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Wednesday, August 9, 2017

Metabolic energy pathways provide the energy for your workouts

The human body utilizes three metabolic energy systems to replenish adenosine triphosphate (ATP). ATP provides energy for all movements, exercises, and muscular activity. These energy systems include the phosphagen system, the glycolysis system, and the oxidative system. The phosphagen system, and the glycolysis system are anaerobic, and do not require oxygen. The glycolysis system is subdivided into two sub systems, which are fast glycolysis and slow glycolysis. The oxidative system is aerobic and requires oxygen. The three macronutrients, proteins, fats, and carbohydrates, are the main food sources for energy. Carbohydrates are the only macronutrient that can be utilized for energy, without oxygen. All physical movements and exercises require the use of one or more of the three energy systems. The type and duration of the movement, or exercise determines which energy system will be utilized (Coburn & Malek, 2012).

For short duration, high intensity, explosive, power movements, the phosphagen system is utilized. The phosphagen system is used for about the first ten seconds of activity. Examples of activities that rely on the phosphagen system include short sprints, plyometric actions such as box jumps, and powerful actions such as an Olympic lift. Even if the physical activity lasts longer than ten seconds, the phosphagen system acts first. ATP and creatine phosphate are the primary elements used by the phosphagen system. The enzymes myosin adenosine triphosphatase (ATPase) and creatine kinase play a role in the breakdown, and regeneration of ATP. The ATP is broken down to release its energy. Myosin ATPase facilitates the breakdown of the ATP into adenosine diphosphate (ADP) and phosphate. This catabolic action releases the energy from the ATP so that it can be utilized by the body. Once this occurs, the ATP must be regenerated. The adenosine diphosphate (ADP) levels rise, which activates the creatine kinase. The creatine kinase breaks down the creatine phosphate, which provides a phosphate group to combine with the ADP. This anabolic sequence produces ATP. This cycle repeats rapidly, and provides a high rate of energy, for a very short duration (Karp, 2009).

As physical exertion moves beyond the first ten seconds, the glycolysis system activates. Glycolysis breaks down carbohydrates from blood glucose, and stored glycogen to form ATP. The glycolysis system provides energy for high intensity activity, which lasts up to two minutes. For intense activities lasting for about thirty seconds, fast glycolysis is used. With fast glycolysis, pyruvate is converted to lactate, to produce ATP. The lactate can be then sent to the liver to be converted back into glucose through the Cori cycle. As the activity goes beyond thirty seconds, slow glycolysis begins to take over. With slow glycolysis, the pyruvate is sent to the mitochondria, through the Krebs cycle, to produce more ATP. This occurs from approximately thirty seconds to two minutes of intense exercise. The intensity of the activity or exercise will decline as the sequence moves through the phosphagen system, to fast glycolysis, and on to slow glycolysis. Examples of activities that could utilize the glycolysis system include a four hundred yard run, a high repetition set of a barbell exercise, or a full court press in basketball (Kelso, 2017).

At approximately the two minute mark and beyond, the oxidative system takes over from the slow glycolysis system. This is where the activity switches from anaerobic to aerobic, and oxygen is required. The oxidative system utilizes both carbohydrates and fats to fuel ATP production. Protein is not usually metabolized as fuel, unless the activity lasts for over ninety minutes. With the oxidative system, ATP production can occur through the Krebs cycle, electron transport chain, and beta oxidation. In addition to being used to produce ATP for longer, slower activities, the oxidative system also works while at rest. While at rest, fats are predominantly used as fuel. Once activity begins, it switches to carbohydrates. Once glucose levels start to become depleted, the system reverts back to fats. As the glycolysis system fades and the oxidative system takes over, the pyruvate is sent to the mitochondria, and converted into acetylCoA. The acetylCoA is then sent through the Krebs cycle for more ATP production. As glucose levels decline, fats are metabolized through the electron transport chain, and beta oxidation process, to be converted to acetylCoA. The acetylCoA then enters the Krebs cycle to produce more ATP. If the activity is very long in duration, protein can assist in energy production through gluconeogenesis and the Krebs cycle. The protein is broken down into its amino acids, which are then either converted to glucose or acetylCoA. Protein breakdown is usually minimal, as glucose and fats are normally present in sufficient quantities. for most activities. Examples of activities utilizing the oxidative system include rest, medium to long distance runs, triathlons, and manual labor during a work day (Pegg, 2013).


Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.

Karp, J. (2009). The three metabolic energy systems. Idea Fit. Retrieved from

Kelso, T. (2017). Understanding energy systems. Breaking Muscle. Retrieved from

Pegg, A. (2013). What is the oxidative energy system? Steady Strength. Retrieved from

Eric Dempsey
MS, NASM Fitness Nutrition Specialist

Saturday, August 5, 2017

The sliding filament theory and muscular contraction

Muscular contraction begins with the nervous system sending a stimulus to the muscle fibers. This stimulus occurs at the neuromuscular junction. There is one neuromuscular junction for each muscle fiber. The neuromuscular junction contains structures that include the axon terminal of the neuron, the motor endplate, and the synaptic cleft, or neuromuscular cleft. From this point, the actions of the sliding filament theory begin. The actin and myosin filaments slide past each other, causing the muscle to shorten or lengthen. The filaments do not change in length during this action. Excitatory neurotransmitter acetylcholine (ACh), is released at the neuromuscular junction, after an action potential passes along the length of a neuron. ACh is released into the synaptic cleft between the axon terminal of the neuron and the muscle fiber. This occurs in direct response to the action potential (Coburn & Malek, 2012).

From that point, the Ach binds with ACh receptors, on the motor endplate of the muscle fiber. This occurs after the Ach moves across the synaptic cleft. This causes another action potential to be created. This new action potential moves along the sarcolemma of the muscle fiber. T-tubules are utilized by this action potential to travel to the interior of the muscle fiber. This movement causes the release of stored calcium from the sarcoplasmic reticulum. The calcium is released into the sarcoplasm. Once in the sarcoplasm, the calcium moves to the troponin molecules. The calcium then binds with the troponin molecules. These the troponin molecules are located along the length of the actin filaments. The shape of the troponin changes after the calcium binds to it. Tropomyosin is attached to the troponin. This change in shape causes the tropomyosin to expose the binding sites on actin, to the myosin head (Krans, 2010).

The exposed binding sites on the actin are able to attach to the myosin, forming a cross bridge. This myosin head of attachment pulls the actin filament toward the center of the sarcomere. The effort of the myosin pulling on the actin depletes the energy of the myosin. The depleted myosin must then detach from the cross bridge and reenergize itself. This requires a new adenosine triphosphate (ATP) molecule to be bound to the myosin. Once the ATP molecule is bound to the myosin, it can then detach and energize itself. The energizing of the myosin comes from the enzyme myosin adenosine triphosphatase (ATPase). The ATPase splits the ATP molecule. This energizes the myosin and allows for the cross bridge sequence to occur again. This sequence can continue as long as the muscle fiber is being stimulated to contract, by its motor neuron. The myosin pulling the actin toward the center of the sarcomere shortens the muscle. This process is the muscle contracting. The success or failure of this sequence is determined the external forces pulling against the cross bridge (Szent-Györgyi, 2004).

There are three basic types of contractions that can occur when the sliding filament theory is activated. Muscle contractions are usually pulling against an external force such as a barbell. When the myosin is pulling the actin through the cross bridge, the resistance determines the outcome. The contraction generates more force than the resistance, causing a concentric contraction. The external force is greater than the contraction force, causing a lengthening of the muscle. This is an eccentric contraction. The contraction force is equal to the resistance force, which stalls movement. This is an isometric contraction. The muscle will always attempt to shorten as the myosin pulls on the actin. The resistance level will cause one of the three types of contractions to occur. ATP is the primary fuel source for this sequence (Lefkowith, 2014).


Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.

Krans, J. (2010). The sliding filament theory of muscle contraction. Nature Education. Retrieved from

Lefkowith, C. (2014). What does it all mean: Concentric, eccentric and isometric. Redefining Strength. Retrieved from

Szent-Györgyi, A. (2004). The early history of the biochemistry of muscle contraction. The Journal of General Physiology. Retrieved from

Eric Dempsey
MS, ISSA Master Trainer

Wednesday, August 2, 2017

Health & Fitness Radio Tuesday: Meal Planning, Vegetable Oil and More

Health and Fitness Radio Tuesday!

This episode's topics:

Meal planning, home gyms, vegetable oil, and listening to your body.

Also talked about advances in cardiac preventative care.

Eric Dempsey
MS, ISSA Master Trainer