Oscar Pistorius's defence has sought to show the double-amputee sprinter feels highly vulnerable and acted out of fear not anger when he shot dead his girlfriend Reeva Steenkamp.

Mr Pistorius has a "split personality", defence lawyer Kenny Oldwadge told the court.

There are "two Oscars", he said - a world-class athlete and a highly vulnerable individual with a serious disability.

Lawyers defending the 27-year-old on charges that he deliberately shot and killed Ms Steenkamp on Valentine's Day last year are calling their final witnesses.

Yesterday medical expert Wayne Derman took to the stand, testifying that Mr Pistorius, known as the Blade Runner for his j-shaped prosthetic limbs, was not always the fearless superhero depicted in sports advertisements.

"Although he loathes to be pitied in any way, the hard truth is that he does not have lower legs," said Mr Derman, chief medical officer of the South African Paralympic Team at the London Olympic Games in 2012.

"You've got a paradox," he said, "Of an individual who is supremely able and an individual who is significantly disabled."

During five months of the stop-start trial, Mr Pistorius's lawyers have sought to portray him as manically obsessed with safety after a difficult childhood with a mother who intermittently abused alcohol and in the face of high crime levels in South Africa.

Those factors, they argue, help explain his reaction on Valentine's Day 2013 when he allegedly believed his 29-year-old girlfriend to be an intruder and shot her dead through a locked toilet door.

Mr Derman, who has known Mr Pistorius for six years, had testified the runner is conditioned to "react through auditory stimulus" a result of training to starter pistols fired at the beginning of athletic races.

The expert witness, expected to be the last before the defence concludes its case, said it was Mr Pistorius's unusual "startle magnitude" that "culminated in this horrific tragedy."

Prosecutors claim Mr Pistorius shot Ms Steenkamp following a row in the dead of the night, arguing neighbours living close to him heard a woman screaming the night he shot the model.

The Olympian, who has appeared tired throughout the day, cracked a grin with his defence team during a break.

He then turned to the first row of the public gallery where he greeted two American tourists in court with a polite handshake and a smile, saying "thank you" for their messages of support.

Mr Pistorius faces up to 25 years in jail and an abrupt end to his glittering sporting career if convicted.