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Beginner's Guide to Stamina and Fitness: Energy for the Active Muscles

By Edited Apr 4, 2016 0 0

Endurance Exercise Builds Stamina

Stamina Facilitates the Attainment of Fitness

Fitness is Good for a Healthy Lifestyle.
Credit: Infomatique's Photostream


 Endurance and stamina are the primary substrates for physical fitness.  To achieve fitness one must apply exercise protocols that include activities of suitable intensities and durations.  These activities are powered and sustained by metabolic energy, primarily from glucose and fatty acids. The brain, the active muscles and numerous other organs must be reasonably nourished for stamina and fitness to occur.  Consequently, it is not surprising that attainment of stamina and fitness involves complex physiological processes.  It is surprising, however, that the liver is pivotal for the management of the metabolic fuels that support stamina and fitness.


 Physical stamina reflects the  adjustments made by your body that enables you to withstand long periods of physical activities. Time, patience and endurance exercises are necessary for the development of stamina. Endurance exercises are the adequate stimuli for the development of stamina. Patience is needed for performing the endurance exercises which can be tedious at times. Time is necessary for the body to make the structural,  biochemical and physiological changes that support stamina and  fitness. At the core of these changes is proper management and utilization of energy.

Energy Management and Utilization

 Glucose and fatty acids are the primary metabolic fuels that your body manages during  physical activities.  An increase in physical activity raises your body's demand and utilization of these metabolic fuels.  Your body makes numerous and complex adjustments to meet these  metabolic demands. More metabolic fuels are supplied to organs, like the skeletal muscles, that need more energy during increased physical activity, and reduced amounts are made available to organs, like the intestines, that need less.  Glucose is spared for organs, like the brain, that preferentially or exclusively use only glucose for energy.  Glucose utilization by the skeletal muscles decline while utilization of fatty acids go up.  More insight into the management of these metabolic fuels can be gained by examining the changes in their utilization by some of the major organs during increases in physial activities.

 Energy Utilization by the Brain

 The brain uses only glucose for energy.  Plasma glucose level must be maintained at adequate levels for the brain to function properly. (In some cases, such as prolonged starvation, the brain can use others sources of fuel such as ketone bodies.)  Glucose remains the source of energy for the brain both at rest and during periods of increased physical activity. The uptake and utilization of glucose by the brain is constant at rest and during exercise. The uptake is  independent of insulin, a notable contrast to the heart, skeletal muscles and numerous other organs and tissues. If blood glucose were to drop sharply during exercise, one can lose consciousness.  Consequently, exercise would cease and stamina would not be achieved. The body must maintain adequate plasma glucose levels to support the functional integrity of the brain and exercise.

Energy Utilization by the Heart

The heart can use any available metabolic fuel as the source of energy. Its major fuel of oxidation, however, is fatty acids (1). Therefore, availability of metabolic fuels for the heart is not as restricted as the metabolic fuels for the brain. Insulin plays a minor role in glucose utilization by the heart compared to its role in the skeletal muscle. Glucose transported from plasma is used primarily for glycogen synthesis. Glycogen (a chain of glucose molecules) is an intracellular stored energy from which glucose can be obtained during intense physical activities (2).  Therefore, the heart selectively uses fatty acids for energy, but it will use glucose from glycogen stores during periods of high intensity exercises when fatty acids can not provide enough energy fuel to the heart.

Energy Utilization by the Skeletal Muscle

Energy utilization by the skeletal muscles is more complex than that of the heart and brain. The primary fuel for the skeletal muscle is plasma glucose under normal conditions but it will switch to fatty acids as the primary fuel during exercise of moderate intensity . This is important because the skeletal muscles are big users of energy, and they can deplete the plasma glucose during exercise if they use plasma glucose as the primary source of energy. Under such a condition, the brain would be starved.

Glucose utilization is dependent on insulin in the skeletal muscle, but this dependency declines during exercise. This is a significant difference from the brain where glucose utilization is independent of insulin. This difference makes it possible to maintain adequate plasma levels of glucose to assure that the brain remains functionally intact during exercise.

The three major changes in the skeletal muscles that increase endurance and stamina are shown below.

l  Increase in the size and number of the mitrochodria (1). Mitochondria are the sites of oxygen-dependent energy production. Increase in energy production (production of adenosine triphosphate, ATP) supports sustained activity during exercise.

l  Increase in glycogen storage in the muscles during rest and diminished activity. Glycogen can be broken down to release glucose during increased energy demand. This source of energy is very important during high intensity exercise when aerobic metabolism is insufficient to support physical activity. A small amount of energy can be obtained from glucose, anaerobically, during such high intensity exercise.

l  Increased vascularization of the muscles (increase in the size of the capillary beds). This facilitates the delivery of oxygen and nutrients to the active muscles.

Source of Glucose During Exercise

The liver is the primary source of plasma glucose during exercise. Glucose released from the liver is important for maintaining adequate plasma glucose levels. Glucose from the liver occurs largely by the break down of glycogen (glycogenolysis).  Glycogen is a stored energy fuel in the liver and the released glucose from this source helps to maintain adequate blood level of glucose which helps the brain to remain functionally sound.

Energy from the Fat Cells

Fat is an important source of metabolic fuel for the skeletal muscles during endurance exercises, such as jogging.   Fat is the  largest source of stored energy in your body. There is enough fat in the well-conditioned lean athlete to run over 20 marathons. During exercise of moderate intensity, fatty acids become the primary source of energy for the skeletal muscles and glucose becomes the secondary source. The opposite is true for high intensity exercises.

Fatty acids are not readily available to the muscles in adequate amounts during high intensity exercises because they go through too many steps that slow down their  availability for energy production.  Endurance training improves the processes of delivering energy from fatty acids to the muscles.

Role of Insulin in Energy Management During Exercise

Insulin is the most important hormone for energy management that facilitates the attainment of stamina. During periods of increased physical activity plasma level of insulin declines, and  insulin sensitivity rises.  Some of the consequences of these changes in insulin are shown below.

 l  Increased degradation of fat from the storage sites and release of fatty acids into the plasma. This makes fatty acids available for oxidation by the active muscles during exercise.

l  Diminished utilization of plasma glucose for energy metabolism by the skeletal muscles

l  Increased utilization of fatty acids  by the active  muscles

l  Maintenance of adequate plasma glucose level because of the preferential utilization of fatty acids by the skeletal muscles.  This spare glucose for the brain. The plasma glucose level is usually maintained at normal or near normal levels during moderate exercise in the non-diabetic individuals.



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  1. . J.F. Horowitz and S. Klein "Lipid Metabolism During Endurance Exercise.." American Journal of Clinical Nutrition. 72 (2000): 538s -563s.
  2. . G. W. Goodwin et al "Preferential Oxidation of Glycogen in Isolated Working Rat Heart." . Clin. Invest. 97 (1996): 1409 - 1416.

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