OSCAR Pistorius is a "paradox of an individual" whose past triumphs as a sprinter contrasted with the daily, severe limitations he endured because of his disability, a physician has told the athlete's trial.

Wayne Derman, a professor of sport and exercise medicine at the University of Cape Town, said in court the contrast likely contributed to stress and anxiety for Pistorius, who fatally shot girlfriend Reeva Steenkamp through a closed toilet door in his home.

He testified for the defence, which wants to show the athlete had a deep sense of vulnerability and it was a factor in what he has described as a mistaken shooting.

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Professor Derman said: "You've got a paradox of an individual who is supremely able, and you've got an individual who is significantly disabled.

"He has a specific fear of being trapped somewhere without being able to move very rapidly."

Pistorius, 27, says he killed Ms Steenkamp on February 14, 2013 by mistake.

The trial continues.