Why “slow” fibres should not be neglected

Zoznik has translated a recent article by renowned fitness scientist Brad Schonfeld. The text on how the weight on the barbell affects muscle growth is written with Dan Ogborn, PhD and CSCS certified trainer.

“The heavier the barbell, the bigger the muscles” is what many trainers and trainees say, or so they say. Seemingly correct, because the higher the load, the more type 2 muscle fibres are involved (the so-called “fast” muscle fibres responsible for strength work, which fatigue quickly), and it is these that have significant potential for growth.

So hypertrophy only requires strength training with excessive weights? It is not that simple.

Type 1 fibres should not be neglected.

In bodybuilding they are usually dismissed as weak, slow contracting and simply smaller in size than their “fast” colleagues (type 2 muscle fibers). But type 1 fibres can work for much longer, resisting fatigue. Marathon runners are grateful for this peculiarity, while bodybuilders consider it a curse. They work hard to stimulate type 2 fibres in their workouts.

But modern studies comparing the effects of training at different intensities show that it is in vain that we give up working out Type 1 fibres and lose kilos of extra mass .

It’s time to rethink your training philosophy and focus on all muscle fibre types for maximum hypertrophy.

“‘Fast’ and ‘slow’ fibres

There are 2 main types of muscle fibres: slow-twitch fibres and fast-twitch fibres. Fast contracting fibres are larger in diameter than “slow” fibres and therefore occupy a more prominent place in your muscles.

“Slow” fibres (type 1 fibres, red in the picture) are also classified as aerobic fibres due to their high oxidative capacity, which enables them to contract for a long time. They are best suited for prolonged activity that requires minimal effort (e.g. long-distance running).

Fast-twitch fibres (type 2 fibres, blue in the picture) are muscle fibres that have a high threshold of excitation as well as a high speed of signal conduction and are better suited for fast efforts (that’s why short-distance runners look like athletes compared to stayers). In other words, these are the fibres you need to successfully rip a heavy barbell.

Training with heavy weights for type 2 fibres

Of course, a huge amount of research confirms that high-intensity strength training leads to an increase in type 2 fibers . Note – we are talking specifically about high intensity. This does not mean that slow-twitch fibres have little potential for hypertrophy; but at high intensity (more than 50% of 1RM), fast-twitch fibres grow better.

Our ideas about hypertrophy of different fibre types are still based on experiments in the laboratory and not on studies of actual training in the gym . The 2004 work of Dr Andrew Frye, in which he compiled data from many studies, strongly suggests that type 2 fibres respond best to vigorous exercise.

However, when the load falls below 50% of 1RM, they begin to overtake type 1 fibres in growth rate (although they do not achieve the same level of hypertrophy as at higher intensity). If guided by this data alone, there is simply no need to revise the training approach.

But the regression analysis that Frye (2) applied has its disadvantages. Firstly, there are not many studies done on low-intensity exercise (2, 3) and there are few scientific papers directly comparing the effects of high and low intensity exercise on the hypertrophy of different fibre types.

If we look at the new data on muscle fibre increase in response to exercise at different intensities (1), it becomes clear that we underestimated type 1 fibres.

Study of type 1 fibres

Although there are very few studies so far, anecdotal evidence suggests that slow-reducing fibres have good potential. For example, the results of a study by Mitchell et al (1) show that in an approach to failure, low-intensity exercise (3 sets at 30% of 1RM) produced approximately the same hypertrophy as high-intensity exercise (3 sets at 80% of 1RM). While the difference is not statistically significant, the high-intensity load stimulated type 2 fibres slightly more (15% gain vs 12%) and the low-intensity load stimulated type 1 fibres more (19% gain vs 14%).

But it is already clear that weight on the bar is not the only growth factor. And science is beginning to approach an idea that has long been intuitively understood: type 1 fibres are maximally stimulated by long, low-weight approaches, while type 2 fibres respond better to short sets of heavy weights.

Most studies are conducted on untrained participants, but the results may be different for more experienced athletes. If we look at studies on trained people, we find evidence to support this assumption. Bodybuilders tend to build up a high training volume by working at an average number of repetitions and building up fatigue (4), whereas for powerlifters (5) and weightlifters, work-weight and/or speed of movement are more important. It makes sense that a bodybuilder has a markedly higher prevalence of type 1 hypertrophy than a weightlifter (2).

Taking all this data into account, it can be concluded that training at different intensities will produce similar overall hypertrophy (1, 6-8), but the rate of growth of the different fibre types will vary.

However, as with many subjects, there is no definitive scientific verdict: two studies (with slightly different conditions) have shown that high-intensity training is more effective for hypertrophy regardless of fibre types (9,10). But there is a nuance. Studies that equate volume of work done show benefits of high intensity for hypertrophy of all fibre types (10,11). If volume is not equated, then training at different intensities leads to similar results.

Byrd et al (12) compared the increase in protein synthesis in response to training with different protocols: working at 90% of 1RM to failure; working at 30% of 1RM the same total volume as at 90%; and working at 30% to failure.

Conclusions: protein synthesis levels were similar when working up to failure, and training at 30% to failure caused twice as much lift as training at 30%, equated in volume with 90%.

Of course, a short-term increase in protein synthesis after a single workout may not provide hypertrophy in the long run, but two studies have already shown that pre-failure work at different intensities leads to similar results (1,6).

Note how muscular long distance runners’ bodies can look – a direct illustration of the effectiveness of both fast fibres (powerlifting) and slow fibres (long distance running).

  • A is Alex Viada, a well-known powerlifter and marathoner. He has a squat of over 315kg and a long-distance run; he’s massive and relievable at the same time. He’s even written a book, The Hybrid Athlete, on how to combine strength and endurance training.
  • B is Linford Christie, who ran the 100 metres in 9.87 in 1993. Slim and very muscular.
  • C – David Goggins, former paratrooper, who runs marathons and ultramarathons for charity. Slim and muscular.
  • D – Christopher Lemaitre is the only white sprinter who has managed to run the 100-metre faster than 10 seconds. He is very fit, but his muscle mass is not great to say the least. Doesn’t resemble any of the previous three.

Does size matter?

Heavy training with heavy work weights leads to significant hypertrophy (without regard to fibre types), as confirmed by many studies (2,9,10,13-17). This is consistent with the Hennemann Recruitment Principle – small motor units are activated when the load is low, and larger units enter later if the load increases (18,19). Lifting heavy loads requires more muscle tissue to be recruited (more motor units) than light exercise.

But this concept does not take into account the accumulation of fatigue that also increases recruitment and stimulation for growth (20). When you perform an exercise with relatively low weight, fewer motor units are recruited at the beginning of an approach than at the beginning of a heavy-weight approach. But gradually, when slowly contracting fibres stop producing enough force, fast contracting fibres get involved as well (21). The same principle applies: the order of entry is determined by size; but eventually you also engage the fast-twitch fibres when fatigue builds up in the low-weight work.

This partly explains why, in the study by Mitchell et al (1), fast-twitch fibres also grew during low-intensity exercise. And why – for the purpose of hypertrophy – it is important to extend the time under load and reach failure.

Extra kilos of muscle?

Do you think I’m exaggerating? Look at the proportion of slow contracting fibres in different muscles. Of course, the composition varies individually and depends on genetics as well as type of training (22); but still, in large muscle groups the proportion of type 1 fibres is high – up to half. It makes sense to devote part of the training time to their stimulation as well.

Alternating ranges for maximal mass

If you want to gain as much muscle as possible, alternate the number of repetitions. Do not limit yourself to ‘bodybuilding’ ranges (6-12 reps), but also use multi-rep (15-20 and more) and low-rep (1-5) protocols.

This variety will not only result in a complete stimulation of all muscle fiber types, but will also help in other aspects. Low repetition approaches will improve neuromuscular adaptations, developing maximal strength and allowing you to lift more weight in an average number of reps. Multi reps sets will push back the lactate threshold, increasing endurance and increasing time under load in the medium range.

Use linear or non-linear periodisation to vary the number of repetitions and ensure steady progress. Both models work fine, the choice depends on personal preference and goals (i.e. the need to peak by a certain point in time).

Another option is to link ranges to exercise types. For example, in multi-joint exercises work in low and medium number of reps (~1-10), while multi-reps (15+) are reserved for isolation movements, in which it is better to use small weights.

There is no one general rule here, the response to training stimuli varies individually. So experiment and find the scheme that works best for you.

The quieter the ride, the farther you go?

So, although type 2 fibres do grow better than type 1 fibres, do you really want to give up stimulation of the latter and additional mass?

The conclusion is simple: to grow muscles as effectively as possible, it makes sense to grow all the types of muscle fibres available – both those that grow well, a lot and from intensive exercise (“fast” fibres) and those that grow from prolonged exercise and low weight (“slow” fibres).

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