Our bodies are composed of three types of skeletal muscle fibers: (1) Type I: slow-twitch, high endurance capacity; (2) Type IIa: fast-twitch, large, powerful, medium endurance capacity; and (3) Type IIb: fast-twitch, large, powerful, and easy-to-fatigue (sometimes called fight-or-flight muscle fibers). Your muscle fiber composition ratio is dependent on a muscle’s function, your age, your genetics, and – something you have control over: exercise type.
See the table below for a quick reference of the three muscle fiber types.
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|Characteristic||Slow-Twitch Type I||Fast-Twitch Type IIA||Fast-Twitch Type IIX or IIB|
|Activities||Marathons, distance running, swimming, cycling, power walking, endurance training||Powerlifting, sprinting, jumping, strength and agility training||Powerlifting, sprinting, jumping, strength and agility training|
|Muscle Fiber Size||Small||Large||Large|
|Force Production||Low||High||Very High|
|Resistance to Fatigue||Slow||Quick||Very Quick|
|Contraction Speed||Slow||Quick||Very Quick|
|Table created by National Association of Sports Medicine, NASM.org.|
We are not permanently assigned dominance or weakness in one muscle fiber type, nor is everyone born with an even distribution of all three. Someone born with a higher ratio of Type I muscle to Type IIa muscle can be better suited for endurance sports, while someone born with the opposite ratio, can be better suited for strength and power sports. However, professional athletes who are born with a predominance of one muscle fiber type over another also engage in sport-specific training to enhance their gifts. The form of training you engage in can create a body that is optimized for strength and power training or for endurance. As shown in the above chart, you don’t have to be genetically gifted to build a specialized body – or a generalized body for that matter; you just have to be mindful about the type of training you engage in.
Age & body composition
I always say: I do regenerative training, not degenerative training. To those who year in and year out try to get me to engage in marathons: I don’t do marathons because I’m informed about muscle fiber types, how they can influence aging, speed, agility, and body composition. As we age, slow-twitch, slimmer, less powerful Type I muscle dominates our bodies, while at the same time, we lose larger, quicker, more powerful Type II muscle. Knowing this, one might wonder why anyone would intentionally contribute to that degenerative process by dedicating themselves to activities that further encourage the dominance of slow-twitch muscle that comes with aging. Marathons require dedication to be done well, but so too does building a quick, muscular, metabolically efficient body. Making the conversion in either direction is not a fast or painless process, and sacrifices and compromises have to be made. You can’t build mass and power and hope to have greater endurance capacity; you can’t spend hours running and hope to be an excellent powerlifter as a result – the aerobic capacities, ATP replenishment windows, contraction speeds, etc., simply aren’t interchangeable. This is called sport specificity and can fall under the general umbrella of “athletic training,” specialized training for mastering a specific sport or activity.
As we age, slow-twitch, slimmer, less powerful Type I muscle dominates our bodies; while at the same time, we lose larger, quicker, more powerful Type II muscle.
For a person who has specialized in fast-twitch muscle development for the past 18 years, I would have to sacrifice my fast-twitch training to develop the endurance capacity to perform well in a marathon. That means losing the metabolic efficiency my Type IIa muscle gives me along with losing power, speed, agility, and strength. Further, I’ll lose the physique I spent years crafting.
I won’t be destroying my hard-earned muscle because a marathoner believes marathons are the best form of exercise or some great challenge. Marathons – and most endurance exercises – are simply repetitive motion. When it comes to marathons, it’s putting one foot in front of the other. The reason many people focus on running to be fit is because it’s a low-learning curve activity; far less challenging than developing a strength programming routine to improve your lifts six months down the line or embarking on a gymnastics program to proficiently perform handstands. But that’s just the start of the low learning curve of endurance training.
The primary ease to running is that the body doesn’t have to learn anything new. Unlike having to train your core, wrists, breathing, and proprioception for months before being capable of performing a single handstand, for example, running is something most of us can do a year or two after birth. Does this mean there aren’t some lessons runners can learn about good form: no, but many never learn proper running form either way, let’s be honest. This only further contributes to the degenerative effects of that form of exercise, i.e., posterior chain breakdown, muscle imbalances, joint issues. Running is a simple activity to take up and perform well at because it’s as simple as developing a tolerance for repeating the same motion over and over again. I won’t get into the issues with single-direction repetitive motion because that necessitates its own post. Ultimately, I beg to differ with marathon enthusiasts claims; marathons are not an ultimate form of exercise. Rather, they are simply very accessible to the general populace and, therefore, more people have experience with that type of challenge than with activities that test overall physical capability and functionality via multi-directional, multi-joint movements, and metabolic training. Regarding single-direction endurance cardio, anything is challenging if it’s repeated hundreds and thousands of times. The question is: is that challenge worth the repercussions, and will you gain the capabilities and physique you desire from it? For me, definitely not. Been there, informed myself out of that.
Now it’s time to cover my bases by stating: for us non-pro-athletes who don’t need to be specialists, it’s important to avoid training that is too lopsided in one direction: fast-twitch or slow-twitch. Instead, we should strive to be closer to well-rounded for long-term overall fitness. However, it is unrealistic to believe you can master every sport because your body will adapt to the training loads, durations, pace, etc., of activities you do most. As such, switching from fast-twitch to slow-twitch activities after specializing in either one, will take time. You will have to choose a side or be a generalist.
While focusing on being as functional, strong, and flexible as my schedule and life allows, I emphasize training my fast-twitch muscle. I’m pleased with my metabolically efficient and larger muscles vs when I fell for endurance-training misconceptions before I became a certified trainer. After which, I learned to train in ways that enable me to develop the body and abilities that inspire and motivate me. When it comes to lifelong fitness, inspiration and motivation are imperative. On the other hand, following the crowd because the crowd says you should do what they’re all doing, can lead you into the type of training that is unmotivating, uninspiring, and unable to deliver the results and grant the abilities you most desire.
Sources "Fast-Twitch Vs. Slow-Twitch Muscle Fiber Types - NASM." https://blog.nasm.org/fitness/fast-twitch-vs-slow-twitch. "Here’s What Proper Running Form Actually Is - SELF." 02 Apr. 2019, https://www.self.com/story/what-proper-running-form-actually-is. "Multi-Directional Strength: Why You Need It, How to Get It ...." 28 Sept. 2016, https://www.stack.com/a/multi-directional-strength-why-you-need-it-how-to-get-it. "Running Injuries Health Center - Sports-health | Trusted ...." https://www.sports-health.com/sports-injuries/running-injuries. "Stop Loading and Start Exploding: Power Training for ...." 26 Sept. 2017, https://www.acefitness.org/education-and-resources/professional/expert-articles/6572/stop-loading-and-start-exploding-power-training-for-powerful-aging/.
As an athlete for over 19 years and a broke single mom for most of that time, I created brokesinglemomfitness.com, now LLAFIT.com, to aid anyone who believes the road to fitness requires a lot of cash or time. In reality, the way to fitness is paved with knowledge and firm principles; teaching readers how to master both is the goal of this site. LLAFIT – Lifelong Affordable Fitness