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Posted by Barry on Apr 19, 2011

Why Do We Bonk?

You are on your third set in Plyometrics and you feel pretty good. You feel like you are going to have a good day. All of a sudden, within the space of five minutes or so, you go from feeling pretty good to awful. Your arms and legs feel heavy and it becomes hard to catch your breath. You are forced to push pause and slow down but really you feel like you need to lie down. What happened? How can you go from feeling good to feeling terrible in such a short period of time? It is what I refer to as “bonking”.  It happens to all of us at some point during our P90X journey and in this article I will explore the physiology of bonking and discuss what you can do to avoid it.

Your body stores carbohydrates in muscle and the liver in a form that is referred to as glycogen. During exercise, your working muscles can tap these glycogen stores for energy. The working muscles can also use circulation blood sugar, which essentially comes from the liver, glycogen, and circulation fat, which comes from our body fat stores. This gives your working muscles three fuel depots to draw from during exercise. Next we will discuss each depot briefly and then wrap up with some recommendations on how to avoid bonking.

Circulating Blood Sugar: Although blood sugar can be used by working muscles during exercise, it is a secondary energy source only. This is because glycogen is located inside the muscle cells and therefore is more readily available than blood sugar. Blood sugar alone cannot provide enough carbohydrates to sustain a high power output during exercise. The liver, the primary source of blood sugar, contains only about 100 grams or so of glycogen compared to the 600 grams or more in skeletal muscle.

Body Fat Stores: Your body has enough stored fat to supply your muscles with the energy for a dozen or so P90X workouts. So why do care about stored carbohydrates? The reason that carbohydrate burning is key is that fats burn more slowly than carbohydrates. Since fat burning is slower than carbohydrate burning, we cannot sustain as high a power output when using fats alone. If you are going to step through the workouts, you can get away with burning primarily fats because your power output is low. However, as soon as you start “Bringing It”, your power output increases and you need a faster burning fuel – carbohydrates – to sustain the higher energy burn rate requirements. The optimal situation is to use a combination of fats and carbohydrates while working out. In fact, optimally, you would want to burn as little carbohydrate as possible to sustain your work rate. Remember, you have plenty of fat to use for energy, but carbohydrates can be limiting.

Muscle Glycogen: A physically fit individual typically has enough stored muscle glycogen to sustain 1-2 hours or so of moderate intensity exercise. Therefore, with a little added fat utilization, trained individuals can usually finish the P90x workouts without running out of carbohydrates. Most of us as we are working to get into shape typically store 1/2 to 1/3 as many carbohydrates as a physically fit individual.

So, let’s review the physiology part briefly. When you run out of stored muscle glycogen, you bonk! The reason that you bonk is that the only sources of fuel remaining are stored fat and blood sugar. The stored fat is burned too slowly to process the necessary energy to continue the intense workout and circulation blood sugar is inadequate in terms of total amount of delivery rate to sustain a high work rate. As a result, you must revert to a lower work rate (Push Pause and take longer breaks) in order to continue.

So, how do we keep from bonking? You can see from the information above that having carbohydrates in your diet is critical.  Ideally you should consume 300-400 carbohydrates one to two hours before your workout to help load up the muscles with glycogen.

Also, you can consume carbohydrates during the workout. This can help to keep blood sugar levels from dropping and can help prevent liver glycogen depletion. Remember, your body cannot continue the workout by burning fats alone. Consuming carbohydrates during the workout can slow the rate of muscle glycogen depletion and help you sustain a higher work rate while burning stored body fat. The trick is to slow the rate of glycogen depletion so that you can complete the workout with high intensity.

However the very best time to consume carbohydrates is right after training. The muscles soak up carbohydrates like a sponge the first two hours after training. This is the whole reason the P90X Results and Recovery Drink was made. Consuming carbohydrates right after training can speed glycogen repletion and muscle recovery.

A good fuel consumption plan is critical. You must start the workout with your muscles already packed with glycogen and refuel along the way to delay glycogen depletion. Being proactive is the key. Once you bonk, it is too late.


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