In a previous post, I talked about using low-level cardio in conjunction with insulin-carried glucose can kick start the protein synthesis process post resistance training. What I want to talk about here is the relationship between intensity and the metabolic pathways, so that we can learn to engage the desired pathway.

When you begin any activity over and above resting metabolism, the energy required to perform that activity will first be supplied by anaerobic metabolism, as it takes several minutes to mobilize fat in sufficient quantities to sustain an activity. As activity is continued for a sufficient amount of time, the body “catches up” as oxygen uptake increases to allow greater utilization of oxidized carbohydrate and fat for energy.

This transition from an anaerobic metabolism to an aerobically dominated metabolism is dependent on two major factors: intensity and duration. Obviously, if an activity requires near maximal effort, such as weight training which cannot be sustained for long periods of time, your exercise metabolism will remain primarily dependent on carbohydrate sources (intramuscular glycogen) without a need for a significantly increased oxygen uptake. This is called anaerobic metabolism. Performing an activity which is of low intensity, such as a fast walk or a slow job maintained for long periods, allows for the increase in oxygen uptake and release of fat stores necessary to utilize fatty acids as the primary energy source. This is called aerobic metabolism. As aerobic intensity increases, as in the case of a more intense run, a growing percentage of energy comes from carbohydrate sources thus bringing the pathway closer to the aerobic/anaerobic threshold.

To illustrate, the metabolic continuum looks like this:

• <--------------Aerobic----------------------I----------Anaerobic----------------------> +

+/- = Intensity
I = Threshold
Aerobic = Fatty Acids / Glucose w/ O2
Anaerobic = Glucose / Glycogen w/o O2

In summary: as intensity increases, the energy substrate switches from fats to carbs. This is becaue sufficient oxygen is unable to be delivered to the working muscles to oxidize the fatty acids. This can be gauged by either heart rate or rate of respiration; the more labored those are, the greater the demand for oxygen (the muscles are sending the signal that they are not getting enough so the heart and lungs speed up to compensate). As a result the metabolic pathways transition from aerobic to anaerobic. This is not desirable in the case of "force feeding" post resistence training, as the fat cells will no longer be releasing their stored energy; therefore rendering them receptive to insulin-carried glucose.