Energy Systems

 In Exercise

Understanding Your Bodies Systems

One of the lesser talked subjects is our bodies energy system, how it operates, its different parts and how it relates to reaching a persons fitness goals. Many of us try to incorporate some form of cardiovascular activity into our workout program, this could be circuit training, high intensity exercise (HIT), sprints, bike riding, running etc. but is the form of training you’re choosing really the best for you to reach your fitness goals? Or is it working against you?

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Our Fuel

It’s important to understand how our body provides energy to function properly and maintain health. Exercise forces the body to meet unique and demanding requirements to provide energy and remove metabolic by-products. In order to supply our muscles with the energy they need to perform exercise our bodies must break down food. However, before food can be transformed into energy it must be broken down into smaller units called substrates. The energy contained in these substrate molecules is then chemically released in cells and stored as a high-energy compound called ATP (adenosine triphosphate).

ATP is the primary source for immediate energy. When the chemical bonds that bind ATP are broken energy is released providing the energy for things such as muscle contraction. Once the chemical bonds that bind ATP are broken the remaining molecule is called ADP (adenosine diphosphate). The body must then have enough energy to reattach a phosphate group to the ADP molecule (creating ATP) restoring the ATP levels back to normal, giving us energy the to perform.

The Three Energy Systems

The three ways the body creates ATP are using the ATP-PC System, through Glycolysis, and using the Oxidative System. Which system(s) the body uses depends on the intensity/duration of exercise. Once an ATP molecule has been used the body must replenish it to provide more energy to the body.

By using a phosphate and other high energy molecule (phosphocreatine) to an ADP molecule the body can create a new ATP molecule. This system is the ATP-PC system, it is the simplest and the quickest of the energy systems. This system is the primary energy system used during short, high intensity exercise such as a sprint. This energy system is capable of providing energy for roughly 10-15 seconds of high intensity exercise. It is also used at the beginning of slower activities until the other systems take over.

Glycolysis is another means of producing ATP by breaking down glucose/glycogen that is stored in our muscles. In order to turn glucose/glycogen into ATP the glucose/glycogen must first be turned into glucose-6-phosphate. The body must then use 1 ATP molecule to convert glucose-6-phosphate into ATP molecules. This creates 2 ATP molecules but it also creates a by product, lactic acid if the process is done without oxygen, or pyruvic acid if the process is done with oxygen. This system is typically limited to around 30-50 seconds in duration. Most workouts place a greater emphasis on this system than others due to typical repetition ranges.

The Oxidative System is a process that uses substrates with oxygen in the creation of ATP. There are three separate processes in this system that contribute to ATP production, aerobic glycolysis, the Krebs Cycle, and the Electron Transport Chain (ETC). Pyruvic acid which is the end product of aerobic glycolysis is converted into an molecule called acetyl coenzyme A which contributes to the second process of oxidative production of ATP, the Krebs Cycle. The complete oxidation produces 2 ATP molecules and by-products hydrogen and carbon dioxide. The hydrogen released during this process is then combined with other enzymes in the third process (ETC) and provides energy for the oxidative phosphorylation of ADP to ATP. This can create 35-40 ATP molecules from a single glucose molecule.

Fat can also be “burnt” aerobically. The first step is the breakdown of triglycerides into fatty free acids and converting them to acetyl coenzyme A which then enters the Krebs cycle and ultimately produces ATP. To oxidize fat the body needs more oxygen than it does with carbohydrates, so its preferred source is glucose/glycogen for the oxidative process.

Training Your Energy Systems

Many of us may have heard of the “fat burning zone”. The theory is by exercising at low intensity for a long period of time a higher percentage of calories burnt are from fat. While technically this may be true it is misleading and ineffective. For example if you did a walk at say 3 mph for 20 min and the body is getting 67% of the energy from fats and 33% from carbohydrates and the person is expending 4.8 calories per minute, you would assume the individual would burn 64 calories from fat, and 32 calories from carbohydrates over the 20 min. Now if someone was running at 6 mph for the same length of time and the body was burning 54% of its calories from carbohydrates and 46% from fats that would result in 104 calories being burnt from carbohydrates and 90 calories from fat. So really even though you are not exercising in the “fat burning zone” you would be burning more calories from fat and more calories in total by exercising outside this zone.

The most important thing for training any of the bodies systems is specificity. The body will adapt to the specific demands imposed on it. This is important to understand when considering how to train your cardio. Long steady state exercise (for example jogging at 5 mph for 30 min) primarily trains your aerobic systems. This is preferable if you plan on running marathons or are in sport that requires long periods of activity. However, if the sport or activity you take part in requires short but intense bouts of activity, training the aerobic system is not enough. Training the two anaerobic systems, (ATP-PC + Anaerobic Glycolysis) will allow your body to adapt and improve its efficiency in producing and recovering energy needed for short bursts of activity.

No matter what the requirements of the sport, or activity, it is good to remember recovery is an aerobic process. So it is always good to include some training aimed at improving the aerobic system, even for sprinters. The same can be said either way, being able to sprint up a hill or in the last 100 metres before a finish line can be important in a marathon, so training your energy systems to be able to do that and recover after is beneficial as well.

Training Guidelines

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  • ATP-PC: Short burst of high intensity exercises (sprints, heavy weight at low reps) followed by longer periods of rest
  • Anaerobic Glycolysis: Activities lasting 30-50 sec at higher intensities (8-12 reps when weight lifting)
  • oxidative system (aerobic): Long periods of lower or steady state activity (long walks, jogging)

In our next post we will detail different training methods for the bodies energy systems and their applications. Be sure to like our facebook page or subscribe to our blog to find out more!

To your success,

Keegan Marshall CPT, CES

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