Muscle Fiber Recruitment


by Devin Sarno, NSCA-CPT

Our muscles are a little more complex than just the quadriceps or pectorals or biceps; there is something more going on in there that many of us pay no mind to. Unless you are an athlete or very dedicated to fitness, muscle fibers and muscle fiber recruitment may have never even crossed your mind. However, it is important for the average everyday gym-goer to have some basic knowledge on muscle fibers and muscle fiber recruitment. Are all muscle fibers the same? Do different muscle fibers have different responsibilities? Are you using your muscles to their full capabilities? Can you manipulate how your muscle fibers are recruited? 

The first thing you need to know is the different types of muscle fibers and what they do. Type 1 muscle fibers are referred to as slow twitch muscle fibers. They are primarily used for more endurance type exercise that requires less force or speed, such as steady-state cardio (ie: a 2 mile run). Then there are Type 2a and Type 2x muscle fibers. Each of these muscle fibers are fast twitch muscle fibers, with Type 2x fibers being required for movements of the highest force and speed. Type 1 muscle fibers are the smallest of the 3, while Type 2x muscle fibers are the largest.

Muscle fibers naturally fire in the order of smallest to largest. No matter how high the intensity, how much power, force, or speed is required for a movement, type 1 fibers will fire first. Even though they are more prevalent in endurance and low force movement, they will always fire first and continue to fire throughout a movement no matter the force required. The biggest difference is that the Type 2 fibers start to get recruited during those higher force movements with Type 2a getting recruited after Type 1. The type 1 fibers are then doing a smaller percentage of work as those remaining fibers keep continuing to fire. With our way of life, we generally have a greater amount of Type 1 fibers, they are very resistant to fatigue relative to Type 2 fibers, made to maintain posture. Each of these muscle fibers reach their peak power in 100 millisecond or less, which sounds extremely fast especially for Type 1 fibers. Even given this fact, athletes benefit even more from having more Type 2 fibers because they contract quicker, reach their peak power quicker, and exert greater force.

When it comes down to it, the nervous system is really what controls your muscle fiber recruitment. You may not be recruiting all of the muscle fibers that you are capable of, particularly in movements that require a greater fast twitch response. This is especially the case for exercise beginners. In fact, that is generally going to be responsible for much of your initial strength gains. If they are not typically used, that doesn’t mean that your Type 2 fibers are not there, they may just be dormant. As you begin to partake in strength training, which happens to require a fair amount of fast twitch movement for your concentric muscle contractions, you will notice some significant gains toward the beginning of your strength training journey. While you may be getting stronger, probably the largest culprit of your increased strength is actually your nervous system and increased recruitment of Type 2 muscle fibers. This is important for anyone just starting out with a strength training routine to know so that they don't get discouraged when they're 3-6 months into a consistent strength training routine and then notice they're progress slowing. 

One thing that you can't do is turn slow twitch (Type 1) muscle fibers into fast twitch (Type 2) muscle fibers. However, we only use a small amount of muscles fibers, so there's still potential to move quicker and produce more force. So although you may not be able to alter your anatomy and the muscle fiber makeup you currently have. With training, you can make your body recruit a greater amount of Type 2 muscle fibers. Can some individuals have a greater percentage of Type 2 muscle fibers than others? Yes, but that does not mean that you cannot be stronger than somebody with a greater amount of Type 2 fibers. If you incorporate the proper training stimulus, you may be able to fire a greater percentage of your Type 2 muscle fibers than the same person with more Type 2 fibers than you. So although the person with more Type 2 muscle fibers may have a greater potential, at the end of the day hard work will still win out!

There is so much going on beneath the surface that so often overlooked. Knowing how your muscles work should have a direct impact on how you train. Important points to take home: Type 1 (slow twitch) muscle fibers are used primarily for endurance and are more resistant to fatigue, recruited first, and always fire; Type 2a and Type 2x muscle fibers are fast twitch fibers and are more responsible for movements that require greater force and speed of contraction but they fatigue quicker, fire after Type 1 fibers. And you can actually train to have your nervous system fire a greater percentage of your type 2 fibers at a given time. So knowing all of this now, be sure to train accordingly!