From The Coaches: Muscle Fiber Basics and SUF Training

This week’s article is designed to generate discussion by giving some general facts on muscle fiber types and how it relates to SUF training.
There is a huge variability between individuals in their muscle fiber make up due to genetics, circulating hormone levels and training. Muscle fibers will adapt to the training stress placed upon them and can change characteristics as a result of consistent training. Training that involves recruiting more muscle fibers of all types and training them to be more efficient is crucial in improving fitness and performance. For example, training at low force, low velocity will only recruit type I fibers. A better option is to train our Type I fibers to be more efficient by performing steady state workouts with surges above Vo2 to activate the type IIa fibers and dump a load of lactate which the type I fibers have to clear. We can also train our type IIa fibers to be more like type I by increasing mitochondrial density and capillarization from Vo2 workouts. Repeated MAP and AC workouts which activate type IIa fibers trains tolerance to lactate and thus quicker recovery between intervals.

Skeletal muscle is composed of three types of fibers: They each have unique characteristics:

Slow Twitch (Type I) :
● Fatigue resistant
● Used for endurance activities
● Low force production
● High concentration of mitochondria and utilize oxygen to produce energy
● High capillary density to transport nutrients to muscles and remove waste products ● High oxidative capacity
● High myoglobin concentration- myoglobin is the pigment found in muscle cell that aids hemoglobin in producing oxygen for aerobic metabolism
● Low glycolytic capacity - the ability to store and breakdown glycogen for use as a high intensity source of energy

Fast Twitch (Type IIA):
● Moderate resistance to fatigue
● Used for short duration (up to 2minutes)
● High force production
● Moderate concentration of mitochondria
● Moderate capillary density
● Moderate oxidative capacity
● Moderate myoglobin concentration
● High glycolytic capacity- can breakdown glycogen and use for high intensity efforts

Fast Twitch (Type IIx or IIB):
● Low resistance to fatigue
● Used for very short efforts up to 8 second at maximal intensity
● Very high force production
● Low number of mitochondria
● Low capillary density
● Low oxidative capacity

Fun facts:
All muscles have a mix of the three fiber types. Most Type I fibers are located deep in the muscle and most Type IIs are on the surface. The lower body generally has more Type I than the upper body. Muscle fiber type structure may shift in response to training or injury. Any shift in structural change is a long term training adaptation. Endurance athletes have a higher proportion of Type I than Type II in trained muscles. Strength trained athletes have higher proportions of Type IIA fibers and power athletes have a higher proportion of Type IIx/b fibers.

What does this mean for your training:
Neuromuscular power and coordination can be improved through all out sprints and cadence drills which teaches our muscles to turn on/off more efficiently.The higher the recruitment, the stronger the muscle contraction will be. Motor units (a group of muscle fibers and a motor neuron) are generally recruited in order of smallest to largest (slow twitch to fast twitch). And also realize any path to fatigue/failure will lead to recruitment of Type IIa and IIx fibers like at the end of an ironman.
The 4DP model is geared for each individual to ensure that the workout is specific in generating the stimulus to enhance fitness. During a Full Frontal 4DP fitness test the, NM 8 sec sprint will be predominantly type IIx muscle fibers. The 5 min MAP and 20 min FTP segments will recruit all the fiber types due to the fatigue generated by the tests. The 1 min AC test is unique. It does activate predominantly type IIa fibers, but it really tests the ability to produce repeated efforts due to the heavy fatigue load already experienced. It is by no means not an all out fresh 1 min effort.
The goal of our training plans is to improve fitness by increasing density of mitochondria, recruiting more muscle to stave off fatigue, have more matches to burn without draining the battery during high intensity efforts and improving our maximum power in the race to the finish line.


I have “retired” muscle fibers … they are very happy fibers…


I dropped my slow twitch fibers the first time I did The Downward Spiral (my first) and haven’t seen them since. Not saying this like it’s a good thing.


or even the best thing in the worl … oops sorry

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I haven’t seen my fast twitch fibers since high school. Not just M.I.A. I believe they’ve started a new life on a beach somewhere south of the equator.


Likely out in margarittaville, couchlandria.


Great info, thanks.
I am interested in what I can do to increase my NM number. It bugs me that I feel it hasn’t gone up a lot in ages (sits around 850). I know you mentioned that it uses IIx, and that doing sprints or cadence drills will help. My cadence is pretty good (200+ club), but I struggle to actually have much power once I’m over 120rpm or so. I am wondering what else I can do. Do the strength workouts help (Goblet squats, lunges, etc.) or are they different muscle types?

I know it’s not critical, but I’d love to hit 1000w one day! Even just for a couple of seconds! :smiley:


Hey @ChrisMTB,
You are spot on to improve your NM you’ll want to activate those type Iix muscle fiber through sprint workouts on the bike and off the bike movements that are short, and explosive such as box jumps, kettlebell swings, and squats. Be cautious with these movements to avoid injury.
You do best by aiming to hit peak power at a high cadence 100-130 rpm.
Here’s an article you might find interesting:;jsessionid=88625C343EC3FBC0800B51BC99EC5F70?sequence=1



So, what to do then if you’re an easy gainer in the muscle arena?

My optimum weight for frame size/power/healthy living is right around 168. Racing weight is luckily right at 164-165 - so easy to get there within 2 weeks.

However, my problem arises when building strength. I am currently in a MS phase and I’ve put on 8 lbs in 3 weeks. I know a good chunk of this is likely water weight, but I’m still riding each week to the tune of 700-1000 TSS, so I know I’m not holding excess water that much. I work pretty hard to keep a working balance of macros to load.

Problem is, I’m a sprinter by profile, yet I don’t want any more sprint power. I was playing around after squats on Friday and hit the below…

Also, my rep count is stratospheric and weight load is low. Yet I’m still gaining size. Thoughts?

Lastly, I race MTB, so more concerned about having enough strength to thrash the bike on the trails than to be whispy up road climbs.


Hey @SirAlexanderLee,
If you’re an easy gainer, stop doing low to moderate weight with high reps. My recommendation is to do heavier weight and less reps. So with squats, or deadlifts, after a good warm up…do 4-5 sets of 3-5 reps with 2-3 minutes rest between sets. You can still do secondary exercises like lunges, step ups, split squats, goblet squats in the 6-8 rep range. For a strong upper body , bench press ,weighted chins, pull-ups in the 4-6 rep range, bench rows, renegade rows, overhead press in the 6-8 rep range. Limit your sessions to 2x/wk.
If you’re still inducing unwanted hypertrophy, let’s revisit. Also, as you age, you’ll want that muscle😉



Hey Coach S,

Thanks for the quick reply.

So, a bit more history for my body - I used to be an amateur power lifter. Body weight back then was 225-230. So anything in the low rep range, my body says, “oh, we’re doing this again?!? Hell yeah, here you go fat boy!” & will pack it on even faster.

Right now I’ve got a 2 week cycle with chest, back, legs all on one day x3 days in 14 and flexion, extension, lateral movements 3x in 14 days.

These cycles are primarily body weight, calisthenics, isometric movements/holds.

Maybe I’ll try your suggestion of pushing another gap day in there and extending odd-out to 21days on the 6 routine?

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Or, should I not worry about it so much right now? Bulk the hell up in the off season and see how much weight stays when the intensity goes up in spring.

Taking weight down isn’t the only way to make a positive w/kg impact.

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I hear you, I trained as a power lifter all through my college football days. The difference between what you used to do ad now is like night and day. You’re not training to be a powerlifter, you’re just doing some of the same movements not as frequently and in conjunction with endurance training. The only way you’ll gain body fat is through excess caloric intake. Definitely experiment and do what works for you.

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Sumo DL is not part of the routine. Neither is benching and squatting with an Inzer shirt or suit for max reps of 1 :laughing:. There are no max reps of 1.

I think I’m going to let it rip for 8 weeks and reassess. There’s no way I’m going to bulk that much - right? Like it’s not possible to get that big on calisthenics and body weight, right? :rofl::flushed::hushed:

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Monster sprint status! Jeez!

Thanks for the great input @Coach.Spencer.R ! I have one question though:

I do understand how the SUF workouts contribute to these goals muscle-wise and how a shorter but harder workout can lead to the same or even better results as longer and easier rides (I think we all agree, that the once a week longer ride is a must nevertheless?).
However: I’m afraid that, if I shift my training to more intense/time efficient sessions, I will become more glycolytic and my power at fatmax will go down (I know, that is not necessarily a bad thing, but I’m more a steady rider than a cyclocross rider)

Thank you!

Hey @Mowag ,
Thanks for your post. All of our plans are geared to make you a stronger, more efficient, faster and a more fit athlete/cyclist at varying distances. Look them over and see how they dose out the different intensities all based on your 4DP numbers. Also read the articles to get a good flavor of our philosophy.
See what works best for you.


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