Carbohydrates serve as the major food fuel for the metabolic production of ATP (adenosine triphosphate) and is stored as glycogen in both the muscle and the liver. Why is ATP important? When a muscle fiber contracts, the energy used to drive the contraction comes primarily from a substance within the cell called ATP. ATP is utilized by all muscle fibers (Muscle Fiber Types) as well as in both aerobic and anaerobic metabolic pathways. (Energy Systems & Greater Muscle Mass = Higher Metabolism)
Glucose is both readily available in your body as blood glucose or in storage from as glycogen. Glucose trapped in one muscle group cannot be removed to help meet the energy needs in another depleted muscle group. So, energy stores may be abundant in your upper body muscles, but they cannot help out your lower body muscles during a challenging lower body workout. However, you can build up your glycogen storage and thus improve exercise performance by being consistent in your workouts and the types of workouts you perform (such as high intensity interval training).
An article from the March 2012 issue of IDEA Fitness Journal discusses why the body prefers carbohydrates as exercise intensity increases:
As exercise intensity increases from rest to near-maximal levels, the body gradually shifts to using glucose and glycogen as the predominant sources of ATP (Kraemer, Fleck & Deschenes 2012). From a metabolic standpoint, in the mitochondria more ATP can be produced aerobically from the breakdown of carbohydrate as opposed to fat. Most important, however, is the fact that as exercise intensity increases, many more fast-twitch muscle fibers are recruited, and (because of their enzymes) these fibers are much more suited to using carbohydrates for the needed ATP production. In addition, higher-intensity exercise stimulates epinephrine production, which also enhances carbohydrate metabolism (Kraemer, Fleck & Deschenes 2012).
Glucose is both readily available in your body as blood glucose or in storage from as glycogen. Glucose trapped in one muscle group cannot be removed to help meet the energy needs in another depleted muscle group. So, energy stores may be abundant in your upper body muscles, but they cannot help out your lower body muscles during a challenging lower body workout. However, you can build up your glycogen storage and thus improve exercise performance by being consistent in your workouts and the types of workouts you perform (such as high intensity interval training).
An article from the March 2012 issue of IDEA Fitness Journal discusses why the body prefers carbohydrates as exercise intensity increases:
As exercise intensity increases from rest to near-maximal levels, the body gradually shifts to using glucose and glycogen as the predominant sources of ATP (Kraemer, Fleck & Deschenes 2012). From a metabolic standpoint, in the mitochondria more ATP can be produced aerobically from the breakdown of carbohydrate as opposed to fat. Most important, however, is the fact that as exercise intensity increases, many more fast-twitch muscle fibers are recruited, and (because of their enzymes) these fibers are much more suited to using carbohydrates for the needed ATP production. In addition, higher-intensity exercise stimulates epinephrine production, which also enhances carbohydrate metabolism (Kraemer, Fleck & Deschenes 2012).
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