Everything You Need To Know About Fast-Twitch And Slow-Twitch Fiber Distribution

Whether your goal is related to aesthetics, or performance, having an understanding of how your muscles are designed is probably in your best interest if maximizing your time and effort is important to you. There are three primary types of skeletal muscle fibers and they are recruited in order from slow twitch, to fast twitch I, and fast twitch II. Each type of muscle fiber’s ability to generate, and maintain force, is inversely proportionate, and directly related to its size.

Slow twitch fibers are the smallest of the bunch, therefore their ability to generate force is minimal by comparison, although their capacity to maintain force is the greatest of the bunch, given that they get enough oxygen when the muscles relax, as they rely on oxygen to perform work. If they fail to get the oxygen they need because the muscles remain fully contracted for the duration of the set, then the body is left with having to recruit fast twitch fibers by default, since they are capable of generating force without the presence of oxygen.

Fast twitch type I fibers are larger than their slow twitch counterparts, and therefore can generate more force, and they rely on glycogen for energy as opposed to oxygen, resulting in a lower capacity to maintain force.

Fast twitch type II fibers are the largest of the bunch and capable of generating the greatest levels of force, albeit with the lowest capacity to maintain force as they also rely on glycogen for energy. These muscle fibers must be forced into play through heavy lifting, or by intending to move a weight as fast as possible during the concentric portion of a lift. These fibers are also preferentially recruited during eccentric actions, as the body perceives a muscle being stretched under load as potentially dangerous, so to prevent catastrophe it recruits its largest, strongest, muscle fibers. Knowing this enables you to consciously modify the way in which you perform exercises to allow for a more deliberate and intentional response.

Because fast twitch muscle fibers rely on glycogen for energy, the more they are stimulated, the greater growth hormone response there will be due to the acidic pH levels resulting from the increased lactate production associated with fast twitch fiber stimulation. This will positively effect the rate at which muscle is built and fat is lost, so it’s in your best interest to try to recruit and fatigue as many fast twitch fibers as possible through training regardless of your goal.

Since muscle fibers are recruited as needed, slow twitch fibers are recruited first, and maintain producing force until it is deliberately released. However, you can negate their potential influence to overall force production by applying constant tension which prevents oxygen to enter the muscle. When a muscle is contracted, lactate will build up in the cell waiting to be released so blood can be rushed to the muscle to clear it away, but it can’t enter until the tension is released. By default there will be greater fast twitch fiber activation, but the only problem here is that the longer you hold the contraction, the more pain and burning you’ll endure.

As far as fiber type distribution is from person to person, or muscle group to muscle group within the same person, is concerned, that will differ on an individual basis as there is no person, or muscle group, that is exclusively made up of either fast or slow twitch muscle fibers. Muscles, like every other aspect of the human body, adapt over time and some areas will take on a greater percentage of fast twitch characteristics over others. For example, it is not uncommon for a person to have a higher amount of slow twitch fibers in the quad and fast twitch fibers in the hamstring.

Generally muscles have a greater percentage of either fast, or slow twitch muscle fibers based on their primary function, or rather the purpose they serve us. If a muscle’s primary purpose is to endure long periods of time where it must remain (semi) active, as some would more than others during general everyday activities like walking, or running, then it will likely be made up of a greater percentage of endurance oriented slow twitch muscle fibers. If a muscle’s primary function is to assist with getting us out of fight or flight situations, than it will likely be made up of a greater deal of performance oriented fast twitch muscle fibers.

It is also common for the same muscle to have differing fiber types from origin to insertion. For example, the lat usually has a higher amount of slow twitch fibers near the insertion in the upper arm than where it originates in the lower back where it has a higher amount of fast twitch fibers. This is because it is a lot more active during common arm movements through a limited range of motion in comparison to when it is needed to generate maximum force to pull the arm into shoulder hyperextension (think elbow behind the body/shoulder).

So what does all this mean for you? Well, it means that to get the best results, you need to stimulate your muscles with the appropriate number of reps each week based on your genetics. Generally, slow twitch dominant muscles need a greater amount of reps on a weekly basis, while fast twitch dominant muscles respond better to a lower total amount of reps per week (generally because heavier weights are used to stimulate them, and the higher intensity takes longer to recover from). Obviously logic would dictate that a mixed fiber type would fall somewhere in between. For slow twitch dominant muscles, aim for 120 reps per week, 80 reps for fast twitch dominant muscles, and 100 for mixed fiber types.

The question now becomes, how do you know if a muscle is more fast twitch dominant or slow twitch dominant? Strength testing. Performing as many reps as you can with roughly 85% of what you can lift one time will give you a general idea for now, of whether or not the primary muscle groups involved in a lift are either fast twitch, slow twitch, or of a mixed fiber type.

If you get more than 5 you score as slow twitch dominant in that muscle/movement. If you get less than 5 you score as fast twitch dominant. And if you get 5 exactly then you score as a mixture of fast and slow twitch fibers distributed within that muscle/movement. Based on the result, you can effectively select the appropriate amount of reps to stimulate the intended muscle for the best potential to make it grow.

It’s not the end of the world if you score as a slow twitch dominant person that wants to get big, as bodybuilders generally are slow twitch dominant, especially in comparison to their powerlifter, Olympic lifter, and strongman counterparts. Also worth noting is that the results of the testing are not completely indicative of fiber type distribution, as it is the nervous system that is responsible for recruiting muscle fibers, so you may just lack the capacity to currently recruit the fast twitch muscle fibers, but that will change over time with experience.

However, slow, cyclic aerobic work such as long distance cardio, can result in decreased ability to recruit the fast twitch muscle fibers, because activities such as this heavily rely on slow twitch muscle fibers which conditions the nervous system to preferentially recruit them. This is especially bad for athletes involved in sports where speed and power is paramount, as it trains them to be slower and weaker.

It is for the reasons explained above that there is no cookie cutter protocol for building muscle. The amount of weight, reps, sets, and rest intervals best suited for one individual will differ from another, as well as one muscle group to the next. The quads may benefit from sets of 50-100 reps with little breaks in between until completion, while the hamstrings may be better suited for performing sets of 5 with greater rest intervals, unless of course the individual in question is more fast twitch dominant in the quads, and slow twitch in the hams.

If you have any questions regarding muscle fibers, feel free to contact me at ben@paramounttraining.ca. I'm available for online consulting and personalized program design, as well as one on one training if you are located in the Greater Toronto Area (GTA).