How to Avoid Bonking and Cramping During Your Marathon

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In this podcast episode and blog post I will zero in on two common difficulties that can make your marathon a lot more challenging —bonking and cramping. I have heard from many runners who have experienced one of these mishaps. It is time to understand why they happen and what you can do about it.


Bonking is often referred to as “hitting the wall” and this phenomenon happens during long runs and marathons. Bonking can occur when you don’t take in adequate carbohydrate replenishment during your run.

Because the average runner burns 100 calories per mile the body will have depleted its muscle glycogen fuel store after approximately 90 minutes of running (if you haven’t been replenishing the lost calories). When you bonk, your body has burned through its reserve supply of muscle glycogen and what you’ve put into your body hasn’t kept up with the demand. You may feel a sudden fatigue, loss of energy and start taking long walking breaks. Other symptoms may include nausea, dizziness, feeling faint, shakiness and rubbery legs which may indicate that your blood sugar is low.

Bonking may also have a mental component where you feel hopeless or like you can’t go on. If your brain is low on glycogen it logically will be signaling your body to stop whatever folly you’re engaged in.

Here’s what you need to do to prevent bonking:

  1. Refill the tank- The 30-60 minute window post-workout is the ideal time to increase muscle glycogen stores. Take in a high quality carbohydrate and protein combination during this time to replenish and increase glycogen stores. That’s one way to guarantee that your body has ready to use fuel for your next workout. I use Recoverite from Hammer Nutrition.

  3. Time your pre-race meal correctly- If you eat too close to races or workouts of 60 minutes or longer you will accelerate the rate that glycogen stores are used causing them to deplete more rapidly. When you have a morning race the main purpose of your pre-race meal is to replenish liver glycogen stores which are depleted during the night. Your muscle glycogen (80% of total glycogen stores) remains intact even after a night of sleep. Even if your stomach is feeling hungry your muscles are fueled and ready to go. Just take a gel about 5 minutes before your race or long run and you’re primed for a successful race. If you feel the need to eat pre-race make sure that you do it at least 3 hours prior to the starting time. The pre-race meal should be high in carbohydrates, between 200-400 calories, low in fiber and fat and easily digestible. If you eat something too high in fat it slows digestion and any added fiber may create the need for a bathroom stop during the race.

  5. Fuel correctly during exercise- The body needs fluid, sodium and carbohydrates during long distance running in amounts it can handle. When you’re doing aerobic exercise the body’s metabolic rate increases 1200-2000% over the sedentary (resting) state. During exercise the body prioritizes blood volume to working muscles. Fluids are used for cooling and oxygen is sent to the brain, heart and other vital organs. During this time the body simply can’t handle large quantities of fuel, fluids and electrolytes. If you try to replace the total amount of lost fluid, electrolytes and calories you run the risk of your body simply rejecting the unwanted items (inducing diarrhea or vomiting). There are many factors that go into how much the body loses verses what it can replace. Some of these factors are age, weight, level of fitness, stress, acclimatization levels and weather conditions. On average the body can absorb around 150-250 calories per hour during exercise. Focus on replenishment, not replacement. Be sure to practice your fueling strategy during training and don’t do anything new on race day.

  7. Practice even pacing or a negative split. Don’t start out faster than your fitness level can handle or you may hit the wall during your marathon.



Muscle cramps are spasmodic, painful, involuntary contractions of the muscles. Exertional muscle cramps happen frequently in some runners while others will never experience this problem. The risk of muscle cramping increases if you try to run further or faster than you are accustomed, with older age, a higher body mass index, shorter or irregular warm up period and a family history of cramping. The usual muscles affected are larger muscle groups like the diaphragm (your breathing muscles inducing a side stitch), hamstrings, quadriceps and calves.

What causes it?

  1. Under-training- According to Dr. Tim Noakes, muscle cramps tend to occur in people who run farther or faster than the distance or speed to which they are accustomed. If you’re a novice marathoner you may be more prone to muscle cramping at first. Once your body becomes accustomed to running further it should compensate by maintaining homeostasis. Make sure that you’ve appropriately trained for the race you’ll be running. Warm up your muscles before running fast and develop a stretching routine that works for you.

  3. Genetics- If you’re prone to cramping you should also carefully stretch the affected muscle group before exercise. The director of the University of Cape Town Sports Medicine Program, Dr. Martin Schwellnus, has produced evidence that muscle cramps can result from alterations in the sensitivity of the reflexes that originate from muscle and tendon tension receptors. If the muscles are contracting frequently without the chance to lengthen (stretch) it may result in neural short circuiting causing the muscle spindles to be over-stimulated. In other words, some runner’s muscles will “short circuit” after a certain distance or level of exertion causing cramps.

  5. Fluid imbalances- Muscle cramping can be a result of fluid imbalances. If you don’t drink enough, you’ll suffer from unpleasant and performance-ruining dehydration. Drink too much, however, and you’ll not only end up with impaired performance but you may even cause potentially life-threatening water intoxication. One of the most respected researchers on hydration is Dr. Tim Noakes, author of “Lore of Running.” He studied thousands of endurance athletes and noted that fast runners typically tend to dehydrate while over-hydration occurs most often among middle to back-of-the-pack athletes. Both conditions lead to hyponatremia (low blood sodium) but through different processes. One expert, Dr. Ian Rogers, found that between 500-750 ml/hr (17-25 oz/hr) will satisfy most athletes’ hydration requirements in most conditions.

  7. Electrolyte Imbalances- Electrolytes are chemicals that form electrically charged particles called ions in body fluids. These ions carry electrical energy necessary for many body functions like muscle contractions, digestive processes, cardiac function and transmission of nerve impulses. Electrolytes can be compared to the motor oil in a car. They don’t make the engine run but they’re necessary for making it run smoothly. The goal with electrolytes is to keep them in balance. Cramping can be one way that your body lets you know that you’re on empty and it can severely impair your athletic performance. It’s important not to wait until you’re cramping before replenishing electrolytes. Many runners have found it helpful to take supplements like electrolyte tablets before and during running. Make sure that you find a brand that includes a full spectrum of electrolytes and not just sodium (we use Endurolytes from Hammer Nutrition). You may also want to add calcium and magnesium supplements to your diet. Studies have shown that 1 in 5 adults are deficient in magnesium. There are also topical magnesium sprays that can be helpful to use on a cramped muscle. That way the internal effects (like diarrhea) are eliminated and you’re still getting help to the affected muscle.

  9. Calorie imbalances- If you take in primarily simple sugars (like sucrose and fructose) during a workout it takes added electrolytes and fluid to break down the sugar. This will create a demand for more electrolytes and fluid to keep your system in balance. Simple sugars draw fluids and electrolytes from the athlete’s system, across the stomach lining and can often induce gastric stress, cramps, gas and premature fatigue. Simple sugar must be mixed in weak 6-8% solutions or they will sit undigested in the stomach and not pass the gastric lining, possibly creating GI distress. Research has found that complex carbohydrates are digested faster than simple sugars during exercise. One such complex carbohydrate called maltodextrin is composed of multiple sugars units hooked together, allowing a concentration of 18% to 24% solution to immediately transfer to the liver where it is turned back to the energy cycle as muscle glycogen.

How to get through it if it occurs?

  • Pressure and stretch- If a cramp hits you mid-run stop and apply pressure to the affected muscle for 15 seconds (but don’t massage). Then gently stretch (lengthen the muscle) and repeat the pressure/stretch sequence if needed. Start walking when the cramp is resolved then resume running. If you frequently deal with cramps in a specific muscle group try to stretch this area before, during and after the run.

  • Electrolyte Supplements- Carry your tested electrolyte supplements with you on race day. Take a dose an hour before running and then in the amount you need during your race. As the ever quotable Ben Franklin said, “An ounce of prevention is worth a pound of cure.”



  • The Lore of Running-4th Edition, Tim Noakes, MD

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11 Responses to How to Avoid Bonking and Cramping During Your Marathon

  1. Trevor July 2, 2013 at 9:24 am #

    So has anyone experienced either bonking or cramping? Tell us your war story.

    • Sara Craig September 29, 2013 at 10:27 am #

      Bonking? Hitting the Wall? It was WAY WORSE then I ever expected. I actually thought I needed an aid car during the 2013 Marine Corp. Marathon. It was my second marathon- don’t have a lot of experience with this distance. I was running strong, keeping my pace, hitting my splits, felt great at mile 23…….Started celebrating early in my mind that I was going to run my BQ. But little did I know but my lack or consistent fueling earlier on came to haunt me. It came on suddenly. Started feeling a bit dizzy, light headed, eyes were starting to see “black spots……. I remember thinking I could still keep running if I didn’t feel like I was going to fall over. Legs were still feeling strong. I’ve felt the fatigue of a marathon before, but not these symptoms. It was scary. I slowed down, walked it out, fueled up and finally picked up pace and ran it in. Missed BQ by 1:02 but learned a valuable lesson about proper fueling.

  2. Wazz July 3, 2013 at 12:58 am #

    ALL carbs eaten after a workout will help in refilling glycogen store, not just the ones in the tiny little magic window. As long as you eat enough carbs before your next workout you’ll be refilled up to the brim (and then some)

    • Angie August 3, 2013 at 12:44 pm #

      You’re right that the muscles will work to refill glycogen stores even after the “magic” window. However, waiting longer than two hours to eat results in 50 percent less glycogen stored in the muscle. The reason for this is that carbohydrate consumption stimulates insulin production, which aids the production of muscle glycogen. Research shows that combining protein with carbohydrate within thirty minutes of exercise nearly doubles the insulin response, which results in more stored glycogen. The optimal carbohydrate to protein ratio for this effect is 4:1. Subjectively I always find that I’m less hungry (and crave fewer carbs) in the hours after exercise if I’ve used a recovery drink.

  3. Christy July 9, 2013 at 4:54 am #

    I major major bonked on my first and only marathon. The Missoula Marathon that you guys just did, but I did it last year. You guys were the source of inspiration for running it and this podcast makes me want to do another and try it again. Here are my recap links and thoughts about it if you want to read about it 🙂

    I love your podcast, thanks for all your hardwork!

    • Angie August 3, 2013 at 12:38 pm #

      Hi Christy. Thanks for sharing your blog post about the Missoula Marathon. You should definitely try it again. Learning things the hard way is never fun but I’m sure you’ll have a much better experience the second time around. Happy running!

  4. Cheri July 31, 2013 at 9:09 am #

    I ran my first half-marathon last December. I made the false assumption that fueling was for fast, “real” runners not slow ones like me. Well needless to say I paid dearly for it. If anything I needed it more than the fast people becasue I was out there for much longer. Without realizing it, I essentially bonked on all of my long training runs and in the race as well. It took me a long time to recover from training runs and probably a month after the race before I felt 100%. I have certainly learned my lesson and I am carefully planning fueling strategies for my next half in October! Thanks for all of the great information. My kids and I love your podcast.

    • Angie August 3, 2013 at 12:46 pm #

      Hi Cheri. I’m glad that you and your kids have been enjoying the podcast. It sounds like you learned about fueling the hard way during your first marathon. I always find that I recover faster when I’ve fueled properly during and after my marathons (and long runs). Best of luck in October!

  5. Ivy November 2, 2014 at 8:03 am #

    I had just finished the rock and roll LA half marathon, I was doing well until mile 9 or 10, it was uphill, and I thought that I would run a little up it, but my left calf said no. So I stretch and walk the rest of the hill. My plan than was to run the downhill, it was going to be downhill all the way to the finish line, but I must have been bonking, because I couldn’t even run downhill. I was disappointed since up until now my time and pace were on target. Fast forward to half a mile till the finish line, the cramp on my left leg right behind the knee (felt like the size of a golf ball) made it impossible to even walk, luckly there was an aid station right there, and they stretched me out enough for me to get to the finish line. I was so sad watching the runners run past. I was taking all the water at the stations, and I had on my compression sleeves.

  6. william September 8, 2020 at 1:28 am #

    This is a great article. I only ever bonked once during my after work 7K runs. That made me scratching my head as I ate, slept, stretch as usual. How come? Then it dawn on me. I’m a 2-4 cups coffee guy and that day I remember I was in a lengthy meeting and seemingly got by w/o a single cup of Joe!

    And when I hit the road I had a mild caffeine withdrawal. Yeah, that’s my mystery fatigue culprit.

    Anyone experience this before? If yes, maybe the author would have another reason to add for bonking causes.

    • Angie Spencer September 8, 2020 at 11:37 am #

      Hey William, Thanks for sharing your experience! Being in a caffeine deprived state could definitely affect your energy levels. I probably didn’t think of this because I’m not a coffee drinker (crazy, I know) 🙂 I’ve heard of other people who run after work who’ve bonked because it had been a long time since lunch and they were hungry (but didn’t want to eat before the run and get GI distress).

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