Muscle Factor Model

Muscle Factor Model

How muscles function during and adapt to training

In the early 1970s a man named Arthur Jones introduced a revolutionary strength training method to the bodybuilding and strength training world. Jones had been studying muscle physiology for about 30 years and had long understood that the standard training methods of the day were not completely consistent with what was known about how muscles function during exercise or how they adapt to exercise. Many of the training practices of the day were rooted in tradition and contradictory to known physiological facts. Jones, a lifetime strength trainee himself, believed that training would be more effective if it were modified so that it worked in accordance with what was then known about muscles. He figured that a training program based on how the body really functioned would produce much better results than those training methods that ignored, denied, or were ignorant of the true workings of the body.

Utilizing his understanding of muscle physiology Jones spent many years testing and experimenting with different training methods, constantly seeking to discover training methods that produced the best results. Being independently wealthy afforded Arthur both the time and money required to test his ideas and he ultimately spent 20+ years and millions of dollars in his quest. The end result of all his work was a revolutionary training method – High Intensity Training – and a completely new type of exercise machine – Nautilus Training Equipment.

However, there was a problem; Arthur’s high intensity training method was not just revolutionary; it was contradictory to the conventional training wisdom of the day. Humans, being only human, are usually reluctant to abandon long-held beliefs and so many were resistant to Arthur’s methods. Controversy broke out about Arthur’s high intensity training method and two opposing camps formed – one group supporting high intensity training and one supporting conventional (high volume) training. These two groups spent lots of time and effort defending their methods and attacking those of the opposing camp. Even today, more than 35 years after Arthur first introduced high intensity training, the two camps still exist and the debate still rages. In fact, one of the the primary debates in the bodybuilding world is still centered around which method – high intensity or high volume – is best.

Of significance is that Arthur’s high intensity training method was basically the first time that exercise physiology was used as the foundation of a training program. Before Arthur, training was mostly based on tradition and what the top champions of the day were doing. Arthur completely ignored tradition and the training of the top champions of the day and focused on designing training based completely on the functioning of muscles. The fact that his methods continue to be widely used today is a testament to the effectiveness of his physiology-based training method.

The Problem of Two Opposing Theories

All this is not to say that the entire world has embraced high intensity training. As noted above, today the strength training and bodybuilding world basically consists of two opposing training methods – high volume and high intensity. Both methods are currently used and promoted as the best training method by their respective proponents.

The reason both training methods still exist is because both are known to work, at least for some number of people. And therein lies the problem. In science, anytime a theory is shown to be contradicted by even a single observation, then, by definition, that theory is inaccurate. When a theory is shown to be inaccurate it must be abandoned or modified. The high volume training theory and high intensity training theory are, in essence, opposing theories as to how the body works. Since these two theories contradict each other it means that both theories are wrong, at least to some degree.

The body works in one way, not in two contradictory ways. Or, said another way, there is one set of principles/laws by which the body functions, not two contradictory set of principles/laws. We know that both training methods produce results for some people. We also know that, by definition, both theories are wrong to some degree since they contradict each other. What all this tells us is that we are missing some important information as to how muscles function during and adapt to training. Once this missing physiological information is filled in, both of the competing theories will be assimilated and replaced by a new training theory. The missing physiological information is what has allowed the two competing training theories to continue to exist for more than 35 years and has prevented further advances in training methods.

Enter the Muscle Factor Model

In 2006, while conducting background research for an article on strength training for endurance runners, I came across a strength training study whose results were quite startling. The study compared a non-traditional training method to a standard periodized training program and found that the non-traditional method produced 50% greater increases in strength than did the periodized program. The researchers themselves were unable to explain why the non-traditional program produced the best results and noted that the results were contradictory to both current beliefs about the functioning of muscles and classical training methodology.

That particular study caused me to rethink some of what physiology currently teaches about muscle activation during exercise and its adaptation following exercise. In turn, this led to a breakthrough in muscle physiology; a breakthrough I have termed the Muscle Factor Model. I suggest that this new model more accurately explains how muscles function during and adapt to exercise. Furthermore, this new model suggests some significant modifications in training methods for any sport in which strength, power, or endurance is important. I believe the muscle factor model is a key piece of the missing physiological information and will ultimately result in the integration of high volume and high intensity training. The muscle factor model may lead to the most significant changes and refinements in training since the introduction of periodization in the United States back in the 1980s. I realize those are bold claims so let’s have a look at this new model. We begin with a discussion of muscle contractile properties.

Muscle Fiber Contractile Properties

Physiologists generally divide muscle fibers into three basic types – Slow Twitch, Fast Twitch A, Fast Twitch B – each with its own distinct contractile properties.

Slow twitch fibers are the weakest of fibers, contract relatively slow, and have very high levels of endurance.

Fast Twitch A fibers are stronger than Slow Twitch fibers, contract relatively fast, and have high levels of endurance.

Fast Twitch B fibers are the strongest of fibers and have the fastest contraction speed but have the least amount of endurance.

The above description of the contractile properties of each muscle fiber type might lead you to believe that each type of fiber has distinct contractile properties. Nothing could be further from the truth. Muscle fibers of any type are not all alike; they don’t all contract the same; they are not homogenous. Instead there is a broad continuum of contractile properties in all the muscle fibers of any type. Physiologists have measured up to a 129x range of contractile properties in muscle fibers of the same type. What this means is that in any specific fiber type you will find fibers that contract much slower or faster than other fibers of the same type; fibers that contract much more or much less forcefully than other fibers of the same type; fibers that possess much more or much less endurance than other fibers of the same type. For example, physiologists measured the time to exhaustion in a group of fast twitch fibers and found some of the fast twitch fibers fatigued in as little as 16 seconds while other fast twitch fibers were able to contract for 34 minutes before reaching fatigue. The contractile properties discussed earlier tell us what the average contractile properties are for each type of muscle fiber. The average Slow Twitch fiber is slower, weaker, and has greater endurance than any of the Fast Twitch fibers. The average Fast Twitch B fiber is stronger and faster but less enduring than other fiber types. But the broad range of contractile properties across all muscle fibers means that fibers of the same type do not all have the same level of strength, endurance, or speed.

A very important point about muscle fiber contractile properties is that there is a strong inverse relationship between a muscle’s strength and its endurance. The stronger a muscle fiber the less endurance it has and vice versa. Weaker fibers possess much greater endurance than do strong fibers. Stronger fibers possess much less endurance than weaker fibers. This point is critical to understand.

Muscle Activation During Exercise

Not all muscle fibers are activated during exercise because the body only activates the minimum number of fibers required in order to get the job done. Muscle fibers are activated in a very specific order, from weakest to strongest. Physiologists have termed this the size principle of activation. Basically, muscle fibers are recruited based on the amount of force required to complete the task at hand. Recall that there is a wide variation in the strength of muscle fibers; every whole muscle has fibers with different levels of strength, from very weak all the way up to very strong. The weaker fibers are recruited first with the strongest of fibers only being recruited during the heaviest of tasks. Fibers are generally recruited in the following order based on the level of force required to perform the task:

Slow twitch – Fast Twitch A – Fast Twitch B

There are 2 important points to understand about muscle fiber activation – 1) it is a team sport and 2) total force is the sum of the force of all the active fibers.

1. It’s a team sport: Muscle fiber work together. Activation proceeds from Slow Twitch – Fast A – Fast B. It is NOT the case that Slow Twitch fibers exclusively handle the easy tasks, Fast Twitch A exclusively handle the moderate tasks and Fast Twitch B exclusively handle the heavy tasks. Instead, as the load increases from easy to moderate to heavy an increasing number of fibers are activated and all are working together to complete the task.

2. The total force produced by a whole muscle during a task is the sum of the force of all the individual fibers. All active fibers, whether Slow Twitch, Fast A, or Fast B, contribute force during movement and the total amount of force generated by a muscle is the sum of the force of every active fiber. During a really heavy lift, even though the Fast A and Fast B fibers are activated and doing the bulk of the work, active Slow Twitch fibers are producing force and helping lift the weight.

In practical terms this is what it means:

If you pick up a light weight, then only Slow Twitch fibers will be activated because little force is needed to pick up the weight.

If you pick up a heavy weight then both Slow Twitch + Fast Twitch A fibers will be activated because more force is required to lift the weight. Note that the Slow Twitch fibers are still active during this exercise, but since they are unable to generate enough force to get the job done by themselves, some Fast Twitch A fibers are also required to help out.

Pick up an even heavier weight and now you are using Slow Twitch + Fast Twitch A + Fast Twitch B fibers to lift the weight. The Slow Twitch and Fast Twitch A fibers did not possess enough strength to lift the weight by themselves, so the strongest of fibers, the Fast Twitch B fibers, were activated.

The same thing applies to any activity. For example, running at a slow pace activates only Slow Twitch fibers because the force required to run slowly is small enough that the Slow Twitch fibers are strong enough to handle the job themselves. Running at a faster pace activates Slow Twitch + Fast Twitch A fibers because running faster requires more force to be generated. Very fast running (i.e. intervals and sprints) and fast or steep uphill running activate the Slow Twitch + Fast Twitch A + Fast Twitch B fibers due to the high level of force required to run at very fast paces.

Muscle Fiber Activation at Exhaustion

As an exercise proceeds it becomes increasingly difficult to maintain a set amount of force production because of fatigue. The first repetition of an exercise might be reasonably easy but repetition 20 with that same weight might be an all-out effort. Are all fibers activated during the hard to all-out effort that athletes routinely reach during intense workouts? Only in some cases; in most cases not all fibers are activated.

During exercise as a person’s active muscle fibers fatigue some inactive muscle fibers are recruited to assist those active fibers that have fatigued. However, there is a limit to the amount of additional fibers that are recruited. Not every muscle fiber is activated during exhaustive exercise. Instead, the person reaches exhaustion or terminates the exercise.  About the only time that all fibers are active is during the heaviest of tasks, such as during very heavy weight lifting (i.e. about 6 or less reps). Less forceful tasks, such as high rep strength training or distance running, do not result in 100% activation of all available muscle fibers, even at the end of the exercise when the trainee is working as hard as they can in that particular exercise. For example, one study found a little less than 70% leg muscle fiber activation while running to exhaustion on a level treadmill and a bit more than 70% activation during exhaustive running up an inclined treadmill.

Overload and Intensity

One of the primary principles of training is the overload principle. Exercise physiology generally describes overload like this – the application of an activity specific overload in order to cause physiologic improvement and bring about a training response. What this means is that muscles must be trained with a sufficient level of intensity in order to cause adaptation to occur. There is nothing earthshaking in the concept of overload as it has been a principle of training for more than a century.

However, we need to carry the concept of overload a bit further and apply it to individual muscle fibers; what is true for a whole muscle is also true for individual muscle fibers. In order to cause a training response in any individual muscle fiber that muscle fiber must be trained with a sufficient level of overload, with a sufficient level of intensity. This is accomplished by training a fiber reasonably close to its maximum capacity. Or said another way you must sufficiently fatigue a fiber in order for it to adapt and improve. This point is critical in understanding how muscles fibers work and adapt to training.

Let’s examine this principle in training terms.

You put weights on a bar so that you are only able to lift the bar a maximum of 10 times. Since the bar is very heavy you will activate Slow Twitch + Fast A + Fast B fibers while lifting it. After 10 reps (about 30 seconds of lifting) you are no longer strong enough to lift the weight an additional repetition so you set the bar down, ending the exercise. Which fibers did you overload?

You only overloaded some of your Fast B fibers. Specifically, you overloaded those Fast B fibers that fatigued in 30 seconds or less.

There were a whole bunch of fibers that you didn’t overload. Which ones? Those fibers that take longer than 30 seconds to fatigue were not fully overloaded when the set ended.

At the end of the set some of your Fast B fibers were exhausted and couldn’t continue to contract. But a lot of your Fast B and all your Fast A and Slow Twitch fibers were not exhausted at rep 10 because they posses more endurance than the strongest of the Fast B fibers (remember, it has been shown that it can take several minutes to exhaust all the Fast B fibers). The reason you terminated the exercise at rep 10 is because the whole muscle lacked the strength to lift the weight, but only some of the Fast B fibers were fatigued.

This set fatigued, and therefore overloaded, some of the Fast B fibers and those are the fibers that will get stronger. But the remainder of your Fast B and all your Fast A and Slow Twitch fibers were not particularly overloaded and will adapt little to none.

When those few Fast B fibers adapt you will be stronger but you will not be as strong as you could get. Why? Because lifting a heavy weight is a team effort and all your Fast B, all your Fast A and all your Slow Twitch fibers contribute to the total strength of the muscle but you didn’t adequately train all your Fast B or your Fast A and Slow Twitch fibers to get stronger. Only when you train all your fibers to overload will you get as strong as you are genetically capable of getting.

Putting it All Together = Muscle Factor Model

When we put all the above facts together, we arrive at the Muscle Factor Model. In order to cause an adaptive response in a muscle fiber, that muscle fiber must 1) be active and 2) be overloaded; failure to accomplish both of these results in little to no adaptation in that muscle fiber.

Recall the inverse relationship between a muscle fiber’s level of strength and its endurance capacity – the higher the strength the less the endurance, the lower the strength the greater the endurance. If you are going to overload a muscle fiber you must work it to a reasonable level of fatigue. Considering that muscle fibers posses widely varying levels of endurance, this means that only a relatively few muscle fibers are fatigued at the end of any normally conducted exercise session.

In training terms this means:

In order to overload weak muscle fibers with abundant endurance requires long training sessions conducted at low levels of force production.

In order to overload stronger muscle fibers with moderate levels of endurance requires moderate duration training sessions conducted at moderate levels of force production.

In order to overload the strongest of muscle fibers with poor endurance requires short duration training sessions conducted at high levels of force production.

If you want to maximize your performance, then you have to train all the muscle fibers that contribute to force production during your chosen activity. You have to train your weak fibers, your moderate fibers, your strong fibers, and your strongest fibers. Since force production is a team effort any untrained fibers detract from the overall performance of the team (in this case the team is the whole muscle).


The muscle factor model provides a more complete explanation for how muscle fibers work during and adapt to exercise. Only muscle fibers that are active and overloaded during exercise will adapt and grow. The only way to overload a muscle fiber is to train it to a sufficient level of fatigue. Normally performed exercise programs usually do not train all or most of the fibers in a whole muscle due to the way muscle fibers are activated during exercise and because muscle fibers have widely varying levels of endurance. The only way to maximize performance is to train all the muscle fibers that are active during the event; any untrained muscle fibers prevent the athlete from reaching his/her maximum potential.


Muscle Factor Model — 40 Comments

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  2. Thanks for writing this article about the Muscle Factor Model- it taught me a lot about the physiology of muscles and of muscle fibres themselves.

    However, I’m not sure I agree with your conclusion of having to train all muscle fibre types to achieve optimal performance. I say this because you informed us in the article that there is a strong inverse relationship between the strength and endurance of a muscle fibre, and the strength and endurance of a muscle. I would think that the goal of training would be to make muscles as strong as possible, as competitions usually involve lifting the heaviest weight possible for only a few repetitions. Training muscle endurance would reduce the ability to lift such a heavy weight, as increased endurance = decreased strength. Therefore, it doesn’t seem logical to me to recommend training Slow Twitch muscle fibres very much in preparation for a strength contest or event. It seems to me that, while training Fast Twitch A and Fast Twitch B fibres would be beneficial to performance, training Slow Twitch muscle fibres would be detrimental.

    I do make one assumption to arrive at my conclusion that training Slow Twitch muscle fibres is detrimental during training for a strength competition; the assumption I make is that Slow Twitch fibres will always gain increased endurance during adaptation (or at least gain more endurance then strength), and that Slow Twitch fibres cannot be converted to Fast Twitch fibres or gain more strength then endurance during adaptation. I seem to recall reading that this is true in Tim Noakes book, the Lore of Running- if I recall correctly, he said that only Fast Twitch A fibres can be converted to another type of muscle fibre, and that Fast Twitch B fibres and, more related to the point I’m trying to make, Slow Twitch fibres, will always remain as their original type of fibre.

    I acknowledge the possibility (to be honest, near-certain probability) that my thinking is incorrect, and that I have arrived at the wrong conclusion. I don’t have the physiological understanding or experience in the field of physiology to understand these concepts to the degree that you are able to. If I have made an error in my thinking, please explain to me where I have gone wrong, and clarify how training Slow Twitch fibres is beneficial for a strength event.


    • Hi, Jake.

      I think you’re on the right track.

      The practical application of the muscle factor model is that we have to train as many fibers as can reasonably be trained. No one can equally train all fibers all the time. You can’t optimally train for a marathon and at the same time maximize strength levels for a powerlifting contest. High levels of endurance training appears to negatively impact strength and size. Building maximum size will negatively affect endurance performance. So, optimal training at either end of the spectrum (max strength on one end and ultimate endurance on the other) will preclude some amount of training near the other end. But athletes not exclusively focused on the extremes can benefit from both strength and endurance training (athletes such as sprinters, middle distance runners, decathletes, football players, etc).

      The question to be asked is how many fibers can I train before training becomes excessive and negatively impacts gains and/or performance?

      For max strength athletes I would say that the focus should be on training all the Fast and the strongest of the Intermediate fibers. If that is true then we need to know what is the range of fatigue in the Fast fibers. Research indicates it to be from a few seconds to about 2 minutes (though the research is far from definitive on this point). Intermediate fibers range in fatigue from about 1.5 minutes to more than 30 minutes (again, less than definitive research).

      I submit that reps in the 3 – 100 probably get the strength athlete in the ball park. Minimally I think reps of 3-40 will likely get the strength athlete 98% of all the strength they are capable of gaining. That’s my recommendation for “training as many fibers as can be reasonably trained” for strength athletes in order to maximize gains.

  3. I believe you’ll find that most ELITE distance runners tend to follow the idea that they need to train all muscle fibers. (i mean look at the Kenyans-thats how they train-either knowingly or not!). Distance runners may overdue some of the slow twitch but maybe that buys them a little insurance. It seems to me the key to distance running improvement is to find that balance of training which impacts the slow to the fast twitch fibers for each specific distance one is racing WITHOUT overdoing it to the extent that one becomes overtrained. (Optimal training load). I do believe that one runs a risk of overtaining as more training of the fast twitch fibers takes place. In general, as intensity goes up, relatively speaking, volume should decrease with “balance” being KING.

    • Hi, Fred.

      I agree that most ELITE distance runners are likely training all their muscle fibers (or as many as can realistically be trained). Most training programs contain a mix of long/slow, medium/medium, and short/fast workouts, with each workout specifically impacting a set of fibers. I would say that most/all would not reference training specific muscles fibers as the reason for doing those workouts – likely it would be explained in terms of VO2max, lactate threshold, running economy, aerobic, and anaerobic. But even if the physiological explanation is wrong it doesn’t change the effect of any of the workouts.

      You are right that a balance of training has to be found. Each person has their own individual genetic talent. That talent combined with that person’s current level of fitness and capability determine what level of training is currently optimum for that person.

  4. Hi Rich.

    You write some great articles so thank you.

    I have a question about training using your Muscle Factor Model in terms of muscle gain.

    From my limited understanding it seems that “drop sets” would be optimal for muscle/strength gain based on your model, would you agree?

    Also what about a method such as clustering using a heavy weight to complete multiple low rep sets in quick succession? would this adequately overload the weaker more enduring muscle fibers as your model suggests is required or does such a method only serve to increase fatigue of the stronger fibers such as in a traditional set?

    • Hi, Sam.

      “From my limited understanding it seems that “drop sets” would be optimal for muscle/strength gain based on your model, would you agree?”

      That is an excellent question. I’ve given it some thought but haven’t personally experimented with using drop sets within the muscle factor method. I have used drop sets in the past with conventional training methods, just not with muscle factor training. I haven’t tried it because my opinion is that drop sets would likely be less effective than straight sets. My reasoning is that I would more likely reach failure during each drop due to metabolic factors or nerve system fatigue and not muscle fiber fatigue. If that were the case, then the muscle fibers would experience less overload from drop sets, leading to lesser gains.

      “Also what about a method such as clustering using a heavy weight to complete multiple low rep sets in quick succession? would this adequately overload the weaker more enduring muscle fibers as your model suggests is required or does such a method only serve to increase fatigue of the stronger fibers such as in a traditional set?”

      Anytime you complete multiple sets for a particular muscle you will be providing more overload for the weaker, more enduring, fibers than if you had done just one set. I suggest this is the primary reason multiple set training programs have proven to be superior to single set training programs.

      The primary question is “what method provides the most overload to as many of the fibers as possible?” The research says the range of fatigue for the Fast fibers (type IIb) is approx 3 – 120 seconds. That means in order to train all the Fast fibers you will need to do sets ranging between about 2 – 50 reps (using a 2-3 second rep cadence). Clustering would provide additional overload on the weaker fibers as compared to a single set of an exercise. But would it actually fatigue the weaker fibers, the ones that take 1-2 minutes to fatigue? Does it provide the necessary broad range of time to fatigue? It would depend on how clustering was performed. Plus, as with drop sets, the concern is whether the cause of failure is overload on the muscle fibers or the nervous system.

      • Thanks for the reply Rich.

        I re-read parts of the article above and also your Muscle Factor Training article to try and find the reason why you theorise that combination training is optimal for strength/size gain. From what I understand your theory is that training with variety of rep ranges/weights in the same workout leads to greater overall muscle fiber fatigue and therefore adaptation? However taking in consideration your point about drop sets/clustering possibly fatiguing the nervous system more so than the muscle fibers, doesn’t this argue in favor of an approach of periodisation from high reps/light weight to low reps/heavy weight over several weeks as then you are focusing on the muscle fiber types and lessening the impact of the nervous system fatigue?

        Also do the different types of muscle fibers have different recovery times? do fast twitch fibers recover faster than slow twitch or do all fibers recover at the same rate?

        • Hi, Sam.

          “From what I understand your theory is that training with variety of rep ranges/weights in the same workout leads to greater overall muscle fiber fatigue and therefore adaptation?”

          A variety of reps will train more total muscle fibers, causing an adaptive response in those fibers. The point being to fatigue as many fibers as is practical. But this is different than training so as to create greater fatigue in a particular fiber.

          Typical training programs do include a variety of reps/weights but typically the range of reps/weights is very narrow. For example, most training programs consist of multiple sets, but all sets are done with reps in a range of 5-15 reps. This, in my opinion, is too narrow of a range in order to train as many fibers as is desirable or practical. Certainly, multiple sets within a narrow range of reps/weights will create greater overload on the fibers being trained (perhaps too much overload, but that is another question). It just won’t train as many total fibers as would be trained if a broader range of reps/weights were used.

          “However taking in consideration your point about drop sets/clustering possibly fatiguing the nervous system more so than the muscle fibers, doesn’t this argue in favor of an approach of periodisation from high reps/light weight to low reps/heavy weight over several weeks as then you are focusing on the muscle fiber types and lessening the impact of the nervous system fatigue?

          Great question. One of the more interesting findings from periodization research is that nonlinear periodization (also called daily undulating periodization by some researchers) has been found to be more effective than other forms of periodization. Nonlinear periodization doesn’t include the typical training periods (i.e. hypertrophy phase, strength phase, peaking phase), instead it modulates training on a weekly basis (first workout of 3-5 reps, second workout of 6-8 reps, third workout of 10-12 reps). Researchers have not identified physiologically why nonlinear periodization works better, they just know it does work better. I suggest the muscle factor answers the “why” question. And I suggest that the reason nonlinear periodization doesn’t need to include the standard training phases is that individual muscle fibers are getting trained less frequently, thus giving them more time to recover, negating the need for standard training phases.

          “Also do the different types of muscle fibers have different recovery times? do fast twitch fibers recover faster than slow twitch or do all fibers recover at the same rate?”

          Not that I’ve been able to determine. I’ve not seen any research indicating different recovery rates, but this isn’t something that has been extensively researched. My personal observation of athletes and in myself is that it takes me about as long to recover from any type of workout. It takes about a week for my fast fibers to recover from a hard strength workout and it takes about a week for my intermediate fibers to recover from a hard power (tempo) run workout.

  5. Hi,
    My power and strength is coming on quite a bit along with my muscle size, however my stamina is going downhill i can only run about 2 miles without stopping completely, how can i improve my stamina ( activate slow twitch muscles without burning muscle – strength and size? ). I also want to increase my fast twitch muscle, and increase my overall health and overall lifting capacity. How do i use science and physiology to succesfully train each muscle type at the same time, and make good proggressions?


    • Hi, Charlie.

      The way to improve your stamina is via endurance workouts. However, too much training results in overtraining and a decrease in performance. If your stamina is decreasing the first thing I suggest evaluating is whether you are training beyond your body’s ability to recover.

  6. considering this article, and one trainers particular training method, what are your thoughts on Dramatic Transformation Principle?

    where you do 5 sets of decreasing reps and higher weight, then do another 5 sets in reverse process ( IE start at say 40 lbs and 50 reps then 50 and 40 on down to 10 reps then 10 reps and back up the range and down the weight)

    • Hi, Curious.

      I am against it for 2 reasons. First, the volume of sets is much higher than research indicates is optimal (or effective) for those with average levels of genetic talent. 10 sets is significantly higher volume than necessary to fully stimulate a growth response and for the average person could very well lead to overtraining.

      Second, the intensity of effort will necessarily be low during the majority of sets performed in order for the trainee to have sufficient energy to complete the entire 10 set process. Lower intensity means few active muscle fibers. Muscle fibers that aren’t activated don’t grow and improve.

      • thank you for the reply, I was curious as I decided to give this method a try and while I have noticed gains in bicep/tricep size primarily, it doesn’t seem to be all that it is hyped up to be.

        I’ve also noticed through additional research that it may primarily be an increase in sarcoplasm that is creating the size increase.

        it also seems that for me, more high weight low rep training would be preferential as it produces more contractile based muscle fibers or myofibriol and would increase strength moreso then size. (IE a higher density)

        now I just need to find my 1RM

  7. I’ve recently read Body by Science, and it seems that the Muscle Factor Model directly contradicts one of the foundations of muscle physiology that McGuff and Little use to show that high intensity, low volume, low frequency training is optimal: the theory of sequential recruitment of muscle fibers. In Body by Science, page 47, sequential recruitment is the process whereby motor units of muscle fibers are recruited only as lower-force-producing motor units (small motor units consisting of Type 1 fibers) fatigue. If all motor units have been fatigued before the low-force-producing motor units recover (which they do relatively quickly), then all fibres can be fatigued over the course of a high intensity single set (measured in Time Under Load (TUL) not reps).

    You talk about fiber teamwork, which seems to refer to the what McGuff and Little call simultaneous recruitment. They say that simultaneous (rather than sequential) recruitment happens when the lower-force motor units are not able to lift the load at all, without recruitment of higher-force motor units. They also say that simultaneous recruitment would not be effective at fatiguing all muscle fibers, for the same reason that you do: because higher-force fibers would fatigue, resulting in concentric failure, leading to the exerciser ending his or her set before lower-force fibers had been fatigued.

    So, can slow fibres fatigue quickly (even if they recover quickly)?

    • I haven’t read Body by Science so I can’t reference it directly. However, I can point out the following facts. Simultaneous and sequential recruitment are both common and frequent occurances. Anytime you lift weights both simultaneous and sequential recruiting of muscle fibers occur.

      When you begin a typical set in a typical weight training program, you sequentially recruit a high percentage of the muscle fibers in the working muscle. Let’s say you have a weight on the bar such that you will only be able to lift that weight a max of 10 reps (I choose 10 reps because it is a common # of reps in most programs, but we could choose any # of reps from 1 – 100 and it would not change how the process works). In order to lift that weight for the 1st rep requires a simultaneous recruitment of thousands of fibers in the working muscle. All of your slow twitch fibers will be activated, all or nearly all of your intermediate fibers will be activated, and some of your fast twitch fibers will be activated on that first rep. The reason all those fibers are activated on the first set is that the slow twitch fibers are not strong enough, by themselves, to lift the weight. Therefore your brain had to recruit all the intermediate fibers too. But, all the slow + all the intermediate fibers were not strong enough to lift the weight so the brain had to recruit some fast twitch fibers so that the weight could be lifted for the first rep. Therefore, we see that simultaneous recruitment slow, intermediate, and fast twitch fibers is occurring at the very first rep.

      After a few reps, some of the fast twitch fibers are now fatigued. Have you ever noticed that as you conduct a set each successive rep gets harder than the one preceding it? Well, that’s due to accumulating fatigue in the active muscle fibers. As those fibers fatigue they are no longer able to generate sufficient force to lift the bar. In order to continue lifting the weight the brain has to recruit additional muscle fibers – sequential recruitment. As you continue to perform reps, more and more active fibers are fatiguing so additional muscle fibers must be recruited in order for sufficient force to be generated to lift the weight. Eventually, though, the brain stops recruiting additional fibers (even though some fibers are not activated), at which point the active fibers are too fatigued to lift the weight another time. The set is terminated assuming you don’t revert to cheating or assistance to continue the set for a few more reps.

      This simultaneous and sequential activation occurs every time we lift weights.

      Let me ask you a question – what if at the point of rep failure, instead of terminating the set or reverting to cheating or assistance to continue the set, you had a buddy instantly strip 50% of the weight off the bar. Would you be able to do one or a few more reps at that point? If you aren’t sure of the answer go try it. You will find that you can, indeed, do a rep or a few reps more. Let’s say you do those extra reps and then your buddy cuts the weight on the bar in half again. Would you be able to do even more reps? If you aren’t sure, try it. You will again find that, yes, you have the strength to a few more reps.

      Why? What is happening within your muscle fibers that allows you to lift a lighter weight even though you are too fatigued to lift a heavier weight. The answer is that some of the active fiber must not be fatigued to the point of being unable to lift the lighter weight. When your buddy stripped off half the weight on the bar, the brain de-recruited the largest and strongest active fast twitch fibers, leaving the slow and intermediate active and available to continue working. The fast twitch fibers were too fatigued to continue working but the slow and intermediate fibers were not at the point of exhaustion. But since they aren’t as strong as the fast twitch fibers the only way you can continue the set is by stripping off some of the weight. Once you have decreased the weight on the bar such that the active slow and intermediate fibers can lift it, you were able to do more reps. At that point, many of the intermediate fibers fatigue and you, once again, have to decrease the weight on the bar if you want to continue the set.

      I don’t know of any research that shows that slow fibers can be fatigued quickly. If McGuff and Little are claiming that slow fibers can be fatigued quickly you should ask for and carefully review the research. Can you think of a lifting method whereby you are unable to lift any weight whatsoever at the end of a short, high intensity set? If not, then it indicates that a pool of unfatigued fibers remains available.

  8. I know I mentioned I was going to revert back to strength training, but my usual method of training was find a 5 rep max for sets of 3 that I can do with good form, and lift it until I can up the reps to 3 sets of 20, then up the weight to another good form rep of 5 set of 3

    what do you think of this method of lifting?

    • Curious,

      I think your method will result in strength increases. However, I don’t believe it will be as effective as regularly performing a variety of different reps.

      Are you familiar with the Theory of Constraints? Basically, this theory says that at any one point in time you have something that limits your strength, some physiological component that prevents you from lifting more weight (something always limits your strength or you could lift any amount of weight as many times as you wanted). In order to increase your strength you have to train that limiting physiological component so that it is no longer the thing limiting your strength. When you do that, your strength increases. However, something else immediately becomes the constraining physiological component, limiting your (now higher) strength. That being the case, you would now need to train the new physiological constraint so that it no longs limits your strength.

      One could argue that the first physiological constraint is also the 2nd physiological constraint – in other words, the physiological element that initially limited your strength adapted with training so that your strength increased but it still remains the limiting physiological constraint. However, that argument is not supported by the observable facts – when trainees perform the same workout and the same number of reps every workout strength gains slow and then stop over a short period of time.

      As Arthur Jones, the inventor of Nautilus, used to say, “Once you learn to spell your name, writing your name over and over won’t make you a better speller.” In other words, variety is not just the spice of life but is the way to higher strength levels and better performance.

      With the program you have described I suspect you will reach a plateau below your ultimate potential.

  9. That’s a great thought experiment that you propose, and thank you for the very detailed reply! You describe the blurriness of fiber types very well. There are a few resources cited for this section on muscle physiology, which I haven’t read yet (excuse my poor format).
    Milner-Brown, Stein, and Yemm, Journal of Physiology, April 1973, two articles: 230, no. 2: 359-70 The orderly recruitment of human motor units during voluntary isometric contractions”, and 230, no. 2: 371-90 “Changes in firing rate of human motor units during linearly changing voluntary contractions”. Also, two articles from the Journal of Neurophysiology: 55, no. 5 (May 1986): 1017-29, and 57, no. 1 (January 1987): 311-24.

    One potential other reason that you may be able to continue lifting lighter weights is that slow fibers recover from fatigue quickly. That’s one of the key ideas that McGuff and Little try to get across (p49):

    “This is why it’s desirable to employ a moderately heavy weight that allows you to progress through all three motor-unit types quickly enough to recruit them all, but not so quickly that only the fast-twitch fibers receive the bulk of the stimulation, and not so slowly that the slow-and/or intermediate-twitch motor units can recover and you end up cycling through the same lower-order motor units again. That would leave the bulk of the fibers in the muscle that you are training largely unstimulated.”

    In their follow up book, The Body By Science Question and Answer Book, McGuff and Little do suggest the possibility that type-I muscle fibers may need more frequent training to maintain their condition than do type-II fibres, and they’d like to do further research on that question. The degree of overload that type-I fibers receive in a single high-intensity workout per week may also be yet to be determined, so adding some lower intensity training may result in more significant overload for them, which would help to explain the Spanish racquet-sports study results. (I also recognize that they were training more frequently than once per week.)

    So, I’ll check those citations. The model presented by McGuff and Little states that type-I fibers are recruited first and while they fatigue quickly, they recover very quickly too. (This needs questioning). Also, the implication is that a single cycle of fatigue at a time is sufficient overload to trigger muscle growth for all types of muscle fibers. (While this may be true for type-IIb fibers, is it true for lower order fibers? This also needs questioning).

    • makelemonade,

      My personal bias is that McGuff and Little’s assertions are likely not supported by the research. So, I agree with you that their suggestion that slow twitch fibers recover rapidly (and during the performance of a set) is questionable. They would also need to show that a limited subset of slow + intermediate fibers are cycling throughout a set such that other slow and intermediate fibers are not being stimulated.

  10. well I know my 5 rep max ( good form) is definitely NOT my max.

    right now my every other day routine consists of individual bicep curls 3 sets of 5 , back rows( currently increased weight and at 3 sets of 10), Arnold press one side at a time ( the one for the shoulders) and flat bench press with each set increasing incline to target different sections of the pectoral mucles which right now it’s 1 set of 10, 1 set of 8, and 3rd set of 7 to 8 on bench

    I also do core exercises and use resistance bands for warm up and cool down, with resistance I do open arm pull backs, back rows, and open arm chest( arms fully extended outward and pull together in front), as well as core based excersises. and bicep/tricep isolation. I usually do 1 set of 10 for warm up, then pull the bands further after my main routine for more resistance and do 1 to 2 sets of 10

    what would you suggest to make it more effective? so far this method has given me much more noticeable size increases on both arms and chest and back. and the muscle is much harder then my previous experimenting with the DTP routine.

    • Curious,

      Personally I have found that training each muscle once per week works best for me. For the first 20 years that I lifted I always trained each muscle multiple times per week. Most of the time I trained each muscle twice weekly but for about 2 years I trained each muscle 3 times per week (20 years provides lots of time to try different training methods). I discovered once per week per muscle by happenstance (back then no one recommended once per week per muscle training since it violated the “96 hour rule”) and found it to be dramatically superior to training 2 or 3 times per muscle per week for me

      I’m a very average guy when it comes to strength building genetics which makes me suspect that the majority of trainees would do better on a once per week per muscle program than on any multiple workouts per muscle per week program.

      Further I found that 4-6 sets total per larger muscle (biceps, back, and legs) and 2-3 sets per smaller muscles (biceps, triceps, and delts) produced better results than doing more total sets or fewer total sets. The first 2 years that I lifted I mostly did 15 sets per muscle (for example, 5 sets of bench press followed by 5 sets of incline press followed by 5 sets of flyes) using a pyramid scheme of reps (10, 8, 6, 8, 10 reps). I quickly stopped growing following this plan. After about 1.5 years of few gains I adjusted my workout to 6-8 sets total per muscle. That program worked much better. Several years later I went to a HIT program of 1-2 sets total per muscle, which worked marginally better for me. I made my best gains ever when I started training each muscle just once per week, for 2-6 sets total per muscle, with a mix of reps (8-10, 25-30, 45-50).

      You might design a similar program and see how it works for you.

  11. so it is pretty much a genetics disposition, along with application of research and then experimentation to find your genetic ability.

    I was mainly reading a lot of your articles, and deduced that higher weight lower rep causes the CNS to recruit MORE muscle tissue/fiber, then a light weight high rep, this in turn uses up more muscle fibers and creates a need to more fiber production to increase strength. this also increases size. your strength article mentions a 13% increase in muscle size if I recall correctly when compared to lighter weight training which yielded a 2.6% increase in size.

    I should have mentioned probably that I am 28 so still young in that respect, and I work a manual labor job for the most part ( sometimes we have really slow days at the shop) I find it hard to figure my exact genetic disposition in regard to muscle gain/strength. but I have noticed that I seem to be slightly above average for muscle gain/work out intervals. as I typically seem recover from a high weight low rep set in 24-48 hours.

    however I will take your advice into consideration and switch it up and see what happens, thanks for your time and knowledge on this subject.

    • Curious,

      “So it is pretty much a genetics disposition, along with application of research and then experimentation to find your genetic ability.” Well said. There is such a wide variance of genetic talent across humans that no single program works for everyone. Genetics determine just how big and strong you are capable of becoming. Training and nutrition determine how much of your genetic potential you will achieve. Experimentation will reveal which training method works best for you.

      You are right, heavier weights cause the recruitment of a greater amount of muscle tissue. However, the key is not simply that heavier weights recruit more muscle fibers. Heavier weights cause the recruitment of fast twitch fibers – the fibers with the greatest potential to increase in size and strength. Fast twitch fibers adapt to training by becoming bigger and stronger, which is why heavy weight and low reps produce the greatest increase in strength. The heavy weight is required to specifically activate and overload the biggest and strongest fast twitch fibers.

      However, not all fast fibers are exactly the same. They exist on a continuum – some are very strong and fast, others are less strong and fast. A program consisting exclusively of heavy weights and low reps (which is what almost all strength training programs recommend) does not adequately overload all of the fast twitch fibers and few of the intermediate fibers. Moderate weights and somewhat higher reps (20 – 50) are necessary in order to overload the undertained fast and intermediate fibers.

      • How exactly does a program consisting exclusively of heavy weights and low reps not adequately overload all of the fast twitch fibers and few of the intermediate fibers? As a powerlifter, the goal is to attain the greatest amount of strength to lift the greatest amount of weight at a competition for a single rep at a time. Wouldn’t solely training fast twitch fibers be of my benefit? Thanks!

        • Vince,

          Yes, only training fast twitch fibers through a traditional workout of heavy weights and low reps is beneficial. However, such a program does not train all the fast twitch fibers. In particular, the Fast Twitch IIA (what I call intermediate fibers) are undertrained on programs consistently only of heavy weights and low reps.

  12. Hello. After studying, reading and practicing strength training for 20 years and trying various programs and methods, it is TRULY refreshing and a blessing to have found your site. I have started to use your Muscle Factor Training suggestions. Gordon Lavelle’s book “Training for Mass” has also helped me greatly. Thank you for all the enlightenment and excellent research.

  13. thank you for clarifying that particular lack of activation. I like using my resistance bands for higher reps/warmup sessions, I may recruit it more for variation, and move strength based only training to work one two off, and on one of the off days from strength use resistance for high rep training. will give this a few months worth of experimentation and see what results I get. thank you again, you have been extremely helpful.

  14. Hello again, I’ve incorporated the resistance bands for lighter training( and I can also use them for medium as well by stretching them or doubling them up and doing isolation) and I’ve expiremented some as well with training frequency, here is the results I’ve noticed.

    I’ve started to get more definition in muscle groups, but no real size increase I can notice. I’ve noticed if I go with a 3 to 4 day training frequency it seems my heavy lift days are harder then before to push for that extra rep then when I was doing a every 2 day training frequency with heavy alone. but it seems I “peak” growth/recovery on the 2nd to 3rd day so within 48 to 72 hours. I also do my light/medium training with 1-2 sets at an 8-10 rep of heavy afterward.

    so I think my frequency should be heavy then off/cardio, then a mix of light medium with isolation, and heavy with isolation, then off two days then heavy only again. I guess I just have a weird genetic makeup.

    thank you for the help and clarification.

  15. after more experimentation, I’ve noticed that actual full recovery occurs after 5 days after a heavy lifting period, and 2-3 days on medium/light lifting.

    I’ve also noticed better gains with mixing up off/on days, for example do a workout set, take 5 days off, do another workout set, take 3 days off, do another take 4 days off. ect. ect. but I never take less then 3 days off.

    I’ve increased my curl weight to 100 from 80 in about 3 weeks by mixing up on/off days and light/medium with heavy training ( trying to take advantage of the muscle “continuum” and hitting the fiber on light weight most on one but ending with hitting the fast twitch for heavy on light medium workouts for at least 1 set of 5 rep)

    I believe another part is that is is also starting to train my CNS to recruit more fibers then before as well.

    I’ve also varied my lifting between group sets and isolation sets, and am also currently able to do military press at 100 but it feels like I could probably go up to 110-120. ( just don’t have enough weight plates)

    like you said experimentation for individual genetics will yield the best results.

  16. Something just seems not entirely right with this picture…a minor point is the division: endurance vs. strength..

    But if one could actually measure the amount of work performed by each type of muscle fiber? It would seem that the division might not be so very strict between endurance and strength, as it is not between the muscle fiber types themselves..

    What I feel is very much neglected is what most just refer to as the “CNS” because there is no easy way to measure motor-neuron activity/adaptations in relation to the muscle.. number of synapses, amount of neurotransmitters, and their receptors in neuromuscular junctions, the same thing in respect to the brain vs. motorneurons in the spinal cord, etc. the “neuron factor”, if you will…

    From what I have read so far I am not such a firm believer in this strict division between slow, fast A and B fibers either.. It would seem to me that they are rather plastic, as observed in tapering for example, when fast A fibers seem to convert to fast B fibers.. Are there any respectable studies that show, on a cellular level, that slow fiber is slow, and nothing else, that fast fiber is fast and nothing else, or could these be just particular adaptations that single fibers are capable of in response to the tasks put before them? Would very much appreciate a quote..

    I also have some practical quibbles with the order in which these fibers are supposed to be activated during training. The theory states you do not activate many of your fast fibers when your muscle is fatigued (intensity vs. effort).. but after some reasonably long light to medium activity- for example.. 3-4 mile run many people find it hard to match even 50% of their maximum reps in pullups or pushups..
    How were their lats or pecs fatigued during that light run? Were their fast fibers in these muscles fatigued or the slow ones? Which ones are activated in these subsequent exercises. Personally, I have observed great increases in strength and speed working at max. effort when “pre-exhausted”..

  17. Hi! I need some advice. I am a competitive crossfit athlete. My cardiovascular and muscular endurance (fitness) is excellent, but I am struggling to get my strength where it needs to be (like for instance in my 1RM attempts for squats, deadlifts, snatches, cleans and jerks, bench press etc.) Unfortunately, to perform well in crossfit, you need both strength AND endurance. My strength is good but it’s not where it should be, I’d say it’s a limiting factor in me reaching my full potential or winning crossfit competitions. The 5×5 or 5×3 ans similar set / rep ranges just doesn’t seem to do the trick for me. My coach (just a crossfit coach, not a professional in the field of sport science or anything like) tells me that I can only focus on one of these (strength OR endurance) at a time and not both – so in essence he is saying that you can’t build strength while training for fitness / endurance, and vice versa. I’m not sure I agree with him, but I need professional advice from someone who knows the science behind it. Maybe I’m wrong and he’s right, but then I want sound evidence. I would really appreciate it if you could shed some light on the matter? I don’t want to “focus” on strength and then my fitness / endurance goes down… Thanks so much! Looking forward to see your reply!

    • Magda,

      It has been my experience and the research supports that it is possible to gain both strength and endurance at the same time. However, the research on this topic has shown an “interference effect”, where trainees doing aerobic training have not increased strength as much as trainees doing strength training only. What this means is that if your training program includes both strength and endurance training you will both increase strength and endurance. But your strength gains won’t be as high versus if you had only trained for strength.

      That being said, there are some weaknesses in the interference studies. We don’t know if different types of endurance training (say running versus cycling) have different interference effects (my guess is that different types of endurance training have different interference effects). We don’t know how much aerobic training a person can do before they interference effect shows up (can you run 3 day a week with no interference effect?). And, finally, we don’t know if the interference effect goes away over long periods of time (As I recall all the interference studies are all short term studies lasting 12 weeks or less. What happens when a trainee does an entire year of strength and endurance training?).

      In summary, the research on this topic is incomplete. You can gain strength and endurance at the same time, but there is an interference effect. Beyond that, each trainee has to experiment to figure out what works best for him/her.


  18. Here is what my wisdom tells me.I should split my workouts for different fibers the same way with different muscles. If I have pull-> push->leg split I think it would be best to alternate reps every week for fast A and B fibers. Doing 4 heavy reps one weak and maybe 20 moderate next for a total of 5 sets per workout. thoughts? Also suggest rest between sets I use 5 minutes

  19. Great insights overall. I am small boned and have never gotten good results from sets of under 15 reps (5 second reps). When I read about Kearney J. Andersons work on the effects three different rep ranges I got excited about the 36 rep workouts. With a 5 second rep (optimum growth hormone stimulation) this leads to 3 minute sets. I began doing all of my strength training using a timer for 2 to 4 minute sets. In 14 months, my progress has been quite good. As a personal trainer with sleep apnea, low thyroid and spinal injuries, I am thrilled.

  20. Hello sir i absolutely loved the article i have a question.
    do we know what percent is the contribution of slow twich fibres in strength activities or will it change depending on the proportions of fibre types in a person?
    Also since slow twich fibres are somewhat little exhausted after heavy low rep sets wont performing high rep sets after heavy sets be more time effective to exhaust slow twich fibres instead of starting the workout itself with high rep sets?
    Thanx a lot in advanve!

    • Saurabh,

      As best as we know, the slow twitch fibers are 100% activated during typical strength training activities.

      People have varying amounts of slow twitch fibers. Some people are born with relatively high amounts of slow twitch fibers while other people are born with relatively low amounts of slow twitch fibers. Training does not seem to change fiber type. In other words, endurance training doesn’t increase the amount of slow twitch fibers a person has and strength training doesn’t increase the amount of fast twitch fibers a person has.

      I suggest doing high rep sets after low rep sets. This way you maximize the amount of weight used during the low rep set and pre-exhaust the slow twitch fibers for the coming high rep sets.

  21. Hello mate nice article, one question i have to get the max of Both worlds ie all the twitch fibers do u think when i train chest? Say two Sets of 8’10 reps then 1set of 12 to 15 then 1 set 15’20 reps to build Muscle?. What are your thoughts about this ?

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