Underestimating Type I Fibre Hypertrophy

After writing a recent T-Nation article on the use of multiple repetition ranges, one question from the comments afterwards stuck with me. If fatigue is essential for hypertrophy following training, why even bother with multiple repetition ranges? Couldn’t you train exclusively light, or heavy, as long as it was to failure (or fatigue) and still enjoy the same hypertrophic benefits regardless of load?

I offered up an answer, readdressing some of the points made in the article (conflicting research base, suitability of intensity ranges by specific exercises, high-load training optimal for strength development, training practices of experienced athletes, enhanced training variety). After thinking about it for a few weeks I feel that it’s possible that the issue has more to do with the interaction of load, fatigue, and the supposed preferential (or superior) hypertrophy of type II fibres, than the points in my initial answer to the question.

You dare question the almighty Type II fibre?

Type II fibres have long displayed greater hypertrophy following high intensity strength training (1-67), but that’s not the same thing as saying they have greater growth potential than the type I fibre. You see, the first statement is context specific, that they have higher growth potential with HIGH INTENSITY training, not necessarily this innate ability to outgrow type I fibres across all conditions. This relative relationship has gone unquestioned for decades; however, recent research suggests we’ve systematically underestimated our slow-twitch fibres, a consequence of studies that stack the deck in favour of the type II fibres. At this point, I’m willing to go as far as to say that the preferential growth of type II fibres is more an artifact of how we’ve studied them than a law of physiology to be etched in stone.

Type II fibre hypertrophy with high-intensity training

In a comprehensive review, Fry (1) collected a number of training studies, combined the hypertrophy data (by fibre-type) and performed a regression analysis, resulting in an expected percentage change in fibre size when trained at a given percentage of 1RM (adapted below).


These results demonstrate two main points critical to our current understanding of fibre-type specific hypertrophy:

  1. Type II Fibres (solid black line) have a higher peak percentage growth as compared to type I fibres (solid yellow line), and display greater growth at the majority of training intensities (>50%-1RM)
  2. At a given intensity however ( <50%-1RM), type I fibre hypertrophy can exceed that of the type II fibres

This indicates it’s not that type II fibres have a universal, superior growth potential, but that they display greater growth when trained on their own turf (high intensities). Give them home-field advantage, and more often than not they’ll win, but the same goes for the slow-twitch. Head over to the land of the light-weights, and we see that their growth can trump that of the type IIs. This collection of data makes a strong argument for the use of heavy weights in training for hypertrophy; greater growth of both fibre-types overall, but still demonstrates that fast-twitch fibres don’t have an innate ability to universally out-grow their type I brothers.

As it stands, we probably wouldn’t change much about our training, emphasizing the heaviest of weights (to 95%-1RM) seems to not only be best for the type II fibres, but also the type Is. However when you look at the characteristics of the studies used in Fry’s (1) analysis, you’ll see we did in fact stack the deck for type IIs, with the majority of studies using exclusively high intensities of training (8), and an under representation of studies using lighter intensities. Take this graph from Wernbom (8) who analyzed the increase in quadriceps cross-sectional area (CSA) across 46 studies (each point on the graph is a study). There are only 5 studies at or below 50%-1RM. When you look at the three studies between 20-30%-1RM, they more or less define the range of growth for all the higher intensities (shaded region) and this isn’t including the more recent study from Mitchell et al (9). While this is whole muscle CSA (not fibre-type specific), it’s still interesting how the hypertophy/intensity relationship for single fibre-types doesn’t necessarily pan out when looking at whole muscle CSA.


When you look at the breakdown of the individual studies by fibre-type specific hypertrophy from Fry (1), you see a similar pattern: many high-intensity studies and a relative dearth of low intensity training (8). There’s also another limitation to the regression analysis used by Fry (1); it’s difficult to put all the different training studies on equal footing. Despite the author’s best attempts, studies use different numbers of exercises, sets, durations of training, and often don’t account for whether sets were taken to failure or not (an important distinction). In this case, any investigation that measures fibre-type specific hypertrophy (type I vs type II) AND directly compares different training intensities (high vs low) is the gold standard, and while there are far fewer of these studies, there’s enough data to point us in the right direction.

Direct comparisons of Training Intensity and Fibre-Type Hypertrophy

Looking at the direct comparison of various training intensities on fibre-type specific hypertrophy, we see that the Type II fibre is not the superior grower that we thought. In a recent, now infamous study, Mitchell et al (9) compared low-load training (3 sets @ 30%-1RM) to failure against two higher-intensity conditions (3 sets @ 80%-1RM, 1 set @ 80%-1RM). At the end of the 10 weeks, whole muscle hypertrophy was similar across the groups (although half for the one set condition), and when looking at the specific fibre types, we see that the type I fibres more than kept up with the type IIs (see Table 1 from the original paper, adapted below). The robust increase in type I CSA while training at 30%-1RM seems supportive of the home-field advantage assertion, although no statistical differences by intensity or fibre type are stated.

[table caption=”Percentage change in fibre area from Mitchell et al 2012″ width=”100%” colwidth=”50|25|25″ colalign=”left|center|center”]
Training Condition,Type I,Type II
80%-1RM 1 set,16±7%,20±5
80%-1RM 3 sets,17±4%,16±4%
30%-1RM 3 sets,30±12%,18±8%

Up until Mitchell et al’s (9) study, Campos et al (3) had the definitive investigation of training intensities, that more or less agreed with the relationship in the graph above (1), higher-training intensities (4×3-5RM, 3 min rest or 3×9-11RM, 2 min rest)  produced greater growth in both type I and II fibres than lower intensity (2×20-28RM, 1 min rest). I’ve adapted their data to percent change based on their averages (excluded deviations) to fit with Mitchell et al (2012), and preserved the indicated statistical significance from the original data (bolded).

[table caption=”Percentage change in fibre area from Campos et al 2002″ width=”100%” colwidth=”40|20|20|20″ colalign=”left|center|center|center”]
Training Condition,Type I,Type IIA,Type IIb
Low Rep,12.45%*,22.94%*,25.27%*
Int Rep,13.14%*,16.27%*,27.26%*
High Rep,10.35%,7.97%,13.52%


These results are more or less in direct conflict with Mitchell et al (9), however in a recent attempt to reproduce these results, albeit NOT at the fibre-type level (whole muscle hypertrophy), a subsequent study (10) demonstrated no difference under the exact same training program (3). This agrees with the Mitchell et al (9) whole muscle data, and combined in the context of the fibre-type discrepancy makes a strong case for using multiple training intensities in your training and that we’ve systematically neglected, or at least underestimated our type I fibres. The one caveat however is a recent study from Schuenke et al (7), where high intensity training (Traditional Strength; 3×6-10RM @ 80-85%-1RM,) resulted in superior hypertrophy of type I, IIa, and Iix fibres, with no change following 6 weeks of low-intensity training (Traditional Endurance; 3×20-30RM @ 40-60%-1RM).

[table caption=”Percentage change in fibre area from Schuenke et al 2012″ width=”100%” colwidth=”40|20|20|20″ colalign=”left|center|center|center”]
Training Condition,Type I,Type IIA,Type IIX
Traditional Strength,22.9%,31.03%,37.2%
Traditional Endurance,-0.02%,8.93%,8.8%


 Evidence from the real world

Bodybuilders (11), powerlifters (12), and strongman (13) all use multiple repetition ranges to develop superior levels of muscle mass, the premise of my T-Nation article with JC Deen, however the amount of work devoted to different rep ranges varies between the groups. Powerlifters emphasize load (intensity) while bodybuilders emphasize volume; however, both pay respect to each others domains, borrowing principles as necessary to maximize results. The result of these focused efforts is fibre-type changes respective of these intensity domains. Powerlifters (and olympic weightlifters) have larger type II fibres than their bodybuilding brethren, but bodybuilders display a greater balance between the type I and IIs. This is best summarized in a figure from Fry (1), that combines multiple studies to show the relationship between fibre-type area (type I fibres yellow, type II fibres black) by sport (olympic weightlifters, powerlifters, and bodybuilders).


This data alone doesn’t indicate the responsiveness of the type I fibre for growth as it could certainly be possible that people with large type I fibres tend to gravitate towards bodybuilding as compared to their fast-twitch counterparts who prefer powerlifting and the olympic lifts. When combined with the fibre-type hypertrophy data from the training studies above however, we start to see those that train with heavy weights (or somewhat lighter weights with maximal speed) tend to end up with larger fast-twitch fibres, and those who have a tendency to emphasize volume or fatigue show a greater balance between the fibre types, and large type I fibres. This suggests that the supposed enhanced growth of the type II fibres may be more a consequence of training intensity (and how we study them) than an intrinsic ability of the fibre-type itself.

How could high intensity training limit Type I fibre hypertrophy?

The size principle of motor unit recruitment dictates that motor units are recruited on the basis of their size where small, slow motor units (slow twitch fibres) are recruited first, followed by progressively larger units (eventually fast-twitch fibres) until the required force for the task at hand is met (15,16). This principle is the basis for high-load strength training. Heavy weights maximize muscle recruitment from the get-go via the size principle and allow you to tap the growth potential of the type IIs (conscious manipulation of activation independent of load is a confounding factor here).

[table caption=”Characteristics of various motor unit types” width=”100%” colwidth=”40|20|20|20″ colalign=”left|center|center|center”]
,I (Slow Oxidative),IIA (Fast Oxidative Glycolytic),IIB (Fast Glycolytic)
Twitch Force,Small, Intermediate,High


In a high-load set to failure, just like the size principle dictated the order the motor units are recruited, it also dictates how they drop out when they fatigue. Under fatiguing conditions, the principle operates in reverse. Those strong, fast motor units that were late to the party, after the slow units started doing their thing, also leave early. The fast motor units fatigue first, dropping out and leaving all the force-generating duties to their weaker, type I counterparts.

As fast motor units are larger and produce more force than the slow units (16-17), when they drop out under high load, you create a situation where the load on the bar is too great for the remaining active fibres to lift, even though they haven’t been fatigued (to the same extent as the high-threshold, fast motor units). So our fast-twitch motor units are worked to fatigue, but our slow motor units still have a ways to go. In this case, the insufficient force production results in concentric failure and the set stops with ideal, fatiguing stimulation of the fast motor units and the slow units left unsatisfied. This explains why the majority of our studies on the intensity/hypertrophy relationship favour type II hypertrophy, since they tend to use higher training intensities (>60%-1RM) that may fail to sufficiently fatigue specific motor unit/fibre types, and ultimately stimulate hypertrophy in the type I fibres.

Conversely with low-load training, as fatigue is induced, higher-threshold motor units are progressively recruited, in the order dictated by the size principle (18). As fatigue increases motor units eventually drop out (fail to produce force) or remain active with compromised force production. The size principle is still preserved, and while the fast units may influence when failure ultimately occurs (19), the slow motor units will have been active for a longer duration within the set regardless. In this case, with a lower-load even if drop out of the fast-twitch units ultimately limit when the set ends (failure) the slow-twitch fibres have been active for a greater amount of time, receiving greater stimulation than under the high-load condition. This potential recruitment strategy can explain recent data finding equivalent hypertrophy with low-load training (9), and that the existing literature base (1) using exclusively high-intensity training may have under-estimated our slow-twitch fibres.

The case for fatigue in hypertrophy, reconciling the differences between studies

The argument that we have chronically under stimulated, and ultimately under-estimated our type I fibres hinges on the fact that a certain minimal threshold of time-under-tension is required, and that this is higher for type I fibres than type IIs and for lower intensity training in general. Up until now, we’ve seen data in support of both arguments, and when you look at direct comparison of fibre-type specific hypertrophy by training intensity, the data is split. But considering the neural mechanisms, the specific recruitment strategy behind both high and low intensity, we can explain the equivocal nature of the existing literature.

These divergent results are easily explained in how each study addresses the differential in exercise volume that occurs when training at each end of the RM continuum. In fact, studies that show equivalent results between training intensities make no attempt at matching work or volume (9,20), while studies that show the relative superiority of higher training intensities do (3,21), (with the one exception being Schuenke et al (7)). This is the oft repeated mantra of any periodization course (inverse relationship between volume and intensity), and as I’ve stated previously, time-under-tension can compensate for reduced intensity when hypertrophy is the desired outcome in training.

Looking at the acute protein synthesis data from Burd et al (20), we can see that attempting to work-match low-load training impairs the protein synthetic response. The authors tested three conditions (90%-1RM to failure, 30%-1RM to failure and 30%-1RM work-matched to the 90%-1RM condition) using a leg extension and four sets per exercise, three minutes rest between sets. I’ve discussed this study previously on this site, so I don’t want to belabour the point, but the 30%-1RM work matched condition failed to augment mixed muscle fractional synthetic rate as well as myofibrillar and sarcoplasmic protein synthesis whereas the 90%-1RM and 30%-1RM conditions did (albeit with slightly different time-courses).

This work does not necessarily implicate failure as a required end-point, it’s possible that there may be a ‘sweet-spot’ between the point of fatigue, a noticeable decrement in a performance variable (i.e. bar speed) and concentric failure. It does support that, when lower intensity training is used, training volume has to be higher to support hypertrophic adaptations similar to that of high intensity training. We can’t discount the ability of failure to maximize fatigue-induced motor unit recruitment and maximize metabolic stress that may be important for hypertrophic adaptations to training (22,23).

Forming evidence-based hypertrophy guidelines

Given the possibility that we’ve underestimated type I fibre hypertrophy an approach that varies training loads, over-time (periodization), within the session (different intensities by exercise type), and even within the set (rest-pause, dropsets) represents the best of both worlds. Using multiple intensity ranges acknowledges the existing literature base (3,7,21), summarized best in Fry’s (1) regression analysis (high intensity means more growth for both fibre-types), while also taking recent literature into account emphasizing the pivotal role of fatigue/failure (and relative independence of load) (9,20,24,25).


    Tony says:

    Great insights into prospective metabolic stress discrepancies and hypertrophy. However, we know there are a variety of different hypertrophic stimuli such as mechanical tension and muscle damage. I feel that many people often obsess over trying to find a specific stimulus that will induce greatest hypertrophy. Hypothetically, if someone feels they have discovered that range of intensity, repetitions, sets, and rest, would you constantly train within these recommendations? Would your body lose sensitivity to this type of stimulus?

    Dan Ogborn says:

    Hi Tony, Glad you liked the post.

    I agree there are various stimuli that are related to hypertrophy, but I think we've gone too far by separating them into discrete events that we can easily manipulate with programming variables. Take fatigue for example, which we usually lump into the metabolic stress category, however we can easily make a case for the role of fatigue in failed EC coupling, which is thought to be a preceding event to muscle damage (failure of the sarcomere to resist lengthening via myofibril links to sarcolemmal membrane via DGCs). As fibres and motor units drop out or at least suffer reduced force production, then we have an issue with force/tension in the muscle and how this is distributed across the muscle. I agree multiple factors affect hypertrophy, but I think we often discount the relationship between the variables and that they are often intimately related.

    I'm a proponent of multiple rep ranges that vary both over time (periodization) but also based on training goals and the characteristics of the various exercises (compound, isolation, etc…). Theoretically, as long as you account for increased strength and adjust the load accordingly (while staying in the same intensity range) what you mentioned should work, although I favour a varied approach, if not just to prevent monotony in training. If you were to maintain the same load then I suspect you would see a diminished anabolic response as you would use less motor units over time to move the weight (as individual motor units become stronger over time).

    Tony says:

    Again, very helpful explanation of events potentially leading to muscle damage. Correct me if I am misinterpreting this… Muscle damage due to fatigue will occur as long as failure is approached or reached. So, load (say 30% 1RM vs 90% 1RM), may dictate which fibers (higher threshold MUs or lower threshold MUs) have greater damage?

    Dan Ogborn says:

    Hard to say exactly, I think the data supports that Type I fibre hypertrophy is higher when greater work is performed with low-intensity loads, but the link to damage isn't in the literature yet. It's hard to find any definitive evidence of low threshold (type I) motor unit drop out in the literature, so I suspect that Type IIs units drop out in either the high or low condition, and may have a higher susceptibility to damage. As you mentioned in your initial comment, it may be that we have different mechanisms at play in differing fibre-types, but as per my post that overall hypertrophy requires a minimimum amount of work or time-under-tension, and that this is greater in Type Is.

    Ironthumb says:

    That is the reason why I always advocate the use of "NON-LINEAR" Periodization, for me that seem to work the best
    (non linear- change in sets and rep schemes every session or every week instead of every month like your usual linear counterpart) Are you getting the same results with your clients?

    Dan Ogborn says:

    I do tend to favour non-linear for most of my programming, but take this on a case by case basis. Considering that there's room within the training session to address multiple training intensities across various exercises makes session-by-sesesion loading fluctuations less of an issue for hypertrophy training IMHO.

    Ironthumb says:

    I can see where your coming from. BTW Dan, would that be considered "non-linear" as well if I am not mistaken. Since we do no formal periodization (I apply that only to my own training ; with clients I give them a more "formal" periodization) so would that not be some kind of a non linear as well to vary the schemes of reps within one session.
    And one more thing I would like to one day see a study the muscle types trained and recruited when you apply HIT techniques like drop set.
    Obviously you start out with a heavy load and then as you fail you drop the weight and extend the single set I suppose that would train a lot of TYPE 1 fibers big time.
    would the momentary anaerobic requirements of fatiguing muscles as you drop set let say 5 to 8 times activate and train more type 2 fibers.

    jerry Telle says:

    Holy cow!! I feel like I just had an entire upper level graduate course “crammed” into 20 minutes. Great stuff.

    In my experience and more generally in the research I have done and perused and how my mind plays with facts there’s many scenarios not represented here — as if it would be possible to generate said information in a volume of books!

    My humble truncated offerings:

    Plyometric rep style combined with explosive positives at 6-8 reps and
    one drop set of the same,
    then maybe a set of 120% 1RM eccentrics with 75% 1RM fast but focused positive reps,
    then 60% – 80% 1RM full rep resistance graduating to a resistance for longest muscle lengths partial force rep positives and partial negatives that hopefully can be stopped but cannot be reversed,
    and finally 30% 1RM full rep resistance “occlusion” graduating to a resistance for shortest muscle length “occlusion” partial force rep positives (if needed) with a 3 second fully contracted top position and partial negatives, if the athlete has any viable (not exhausted) neural energy,
    a drop to 15% and repeat!

    When I was a trainer a really good working paradigm for most trainers included:

    1 set of 75-80% 1RM positives readjusting the inertial geometry to approximately 110%-120% for negatives, including force rep partial longest muscle length positives and as much increase in resistance with the same weight for approximately 100% -110% for negatives. I realize this is not as

    Then 1 set of ~ 50-60% full range positives again readjusting the inertial geometry to approximately 90-100%1RM for negatives, including force rep partial longest muscle length positives and as much increase in resistance with the same weight for approximately 100% -110% for negatives:

    then 1 set of ~ 30% full range positives at a 3243 again readjusting the inertial geometry to approximately 50% 1RM for negatives, including force rep partials at shortest muscle length “occlusion” positives and as much increase in resistance with the same weight for approximately 50% negatives.

    One cycle of this seemed to be as effective as 2-3 cycles of the same or a similar paradigm.

    It’s much simpler than the rubric would indicate once one gets used to adjusting the inertial emphasis.

    occlusion fully contracted with loading commensurate loading

    Arne says:

    Although slow fibers might not be the best when it comes to max effort, having them in addition to the fast ones should make a difference.
    When bodybuilders max out the size of the fast AND the slow fibers, Why aren’t they the strongest?

    Dan Ogborn says:

    Hi Arne, You ask a good question that is unfortunately impossible to answer with the data we have available. Strength is dependent on many factors in addition to fibre CSA, but there’s just too many variables to control to ever come up with a definitive answer to that question, or even establish that they are in fact systematically weaker.

    DoYouEven? says:

    This article was superbly written, and imo whose interal logic too seldom used by the new gurus of the interwebz (aka Jason Blaha). Training the Type I fibers seem like a very clever idea indeed.

    But how come drop-set studies haven’t shown much benefits? Is it because they didn’t study fiber-type specific adaptations or because volume wasn’t controlled? Intuitively, it should benefit Type 1 fiber recruitement/exhaustion, right?

    Dan Ogborn says:

    It’s tough to say, anecdotally we know that bodybuilders have had a love affair with drop sets for years, but the scientific validation is lacking. I suspect you’re right that it may be a lack of consideration for fibre type specific adaptations but in reality I just don’t think we’ve studied them enough to really say for sure. I’ve written a post on “drop set science” that is up on another site that covers the issue, I’ll probably get a post up linking to it in the next few days.

    Maitland Barbell says:

    Hey Dan, great write up!!!

    Id be interested to here your thoughts on the training status of the subjects in these studies (including Fry’s) and how this may have effected the outcomes presented??? I havent read the latest literature yet and only vaguely remember Fry’s review, but if most of these studies did rely on untrained subjects could this explain some of the possible discrepancies found e.g. high intensity stimulating superior growth in both fibres in one study but not in others???

    On a more practical note, this seems to support the contention of multiple set training and its superiority for growth, as even when using heavy to moderate intensities (5-15rm), additional sets usually necessitate a decrease in load and hence subsequent increase in time-under-tension for the type I’s.

    Looking forward to your response 🙂

    Dan Ogborn says:

    The Fry study was a compilation of other studies, which for the most part (haven’t looked at them all specifically for this) likely used untrained subjects. It is always possible that training status influenced the results, and I do have plans to write a post about this, but we don’t have any evidence to suggest that the recruitment mechanism of type II fibres with fatigue that is likely contributing to these results should be any different.

    I prefer multiple set training programs myself, but I think this data makes a stronger case for ensuring there is a at least a degree of fatigue during the set. This could certainly be accumulated over multiple sets, but if those rest intervals creep too high, then I suspect you’d lose some of the benefit.

    Maitland Barbell says:

    I certainly agree with the fatigue notion; studies that have examined continuous vs intermittent reps have suggested fatigue is important for both strength and hypertrophy.

    Your second notion, however, is quite interesting. Some training protocols like Borge’s Myoreps use very short rest intervals between sets (rest-pause) and state specific reasons for doing so, but i havent yet encountered reasons for why having too long a rest period could be detrimental to hypertrophy (or strength for that matter). In fact if a longer time under tension may be required for better type 1 growth, straight sets using long rest intervals (to ensure best performance per set) would intuitively make sense, provided the set itself is taken to failure or somewhat close to.

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