There you are, running along, doing fine. Suddenly, your legs feel like they’re encased in cement. Your energy is gone, and you’ve quickly dropped to the back of the pack. What you’ve just experienced is commonly known as hitting the wall, or “bonking.” It’s a frustrating feeling, and one that—if you’re a distance runner—you’ve probably experienced first-hand. But, fortunately, with the right preparation, it can be avoided.
Why We Bonk
Scientifically speaking, hitting the wall happens when your body runs out of glycogen. According to Harvard-MIT scientist Benjamin Rapoport, when this occurs, your body is forced to start burning fat as fuel. This can drop your pace by roughly 30 percent. This also creates ketones—the byproducts of fat metabolism—to build up in the body, which causes pain and fatigue.
You’re not being realistic.
To keep the above from ruining your run, you need to train properly. Also, you need to ensure that you’re getting enough fuel before and during runs. How fast you burn that fuel, however, is largely dependent on your pacing and personal capabilities. For Rapoport, that means listening to his body and setting a target pace that’s sustainable. If you start too fast or set an unrealistic pace, you’re more likely to pull up short of the finish line.
Identifying your personal VO2 max, or aerobic capacity, can be useful here. By measuring how much oxygen your body can transport to your muscles during exercise, you can better calculate your ideal pace. Doing so accurately usually requires a treadmill stress test. However, you can also reasonably estimate it by tracking your heart rate during a steady run.
You’re not fueling properly.
“Hitting the wall is typically a multi-pronged problem,” says Aaptiv Trainer Wes Pedersen. He notes that some of the biggest culprits are improper nutrition and lack of calories, as well as lack of hydration and electrolytes. When you’re not fueled properly, your legs lose their ability “to continue with rapid depolarization due to fatigue,” he adds. “Your muscles become less powerful, so your stride becomes less springy, meaning that your foot loses the ability to rebound quickly off the ground.”
He mentions that, when your foot is on the ground, you are actually slowing yourself down and essentially braking. In order to run at your most efficient pace, you want to spend as much time off the ground as possible, which can be accomplished via those springy strides.
Three Tips for Avoiding the Wall
According to Pederson, avoiding that dreaded bonk typically boils down to three factors: nutrition, hydration, and training. Below, he outlines strategies for using each to your advantage to ensure that you can keep on running.
In addition to getting plenty of carbs before exercise, it’s also important to consume carbs and calories throughout your run. The American College of Sports Medicine advises consuming between 30 and 60 grams of carbohydrates per hour during endurance exercises that last longer than one hour.
Pedersen suggests using the standard calculation that you burn approximately 100 calories per mile, and that your body has enough stored glucose to go continuously for about an hour. Replenishing those carbs and calories on longer runs will not only make you feel better but also feed your body’s furnace so you’re able to keep moving forward at your desired pace.
“Use your long training runs to practice what to eat come race day,” says Pedersen. The longer your run, the more fuel you’ll need to bring with you. Such fuel can include sports drinks, gels, blocks, bars, or anything else you like. As Pedersen mentions above, it’s smart to adhere to your nutrition regimen during training runs, so that you’re not surprised on race day by the volume of food or stomach distress.
“Stay ahead of your hydration needs,” says Pedersen. “The general rule is that if you’re thirsty, then you’re already slightly dehydrated. Aim to replace electrolytes and even carbohydrates with your beverages.”
He notes that everyone has a different sweat rate, so it can be helpful to weigh yourself before and after long runs. Doing so will help you understand how much sweat you’re losing and how much fluid you need to replace during your runs. “I sweat more than average,” says Pedersen, “so I drink small amounts often and then use salt tabs to keep my electrolyte levels high.”
You’d never just go run a marathon without training—well, you shouldn’t. You’ve got to prime your body for logging that many miles. Pedersen says that long training runs help your legs get used to functioning when they are fatigued. But, you don’t have to spend all of that time pounding the pavement.
“One strategy that I employ is spending time on the spin bike, up to an hour, before doing a mid-distance run,” he says. “This will simulate the longer duration of a long distance run without the increased volume and impact force that running provides.”