3 Energy Pathways* (part 1)

What I outline here are the 3 most important levels of exercise intensity and how your muscles get energy in different ways for those differing levels. This is important to understanding how and why you want to train these various systems with a wide variety of exercise forms and levels of intensity. It gets a little wordy so I'll try to summarize at the end of this section.

In order for a muscle to move (contract or "twitch") it needs a signal in the form of a neuro-electrical impulse and it needs a source of energy to propel the contraction (movement). ATP "Adenosine Tri-phosphate" is the molecule that is the direct source of that energy.

When an intact ATP molecule breaks down (loses a phosphate Ion and becomes ADP "Adenosine-Di-Phosphate") energy gets released. In a nutshell, this energy (acting with specialized "motor proteins" that can convert chemical energy to mechanical) is what propels the contraction of the muscle cell.

ATP is not stable within the body however (stores lasting mere seconds) and so ATP must be remade continuously. Generally speaking, ATP is made from the breakdown of nutrients (example: glucose, amino acids, fatty acids) during a process called "cellular respiration". During cell respiration, chemical energy is taken from the nutrients and stored in the chemical bonds of the rebuilt ATP molecule.

There are basically 3 different paths (types) of cellular respiration and thus 3 "energy pathways" for a muscle to get its energy to contract.

One of these energy pathways is "aerobic" because it utilizes oxygen to metabolize the nutrients into ATP.

The other 2 pathways are "anaerobic" because they DO NOT utilize oxygen to metabolize the nutrients into ATP.

AEROBIC ENERGY

Put simply, the aerobic pathway involves a process of breaking down glucose (blood sugar) while in the presence of oxygen to ultimately produce ATP.

This aerobic pathway is slow when compared to the anaerobic paths in that it relies on breathing and circulation to "replenish" the ATP supplies. This is one reason you increase heart and respiration rates during exercise.

This is the main system for fueling and re-energizing what are called "type 1 slow twitch" (slow movement) muscle fibers.

Because the aerobic pathways take a lot of time to do their thing, these paths mostly operate when you can give them time by keeping the ATP demand at a relative low rate - as in moderate to low intensity endurance type exercise.

In terms of ratio, a larger percentage of fats may be "burned" to make ATP if the exercise intensity (metabolic demand) is kept low on the aerobic level. However, this doesn't mean you burn off "more" fat overall with low intensity exercise. None the less, low level aerobic exercise is often labeled as "the fat burning zone" because of this fact.

Because aerobic pathways dominate the muscle metabolic system at this particular level of demand or intensity, this level of exercise is generally called "aerobic" exercise.

If you step up the intensity however, to a point where the aerobic pathways can't produce the required ATP to energize the muscle, your body begins to switch on the ANaerobic pathways which draw on more immediate sources of fuel.

ANAEROBIC ENERGY

As mentioned earlier, muscles fulfill their refueling needs in different ways depending on exercise intensity and duration - the higher the intensity and shorter the duration, the more the AN-aerobic (without oxygen) pathways predominate. The two major ANaerobic pathways are the ATP-CP system and the Glycolytic system.

ATP-CP (ATP+"CREATINE PHOSPHATE") SYSTEM

The "ATP-CP" or ATP "Phosphate System" predominates in exercises that last a fairly short amount of time. These exercises usually involve high intensity and what are called the type 2 "fast twitch" (fast movement) muscle fibers. This muscle energy system works well for intense exercise lasting 15 or fewer seconds such as lifting, sprinting or pulling a powerful climbing move.

For the first 2 seconds of exercise, direct ATP stores in the muscle are used up, but then the ATP gets "re-built" (resupplied) by "borrowing" phosphate from the Creatine Phosphate stores in the muscle.

Beyond about 10-15 seconds of exercise, a different ANaerobic mechanism begins to dominate if the exercise remains sufficiently intense.

This system is called by many names from the "lactate pathway" to "glucogenic pathway" to the "fermentation pathway" etc.. I call it the Glycolytic or ANaerobic Glycolysis Pathway.

GLYCOLITIC SYSTEM

The ANaerobic Glycolytic pathways are what predominate when both exercise intensity AND duration are at very high levels. This is the principle energy system for intense exercise lasting up to about 2-3 minutes (beyond that, aerobic paths predominate). At this level, the ATP cannot be rebuilt fast enough by the slower aerobic system and the short lived ATP-CP system has been used up, so the ATP gets re-built from the breakdown of glucose (from carbohydrates) but without the presence of oxygen.

One negative of this system is that it allows a build up of "waste" byproducts in the muscle such as lactic acid which is what causes the "burn" feeling during the intense exercise. This is a hall mark of LT (Lactate Threshold) or AT (Anaerobic Threshold) training.

This is also the land of "Intervals" (interval training) and is where you can greatly improve performance by training (repeatedly) this system for efficiency and by building a tolerance for the suffering that often accompanies such hard levels of effort.

TO SUMMARIZE..

Muscle gets energy for movement (contraction) from a molecule called ATP. ATP is made via different ways or "pathways".

If muscle motion is of relatively low intensity (for example walking) the ATP can be supplied through aerobic (with oxygen) pathways.

If the muscle motion reaches certain "thresholds" of intensity (for example sprinting or weight lifting) ANaerobic paths (without oxygen) become the predominate mechanism for resupplying the ATP.

ANaerobic paths are much more time limited than aerobic pathways and once this ANaerobic threshold (AT) is reached these paths cannot be the main energy producing mechanism beyond about 2-3 minutes duration. Beyond that time, muscle motion must either stop or be reduced to a level of intensity that can be supported via aerobic pathways.

Importantly however, the ANaerobic Threshold (AT - often used interchangeably with the term "Lactate Threshold or LT) can be trained or "pushed back" to occur at a higher level of exercise intensity. What this means is that an athlete can (with proper "interval" threshold training) go longer and harder than they could if they do not "train" these pathways.


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