That’s a great question! Yep, there’s definitely a physical limit to how fast Usain Bolt – or anyone! – can run. There are a few reasons for this; one main one is that when you run, you apply pressure to your heels and feet. The faster the run, the more force you’re exerting – so eventually, you’d be applying so much force that you would break something! The other reason though, is that there is a limit to how quickly your muscles can contract. That’s just due to the way the fibres are constructed, but I think that the theoretical top speed for a human would be around 40 mph. Usain Bolt’s been clocked at about 38mph, so he’s not far off!