The muscle fibre growth formula

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

If you know which upper-body muscles are predominately made up of either fast-twitch or slow-twitch muscles fibres you are better able to train your chest, shoulders, arms and lats to elicit maximum gains.

First, here’s a brief recap on fibre types. Muscles with large proportions of type I (or slow twitch) fibres are highly resistant to fatigue and respond best to sets of higher repetitions. Muscles with a large proportion of type II (or fast twitch) fibres fatigue far quicker and respond better to heavy-load, low-rep sets.

Muscles with a mixture of both type I and type II muscle fibres will need a combined approach, perhaps in a programme where some days are devoted to high repetitions and others to low repetitions[1]. Here’s what the research tells us about the different fibre make-up of each upper-body muscle group so we can build up a picture of how you should train these muscles optimally for hypertrophy. For the muscle fibre make-up of your lower-body muscles click here.

Not many studies have explored the muscle fibre types of the pectorals, but those that have reported very consistent results. The first found that type I fibre proportion of the pectorals ranged from 42% to 43%[2], while a 2007 study similarly reported a low proportion of type I fibres of around 35%[3], making the pecs predominantly type II. This is probably why guys who bust out thousands of press-ups can sometimes look like they don’t even train, while powerlifters who bench big weights for low-rep sets have huge chests. Therefore, the pectorals are likely to respond best to lower-rep, heavy weight sets. Press-ups can still be effective, but you may need additional weight to elicit hypertrophy, either with a weighted vest or using heavy resistance bands.

A 1971 study looking at the general population found that type I muscle fibre proportion in the shoulders was >50%[4], while one conducted two years later reported a range of 53% to 61% in similar subjects[5]. However, a 1982 study assessed the middle deltoid of international-standard canoeists and reported a much higher type I fibre proportion[6], while a study investigating wheelchair basketball players, wrestlers, soldiers and kayak athletes and found a range of 48% to 70%[7]. More recently, a range of 47% to 56% was found across the anterior, middle and posterior deltoids[8], while a similar figure of 55% was found in elite tennis players[9].

Therefore, the shoulders tend to display a leaning towards type I fibres, although to reach the upper end of that range probably requires some degree of sport specialisation and extensive training. So even though most trainees prefer to lift overhead with as much weight as they can handle, the shoulders are likely to respond better to a strategy of high-rep sets, lower weights and plenty of fatigue.

The first batch of studies found an even balance of muscle fibre types in the biceps. One found that in the general population type I muscle fibre proportion in the biceps was around 50%[9]. Another reported a slightly lower range of 42% to 51%[10], while two separate studies reported 52% to 53%[11,12].

More recent studies have found that the biceps contain a greater proportion of type II fibre: two reported a low type I proportion of just 39%[13,14], while a 2007 study found that type I muscle fibre proportion was 37% to 39% across various parts of the muscle[15].
Therefore, the biceps display a leaning towards type II fibres and are likely to respond better to lower-rep sets and relatively heavy loads.

Almost every study looking at muscle fibres in the triceps have reported a low percentage of type I fibres, with figures ranging from 32% to 33%[16], 35%[17], 44%[18], with the highest ratio of 49% to 50%[19].

So the research is overwhelming that the triceps display a strong leaning towards type II fibres, so are likely to respond best to lower-rep sets and plenty of weight. Benching like a powerlifter, with a close grip and tucked elbows is probably a good bet, as are heavy weighted dips if your shoulders can handle them.

Studies that have investigated the latissimus dorsi fall into two camps. The main group of studies consistently report a balanced muscle fibre type, with ranges of 51%[20], 49% to 51%[21], 48%[22], and 44%[23].

More recently a new technique of muscle fibre measurement reported a figure of just 30%[24], however, this aberrant result could easily be a feature of the different methodology. The latissimus dorsi displays a balanced fibre type, although there is an indication that it may lean towards type II fibres. The balanced nature of the fibre type in this muscle might explain why big backs can be built in many different sports.

Climbers who develop a lot of force in vertical pulling movements tend to have great back development. Similarly, swimmers also have sizeable latissimus dorsi musculature, even though the forces they develop in the water are relatively small and they do countless repetitions. So for building a big back, both high-rep sets and plenty of fatigue as well as lower-rep sets with plenty of weight will produce good results for the majority of people.

Wrapping up
When you’re designing your next upper-body training programme, bear in mind the following.
Your shoulders need higher reps and plenty of fatigue, while the biceps, triceps and pectorals need heavy weight and low-rep sets, if your joints can take it. The latissimus dorsi doesn’t seem that fussy and you can hit it either with higher reps or with heavy weights, or ideally a combination of both.

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, 5, 10, 16, 19 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.
3, 8, 15, 18, 22 Srinivasan, R. C., Lungren, M. P., Langenderfer, J. E., & Hughes, R. E. (2007). fibre type composition and maximum shortening velocity of muscles crossing the human shoulder. Clinical Anatomy, 20(2), 144-149.
4, 9 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.
6 Humphrey, K. E. (1982). An investigation of the fibre composition in the deltoid muscle of elite British slalom kayak competitors. BA Thesis, University College of North Wales.
7 Tesch, P. A., & Karlsson, J. (1983). Muscle fibre type characteristics of M. deltoideus in wheelchair athletes: comparison with other trained athletes. American Journal of Physical Medicine & Rehabilitation, 62(5), 239-243.
8 Mavidis, A., Vamvakoudis, E., Metaxas, T., Stefanidis, P., Koutlianos, N., Christoulas, K. & Mandroukas, K. (2007). Morphology of the deltoid muscles in elite tennis players. Journal of Sports Sciences, 25(13), 1501-1506.
11 Nygaard, E., & Sanchez, J. (1982). Intramuscular variation of fibre types in the brachial biceps and the lateral vastus muscles of elderly men: how representative is a small biopsy sample?. The Anatomical Record, 203(4), 451-459.
12 Dahmane, R., Valenčič, V., Knez, N., & Eržen, I. (2001). Evaluation of the ability to make non-invasive estimation of muscle contractile properties on the basis of the muscle belly response. Medical and Biological Engineering and Computing, 39(1), 51-55.
13, 17 Dahmane, R., Djordjevič, S., Šimunič, B., & Valenčič, V. (2005). Spatial fibre type distribution in normal human muscle: histochemical and tensiomyographical evaluation. Journal of Biomechanics, 38(12), 2451-2459.
14 MacDougall, J. D., Sale, D. G., Alway, S. E., & Sutton, J. R. (1984). Muscle fibre number in biceps brachii in bodybuilders and control subjects. Journal of Applied Physiology, 57(5), 1399-403.
19 Schantz, P., Randall-Fox, E., Hutchison, W., & Tyden, Astrand, P. O (1983). Muscle fibre type distribution, muscle cross-sectional area and maximal voluntary strength in humans. Acta Physiologica Scandinavica, 117, 219-226.
21 Baker, S. J., & Hardy, L. (1989). Effects of high intensity canoeing training on fibre area and fibre type in the latissimus dorsi muscle. British Journal of Sports Medicine, 23(1), 23-26.
23 Hards, J. M., Reid, W. D., & Pardy, R. L. (1990). Respiratory muscle fibre morphometry. Correlation with pulmonary function and nutrition. CHEST Journal, 97(5), 1037-1044.
24 Paoli, A., Pacelli, Q. F., Toniolo, L., Miotti, D., & Reggiani, C. (2010). Latissimus dorsi fine needle muscle biopsy: a novel and efficient approach to study proximal muscles of upper limbs. Journal of Surgical Research, 164(2), e257-e263.



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