And the Belgian coach questioned the position of the player in the cage.
"The fact the performing player always stays in the central position is also a disadvantage. In a game he has to move and search for the best position to receive the ball.
"I am also curious to know how one-footed or technically less skilled players are going to react tn the 'Footbonaut'. For me you need to prepare a player for this kind of resource."
Bruyninckx also raised concerns over grueling physical workout which the cage gives a player.
"Using sweating as a parameter is not a good approach," he said. "Too often coaches think that high intensity delivers high performance. Brain research tells us that if energy is low, learning will go down."
So, while the "Footbonaut" is one small step for Dortmund, it could take a giant leap for the device to become a fixture at training grounds across the world.