Last year, a group of eight Royal Marines broke the Guinness world record for speed-marching. This is how they did it.

Filmmaker Chris Terrill, who, as a civilian, has undergone the 32-week Royal Marines Commando training course and graduated with an honorary Green Beret, filmed the group’s preparation and attempt. The group of active and former commandos completed the speed march in four hours, 16 minutes and 43 seconds, shaving two minutes out of the previous world record.

This was their third go at the Guinness world record. In 2012, they came close to success, but overshot the time by two minutes. The next year, in 2013, they broke the record but were disqualified because they didn’t finish as a group — one of their numbers, Dave Perret, succumbed to the rigours of the exertion and collapsed, avoiding death only because of the prompt response of his teammates and the paramedics.

“That moment when he collapsed, was a difficult one for me,” said Major Scotty Mills, the officer in charge of the group. “Because he pushed himself beyond the limit of what was humanly possible for him. It was down to me to decide what to do. I’d the paramedics with me and the consulting doctor … but I took one look at Dave. It was a very easy decision for me … He’s one of my brothers. I’ve served in Iraq with Dave, and there’s no way I’d his life in unnecessary risk.”

Despite breaking the record, however, the Guinness team annulled their effort because of the pants they were wearing. The previous record-holders were wearing less flexible military denim trousers, which have been out-of-production for decades. But the Royal Marines were wearing modern-day issued fatigues.

“I know in my heart of hearts that the boys have done the 26.2 miles carrying 40 lbs of weight, broken the world record time in good order, as a team with their brothers by their side. What trousers we were wearing is irrelevant,” added Mills.

A representative for Guinness said that “We can confirm that an appeal has been made and further evidence has been received for review.”

Countless hours were spent in preparation for the event. The group, which initially was comprised of more commandos, underwent a selection process to determine who really was committed to the endeavour.