So far, the quickest time has been around 27 1/2 miles per hour, which sprinter Usain Bolt achieved (briefly) shortly after the midway of his world-record 100-meter dash in 2009. The strength of our bones and tendons most likely does not enforce this speed restriction. Humans can easily accelerate to dangerous levels if we want to go faster than this. In fact, scientists have proven that even sprinters like Bolt can be driven to extremes of speed if necessary. In 2004, researchers from McMaster University in Hamilton, Ontario, monitored Bolt as he ran several hundred meters at a time on a special treadmill. They found that despite being a skilled runner, Bolt could only manage a top speed of 20.5 miles per hour - about equal to walking pace - for about two minutes before he started to struggle. This is because muscles need time to recover between uses so they don't get injured. If we try to run at a high speed for a long period then the muscles will be stressed beyond their limit, which can lead to injury. In conclusion, humans are limited by biology when it comes to speed. We can run very quickly for a few minutes if necessary but cannot maintain this level of speed for long periods. This is why athletes like Bolt who are highly trained can run so fast; they have overcome this natural limitation through practice.