The World's Toughest Ultra

First of all, I should state that I wrote this article due to the irritation of seeing so many race directors claiming their race to be the toughest in the world.  Often such people have little grasp of what makes a race truly challenging, have limited personal experience compared with the racers themselves, and they could simply be using the unfounded, unverified claim as a marketing tool.  

Many of us in the ultra-running community have become quite bored of the dreary claims and endless articles about the 'world's toughest ultra'.  The following is really an exercise in how these claims could be addressed from an exercise physiology perspective, and the races included are there purely to give an impression of what we could consider and what should be ruled-out.  It is not intended to be an encyclopedic account of every race, more a bit of fun with some thought experiments.  Hence, many racers will claim others to be tougher, but that in itself is the point - the people who write articles on the world's toughest ultras are not the people who have competed in all of the potential races, nor are they physiologists and sports psychologists who have fairly assessed each and every race equally.

To avoid the immediate trap of 'some people are better trained', 'some racers have an off-day', 'some are better acclimatised', and so on, this should be considered from the perspective of Runner X.  Our Runner X is an average ultra-runner, without any particular specialisation to a particular type of terrain or environment.  Runner X lives and trains over variable terrain - a mix of trails and roads - at close to sea level.  What we are considering is what happens when Runner X is placed at the start line of a race, having had a reasonable amount of training, given the appropriate clothing, footwear, nutrition and so on.  Of all the races available - that seem to claim being the toughest in the world - which ones are most likely to be the toughest for our Runner X.

I am writing this article from the perspective of an exercise physiologist, as that is my background.  I am considering what would be the toughest physical challenge, in terms of demands on the aerobic system.  If we could measure Runner X in each race, looking at markers of muscle breakdown, muscle stress, heart rate, breathing rate, oxygen saturation, temperature, fuel utilisation, electrolyte balance, organ damage, and so on, what would we find?  There are emerging races that have been designed to test a runner's psychology and motivation to continue, but as a physiologist I do not believe in that sort of thing.  In any case, even for those who do believe in the dark arts, the testing is currently subjective, meaning it is opinion-driven and therefore open to individual interpretation.  What I like about the physiological approach is that nobody can fake acute renal failure.

I ran my first ultra in 2007. It was the Marathon des Sables, famed as the world's toughest footrace (by Discovery Channel UK, it later emerged). At breakfast the day after it was mentioned how it was not the hardest after all, but the real toughest race was the Jungle Marathon.  I went along as support to the Jungle Marathon in 2007, having signed-up to compete in 2008. By that time, I had signed up for yet another 'world's toughest race'; the 450-mile Yukon Arctic Ultra (YAU).

In addition to these races, I have competed in and finished many others, including the Hardmoors 110, the Transalpine and La Ultra / The High (222-km). Having been a part of the ultra-running community since 2007, I have seen, heard of, and experienced many races that claim to be the toughest. What I have come to appreciate, after all these years, is that so many of the claims are utterly absurd. The nonsense is generated for the sake of marketing, because people like the idea of claiming to have finished the world's toughest race, or else it is a good spin for a charity or for self-promotion.

We need to decide what we mean by 'toughest'. Do we mean, for example, it is hardest to reach the finish line, or hardest to travel a single mile? If it is the toughest to reach the finish, then we ought to consider the Race Across America (3200 miles in 70 days), and the Trans Europe (2500 miles in 60 days). A race can be a long, tough slog to the finish, without the environment being particularly arduous at all. By contrast, a mile of running in a jungle or at very high altitude is incredibly tough, but such races are not even 5% of the distance of the Race Across America.  It becomes more complex the more factors we consider. 


Single-Stage Racing


In single-stage racing, there is a start and a finish, and no pre-determined rest points along the way. Some of these races can be around 100 miles or so, such as the Hardmoors 110, Lakeland 100, Viking Way, Spartathlon, La Ultra/The High, and so on. Competitors rarely rest, perhaps at most walking some sections and running others, without breaks to sleep. These races are a true endurance challenge, as the joy of running is often lost in pain and sore muscles and joints, with many racers staggering and shuffling to the finish line. There is no comparison between a 150-mile multi-stage race and a single-stage one. It is far harder to complete the Spartathlon (~150 miles in 36 hours) than the Marathon des Sables (~150 miles in 7 days), for example. 

Another twist on the single-stage theme is the multi-day variety. The 100-150 milers are continuous, perhaps across 24, 36 or even 48 hours.  Competitors do not sleep during these, and even sitting down for a few minutes can be scorned upon. However, some single-stage races are in such environments, or over such great distances, that sleep must be planned. Over a few days the competitors can get by on less than an hour a night, but over a week or more there is a requirement for more sleep. In a single-stage race though, as one racer sleeps another catches up or extends their lead. Further, as a racer sleeps they lose time when they could be making progress along the trail. Too little sleep and they will suffer with thinking tasks, such as planning, managing timings and pace, and navigating. Too much sleep and they might run out of time to reach the finish within the cut-off.

Contenders for the toughest continuous single-stage race would be the UTMB, La Ultra/The High, The Spartathlon and Badwater. New races are appearing every year though, so James Adams' Piece of String and Mark Cockbain's Hill and Viking Way are worth consideration.



Finishing Stats


Some organisers like to use finishing statistics as 'proof' of difficulty. That is, of the number of people who start, how many finish. This is an incredibly unreliable tool though.  When I first competed in the YAU in 2009, there were 16 who began the 450-miler, and 14 in the 300-miler. Of those, 9 finished the 430 and 1 finished the 300. In 2011, 6 out of 20 finished the 430 and 7 out of 18 finished the 300. In 2013 there were 14 finishers from 29 in the 430, and 2 from 18 in the 300. Overall all it appears the 300 is a harder race. Actually, all racers use the same route until about 280 miles, at which point the 430 racers enter a region of the toughest terrain of the race.

The reason there are so few finishers in the 300 is that so few have any cold-weather race experience.  Most in the 430 would have been required to demonstrate some cold-weather racing pedigree, with very few attempting it as their first race. It is more complicated still, as in 2011 the temperatures plummeted to -50 Celsius, during the last days of the race, by which time the 300 competitors had all finished and the remaining 430 racers were mostly wiped away. In 2013 it was much warmer, making ground conditions very soft and seeing off competitors with overuse injuries about a week into the race.

One reason I have heard the 6633 race to be claimed as the toughest, is due to the low number of finishers, yet there is no requirement for racers to have any cold-weather racing experience. In the 2013 edition of the race, some competitors claimed it was their first ever ultra! Although the organisers also claim the 6633 is the toughest due to the cold and wind, this is not an appropriate marker. Conditions during the Iditarod Trail Invitational (1000 mile race) will be consistently far more challenging (for both trail and weather conditions).

The 6633 is also entirely on road, making progress far easier than on a trail. The temperatures claimed to have occurred during the 6633 one year are based upon windchill factor, which Canadians in the north do not use. Temperature without windchill is far more appropriate, as clothing counters the wind. Temperatures in the YAU have been below -50 Celsius with the race continuing, and well below -60 one year when the race had to be halted. In 2013, the average temperature during the 6633 was -20, similar to what it was when I first raced the YAU in 2009.  In any case, surely 'average' conditions are more valid than a single, one-off fluctuation in one edition of the race?

I do not regard temperature alone as being sufficient to qualify a race as the toughest.  I do, however, think the 6633 is a fascinating and iconic race, as it takes in a variety of landscapes (mountain passes, woodland, open land, an ice road, a river delta and the Arctic Ocean).  I think it is a good cold-weather race to aim for a good time in, and a safer environment than others because it is along an easily accessible road. I just do not think it can compare to the YAU or Iditarod, in terms of conditions or overall difficulty. As with the MdS, it is a shame that the recognised false hype of 'the toughest race' tarnishes the 6633. As a race in its own right it is extraordinary, but to claim it as the toughest race actually harms its reputation, in my opinion.  Personally, I think the 6633 should focus its marketing on how it is along one of the roads featured in Ice Road Truckers, that it is mostly within the Arctic Circle, and that it leads into Fort McPherson, from where the story of The Lost Patrol originates.  Similarly, I think the YAU should focus its marketing on the Yukon Quest and the history of the gold rush around Dawson City.

If we were to consider temperature alone, would it be a combination of the extreme of temperature and the length of time it lasted? Surely 3 days at -40C is worse than 6 hours at -60C, for example? How would we objectively judge? We could not use drop-out rates or the number of cold-related injuries, unless we had some confidence competitors we want to compare are of a similar level of experience and in equally-effective clothing.

There are some unique challenges involved in cold weather racing. The self-confidence, competency in the environment, and survival skills necessary to be safe are far greater than in any other extreme environment.  This is matched with the amount of clothing and equipment required, often totalling thousands of pounds to cover down jackets, sleeping bags, sleds, harnesses, snowshoes, overboots and technical clothing. However, it is a shame if a cold weather race should be judged the toughest race in the world, and for one simple reason: they can all be walked. For me, the toughest endurance race ought not to be one that can be finished at a plod - it has to be a true test of aerobic fitness. Unfortunately there are no other multi-day, single stage races I am aware of, that compete with the hardships of cold-weather races, whilst being of an equal distance. A 500-mile, single-stage race at either very high altitude or in a jungle, with an appropriate cut-off time would be suitable. Safety factors will probably prevent any such race from appearing.



How Difficult is it to Move a Mile?


This is challenging to summarise within only a couple of paragraphs. To convey the real hardships and challenges to physiology, there are review articles on this website under the 'Online Chapters' tab. In brief, a desert environment is the easiest of the extreme environments, because the body is able to sweat efficiently to allow cooling. The jungle, by contrast, is exceptionally difficult, due to the humidity preventing evaporation of sweat. This means it is easier to suffer with heat-related sickness in a jungle than in a desert. This effectively rules out the desert ultras for consideration - the jungle races are far more physiologically demanding.

The challenge of Arctic and other cold weather racing is not in travelling a mile. A mile can be covered easily, if not very quickly, due to the nature of soft trails. Very high altitude, however, is truly the toughest environment. At the highest point of La Ultra / The High, racers are around 18,000 feet above sea-level, at which point the oxygen levels are so low breathing is heavily laboured, recovery needs are extended, and progress very slow. In terms of sheer physical discomfort, a high-altitude ultra, above 3000 meters, is the toughest.

A more scientific appraisal could assess acclimatisation. In a desert or jungle, it takes 5-8 days to acclimatise to the heat, during which time the body improves heat tolerance and sweat efficiency. In a cold environment, it takes a couple of days to improve cold tolerance, perhaps with a slight drop in core temperature to aid this. Acclimatisation to high altitude begins immediately but takes weeks to complete, with improvements continuing at a lower rate across a month or so. The main adaptation is an increase in haemoglobin and red blood cells.

As an extreme point, athletes lacking in high-altitude acclimatisation require several days of rest when arriving in Leh ahead of La Ultra. The flight to Leh brings people from low altitude (i.e., New Delhi) to approximately 3000 metres. Backpackers have been known to leave the airport and head directly to the hills, some of whom have died due to the lack of acclimatisation. No other extreme environment is so restrictive on human physiology as high altitude. Thus, the toughest environment is very high altitude, followed by jungles, followed by either Arctic or desert.




Toughest to Reach the Finish


So, does this make La Ultra the toughest single-stage race in the world? Well, the 333-km La Ultra might well be the toughest, both for traveling a single mile and for finishing. This is particularly the case as the 72-hour cut-off is so ambitious.

The Spartathlon, as another potential candidate, is 153 miles to be completed in 36 hours, which is also particularly tough. However, the cut-off for the Spartathlon is deceiving. The cut-offs are hardest early on, and extend out to a 36-hour finish overall. The first 50 miles must be completed within 9.5 hours. Competitors, who could finish within the overall cut-off, make it harder by trying to build a good margin within the cut-off times along the route, often working too hard during the first 50 miles, then not making the rest. It is worthy of consideration, despite not being an extreme location.

As for a scientific consideration, researchers investigating rhabdomyolysis in Spartathlon competitors recorded the greatest levels of muscle and liver damage reported from physical activity. The damage refers to muscle cell membranes rupturing and their contents entering the blood. The large protein in muscle cells, myoglobin, enters the main circulation, overwhelming the liver and kidneys. The extreme consequence of rhabdomyolysis is death from kidney failure. It is the most likely condition to kill an ultra-runner, around the time of a race, other than hyponatraemia (although the two might be related and both are exacerbated in hot climates). New research is emerging on the irreversible scarring to the heart muscle during marathons and ultra-marathons, which may be related to severe consequences of long distance running. However, due to the reduced blood perfusion at very high altitude leading to a lowered maximal heart rate, there is a possibility (unverified) that high altitude may be both tougher and safer than exercise at sea level. In any case, none of the contenders for toughest ultra has been assessed for myocardial fibrosis (scarring).

Overall, I am inclined to propose the 333-km, 72-hour La Ultra race as the toughest single-stage, continuous ultra. The 222-km version may have been harder than the Spartathlon, in terms of physiological demands, but the generous finish time made it too accessible. This was changed in 2013, but with the addition of the 333-km distance in 72 hours, this, I think, will prove itself as toughest. As for other possible contenders, such as the UTMB and Badwater, it seems that the challenges of the terrain and environment are too balanced-out by the generous times to finish (particularly in light of new events, such as Mark Cockbain's Hill Race). This point is only in the interest of considering what the toughest ultra-races might be, as my preference is for races to be accessible to as many individuals who might enjoy them.






When considering what really qualifies as the world's toughest race, it is important to categorise whether it is single-stage or multi-stage, and if single-stage across how many days. Overall, a single-stage race will be tougher than a multi-stage one, but then there are races such as the Race Across America and Trans Europe that cloud the issue.  

The 'toughest' often tends to be a reference to the environment, climate and/or terrain. In these terms, the very high altitude races should be hardest, followed by jungle, cold weather and desert. What clouds the issue here is the mix of time and distance.   Is it fair to compare a 138-mile race in the Himalayas with a 1000-mile race in the Arctic, for example? This is what makes the 'toughest to complete a mile' a useful marker. But then the overall toughness has to take into account total distance and cut-off times.

In conclusion, the only real way to determine the world's toughest race would be to replicate the experiments done during the Spartathlon. By assessing the level of muscle and organ damage induced by racing, it is possible to get an objective measure of just how strenuous the event is.  Such information should need to be scored along with the racers' fitness, training mileages and intensities, speed-distance data, and other variables such as heart rate, breathing rate and body temperature. Without such measures, any claims are mostly speculation.

There are, of course, plenty of races that have not been considered here, mostly because they either do not claim to be the toughest, or else I just have not come across them.  A full review of every tough race would be a book in itself, and by the time such a book had been written, a great number of new races would have been invented.

What makes running a physical challenge is the availability of oxygen to the working muscles, and the availability of substrates for fuel.  At very high altitude the oxygen levels are drastically lower than at sea-level, and substrate availability is similarly impeded (utilising anaerobic energy sources more than at sea-level).  Thus, energy substrates are depleted more rapidly at very high altitude, and oxygen availability can be half that at sea-level.  The longest footrace with the tightest cut-off time at very high altitude is the La Ultra-The High 333-km.  I am unable to think of a greater challenge to human physiology than this event, that exists as an established footrace.


Scientific Study Y:


This article was written from the perspective of a physiologist assessing Runner X.  As mentioned at various points in this, it is possible to objectively measure each race and rank it in order of difficulty.  The reality is that it becomes a very long-term, involved process, or at least it is to do well.  The point is that it can be done, and many races have been assessed by researchers, meaning that cop-outs like 'it's individual' or 'it's opinion' can be ignored.


These are some considerations for establishing a true ranking of races, based upon a hypothetical scientific study (Study Y):

1. Each race ought to be assessed across a minimum of three years, so as to control for fluctuations in environmental conditions.  These would include temperatures, air pollution and ground conditions.

2. As many racers should be involved in the study as possible, so as to generate a good spread of data and make the study's findings more meaningful and relevant to future racers.

3. Data recorded before the race should include all the blood and urine tests to be taken during and after the race, for true comparisons.

4. Data should include training and dietary habits in the year and weeks before the race.

5. Testing during the month prior to the race should include exercise heart rate, breathing rate, oxygen saturation, core temperature, sleep habits, training mileage, frequencies and intensities, and blood biomarkers of muscle and organ health.

6. Testing during and after the event would include blood biomarkers of muscle health, vascular health, organ health (liver and kidneys), heart rate, breathing rate, sweat composition, oxygen saturation, blood glucose, substrate utilisation (carbohydrates, fats and proteins), urine composition, core temperature, hydration (all as examples).

7. Just for fun, and to humour them, the sports psychologists can bother athletes by asking them 'how do you feel about that' at inconvenient intervals.


This level of testing is perfectly achievable, and already takes place.  If anyone (company/institution) would like to offer-up some funding to assess races in an unbiased, scientific manner, then please do get in touch. If a race organiser would like objective testing of their racers, again I am open to invites but will require funding. The investment would be for the research team, their time and personal expenses, and the equipment.  A good level of testing has already been conducted at both the Western States 100, and Spartathlon events, as well as various others.  Not only is this testing of scientific interest, but it helps guide athletes in their race preparation, nutrition and hydration strategies, as well as aiding medical support in diagnosis, treatment and equipment needs.  Long may it continue.

Until such testing is conducted, however, we shall have to assume all organiser- or media-appointed claims must be regarded thus:

"That which can be asserted without evidence, can be dismissed without evidence"

- Christopher Hitchens

I give lectures and courses around the UK on subjects relevant to endurance athletes, coaches, personal trainers and therapists.  Details of these can be found here.



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