Fast-Twitch Muscle Fibers and Autonomic Nervous System States

Ok, so here I am finding myself training a lot of people with passionate lifestyle pursuits. Backcountry hunters, backpackers, triathletes, skiers, mountain bikers…people who love what they do and are motivated to be able to do it for a lifetime. Folks who have goals, trips, and events targeted this year for which they are preparing. 

Our discussions will often arrive at the intersection of muscle fibers and movement proficiency. These topics can be equally interesting and complicated. I’d like to parse out just one segment in the subject area, and it is the one featured in the title. My comments will focus upon several FAQ’s. Here they are.

  1. What are fast-twitch muscle fibers?
  2. Why are fast-twitch muscle fibers important for every Lifetime Athlete?
  3. How do autonomic nervous system states relate to muscle fiber recruitment?
  4. Are there a few simple tips anyone can use to target fast-twitch fibers effectively and safely?

I’m going to do my best to be relatively concise here, but you can always ping me with questions or take a peek at some TLA training methods @thelifetimeathlete on Instagram or on the YouTube Channel.

What are fast-twitch fibers? The name actually says it all. They have the ability to twitch, or contract, quite rapidly. In the simplest terms, our bodies have slow-twitch (Type I), and fast-twitch (Type II) fibers within all our muscles. The ratio between Type I and II varies among muscle groups depending on location and function within the body. It’s also highly impacted by genetics and significantly impacted by training (or not) behaviors. 

The Type II fibers, in humans, can be subdivided into IIa and IIx fibers. The IIa are most common and contract 5-6 times more rapidly than Type I’s. IIx’s are the most explosive and more rare. They are called upon for our most intense efforts and are present to a higher degree (although according to the research available they are still a minority) in the fastest and most powerful athletes. Type IIx’s contract about 20x faster than Type I’s. For the sake of simplicity, I’m going to talk generically about Type II’s and not worry about the distinction.

Fast-twitch muscle fibers tend to be larger in diameter than slow-twitch, and have the capacity to produce more force. They are generally part of what are known as “high-threshold motor units.” You can think of this as the continuation from the motor cortex in the cerebrum through the alpha motor neuron, to the synapses at the neuromuscular junction and into the contractile machinery of the muscle fibers. High-threshold motor units require a greater amount of stimulus to depolarize and this is of course by brilliant human design. If you contrast this with low-threshold motor units, you can think of those as being a bit slower and more precise, such as the typing I’m doing right now. They are great for fine motor control applications. But when we need to move fast or lift heavy objects…the demand there is high enough that our intention spills over the threshold to trigger the Type II connections. This is probably one of the most important points in the discussion because our intentions and task demands drive the muscle fiber recruitment in our bodies.

Why are fast-twitch fibers important for every Lifetime Athlete? This question comes up more often than I like, or probably than you’d think. It’s actually why I’m offering this little missive. There are many athletes who want to be strong, fast, and muscular…and that is the primary focus or at least one of them) in their training. But quite a few will say “I’m an endurance athlete and I don’t really need speed or strength, and I also don’t want too much size.” I totally understand where they are coming from but here’s where the conversation gets perhaps more valuable. 

Type II fibers indeed provide the bulk of our max speed and absolute strength, but they also are intricately linked to long term performance, health, and longevity. The component they provide in your athletic arsenal (when you need to call upon any surge of effort as one example) will atrophy if it is not trained, and it’s this loss of fast-twitch fibers most associated with the loss of speed and strength in older athletes. Simply adjusting the training to not neglect Type II muscle fibers can make performance declination incremental in most athletes and significantly extend their athletic careers. But I need to reiterate that these muscle fibers really atrophy (go away) if they are not used. They can be retrained, but it’s way easier just to keep them functioning all along. And don’t worry about looking at a weight and becoming too big. That just doesn’t happen. You probably need to have incredible genetics, a killer work ethic, performance-enhancing drugs, and years of all-out training to even approach such a possibility.

Health is affected by myriad factors. But if we isolate out the role of Type II fibers, they are highly adept at glucose management. These fibers store sugar as glycogen, and “burn up” excess amounts rapidly when they are regularly used. They also give you that extra umph to be able move things around in your garage or yard or play with your kids or grandkids. This means you don’t have to “hire out” some tasks or “sit out” when everybody else is doing something fun.

The longevity picture is even more dramatic. It is our balance, righting reactions, fall management, and functional independence that is most dependent on fast-twitch fibers. Much of our societal trend to “go light and slow” as you get older actually drives sarcopenia (muscle loss), incapacitation, and institutionalization. The classic example is the hip fracture. Older folks fall, break a hip, end up in the hospital…and if they don’t perish from pneumonia there they end up in a nursing home and are statistically likely to die within a year. This is staggeringly shocking to me. Of course we can’t prevent every episode, but I am certain we can positively address life function for many if not most of our population.

How do autonomic nervous system states relate to muscle fiber recruitment? This goes back to the most important aspect of this discussion. Intention is everything. It drives muscle recruitment and contraction. In order to utilize your Type II fibers, you have to try hard. Your brain won’t select those fibers unless it knows they are needed.

The two sides of the autonomic nervous system, the parasympathetic and sympathetic, are well known. Parasympathetic is rest and digest (or repose) and it’s the calm side of the equation. We are evolutionarily wired to be in this state most of the time. The sympathetic is fight or flight and it’s the elevated, activated state of our inner beast. If we look at the condition of being turned on and up, amped and ramped, excited, aggressive, “cocked, locked, and ready to rock, Doc!” or any other term you like…that’s a sympathetic state and it’s when the brain will preferentially utilize those Type II fibers. Jumping and landing, sprinting, throwing a medicine ball, or getting under a heavy bar are all examples in training or sport (and there are so many more) which you may visualize. Humans are designed to be able to easily recruit or manufacture this state, and then return to that parasympathetic baseline once the event is over or the danger has passed.

Here’s where a major disconnect comes in. Our society, at least for many, is a bit messed up in this relationship. There is often a moderately high, chronic sympathetic drive in our day-to-day existence. Stress, deadlines, traffic, poor sleep, crappy food, tense relationships, and more raise the baseline autonomic state to a higher level than it should be. Then the message is almost always to seek parasympathetic input as a correction and many people do that with exercise. Yoga, low-intensity steady-state cardio, and other meditative practices are emphasized. These are valuable and necessary in the Lifetime Athlete’s training mix, they just shouldn’t be the only thing. We need to use lifestyle practices to keep that parasympathetic baseline nice and low and frequent, but we also need to facilitate sympathetic states in training and competition a couple times a week (relatively briefly). Not only can this have a rather cathartic effect for stress, it applies the correct stress to the system to activate and develop the fast-twitch muscles. It simply does not happen if we don’t do this.

This is probably a great place to inject a little humor. Let’s call it the “F**k Factor!” I’ve seen this so many times that I think it is common. When working with athletes, clients, trainees (or whatever you want to call these awesome humans), we might go through a variety of preparatory components in a session. But if, on the day, we are targeting strength, power, or speed (AKA max Type II recruitment), there is a point when a lot of people will scream “F**k!” either before, during, or after an explosive effort. It takes that kind of intensity to summon the appropriate neurologic and physiologic response. Less than that doesn’t quite cut it. So I encourage folks to hit the “F**k Factor!” a couple of times a week in most cases. And we are talking seconds here, maybe for a few bouts. You can’t really use max intensity for more than a few seconds, so a 2-minute grind on a bike is not the proper training application.

Are there a few simple tips anyone can use to target fast-twitch fibers effectively and safely? This strongly relates to that F2 thing I just mentioned. We never do anything that is downright stupid. You should never come off the couch and hit a max sprint. This takes months of training to get to that point. Same is true for max lifting. All training can and should be scaled and progressed appropriately for each individual, taking into account such elements as training history and age, injury status, genetics, sports played, and many more.

However, there are actually a lot of ways to explore sympathetic states and fast-twitch behaviors. It’s fun. That alive, buzzing, almost electric feeling you get is amazing and it’s a natural thing everyone needs to feel from time to time. We just have to be a little sensible in how we work up to it, how far we take it, and how much we do.

First of all, I’m going to tell anyone without extensive training experience to simply perform basic fitness exercises for several months. Don’t even worry about going really fast or heavy. Get some good, basic Zone 2 cardio training to develop your aerobic system. Work on simple flexibility and mobility. Do resistance training using a few compound movements with moderate resistance and keep RPE 7-8 (leaving a few reps in the tank on each set). Learn movement patterns. Create resiliency in your connective tissues. Establish consistency. Then…you will be ready for the next, and necessary level, in your long-term (as in lifelong) athletic development plan.

OK…so what might this look like? Probably one of the best ways to get some fast-twitch activation into your system is to place a small amount of some heavy explosive work into your training sessions. Not every day…maybe only one exposure to each in a given week. You’ll want to do this early in your workout, once you are fully warmed up but before fatigue can lessen (and it will) the recruitment process. Use only movement patterns that you have practiced a lot and are good at. Several bouts of several seconds is what you are shooting for. Rest periods should be ample. You want to be fully recovered and fresh so you can blast it.

In the strength realm, you should select from just a few of the major compound lifts. Ones that you know and like and with which you feel very competent. Examples might be a squat or deadlift variation, and particularly the horizontal press and row options. After a few progressive light to moderate warmup sets, try for 1-3 heavy working sets in a rep range of 4-8. Another great trick is to simply use a rack to set up a mid-range isometric doing a few rounds of 5-7 seconds.

For power, consider the sustained output which represents Force x Distance/Time. This can take on many forms but perhaps my favorite is the “drag rig.” No, this isn’t an outfit you wear, it’s any sled, sandbag, or other apparatus that you can drag, forwards or backwards, all out for those same 5-7 seconds. Just a few reps as well. If your gym has a Prowler sled and turf, plus maybe a harness, that’s fantastic. You can go forwards or backwards, pushing or pulling with your hands, or using the waist harness. There are benefits to all. With no gym, one of the ways we’ve done it a lot in the Tribe is to throw a rope around an old tractor tire in any field or yard. Right now I have a couple sandbags wrapped in an old blanket that I’m dragging a few feet on a concrete floor. Works great.

Speed…ahh yes. Perhaps my favorite and literally the fountain of youth. Whether you watch the athletes who win, or any bunch of kids at a playground – speed defines athleticism and youthfulness. We don’t want to give it up. As much as I love sprinting…it’s not where I usually start most folks. Low box jumps (jump up and on but then step off slowly) and med ball slams are some of my favorite exercises to get an athlete moving fast and explosive. For some trainees, simply beginning by adjusting exercise tempo, and teaching them to first perform a fast concentric (like the pushing bar ascent on a bench press), or ultimately a fast eccentric like a snap-down or lightweight descent into a squat…are excellent. For those inclined, I love progressing them to more advanced plyometrics, change of direction drills, and actual sprinting. It takes time but it’s completely worth it. 

In summary, I want to thank you for joining me today and I hope I’ve given you some useful information and ideas. Understanding how those Type II fibers really function, and why we need to work on them, was my mission. If you’d like to dive deeper into how you can optimize your muscle fiber development and overall training for your passionate life pursuits, consider joining our Training Tribe or working with me in a coaching capacity. Have a great day!

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