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Which muscles are fast-twitch and which are slow?

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Chris Beardsley is a biomechanics researcher. He is also the editor of Strength and Conditioning Research. He is based in Loughborough, Leicestershire.

Most people associate slow-twitch muscle fibres (or type I) with marathon runners and Tour de France cyclists and fast-twitch muscle fibres (or type II) with Olympic sprinters and weightlifters.

But like most aspects of the human body, things are never that black or white, and muscle fibre type actually varies a lot more between muscles than between individuals training for different sports.
A long-accepted and widely-believed training theory is that focusing on developing the type II muscle fibres, which respond most significantly to resistance-training, is the best approach to build bigger, stronger muscles. However, research published this year found that both type I and type II muscle fibre areas can contribute to muscle growth and should be trained wherever possible[1]. If this is the case, then how do we train the type I and type II muscle fibre areas optimally?

Muscle fibre functions
Muscles with large proportions of type I fibres are highly resistant to fatigue and respond best to sets of higher repetitions. Muscles with large proportions of type II fibres fatigue quickly and respond best to high-load, low-repetition sets. Muscles with a mixture of both type I and type II muscle fibres will probably need a combined approach, perhaps in a programme where some days are devoted to high repetitions and others to low repetitions. Here’s what the research tells us about the different fibre make-up of the lower-body muscles so we can build up a picture of how you should train the legs optimally for hypertrophy. For the muscle fibre make-up of your upper-body muscles, click here.

Perhaps because of the huge quads boasted by Tour de France cyclists, many have fallen into the trap of assuming that the quadriceps are predominantly made up of slow-twitch muscle fibres and so big legs are built using higher repetitions. However, such anecdotes must be balanced by the huge quadriceps owned by most Olympic weightlifters, whose low-rep squatting routines build these muscles fast and effectively.
The quadriceps are so-called because they are a muscle group comprising four individual muscles: the rectus femoris, the vastus lateralis, the vastus medialis, and the vastus intermedius. That means there is a lot of research to look at.

The rectus femoris – the only two-joint muscle – seems to be fairly fast twitch, with one study  finding that the proportion of type I fibres was <50%[2]. Another found a range – depending on location measured – of between 30%-43%[3], with another reporting around 42% type I[4].

The fibre type of the vastus lateralis seems more balanced. A recent systematic review found that the mean type I fibre proportion was exactly 50%[5]. The two other quadriceps muscles have rarely been explored. Studies done on the vastus medialis have reported very mixed results: a range of 44%-62%[6], and a value of 51%[7], which is broadly in the middle of that range, although another study reported values at the top end of the range of between 60%-64%[8]. The vastus intermedius, like the rectus femoris, seems relatively fast twitch: one study reported a value of 47%[9], with another reporting 46%[10].

To summarise, the rectus femoris and vastus intermedius seem to be definitely quite fast twitch; the fibre type of the vastus medialis is unclear, and the vastus lateralis definitely has a mixed fibre type. So while we still need a range of loading protocols, we should certainly lean towards preferring heavier loads and lower-rep sets for building bigger quads.

Many strength coaches starting out believe that the hamstrings contain mainly type II muscle fibres and should be trained with low repetitions and high loads. But does the research support this?

One of the first studies found that the lateral hamstrings displayed a greater proportion of type I fibres, at 67%[11]. Later research found more balanced results: one reported 55% of type I[12], another found an even balance in the medial hamstrings, but slightly more type I fibres in the lateral hamstrings[13]. More recent research also reported an even balance with 51% type I[14].

Therefore, contrary to popular believe, the research tells us that the hamstrings contain a fairly even ratio of type I to type II muscle fibres. That means we should train them with a mixture of high and low repetitions for the best-possible results.

Gluteus maximus
Not many people think about training the gluteus maximus with targeted movements, which is odd when you think that it is the largest muscle in the body.

It’s rare to find bodybuilders who don’t perform leg extensions as well as their squats or leg curls on top of their deadlifts. But ask about what targeted work they do for the glutes and you tend to get blank stares.
So the question of what fibre type is most common in the gluteus maximus isn’t one that you hear very often. But, for the more enlightened lifters, there’s a lot we can learn from the research.

One of the earliest studies found that the glutes displayed an even mix with 52% type I and 48% type II[15], while another found a greater proportion of type I fibres at 68%[16]. So it appears that this huge muscle contains a fairly even balance of both type I and type II muscle fibres. That means we should do our glute bridges and hip thrusts with a mix of higher and lower rep sets.

The soleus and gastrocnemius together make up the calves. The soleus is a single-joint muscle that lies underneath the multi-joint gastrocnemius.

This difference is important, because it means that when we are sitting down with our knees bent – such as when doing seated calf raises – the gastrocnemius gets too short to contract properly so the soleus has to do all the work. On the other hand, when we are standing up, the gastrocnemius tends to take over, especially if the knee extends and lengthens the gastrocnemius, as it does during gait.

Many advanced bodybuilders consequently train these two muscles separately, by performing calf raises seated for the soleus and standing for the gastrocnemius. But what repetition schemes are best? One of the first studies found that the soleus displayed the highest proportion of type I fibres of any muscle measured, at 89%, with the gastrocnemius having a fast-twitch range – depending on location measured – between 44%-51%[17].

Another reported 70% fast-twitch for the soleus and 50% for the gastrocnemius[18], while a more recent study reported 90% and 70% respectively[19]. So while the soleus does display a strong trend towards type I dominance, the gastrocnemius displays more of a blend of type I and II fibres. That means we should probably always perform seated calf raises with very high-rep work, but standing calf raises and farmers’ walks with a mix of different loading schemes.

Optimal workout design
When you’re designing your next lower-body training programme, spare a thought for muscle fibre type. The quadriceps benefit from some high relative loads, so multiple sets of low repetitions like Olympic weightlifters use will probably work pretty well. The hamstrings and gluteus maximus need a balanced programme of high and low repetitions. The soleus needs hammering with high repetitions, most likely using the seated calf raise machine, while the gastrocnemius needs a blend of high and low repetitions.

1 Ogborn, D., & Schoenfeld, B. J. (2014). The Role of fibre Types in Muscle Hypertrophy: Implications for Loading Strategies. Strength & Conditioning Journal, 36(2), 20-25.
2 Jennekens, F. G., Tomlinson, B. E., & Walton, J. N. (1971). Data on the distribution of fibre types in five human limb muscles. An autopsy study. Journal of the Neurological Sciences, 14(3), 245.
3, 6, 11, 15, 17 Johnson, M., Polgar, J., Weightman, D., & Appleton, D. (1973). Data on the distribution of fibre types in thirty-six human muscles: an autopsy study. Journal of the Neurological Sciences, 18(1), 111-129.
4, 7, 10, 12  Garrett, W. E., Califf, J. C., & Bassett, F. H. (1984). Histochemical correlates of hamstring injuries. The American Journal of Sports Medicine, 12(2), 98-103.
5 Gouzi, F., Maury, J., Molinari, N., Pomiès, P., Mercier, J., Préfaut, C., & Hayot, M. (2013). Reference values for vastus lateralis fibre size and type in healthy subjects over 40 years old: a systematic review and metaanalysis. Journal of Applied Physiology, 115(3), 346-354.
8 Travnik, L., Pernus, F., & Erzen, I. (1995). Histochemical and morphometric characteristics of the normal human vastus medialis longus and vastus medialis obliquus muscles. Journal of Anatomy, 187(Pt 2), 403.
9, 18 Edgerton, V. R., Smith, J. L., & Simpson, D. R. (1975). Muscle fibre type populations of human leg muscles. The Histochemical Journal, 7(3), 259-266.
13 Pierrynowski, M. R., & Morrison, J. B. (1985). A physiological model for the evaluation of muscular forces in human locomotion: theoretical aspects. Mathematical Biosciences, 75(1), 69-101.
14, 19 Dahmane, R., Djordjevič, S., & Smerdu, V. (2006). Adaptive potential of human biceps femoris muscle demonstrated by histochemical, immunohistochemical and mechanomyographical methods. Medical and Biological Engineering and Computing, 44(11), 999-1006.
16 Sirca, A., & Susec-Michieli, M. (1980). Selective type II fibre muscular atrophy in patients with osteoarthritis of the hip. Journal of the Neurological Sciences, 44(2), 149-159.


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