We wired up the teenagers from the U20s side to some GPS and heart rate monitors and told them just to play free at the Canterbury Kikoff centre in Sydney while we monitored their vital signs.

But little did they know that while they were having a kickaround and barely getting out of their comfort zones, we had Western Sydney Wanderers assistant coach Andres Carrasco hiding in the shadows...

Check out our exclusive video to see what happened next...

The impact was instant. As soon as we stopped them playing and brought Andres onto the pitch, heart rates soared immediately.

One player's resting heart rate shot up from 103bpm to 144bpm just while Andres was being introduced.

Straight away, we told them to go back to playing exactly as they had been before - no rules, no score, just play free...and then looked at the results to see what was happening on the field.

Suddenly the tempo racked up into top gear. Tackles got tastier, speeds increased, runs got longer – and the heart rates soared into the red zone and beyond their 100% max level.

On average, speed rose by 64% from 3.25km/h to 5.32km/h while distance travelled over the same period increased by 23% from 1.85km to 2.29km – and overall heart rates increased 13%.

But in their rush to impress, the pressure got to them and mistakes set in, with rash challenges and missed passes.

Afterwards, Andres - former youth coach for 14 years at Barcelona's famous La Masia academy  - told the teens to try to tame their nerves when under pressure...and just play free.

"Your heart rates went up when I arrived," he told them. "That's something that can punish you. A very high rate because you are nervous means you can make more mistakes.

"On the other side though, if it was because you were doing a little bit more, that's something positive.

"But maybe you should think that you should do that little bit more ALWAYS...!"