Forensic pathologist Dr Marjorie Turner told a jury that "text book evidence" suggested the teenager would have passed out after about 10 or 15 seconds.
During that time she might have scratched her own neck, trying to claw at the ligature.
The High Court in Edinburgh also heard that it was a mystery why Elaine did not appear to have fought her assailant at any time before the deadly ligature tightened across her throat.
The naked body of Elaine, 16, was found yards from her home in Ardgowan Street, Greenock, on June 2 1986. A post mortem later that day, performed by hospital doctors concluded she had been strangled.
There was a long scratch on her thigh which might have happened as her clothes were taken off.
But there was a lack of obvious "defensive injuries". "It would appear she has not been able to defend herself to any extent," said the pathologist.
The ligature had left a red-brown mark - three-eighths-of-an-inch wide - round her neck.
Defence QC Donald Findlay, suggested Elaine may have gone to a remote area with someone she knew.
He said: "There, some form of sexual play took place and she undressed voluntarily and something catastrophic occurred and she was attacked."
He asked Dr Turner if there was any evidence that the girl's clothes had been torn off and the pathologist said there was not.
John Docherty, 49, of Hunters' Quay, Holiday Village, Dunoon denies murder. The trial continues.