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s15e5 Metabolic Systems, Francis Fessler.jpg

Metabolic Systems

This week we will discuss Metabolic Systems with our guest,  Francis Fessler.


A certified personal trainer and conditioning coach for the last 25 years, Francis Fessler has built a career by designing programs and coaching professional and amateur athletes, celebrities, business professionals, parents and children to achieve their wellness and fitness goals. Throughout his time in the health and wellness world he evaluated, tried and tested countless ‘diet and nutrition plans’ and could not find one that had consistent results for both women and men- so he built one. Francis created F2 Wellness and the highly successful F2 Weight Loss Program for not only his clients, but for anyone looking for a simple, successful and sustainable weight loss program.


Have you ever experienced tiredness in your muscles while working out and you couldn’t continue exercising after a certain point? Have you wondered what can make you lift heavier weights or run longer than you can today? If you have, understanding the mechanism of the body's energy system can help you find answers to these questions.

Three metabolic pathways generate the energy required to perform an exercise: the phosphagen pathway, the glycolytic pathway, and the oxidative pathway, together known as the energy systems. Although your body is always using all three simultaneously, depending on the intensity and duration of the exercise, your body will choose from which pathway it will use the largest percentage of its energy.

As you may know, all energy used by our bodies is generated from the breakdown of food and drink. The three macronutrients are protein, carbohydrate, and fat. Those are metabolized to create adenosine triphosphate, which is the source of fuel for all body processes, including muscle contraction.

Unfortunately, the supply of readily available ATP is very limited. It means our bodies constantly have to produce the substance; otherwise, muscle contraction would stop. This re-synthesis of ATP is done by the three energy systems.

The first 10 to 20 seconds of high-intensity physical activity is fueled by the “ATP-CP,” also known as the phosphagen energy system. Once the available ATP is used up, which occurs in a few seconds, a molecule called phosphocreatine is used to re-form ATP in the muscle. This energy system operates very quickly and can bring the highest output of the three systems. However, it is limited by the availability of creatine phosphate, which is usually consumed within 15 seconds.

Your body can eventually refill these stores when you rest. This is why this system is most active for athletes who engage in short bouts of very intense, explosive movement, such as a the 50-meter dash or powerlifting. This is also the reason we can sprint at full speed for only a few seconds or lift maximum loads only 1-2 times before requiring rest or a decrease in exercise intensity using another metabolic pathway.

The second pathway, the glycolytic pathway, is the primary energy system used for exercise lasting from 15 seconds to three minutes. People running an 800-meter event, for example, use this pathway the most. This energy system uses the glucose stored in the muscle, broken down primarily from carbohydrates, to form ATP. The benefit of this pathway is that it kicks in quickly, but it doesn’t make very much energy; it can only supply a maximum of about three minutes of energy. This pathway is responsible for the buildup of lactic acid in our muscles, which contributes to fatigue.

For exercise lasting longer than three minutes, the oxidative pathway is used. Unlike the others, this energy system requires oxygen. The increase in respiratory rate meets the oxygen demand during physical activity. The oxidative system is slow, but is also the most efficient. Using fat as its primary energy substrate, it produces enough ATP to sustain longer duration activities, but only at submaximal exercise output. It means fat is the predominant fuel source used during low to moderate-intensity activity, like biking or jogging long distances.

Now you are more knowledgeable on how your body relies on each of these systems working together to meet the energy demands needed for activities of daily living and exercise.

The system your body will use primarily depends on the type of activities you mostly engage in. The more you train in that particular type of exercise, the better your body adapts to being able to efficiently use that energy system.

For example, individuals who have trained in powerlifting can store more phosphocreatine and ATP than a marathon runner or sedentary individual. On the other hand, endurance-trained individuals have better ventilation ability, maximizing oxygen availability for the oxidative pathway. At the end of the day, consistency is key. If you want to excel at a particular type of exercise, just keep doing it and in time, your body will adapt. (Credits: US Army)




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