The events are often pitched as the ultimate challenge, but participants in a recent French Obstacle Course Race (OCR) faced an unexpected hurdle after their big day, as more than 1,000 runners contracted diarrhoea.

It’s suspected the mud that participants were trawling through as part of the Mud Day race in Nice on 20 June may have been contaminated with animal faeces. French regional health authorities have confirmed that over 1,000 runners had contracted norovirus – a bug that causes fever, vomiting and diarrhoea.

More than 8,400 contestants took part in the race, amid reports that many have since complained that there was a “distinct smell of horse manure” on the course.

Mud Day organisers have stated they are currently conducting an investigation. They said: “We thank the health services for their efficiency and offer all the "Mud Guys" and "Mud Girls"who fell ill our support and wish them a speedy recovery.”

Concerns over contaminated water at OCR events have previously been raised in the US, with the Centers for Disease Control and Prevention finding a "statistically significant association" between accidentally swallowing muddy water and the illnesses of 22 participants laid low after an event in Nevada in 2012.

Endurance events have become increasingly prevalent in recent years, with extreme fitness identified as a major trend in the Fitness Foresight section of the 2015 Health Club Management Handbook (p.20)

The growing popularity of extreme fitness challenges such as OCRs and triathlons has led to a raft of new training tie-ups, as UK health clubs have been quick to offer their expertise and facilities to a booming market. Although there have been no such incidences reported in the UK, the fallout from Mud Day highlights the potential risks that contaminated mud can pose to participants of outdoor events.