All muscle cells have mitochondria. In fact every cell in your body (besides red blood cells) has mitochondria. Mito provide the life force for……..life. But you are correct that Slow Twitch (ST) have greater mito mass then do FTa and much more that FTb. There is some evidence that FTa fibers can shift toward ST fibers by developing aerobic qualities like increased mito density and better capillary networks. There have been many studies done on rodents that show fiber type shifts but only one long term that I know of, done by Heikki Rusko studying a group of high level Finnish junior XC skiers as they trained and aged. He saw the number of what would fall into the ST category increase and FTa types decrease indicating that the FT had become more like ST and developed better endurance.
OK, Now you got me going so hold on as I have a lot to say about this.
Only the fibers recruited during the exercise get the training effect. So long duration low intensity exercise trains ST fibers because the force requirements are low. High intensity training, because of the higher forces necessarily recruits FT fibers. If those fibers stay in use for long enough (close to their endurance limit) then they will see an endurance training effect. So, when you do 15 reps with 80% of your 1RM max squat weight many FT fibers need to be recruited. By the 15th rep many of them are at their endurance limit and will see an endurance training effect.
So none of this is new. We cover it in some detail in our book Training for the New Alpinism on page 125 including the sidebar on that page. There’s a lot of good info in that book :-). There are many ways to train the local muscular endurance that you are referring to. The principles are fairly simple: Recruit the “limit” fibers to do the work. These are the most powerful of the FT fiber pool needed to accomplish the task for the duration required. So requirement #1 is that the load must be relatively high. By relatively I mean it will be higher for short events than for longer distance events. #2 keep those fibers working for a long enough time right at their endurance limit so that get an endurance training effect. Voila, those fibers will develop more endurance. For endurance; duration of the training is the key factor, even for these FT fibers.
This is the training effect we are accomplishing here:
This is exactly the theory that underlies all these fad HIIT exercise schemes like CrossFit although the Kool-Aid drinkers don’t have enough knowledge to know this has been well understood for 60 years and like to claim that they have discovered something new. In fact they badly misapply the principle…….but that is another story.
Yuri Verkhoshansky wrote extensively about this in the 60s and the methods to accomplish it a varied depending on the duration of the event you are training for. The method I have used most successfully is what He called: Specific Locomotion Executed in More Difficult Conditions. There is nothing new in training. Any breakthrough is merely a recycling of old ideas dressed a bit differently.
We only use what Verkhoshansky calls LME (Local Muscular Endurance) training during certain stages of the train cycle. For an XC skier it is often done in the early base period to provide a ME base for more aerobically taxing interval training latter. For mountaineers we use in the later stages of training for the less well trained and for longer periods with the highly trained. Steve did this training in certain phases of his preparation. I used this to good effect with Sadie Bjornsen, her brother Erik and with Torin Koos. Names that skiers may be familiar with.
It is vital that the aerobic base training volume be kept high during this LME period
I’ve written about that extensively on these forums (look under Muscular Endurance on the index to your right) and in the articles linked but you have been warned. As I recall from your recent forum posts, it seems very likely that your have ADS (Aerobic Deficiency Syndrome)> You should NOT be using this training. At least not now until you improve your aerobic capacity so your HR does not go so high when you go uphill. Doing LME will negatively impact your aerobic base training. That’s why for less trained mountaineers we put it late in their programs: We hope to get there AeT up higher before applying it. For the well trained or the World Class XC skier can come early in he base period because they have a high AeT already.
The higher your aerobic base capacity the more you can expect to gain from LME training.