. Fast sprinters do not move their legs dramatically quicker : they apply more force into the ground in the brief moment the foot is down. Contact time at top speed is under 0.1 seconds, so the window to apply it is tiny.
That is also why sprinting is hard to improve much in adults: the trait is heavily influenced by muscle fibre composition and tendon properties, which respond to training far less than endurance does.
Sprinting safely
Sprinting is the most common way recreational athletes tear a hamstring, usually in the first hard effort after a long gap. If you have not sprinted in months, build up over several sessions rather than going flat out cold. Warm up properly first.
Measuring your sprint
Phone GPS updates roughly once or twice a second, which is enough to capture a sprint's peak but not to time a race. Use the running speed checker, start it before you set off, and read the peak value afterwards rather than trying to watch the screen while running.
Do this in open ground. Buildings and tree cover degrade GPS accuracy, and a short sprint gives the receiver little time to settle.
The takeaway
Most adults top out around 24–30 km/h for a handful of seconds. The gap to world class is smaller than the gap in almost any other athletic measure, and the limiting factor is how hard you hit the ground, not how fast you move your legs.