For anyone who has ever bonked or "hit the wall" during a race, this phenomenon needs no introduction. For the rest of you, consider yourself lucky because bonking is always an awful experience unless you're the type of person that likes being punched in the face repeatedly. There are many different forms of bodily discomfort in endurance sports - the slow deterioration of speed and motivation during a long run, the sharp lactic acid burning of an overtaxed muscle, or the fierce cramping of leg muscles often caused by dehydration, but bonking is a singular experience far more severe than these others. It comes on seemingly instantly late in a race, like some omnipotent being decided to cut your puppet strings and play with another toy. One minute you'll be trotting along feeling moderately tired or maybe a little uneasy, then the next you can hardly count to two let alone consider getting up out of the fetal position and starting to walk.

Bonking can take many different forms and even scientists can't agree on exactly what causes it but it seems clear that depletion of muscle glycogen stores is probably the prime factor (if you think your body runs on motivation alone, you're in for a rude awakening). Many factors can influence the severity of a bonk though. Dehydration also often plays a part as well as mental fatigue, which probably plays a bigger role than most people realize. Excessive heat will make it easier to bonk. If you have only depleted your glycogen stores and are mentally alert and well hydrated and not working too hard in the heat, then you might get off easy. You might bonk but still be able to continue moving at half speed and recover pretty quickly if you get some calories into your body. But if you are mentally stressed and severely low on sleep, water, and food, then your race may be over when the bonk hits you.

The worst bonk I ever experienced was actually not while adventure racing but while backpacking with my wife in the Colorado Rockies. We had just flown in from Indiana a few days prior and decided to do a hike that took us up 5000 feet from the trailhead to a 12000 foot lake. There wasn't really a trail most of the way so lots of route-finding and bushwacking was involved. And usually we're good about eating and drinking enough while backpacking but who knew the mosquitos in the Rockies could be so ravenous in June? So long story made short, we made it in one push to our stopping point for the day, but then all of a sudden it hit us.

We realized that we hadn't really eaten or drank anything all day and that not having acclimatized probably was not helping us. I just wanted to close my eyes go to sleep soooo badly but I felt so horrible that I was afraid if I went to sleep I would die. I thought I was probably going to die anyway but just in case I decided to try to force myself to eat something even though I wasn't the slightest bit hungry or thirsty and didn't even feel like I could keep anything down. I was almost physically unable to eat, but somehow managed enough effort to eat one piece of candy even though it took literally 15 minutes. That and lying down did seem to help though and that gave me the energy to go retrieve some water from a nearby pond and start drinking some of that. After about an hour of nursing some food and water, we started to feel like we were going to survive the night. Needless to say if this had happened to me in a race, I would have had no chance of finishing.

In addition to what I listed above, there are some other common symptoms of bonking besides the general feeling that you want someone to shoot you in the head and put you out of your misery. If you're wondering whether you've ever bonked or not, then you probably haven't, but here's a list of the most common symptoms in order of severity:

  1. Extreme irritability
  2. Severe fatigue/weakness
  3. Muscle cramping
  4. Extreme tiredness/sleepiness
  5. Confusion and disorientation
  6. Nausea/vomiting
  7. Temporary loss of vision or hearing
  8. Fainting
  9. Feeling like you are going to die

Now you probably think I threw that last one in there as a joke, but nothing could be further from the truth. The first time you bonk bad enough, you will feel so terrible that you will start to think that there's at least a small possibility that you are dying. Once you experience it for the first time, you'll know. As a great example, check out this video from the 1995 Ironman where legend Paula Newby Fraser bonked very badly toward the end of the run. If you don't want to watch the whole thing, fast-forward to 6:20 where she says "I think I'm gonna die."

To be fair she was probably suffering from heat exhaustion as well, but as I said above there are often more factors than not eating enough that will lead to bonking. As far as I know, after Paula eventually got up and finished the race, she had no serious health repercussions - it was just a really bad bonk and once she got water/calories in her and rested for 20 minutes, she felt a lot better. She was only a few hundred yards from the finish but when you're bonking, the finsh line might as well be on the moon for all you care.

Yes I am aware that in certain cultures bonking is also a slang term for sex. It boggles the mind how one word could mean something so good and so bad at the same time. I wonder if anyone has ever bonked while bonking...

How to prevent yourself from bonking

Now that we've established what bonking is, the question becomes how to avoid it? Not surprisingly the best way is simply to eat plenty of food. Of course that's easier said than done in the intensity of a race or vicious training session. Ask a bonking athlete why he didn't eat more food and you're liable to get smacked (feebly). But other than having a fool-proof plan for getting enough calories in you while racing (check out the Race Day Nutrition page, hint try try to take in liquid calories throughout the day by adding a drink mix to your water), the best way to ensure that you don't bonk is to learn to listen to your body and learn the warning signs that you are getting close to bonking. Also check out the race day food page for info on how to manage your food intake during the race.

Listening to your body is easier said than done as some racers will bonk even in training when they don't have all the pressure and excitement of race day to deal with. But the subtle signals your body sends you during a race and training, despite being easy to ignore, are something that you are going to have to learn to listen to if you want to prevent yourself from bonking. And once you learn to pay attention to them, they start seeming not so subtle. I'm willing to bet that you will never bonk without having ignored several warning signs for a couple hours before it actually happens. Bonking feels like it comes on all of a sudden, but really it's a long process of neglect and overexertion that got you there. Your body doesn't just break down spectacularly like that on a whim. What your first symptom actually ends up being is a personal thing, but here's an example timeline of how it can happen during a sprint adventure race:

  • Hour 0: Race starts, you are well fed and well hydrated, but you got hardly any sleep last night trying to pack your gear and having to get up early to get to the race site. You don't feel tired though because of adrenaline and nerves. (Not being fully rested means your body is already stressed. Also when the adrenaline eventually runs out you will feel drained.)
  • Hour 1: You feel a little hungry so you take a couple bites of an energy bar and your hunger goes away. (Not eating enough already. Hunger is highly muted during a race so you should pay serious attention to it and assume that if you are hungry it means you already should have eaten more and need at least 200 calories immediately, especially if it's early in the race.)
  • Hour 2: You realize your water bottles are still almost full so you take a few sips. (Don't wait until you are thirsty to drink - after two hours you are already getting fairly dehydrated. Again, you have a little more leeway at the end of a race if you hydrated well the rest of the time, but never skimp on water this early.)
  • Hour 3: You spent a lot of the last hour lost, looking for a checkpoint that you totally overshot, and now you realize a bunch of teams passed you and you're not doing as well in the race as you had hoped. You haven't even thought about eating since you took a few bites of that energy bar because you've been so caught up in trying to figure out where you went wrong. (Mental stress is often a big factor in bonking, plus you just traveled a lot further than you had to because you were lost.)
  • Hour 3.5: You notice a slight cramp in your left hamstring and a headache, but shake it off and stretch a little, then keep on running. (Strong warning sign that you are dehydrated and low on electrolytes. You should have drank some water with some electrolyte pills and really eaten something too.)
  • Hour 4: On the bike, you see a group of teams go around a corner well ahead of you and know that your team can catch them if you pump hard enough. You follow them up a massive hill, cranking hard. Eventually you do pass them, but then they turn up the speed trying to pass you back up and you maintain a much higher pace than you normally would on this section trying to stay ahead of them. Of course you have no time to pull some food out of your pack, but you take a few sips of water. (Again, more mental stress, and you're still not drinking or eating enough. You're pushing too hard and there's too much race left.)
  • Hour 4.5: You get off the bike to go find a checkpoint on foot and lose the other team. You are feeling very sluggish, but push through it and are able to climb a hill to the checkpoint and punch it. You relax a little as there are no teams around. All of a sudden you feel terrible. You have to stop walking altogether and lean against a tree for support. Then you mumble something to your teammates about needing to take a rest break and sit down on the ground. You just bonked, and your race may be over.