top of page

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, or else media articles produced by journalists claiming a particular race is the toughest (or their lists of 'top ten toughest races', etc.  Often such people have little grasp of what makes a race truly challenging, have limited personal experience compared with the racers themselves, and are mostly using the unfounded, unverified claim as a marketing tool (whether to increase racer applications or to increase article readership).

Many of us in the ultra-running community have become quite tired 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?  This also allows us to dismiss the 'horses for courses' argument, which is a thoughtless means of dismissing what ought to be an interesting discussion and logical process.

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 and muscular systems.  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 am not required to 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.

The first step is 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 frowned 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, out 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 (the same is true with the 6633).  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 event.

With the Arctic races it is common to claim something remarkable due to weather.  Cold and wind vary hugely from one year to the next, and because the weather was rough a decade ago, that doesn't justify that next year's event will be the toughest race in the world.  Besides, when it comes to brutal conditions, it's hard to compete with the ITI1000, which is famous for its remoteness and the fairly horrendous conditions routinely encountered along sections of the Arctic ocean.  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.

I do not regard temperature alone as being sufficient to qualify a race as the toughest.  I do, however, think these Arctic events should be celebrated in their own right, rather than being tied to claims of the toughest race.  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.  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.  The ITI is the route of the Iditarod and another gold rush legend.  In terms of toughness, the 6633 is on road, which facilitates much easier progress than a trail race.  The trail of the MYAU is consistently packed down like a pavement, whereas the ITI route has minimal traffic and is notoriously soft and difficult.  Out of the Arctic and sub-Arctic races, the ITI1000 deserves consideration as the toughest physical challenge.

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 totaling thousands of pounds to cover down jackets, sleeping bags, sleds, harnesses, snowshoes, overboots and technical clothing. However, none of the big cold-weather races is the 'toughest in the world' - not the Spine, MYAU, 6633 or ITI -  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.  People tend to perform better in the cold; they can run faster than in warmer environments.  The physiological challenge comes from managing cold exposure, which means protecting the body from hypothermia and frostbite, but these are dealt with through proper clothing.  If you can plod a long way in a temperate environment, it is not physiologically harder to go the same distance in the cold (in many ways it is easier, although certainly more dangerous).  Overall, whatever the unique challenges presented by ultra-races in different environments, finishing statistics alone are not appropriate to base a claim of 'world's toughest race'.  We should also be cautious about claims for toughness based upon extreme (but short-lived) temperatures, particularly when these rely upon a freak occurrence in one year, and not the average temperatures year-to-year.  If Runner X can manage a good hiking pace for a week or so, physiologically he will manage these races.  In terms of his safety, that depends considerably on the weather during that particular edition of the race, which is hard to predict more than a couple of days before the event.


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 on exercise in the cold, exercise in the heat, and exercise at altitude. 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 more likely our Runner X will 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 traveling 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 physiological 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.  Again, it is worth reading the article on running at altitude to get a better understanding of the physiological processes. 


In brief, ultra-running is an aerobic activity, meaning we are able to run because we can transport sufficient oxygen to our muscles to permit them to contract, and for us to move.  In a cold environment, oxygen availability is not altered by the temperature, so physiologically it is not more challenging than running in a more temperate zone.  In a hot environment, sweat losses are higher, which means blood content is reduced and the heart must beat faster to compensate for dehydration.  A higher heart rate reduces oxygen supply to muscles (blood moves around the lungs too quickly for efficient exchange of oxygen and carbon dioxide).  Still, this does not compare with the effects of lower atmospheric oxygen availability and lower air pressure at very high altitude.  Further, energy substrates are depleted more rapidly at very high altitude, which also promotes exhaustion.  Considering high altitude races - and La Ultra / The High in particular - are often in hot environments, these are our strongest contenders for the world's toughest ultra, based upon environmental physiology.

A more complete scientific appraisal should consider the effectiveness of 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.  It is obvious that people who live in the different environments ought to fair better than those who do not, but acclimatisation does a lot to level the playing field.  Performance in ultra-races is generally not predicted based upon how close the racers live to the race route. 

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, then deserts and finally sub-Arctic and Arctic.


​The Physiological Effects Of Ultras


The Spartathlon is 153 miles of continuous running, between Athens and Sparta, to be completed in 36 hours.  For a scientific consideration, researchers investigating rhabdomyolysis in Spartathlon competitors have 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).

Ultra-endurance running can cause irreversible damage to the heart (scarring) which may be responsible for altered electrical activity, and deaths, in some ultra-racers.  Rhabdomyolysis is well-documented in runners, which leads to organ damage and can be life-threatening.  Muscle damage (including heart muscle damage) is also well-documented in endurance athletes.  Hormone levels are reduced by even fairly moderate running distances, which have a negative impact upon fertility.  To ascertain which race is truly the toughest, we would need to measure the incidence and severity of each of these, in a controlled group of as many runners as possible, racing in as many potential 'toughest races' across the world.  This would give strong, objective evidence of how physiologically tough an event truly is.  Whilst some evidence has been reported from events such as Spartathlon and Western States, there are no big studies across multiple races. 




When considering what really qualifies as the world's toughest race, it is firstly 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 (where very high accumulated distances are more likely to cause muscle, joint and heart damage).  

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, desert and Arctic. 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.  When it comes to claiming a race is the toughest, unless there is objective evidence to compare it to other races, those claims can and should be dismissed.

Scientific Study Y:


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, however, is that it can be done, and many races have been assessed by researchers, meaning that cop-outs like 'it's individual', 'it's opinion', or 'horses for courses' can be ignored. 

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

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 apply Hitchens' razor to all organiser- or media-appointed claims:

"What can be asserted without evidence can be dismissed without evidence"

- Christopher Hitchens

bottom of page