Despite being one of the fastest runners in the world, Oscar Pistorius' doctor stressed the fact that the athlete still deals with the hardship of being disabled during his murder trial on Thursday.

Pistorius became the first amputee to compete as an Olympic track athlete after his selection to represent South Africa in the 2012 London Summer Olympics. However, his life took a sharp turn when he fatally shot his girlfriend Reveea Steenkamp on Feb. 14, 2013.  The South African track star has pleaded not guilty to first degree murder, arguing that he shot her because he mistook her for an intruder. Prosecutors, however, say that he killed her after the couple had a heated argument in the late hours of Valentine's Day.

On Thursday, Pistorius' defense team called sports physician Wayne Derman to the stand to testify that he acted on a "fight or flight" impulse in which he chose to confront what he a dangerous intruder because his disability prevented him from fleeing, USA Today reported. Derman, who has treated Pistorius for years, also emphasized the difficulties that double amputees experience every day.

"The saddest thing I have learned through my six years of working with athletes with disability is that disability never sleeps. It's there when you go to sleep at night and it's there when you wake up in the morning. It affects nearly every aspect of your life," he said, according to CNN.

Earlier this week, prosecutors announced that the 27-year-old Olympic star was perfectly sane when he killed Steenkamp, based on a psychiatric evaluation, The New York Times reported. 

Nel cited key parts of the conclusion, noting that the mental health experts believed Pistorius was "capable of appreciating the wrongfulness of his act" when he killed the law graduate, ESPN reported.

Pistorius faces 25 years to life in prison if found guilty of premeditated murder. He is currently free on bail.