When the Body Bonks, You Listen
It happens suddenly; after exerting their bodies for two hours or more, athletes feel as if they've sprung a leak in their foot that drains their bodies of energy. Some quit. Some limp to the finish, making spectators wince with sympathy pains.
Skiers and bikers call it "bonking." Runners prefer the traditional "hitting the wall," although phrases such as "carrying the piano," and "throwing out the anchor" also have been used to describe the sensation of how the body sometimes crashes in a long-distance sporting event.
Tom Wells, head of the University of Alaska Fairbanks physical education department and a professor of exercise physiology, is a veteran bonker. He's run nine marathons, and has hit the wall in each one. What sets him apart from most athletes is that Wells can describe in detail what's happening to his body when that terrible sensation of fatigue hits after running about 22 or 23 miles in a marathon.
Over the course of a marathon run, the human body burns about 2,600 calories. Since it's difficult to gain that many calories from eating during the run, the energy required for the marathon must come from within. Muscular activity is fueled in large part by stored glycogen within muscle cells, Wells said. The human liver converts digested carbohydrates from such foods as pasta and potatoes into glycogen, which enters muscle cells with the help of insulin. Glycogen remains stored in muscle cells like money in the bank.
After two hours of constant exercise, the stored deposit of glycogen within muscle cells may begin to run dry. At this stage, exhaustion sets in as quickly as one can say "bonk." Bonking occurs for any number of reasons, including inadequate training or failure to maintain blood sugar levels by drinking juice or other sports beverages along the way.
The bonked body attempts to compensate by drawing glycogen from the liver and sugar from the blood in the form of glucose. After these withdrawals on the body's energy saving account, the runner can succumb to hypoglycemia, or low blood sugar.
Without glycogen to power the process, muscle cells can't contract effectively, and larger muscle groups stop working. Runners react in several ways when muscle glycogen runs out; many give in to the inner voice that's screaming at them to stop. Others continue on, using different muscle groups, such as hip abductors, which still contain stored glycogen. This results in some of the pitiful-looking gaits people sometimes use to carry themselves over the finish line.
Wells said that everybody has a point at which they'll bonk, but there are ways to prevent muscle glycogen stores from vanishing. One is to engage in long-distance workouts that teach the body to burn fat rather than carbohydrates. Another bonking prevention tool, known as "carbo-loading," is to store carbohydrates a few days before a race by eating foods rich in carbohydrates, such as pasta and fresh vegetables, and to eat complex carbohydrates, such as breads and other starches, which release energy much more slowly than simple carbohydrates such as processed sugar. Along the course, Wells said it's a good idea to conserve blood glucose by bolstering it with sports drinks or other food high in stored carbohydrates.
Wells, who will soon move to Seattle after 11 years at UAF, said he's gearing up for yet another marathon this summer. He hopes to avoid the unpleasant sensation of hitting the wall, but he's also a realist, knowing he won't have much time to train between moving and looking for a new job.
"I'll probably bonk again," he said.
