Courtois happy to undergo tests at Chelsea
Courtois is this season on-loan at Atletico Madrid.
"The tests showed that over six months I have become a lot stronger," Courtois told Voetbal Primeur.
"I can jump a foot higher than I could last year and my reflexes have also improved.
"Chelsea were especially impressed by my results in the stress test. Under great pressure I was able to continue coolly."