Inside Story

What exactly is going on under your body's bonnet as you move through the 26.2 miles of a marathon?

Posted: 4 September 2008
by Matt Barbour

No wonder the marathon is our ultimate yardstick – from cramps, to dizziness, to dehydration, those gruelling 26 miles 385 yards (count them) reach parts other distances can't reach. And it can pay to know exactly what parts, says Clare Lane, applied physiologist at Bath University's Human Performance Centre. "Understanding the physiological processes that occur during the different stages of a marathon gives runners a better grip of the underpinnings of strategy and training and can take their running to the next level."

It's now that the pre-race nerves kick in. But it's not just your incessant back-and-forth pacing that's sapping vital energy stores – sizing up the competition and playing your mile-by-mile strategy on a mental loop can be equally draining. "What most runners don't appreciate is just how much energy your brain uses, especially when it's pumped with adrenalin," says Charlie Pedlar, endurance physiologist with the English Institute of Sport (EIS).

"Your heart beats faster, and your brain, being a hugely inefficient and sizeable organ, can use up to 10 per cent of your stored glycogen energy reserves before you've even begun – you'll see the elite runners take time away from the crowds to keep calm and maintain their pulse at no more than 10 beats over their resting heart rate." That rush of 'fight or flight' adrenalin is there for a purpose – to elevate your core temperature and prime your muscles for action – but you need to control it if you're going long, says Pedlar. "Adrenaline is great for sprint events, when you want to shoot out of the blocks, but with marathons, the mantra has to be energy conservation."

An average 40-year-old male runner should have a resting pulse of about 60-70 bpm, with women perhaps 5 bpm or so higher. "In the opening stages of a marathon, the heart rate of both men and women should climb to the plateau of about 140bpm, working at about 70 per cent of your maximum effort, a guide we use for testing new athletes' fitness," says Pedlar. "Any higher than this, and it's more than likely you'll be using the anaerobic energy system, which is up to 18 times less efficient."

Stick to your steady pace and you'll avoid exceeding your RER – or 'Respiratory Exchange Ratio' – he explains, which is when you're breathing out a greater volume of CO2 than the oxygen you're breathing in. The EIS uses state-of-the-art air analysis software to help identify an athlete's optimal aerobic pace, but it's pretty much impossible to gauge on your own in race conditions. "The rule of thumb should be 'go slower',” says Pedlar. "Even a short burst of anaerobic exercise will have an exponentially negative effect on your performance later on."

Your heart rate's not the only thing on the rise; your core body temperature should elevate quickly from around 37ºC to about 39ºC, prompting the hypothalamus, an almond-sized area of the brain, to flick the 'on' switch on your sweat glands to help keep things cucumber-cool. And it prompts the release of more adrenalin. "This helps the mobilisation of fat as a fuel source through your oxidative pathways, replenishing the glycogen energy stores in the liver and muscles you're tapping into,” says Pedlar.

6-12 MILES
By now you're well settled into your pace, and your key readings will start to broadly plateau. From here to the end of the race, you should experience what's called 'cardiac drift'; your heart rate and core temperature steadily rise by about five per cent even though you're maintaining the same pace. Your body is finding it increasingly hard to dissipate the heat your efforts are creating.

"This makes your heart work harder to pump the blood through your system faster to cool you down,” explains Dr Tom Crisp, sports and orthopaedic physician at The Royal Free Hospital, London ( "It's a system that can be terminal if you lose sight of your mile splits, so I always advise marathon runners to do negative splits, speeding up in the second half of the race." Your body will still be relying on the ultra-accessible glycogen stores in your liver and muscles at this point – although increasingly sensing the need to start converting fat to replenish those stores – and your sweat output should peak.

It's now that hydration is needed. Some put it off, feeling too good in the moment to contemplate feeling bad later. But that's not wise, says Pedlar. "The fitter you are, the more efficient your body is at cooling you down with sweat, which means the well-trained are more likely to suffer from dehydration."

Use training runs to gauge personal fluid requirements. Weigh yourself before and after a run. For every kilogram of total weight lost, you need to be drinking at least one and a half litres of fluid. "That needs to be taken on board before you sense any thirst," warns Pedlar. "By that time, your metabolism and energy efficiency will be severely impaired." Typically, 150 to 300ml of fluid every 15 minutes should combat dehydration. "It's a vicious circle – you don't drink enough, your blood volume decreases, so your heart has to pump less blood faster to keep you cool, making you heat up more and making your heart beat even faster."

The hours of training you put in make your body better at transporting oxygen to your muslces, so you stay below your anaerobic threshold

12-18 MILES
Your glycogen stores are now starting to run low, so your body is desperately trying to convert the more abundant and energy-rich fat stores to keep your motor running. "How efficient your body is at doing this is about 30 per cent genetic, and 70 per cent training," explains Lane. "Some people have more aerobic mitochondria that convert fatty adipose tissue to glycogen and blood glucose, but the fitter you are the better your body utilises them."

The effects of over-exertion are kicking in as you try to maintain pace, despite the messages coming from within that something's changing. You could suffer stomach cramps, as oxygen-rich blood has been diverted away from the digestive system towards the muscles, while other runners will get diarrhoea, caused by interruption to the normal bowel movement. But it's not all doom and gloom downstairs, says Lane. "The bladder shouldn't need to be emptied. Blood is diverted away from kidneys during strenuous exercise, slowing urine production."

18-24 MILES
With glycogen stores bottoming out, it's now that you might be facing up to the dreaded 'wall'. Many runners are under the misapprehension that the fuel type you use suddenly changes at different points in the race, but that isn't the case, explains Pedlar. "It's an almost continuous transition, and when you start to run out of easily-accessible blood sugar and glycogen you're also pretty dog-tired, so there's a large mental element to it." It's now that those hours (and hours and hours) of training come into play.

"People who haven't prepared properly can start to go into anaerobic respiration, when there's too little oxygen reaching the muscles," he says. "Your body isn't efficient at taking in oxygen, so you'll hit your anaerobic threshold at very low intensity." And the killer by-product of this is lactic acid, causing acute pain and muscle cramps. Lactic acid production provides another vicious circle. "It impairs the mechanism that breaks down fat for energy," explains Pedlar.

And as if that weren't enough discomfort to be (not) getting on with, your joints should be feeling it now too. Unlike at the start, when your foot spends 200 milliseconds on the ground and 500 milliseconds in the air with each stride, these two figures will become almost equal – around the 300-millisecond mark. "Your stride becomes less efficient, so your foot spends more time on the ground, absorbing more of the two to three bodyweights you drive down with each step, often causing an intense, dull pain in the kneecaps," he says.

24-26.2 MILES
Just when you're feeling like you want to keel over and die, Mother Nature steps in. "It's often after the intense physical low and with the finishing line in sight that runners talk of experiencing a 'high'," says Crisp. "While this is no doubt a pain-management facility, it also presents real dangers as you stop listening to your body and push yourself to absolute exhaustion." The heart rate can soar to 180 and your blood pressure can shoot through the roof, while your body temperature can even rise over 41ºC. "It's no surprise that over 80 per cent of all fatalities in marathons happen within two miles of the finishing line."

As finishers collect their goodie bags and medals, their blood pressure plummets and can even go too low, Pedlar says, leading to fainting. giddiness and even hypothermia. "Your internal temperature-controlling mechanisms can be shot, so it's essential to keep walking around so your heart and blood pressure can normalise in a more controlled, stable way." After 26.2 miles of pounding, there'll be significant micro-trauma damage to the musculoskeletal system, and you'll more than likely have a fluid deficit, which will inhibit recovery. The waste products of metabolism and tissue damage clog up your lymphatic channels, which can prevent fluids and minerals from entering your system and speeding repair, says Pedlar. "The best way to minimise delayed onset muscle soreness and drain your lymphatic system is ice baths, compression and massage combined with 20 to 30 minutes' light cycling or swimming in the days after your marathon."

The Wall Explained...

"Hitting the wall is about running out of energy," says Dr Tom Crisp, sports and orthopaedic physician at The Royal Free Hospital in London. We’re talking about chemical energy, obtained from the breakdown (metabolism) of energy-containing fuel – in other words carbohydrates (blood glucose and glycogen, a polymer of glucose stored in the muscles and liver) and fats (free fatty acids in the blood stream and muscle triglycerides). While fat metabolism requires vast amounts of oxygen the simpler carbohydrate molecules, which eat up minimal O2 when burnt, are your fuel of choice.

"Even if you carbo-load wisely and maintain a reasonable pace, you still only have about 2,000 calories worth of glycogen stored in your muscles – enough to get most runners to about mile 18 or 20," he says. "As glycogen reserves are used up and fatty acid metabolism increases, your heart has to work harder to pump more oxygen-carrying blood to your muscles, making it difficult to maintain pace."

Add in the effects of dehydration – your blood thickens, making it harder to pump round your body – and the micro-trauma you’re no doubt experiencing in your major muscle groups, and you’re fighting a losing battle. "Fatty acid metabolism itself requires glucose, so you’re instantly double-dipping, causing the steep drop into low energy." Non-working muscles can’t even come to the rescue and transfer their un-tapped glycogen reserves to working muscles – once it’s inside a muscle cell, it stays there until it’s metabolised. "This is why many marathon runners prefer courses with periodic elevation changes, which allow glycogen reserves to be shared among a larger group of working muscles."

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Discuss this article

OK, this is all good and interesting, but I am curious as to what happens physiologically when you go beyond 26.2.

What happens to the body when you go well beyond this distance......35miles, 50? 75? 100? 100+????

Is it a case of your body burns fat for a bit, then lean tissue? Or switches between these and the carbs that you desperately try to shove into yourself?


Posted: 09/09/2008 at 11:19

Nick L wrote (see)

OK, this is all good and interesting, but I am curious as to what happens physiologically when you go beyond 26.2.

What happens to the body when you go well beyond this distance......35miles, 50? 75? 100? 100+????

You pop.  Simple as.
Posted: 09/09/2008 at 11:45

Oh dear.

That sounds painful?!

Posted: 09/09/2008 at 12:10

well, beyond 26.2 is just bloody stupid isnt it

who would want to do that?

Posted: 09/09/2008 at 12:19

Pfft I dunno?!
Posted: 09/09/2008 at 12:20


Posted: 09/09/2008 at 12:21

Wouldnt want to argue with you Hipps....might get a slap!.....although ACTUALLY........
Posted: 09/09/2008 at 12:35

Posted: 09/09/2008 at 12:37

.....oooh that it again!
Posted: 09/09/2008 at 12:53

Yes beyond 26.2 you turn into animals with silly names.......
Posted: 09/09/2008 at 14:26

Sorry to interupt your playtime Hipps & Nick but to get us back on track.....

I'm also interested in what happens beyond 26.2 and how best to survive it (other than don't do it!).

Posted: 09/09/2008 at 14:27

Ah thanks Floosie Sue!

Yes....any serious comments???!

...I have a vague idea as how best to survive it (keep eating even when you dont want to).....and ideally have someone nag you to eat.

Posted: 09/09/2008 at 14:34

No great expert on the physiology, but you pace yourself more slowly - for obvious reasons - so you switch to fat burning mode more gradually and so do not as such hit the wall (the ground through total exhaustion is another matter).  Also you will most likely be eating rather than relying on gels/drinks alone.  Time on feet can be very long - 24 hours and beyond for certain events so tiredness can add to the enjoyment with hallucinations as an added fun extra. 
Posted: 09/09/2008 at 14:40

Just my opinion but I reckon the toughest challenges to overcome on longer distances will be mental which doesn't answer the question about physiological changes, sorry.  Ask me four years from now and I'll hopefully have an informed opinion and charge you £100. 
Posted: 09/09/2008 at 14:54

On organised events it is easier to think check point to check point rather than overall distance.  Cannot imagine what it's like doing something like a 24 hour track race, but those who've done it say that it is not so bad as it seems to an outsider.
Posted: 09/09/2008 at 15:01

Ive not contemplated a 24h track race either.......but have done a few ultras. The thought of sooo many laps would drive me bonkers I think!

You are right about the mental aspect of it though dodge.

Posted: 09/09/2008 at 15:44

Yeah, obviously 'mental' to do it in the first place!!

Where do I sign up????  ;--)

Posted: 09/09/2008 at 20:21

There's a nice little 55 miler this weekend - I could do with company for when I (inevitably) get lost.
Posted: 10/09/2008 at 10:06

Oooh where is that Bear?

Not that I will be able to make it I will be at Langdale marathon. Am curious though!

Posted: 10/09/2008 at 11:56

  • At rest, 33% of the body's energy comes from carbohydrates, or glycogen, stored within the muscles and liver. 66% comes from fat.
  • During aerobic work, 50-60% of the energy comes from fats
    • Primarily carbohydrates are used during the first several minutes of exercise
    • For an average fit person, it takes 20 to 30 minutes of continuous aerobic activity to burn 50% fat and 50% carbohydrate
    • There is approximately a 7 fold increase of fat mobilization after 1 hour of exercise
  • Proteins contribute less than 2% of the substrates used during exercise of less than 1 hour.
    • Slightly more proteins are utilized as a fuel source during prolonged exercise.
      • During the final moments of exercise lasting 3 to 5 hours, protein utilization may reach 5-15% of the fuel supply (Berg A & Keul J 1980; Cerretelli P 1977; Hood D & Terjung R 1990; Lemon P & Mullin F 1980; Lemon P & Nagle 1980)
    • Protein can supply up to 10% of total energy substrate utilization during prolonged intense exercise if glycogen stores and energy intake is inadequate (Brooks, 1987)

Posted: 10/09/2008 at 13:53

A bit like marathon running, except you're going more slowly so a higher proportion of the calories used up come from fat metabolism. There is a popular myth (total b*ll*cks) that all marathon runners "hit the wall" because they suddenly switch from metabolising sugars to metabolising fats. Not true, unless they have absolutely no clue about pacing and set off at half marathon speed. Really it's just being undertrained for the distance that causes that, and running out of fuel generally. In fact, if you're going slowly enough, fat metabolism is going full pelt well within the first hour. In an ultra you should naturally be going slowly enough that the remainder of the fuel comes from food you're taking on.

Eg let's assume you're jogging along at 5mph, which happens to be right at the bottom of your aeorobic range. And you weigh about 10 stone. From running you'll be expending ~500 cals/hr (broadly) of which 50-60% can come from fat metabolism, call it 250 cals. So you need 250 cals of sugar, either from food you take on or from glycogen reserves. An average person can eat & absorb something like 240-360 cals/hr, so all can theoretically remain in balance.

(Cf if the same person were doing a marathon at say 7mph, reasonably close to their lactate threshold, they'd expend ~700 cals/hr, with fat metabolism starting to shut down. Call it 20%. So fat is ~150 cals/hr and sugars need to make up the remaining ~550. That's why carb loading and sensible fuelling is key - otherwise they run out of energy)

There are a few additional points to bear in mind with ultras. 1 stomach problems - if you can't eat the ~250, you're boned. 2 depending on length, you don't need to just fuel the running, but also your body's normal operation. Eg 2500 cals a day, or whatever. 3. Some sugar is manufactured by the body from lean tissue by a process called gluconeogenesis. Try to eat a bit of protein, too. 4. For very long ones (12 hours ++), following 3., the protein depletion can hit certain essential (branched chain) amino acids harder than others. This causes fatigue, since a depletion of BCAAs in your brain has a side effect of making you fall asleep. This can be countered by putting BCAA powder in your drink.

In short, run slow, eat a lot, and don't neglect the protein.

Do I win £100?
Posted: 10/09/2008 at 14:15

Nick L - It's the ldwa Shotley Peninsula Challenge so will be mainly wa*kers.  Night section round peninsula so will probably end up in sea. 
Posted: 10/09/2008 at 14:21

That sounds right-William Sichel had us adding protein to his drink after the first 24 hours in his 6 day race and I've read about BCAA in the past. 

Long ultras are more about the will to keep going (or maybe the won't) shorter ones are just long marathons with similar demands that relate to the marathon as the marathon relates to the 10k.

Posted: 10/09/2008 at 14:21

.....and watch out for canals eh Candy? 

 Thanks though makes for some interesting reading!

Take your cossie Bear!

Posted: 10/09/2008 at 15:06

Canals are good for cooling down if you start to feel a bit warm
Posted: 10/09/2008 at 15:39

I've seen a bod swimming in the canal - he's cleared some growth form the towpath and dropped a step ladder into the water. I ran past said hello and carried on......

On the subject of fuelling , I'm running the New Forest Marathon next week and as I've done in previous marathons I'll be taking a GoGel every five miles with one on the start line. I'm wondering how much is actually used by the body and how much just sits there and is used by the body to recover . I've found the recovery to be the key - a litre of Chocolate milk usually does it for me 1500calories straight down.
Posted: 13/09/2008 at 18:13

I had two pints of beer and some crisps after langdale marathon on sunday.....and was a giggling mess!

NF Half for me this weekend Mike!

Posted: 16/09/2008 at 10:16

Nick - I have to make sure I'm not pulled along by the fast half runners , my training has been sporadic and I've only done a couple of halfs in the last two weeks.

I suppose I'll see if the gels will get me round?
Posted: 16/09/2008 at 13:13

That was what happened to me (half mara runners)....but I went with it. Last 3 miles were tough....but steep hills destroy quads.

Take it can always speed up!

Posted: 16/09/2008 at 14:57

 Morning All

All very interesting, but could do with some practical advice. What to actually eat on an ultra? I'm running London2Brigton in two weeks which is 56 miles and have tried differant things, with mixed results. Did a 30 mile training run slowly on Saturday and stopped half way for 10 mins and had rice pudding and peaches, bit later peanut butter sandwhich and at various other times, jelly babies and a couple of mars bars. Trouble was, I finished tired - which I hope is down to the training and the marathon the weekend before - but more importantly I didnt feel like eating more. That was a worry as come race day I will still have a marathon to run.

 Can anyone offer suggestions on suitable high energy foods that will get the carbs back into me which I can eat regularly over what will be a 11-12 hour race, so that I avoid as I understand it my metabolism using my fat stores?

Ps not sure how much to taper. Anyone got a good site they can recommend?

Posted: 19/09/2008 at 10:20

How is a 55-miler a) little or b) nice ?!?!?!

26.2 is just enough for me thanks : 0

Posted: 19/09/2008 at 13:23

Ps Taff did you take all this food with you or did you conveniently know people who lived along the route and popped in for your banquet?
Posted: 19/09/2008 at 13:24

err, isn't that just asking "what's the nicest food" taff??

 take a variety of things you like, some sweet and some savoury.  anything high calorie that doesn't take up much space in your back pack.  maybe a big bag of jelly babies, a tube of pringles and a malt loaf.  that should be more than enough, and will fit easily in most back packs.  some people take ginger cake, as apparently ginger stops you feeling sick so might be easier to get down in the later stages.

Posted: 19/09/2008 at 14:03

 Candy, malt loaf and ginger cake is a great idea. Come to think of it, I did Salisbury marathon about 4 weeks ago I think and there was lots of cake at a 15 mile checkpoint, which was lovely. And the answer to your question is yes. Or what can I eat lots of without barfing!

 TNGM, took most out but also ran so far went through various villages and stopped for supplies at a post office. A good tip for water by the way, is to incorporate churches on your long routes. I went through 4 , all of whom had an outside water tap - most do for the mourners to use on the flowers I assume on the graves. Perfectly drinkable normal water. Saves having to carry loads, especially on hot days. Might need to hunt around to find a tap, but there will be one there more often than not. 

Posted: 19/09/2008 at 15:01

if you sit on the malt loaf first, it goes flat and takes up no room.  well, less.
Posted: 19/09/2008 at 16:03

Well you could always find out what happens after 65 miles. Join us at and join in a run of a lifetime but....... raising money for guys returning from Afghanistan with no legs and horrendous injuries. Our pain will last a few days theirs is for a lifetime.

Whats worse I will be 63 years 10 months the day of the run!!!!!

Posted: 26/09/2008 at 16:10

By the way we already have 40 confirmed runners and 36 pending. The running list closes at 199. Why? Email me and I will explain.
Posted: 26/09/2008 at 16:12

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