What You Need To Know About Metabolic Energy Systems

Whether you are an endurance athlete, a sprinter or a recreational walker, you will be using one of the three energy systems whenever you perform any movement. The nature of these energy sources differs significantly and you will be required to train in various ways in order to make each one more effective.

The three main energy systems are:

The ATP-PCr system - known as the immediate energy system
The Lactic Acid system - known as the short term energy system
The Aerobic system - known as the aerobic system

The Aerobic System

Lastly you have the aerobic system, which is used when you are performing exercise for a much greater duration of time. This energy system relies on a process called glycolysis to produce energy and with more and more training you will see yourself being more capable to go for longer periods. Usually the types of exercise you perform while in this system is 'steady state', meaning you are not altering intensity at all but instead are going at a steady pace the whole entire session. This energy system does not really ever become depleted as it can rely on fats for fuel and most humans have more than enough fat stored to last for extreme durations of time (much past marathon distances). The bigger problem with prolonging this exercise though comes with motivational levels and then keeping fluid and electrolyte balances in check (sodium and potassium levels).

Obviously too as your body depletes itself of carbohydrates you will start to feel slightly more fatigued as running off fats is a slower process in terms of producing the energy for muscle contraction. Note however that the better trained you are aerobically, the more your body will be able to rely on fats when doing this exercise and the quicker it will make the switchover from carbohydrates to fat stores.

One final thing that is good to note is that when it comes to energy expended per session, while it may seem as though you would burn the most calories when using the aerobic system for a longer period of time, especially since you are more apt to using fat as a fuel source, in all reality it is often the ATP-CPr system or the lactic system that will cause you to succeed the most in weight loss. The reason for this being that when you perform these two activities, you are literally performing them without oxygen and this creates what is called an oxygen deficit. It is after you perform these higher intensity exercises where you see a greater number of calories burned due to the fact that your body is recovering from the burst of movement. Secondly, with aerobic work, the body tends to adapt to it quite nicely and over time, you will start burning less on a minute by minute basis when performing the activity. So for example, if you started out burning 8 calories per minute while running at 7 mph and you kept this pace, in a year you may only be burning 7 calories per minute running at that same 7 mph pace. The body has now become more efficient at performing the movement and therefore does not require as many calories to perform it. For this reason, it is important to constantly be increasing the intensity ever so slightly on your steady state work (however not such a great increase that you push yourself out of the aerobic energy system zone). This will keep your calorie burn up and hopefully keep fat loss on track.

The ATP-PCr System

The ATP-PCr system is an energy system that is at work when you are performing very short duration, high intensity activities. These would typically include such things as a 100-m sprint, a 25-m swim or an all out weight lifting set of 1-3 reps.

When you are doing these actions, you are going to be using a fuel source called ATP (adenosine triphosphate) along with PCr (phosphocreatine) that is stored in the muscle cells. Each kilogram of muscle that you have in your body will store a maximum of 5 millimoles of ATP and 15 millimoles of PCr. These stores are rapidly depleted however, which is why the exercise is so short in duration. If you tried to keep up the intensity, you would find yourself quickly fatiguing and the body would stop naturally.

The Lactic Acid System

The next energy system that is used in human body movement is the lactic acid system, or the short-term energy system. This will also fuel activities that are quite intense in nature, however they will not be as intense as those seen in the above situation.

During this intense exercise, intramuscular stored glycogen provides the energy source to phosphorylate ADP during a process called anaerobic glycogenolysis, which then forms lactate.

In order to continually workout at the intensity levels demanded by the activities using this system (for example a 450 m sprint, a 100 m swim or sports that involve significant sprinting components such as hockey and soccer), the body will rely on its stored phosphogens to supply the ATP production cycle. When individuals choose to use creatine supplements, this is partially a reason why. The creatine you ingest will help to replenish these stores quicker so you will be able to prolong the number of repeats of this activity you perform (not necessarily the duration of the activity but more the number of times per session you sprint, lift a weight and so on).

The body will try and remove the byproducts that begin to build-up as part of this cycle, however if the removal rate does not match the production rate, and as exercise time increases, the byproducts will begin to build up and blood lactate will accumulate. This is when you see fatigue beginning to set in and activity stops.

What is interesting to note is that with a fair amount of aerobic training, the body becomes better able to handle the build-up of lactate and can remove it easier, therefore you will be able to sustain exercises of a higher intensity in nature longer. So sprint athletes would show some benefits from partaking in the odd longer run in order to help their body to learn to deal with these issues more effectively.

You will start seeing an increase in the production of blood lactate when you reach your blood lactate threshold and this occurs at the highest percentage of an athlete's aerobic capacity; usually determined by a mix of training and genetic factors.

When you perform a lot of endurance work, you will see an increase in the capillary density and mitochondria size and number in the muscle cells. As these two structures aid in the transfer of energy and oxygen to the muscle cell, an increase in them will help the muscles move oxygen quicker and therefore be more efficient. Additionally when you train for endurance you will enhance the cells ability to produce ATP aerobically through the breakdown of fatty acids, which again will help supply the muscle tissues with more energy.

So look at your current cardiovascular exercise schedule and consider possibly revamping it a little to work your different energy systems. No one says you have to focus on only one at a time and you can most definitely have a sprint session and a few endurance sessions in your weekly plan (in fact this is likely very beneficial). Plus, doing so will help to prevent boredom as well as overtraining. Just be sure regardless of what system you use, you are getting enough overall rest to allow your body a chance to recover.

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