Like the ATP/CP pathway, the glycolytic pathway is also anaerobic. Once you have depleted the readily available ATP/CP stores, the body must break down carbohydrates to produce more ATP. This process uses either glycogen (which is stored in the muscle cells) or glucose (which is found in the blood) to convert ADP back into ATP with the waste product being lactic acid:

Glucose + 2Pi + 2ADP + 2NAD+ 2 lactic acid + 2ATP + 2NAD

(Eq. 1-4)

This lactic acid eventually builds more quickly than it can be flushed out of the muscle to the point of the anaerobic threshold, otherwise known as muscular fatigue. At this point, you must either stop or slow down until the lactic acid is removed. Lactic acid is converted to a less toxic form called lactate which is used either as an energy substrate or to produce more glucose (a process called gluconeogenesis).

Stored sugars are rarely ever depleted (and are never depleted in the glycolytic pathway). However, this is not the limiting factor; the limiting factor is the accumulation of lactic acid. Generally, the glycolytic pathway ends under maximal conditions at around 80 seconds before the oxidative pathway (and lower levels of activity) takes over.

How well your muscles function in the glycolytic pathway is determined by several things:

1. How quickly you can get rid of the lactic acid.

2. How well you can tolerate the pain caused by lactic acid.

3. How far you can go before that pain comes and it becomes vital to get rid of the lactic acid (the anaerobic threshold).
   

Blood lactate levels usually return to normal within an hour after activity. Research shows that training can increase the rate in which lactic acid is removed as well as push back the anaerobic threshold. As for your ability to tolerate the pain, THAT comes with experience!

This system is aerobic, meaning it uses oxygen to produce ATP via the Krebs cycle and electron transport chain. Ultimately, more ATP is produced through this pathway than through the other two; however, it takes much longer. Pyruvate, which is produced through glycolysis, undergoes a long trip through the Krebs cycle to convert several coenzymes that have lost an electron back into their originalstate. It is in the electron transport chain where these coenzymes undergo oxidation to convert ADP back into ATP. In the end, up to 38 molecules of ATP can be produced through the oxidative pathway.

It is only in this pathway that fat can be used for energy. Breaking down fat for energy is also a long process (called beta oxidation), which does not directly produce ATP. Rather, it provides the coenzymes needed for the Krebs cycle. Scientists have estimated that while at rest (and in the oxidative pathway) 70% of energy comes from fat, not carbohydrates or protein.

However, as exercise intensity increases, more and more carbohydrates are used instead of fat (beta oxidation can’t keep up). In fact, at the upper limits of the aerobic pathway, 100% of the energy is coming from carbohydrates — not fat! If at these levels carbohydrates aren’t available, the body will indeed catabolize the very muscle it’s using for energy.

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