He's not the only one to do it but apparently Leo Messi also likes to do the training warm-up with his boots untied. This Thursday at the sports city we could see the Argentinian star with his boots untied.
Once the footballers start the real training then they tie his boots. But even it's a usual thing, not everybody does it. For example Dani Alves, Munir or Suarez didn't do like Messi.
Here we can see pictures of Barça's players boots in this Thursday training session.
Boots Messi and Alves