Young runners who recently completed their first London Marathon are helping doctors to attempt to solve the mystery of whether mass-participation endurance races weaken the heart.

Concerns about the physical impact of running a marathon were reignited when Army captain David Seath, 31, suffered cardiac arrest and died in hospital after collapsing three miles from the finish line during last month’s event.

Researchers are carrying out extensive tests on 120 novice runners aged 18 to 35 to see how their training programme and race-day experience affected their cardiac strength.

Athletes are known to develop larger and thicker hearts, while the main pumping chamber can become “spongy”. Researchers at St George’s, University of London want to know whether this sponginess is merely temporary or a sign of a rare condition, known as left ventricular non-compaction cardiomyopathy.

They have been given access to two of the MRI scanners at the new Barts Heart Centre and are putting the 60 men and 60 women through four hours of tests, comparing their heart health now to tests done when they were selected for the marathon six months ago.

Dr Andrew D’Silva, clinical research fellow at St George’s, said improvements in heart-scanning machines allowed a breakthrough in understanding whether an individual’s “sponginess” levels were normal or increased by high-intensity exercise. The sponginess can cause the blood to clot, increasing the risk of heart attack or stroke.

“To have a lot of heart muscle sponginess does not necessarily mean there is a problem. It’s a clue,” he said. “I would like to think we will be able to give a crystal clear story of what happens in both genders with heart muscle changes: can mass participation endurance exercises harm the heart?”

He said that deaths from marathon-running were “very rare” and often difficult to link directly to the race. Exercise in general was “overwhelmingly good” for the heart, he said.

“It’s unlikely we are going to find any messages to tell people who are considering signing up for a marathon or mass participation event that they shouldn’t do it,” he said. “But hopefully we will understand better what is going to happen to their body and heart at the end of all the training they have done.”

Josh Bryan marathon runner

Josh Bryan: novice marathon runner who took part in the tests

One runner, Josh Bryan, 26, above, a City insurance underwriter from Clapham, achieved his target time of 3 hours 30 minutes after training five times a week for 16 weeks.

He was motivated to take part in the study after a former classmate and a friend of his brother both died from sudden adult death syndrome.

Mr Bryan said: “When you hear of stuff like that – people who are your age who basically drop dead without it being anticipated, who just went to sleep at night and didn’t wake up – it’s quite scary.

“Because all the studies they were doing were so extensive – taking my blood pressure and measuring my heart – I thought if anything was wrong with me, I’m in the best place: on a heart ward in hospital.”