The 20-year-old's sudden death was probed during a coronial inquest that began on Monday, more than three years after the emerging talent fell ill at pre-season training.
Noting body cramps while stretching after a cardio workout, Titmuss suffered a seizure at the club's base at Narrabeen, on Sydney's northern beaches, on November 23, 2020.
He later died at Royal North Shore Hospital after being taken there in an ambulance.
While an autopsy report was unable to ascertain the cause of death, counsel assisting the coroner Adam Casselden SC told the court that Titmuss most likely suffered heat stroke.
Paramedics who arrived at the scene noted Titmuss's temperature was 41.9C, while his heart rate was over 140bpm.
Manly coaching staff noted Titmuss had struggled throughout the training session, when his weight increased from 112kg to 116.9kg.
"The fact is that Keith, relatively speaking, was less fit and less conditioned to the heat than he would have been other times," Mr Casselden said.
"Studies of players of American football have established that the risk of heat exertion are the greatest in the first 14 days of practice after off-season."
His death came just three days into his time as a member of the club's top-30 NRL squad.
Titmuss had been a rising star at the club, having delivered the Sea Eagles their maiden Holden Cup title in 2017 with a try on the siren to beat Parramatta.
He scored the match-winning try for Wayne Lambkin's side in the last cup grand final before being named players' player in the 2019 Jersey Flegg Cup team.
The inquest continues.
9°C