Simon:
Glad you found that sweet spot in weight to carry. That is THE critical step that many people struggle with on these workouts. It can take some trial and error.
Why does excessive ME work lower the aerobic capacity? This gets into some physiology so pay attention. Sit back, put your feet up and even refer back to our book. We cover this extensively.
Following I’ll describe a popular model and while not 100% correct (way oversimplified) does a good job of explaining what is going on. It is called the Size Principle.
When you find that sweet spot, you are limited in speed/effort by the aerobic capacity/endurance of those FT fibers you are able to recruit for the job. The size Principle states that your brain recruits motor units starting with the smallest cross section, slowest, ST fibers and then calling upon more and more ST fibers till the force requirement is met. And if the force required is high enough the brain will call some larger cross section FT fibers off the bench to help get the job done. If the load is high enough (sweet spot) then some FT fibers will get called in. Remember this illustration on p128 of Training for the New Alpinism? There is a ton of info on those pages that concerns this topic.
Now, these FT fibers don’t have much aerobic capacity (aka, endurance). This is why you need to find the sweet spot by creating an artificial load with weight in the first place so you are recruiting the most endurance limited fibers that your brain gets to do this job. These are the fibers that are getting the main training effect. This may not be 1005 correct as there is a theory that the ST fibers also get a strengthen effect. The FT fibers are the ones talking to you as you slog up that steep hill with the slight burn going.
The reason we call this ME (Local Muscular Endurance is the original name) is that your speed is limited by the very localized fatigue going on among these FT fibers. Be repeatedly exposing these fibers to this sort of stress they will actually begin to develop aerobic qualities more like ST and hence improve their endurance. What happens though, during this sort of training, is the acidity of those muscles goes up. When your muscle pH drops (acidity going up) some of the most important genetic signaling for aerobic adaptation is muted or even stopped.
Thats right, training right at the limit of endurance has both positive and negative implications. Too much, too hard or too often training with excessive acidity will actually reduce, especially in the ST fibers (your aerobic base of support) aerobic capacity. Endurance coaches know this phenomenon as “Death by Threshold” it is common among those who engage in too much high intensity training. This is what we see all the time with people who engage in many of the HIT fitness exercise routines so popular right now such as Cross Fit. Excessive or exclusive use of these exercise fads and programs where you are operating at your endurance limit for the whole workout and doing it 3-5 days per week literally destroys the individuals aerobic capacity. I point you to our book again. There is a great deal if physiological info there to support this.
So HIT (High Intensity Training) like these ME workouts, is very important. But, it must be a supplement to a big aerobic base, not a substitute for it. While engaging in HIT (ME) you must maintain a high volume of Z1-2 training to offset the negative effect of all the HIT on the ST fibers aerobic capacity.
This is why we carefully prescribe a high aerobic volume during the ME phase and why we precede it with a big aerobic base building phase. This concept is lost an a great many fitness websites even those that claim to cater to mountain athletes.
I hope this helps your understanding.
Scott
