3 Energy Pathways* (part 2)

ENERGY & MUSCLE IN ACTION

Lets use the example of the act of someone getting up from a chair and going for a run.

In the early stages of any motion (no matter the intensity) or during easy movement (movement = muscle contraction) the ATP demand is fulfilled directly from ATP stored in the muscle.

ATP (up to 2-3 seconds):
In this example this first stage of energy production takes place as the person begins to get off the chair and casually walk toward the running track. At this point most of the energy for their muscle motion comes from the stored ATP in the muscles.

ATP-CP (from 2 to 10/15 seconds):
As the runner takes off (lets say he/she sprints out of the blocks at a super hard pace), from around 2 through the first 10 to 15 seconds or so this hard effort is fueled mainly by the ANaerobic "ATP-CP" or ATP "Creatine Phosphate System" while the main muscles recruited (utilized) are the Type II fast twitch variety.

ANAEROBIC GLYCOLYTIC (15 seconds to 2-3 minutes):
Now lets say our runner continues at a very hard pace (around 90-99% sprint) and goes beyond the initial 10-15 seconds duration (or there abouts) and up to around 2-3 minutes. The energy (ATP) production at this pace is still ANaerobic but can no longer be restocked from the now depleted ATP-CP system because the Creatine-Phosphate has been used up. Here, the ATP must be synthesized mostly from the breakdown of glucose (blood sugar). This pathway will still recruit mostly fast twitch muscle fibers but also with a mix of type 1 slow twitch muscle as well.

A "side effect" of this stage is the production of "waste" products like lactic acid (actually LA has been now declared as a metabolic fuel source) which inhibits and fatigues muscle functioning - ie you feel the burn. So somewhere between 3-4 minutes our runner must slow the pace.

AEROBIC (beyond ANaerobic intensities and duration):
So now going beyong 2-3 minutes, the pace has to be reduced -in this case to a zone 2 jog. Here, our runner begins to "go aerobic" and the main muscle fibers being activated are (or soon will be) the slow twitch type 1 variety.

During this low intensity long duration motion the ATP will be mainly synthesized from the aerobic breakdown of glucose supplied from the carbohydrate glycogen stored in muscles and then later from the liver.

If this movement goes on long enough to diminish muscle glycogen - and is of a low enough intensity to allow the cardio/respiratory system to provide sufficient oxygen - then lipids (fats) will also be metabolized as an energy source - ie fat will become a significant fuel source.

This level of movement can continue as long as the muscles can continue to draw on aerobic pathways for energy. The slow twitch type I muscle fibers that now predominate are the ones best suited for this long term aerobic ATP production.

However should our runner again step up the pace and perhaps do a "hill interval" they will again "go ANaerobic" and use more type II muscles and draw on glycogen and glucose breakdown (to form ATP) WITHOUT the "benefits" (long duration) of optimal oxygen presence.

The predominant fuel/nutrient chronological hierarchy for exercise levels looks something like this:

~ Stored ATP = from 2 to 3 sec
~ ATP-CP = from 3 sec to 10/15 sec
~ ANaerobic Glycolysis = from 15 sec to 2-3 min
~ Aerobic Glycogen & lipids = beyond 3 min ANaerobic durations and below ANaerobic threshold (AT/LT) intensites

Note that you spend the first 15 to 20 minutes of aerobic exercise "burning off" stored muscle glycogen supplies before resupplied glycogen or lipids (fats) significantly come into play as fuel sources.

FAT BURNING ZONE

You may have heard of this but it's really quite the myth.

The idea is that if you keep your cardio exercise intensity levels low enough (zone 2 and below) you will allow the sufficient level of oxygen uptake needed to burn more fat as fuel. It is true that lower intensities DO result in a higher ratio of fats to carbs for ATP generation.

What's wrong with this theory though is simply that you can burn a greater total quantity of fat (not merely a higher ratio) by using higher intensities.

Example: say if you take an hour to complete a slow and low workout and burn 300 calories total at an intensity that provides a 60% fat to 40% carb fuel ratio - you will burn 180 calories of fat.

BUT if you instead do a 50 minute interval workout that burns 600 calories at an intensity that provides a 60% carb to 40% fat fuel ratio you will burn 240 calories of fat and still have ten extra minutes to spare! (not to mention an improved vo2 max, lactate threshold, anaerobic threshold, resting metabolic rate).

To lose more fat (without eating your muscle protein up) short sub 1 hour high intensity cardio is usually the better choice.

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