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and how they're integral to weight-training

Aren't muscle fibers all created equal?

Isn't it simply a case that they contract at different intensities, like when lifting a spoon to your mouth, or curling a barbell?

The answer is a definite no, and here’s why:


Activating Slow or Fast-Twitch Muscle Fibers influences different fitness gains. Because of the difference in they way they produce energy and create force, we distinguish between:

  1. The red, slow-oxidative type (I);
  2. The white, fast-oxidative type (incl. subtypes IIA and IIB).

In general terms though, we talk about slow or fast-twitch muscle fibers.

Muscle fibers are 'recruited' (and contract) differently (depending on the task), when called on by signals to motor neurons from the brain.


Because understanding how your muscular physiology is engaged and impacted by physical activity, and how it adapts – will enable you to know which exercises to focus on for particular fitness outcomes.

Although the Sensible Fitness Program's goal is to provide sustained functional fitness by targeting both slow and fast-twitch muscle fibers – you might want to go for other, more specific fitness goals. 

With that, let's get a technical just for a moment, and look at the structure and characteristics of muscle fibers, so you can consider their uses in sport and exercise:

An overview of muscle structure

'Muscle fibers form 'motor units', which are made up of hundreds of slow and fast-twitch muscle fibers. A motor unit consists of one motor neuron and all the related muscle fibers it activates to contract.

As shown in the image, skeletal muscle consists of cells, nuclei and various proteins combined with groups of fibers.

A group of motor units is separated from other groups of motor units by the epimysium and perimysium – a fascia or membranous sheath covering the entire muscle, continuing all the way to the tendon and onto the bone, where it becomes collagenous (and considerably denser).

Muscle fiber comprises myofibrils (bundles of myofilaments), which are the elements that contract, and in so doing, shorten the muscle to perform an action.

But why are fast-twitch muscle fibers so integral to lifting weights?

Because of the ‘degree’ of muscle-fiber recruitment during muscle contractions – which dictates the type of energy, strength and power output of the muscles.

In other words, you can use your muscles differently. Think of the above example about the spoon vs. the barbell.


Fast-twitch muscle fibers (including subtypes IIA and B) contract quickly for use in short intense activities of brief duration.

As such, fast-twitch muscle fibers are cardinal to strength, power and hypertrophy.

These muscle fibers are recruited in selective degrees when strength and power are needed to move a heavy load. Accordingly, the muscle fibers contract quickly, providing short bursts of energy. And because of the heavy workload, they are recruited in high numbers. This is typical during intense exercise like weight-lifting, sprinting or swinging a baseball bat.

However, it’s worth noting that, unlike the slow-twitch muscle fibers, fast-twitch muscle fibers are rapidly depleted of energy, with the likelihood of pain, ‘muscle-burn’ and cramps, due to lactic acid build-up.

You've probably seen how sprinters literally 'explode' out of their starting blocks and burn up the track.

However, they are totally burned-out by the time they cross the finish-line a mere 100 meters later.

They’re bent-over double and out-of-breath from the after-effects of having ignited their fast-twitch muscle fibers to the max – short and intense

Slow-twitch muscle fibers on the other hand, contract slowly and support smoother, lower

intensity repetitive contractions. This is optimally suited to endurance-type activities.

These slow-twitch muscle fiber-types are capable of sustained workloads at low output and prolonged duration, such as long-distance running.

Keep in mind that no 'one' person has the same muscle fiber ratio and distribution as the next. It's why some people are suited to pole-vaulting but not shot put – and why others have the physiology to comfortably do marathons, but lack the appropriate muscle fiber-dominance to excel at sprinting.

That is why consideration of the various somatotypes and their characteristics, plays such a big role in the choice of sport. 

Athletes fare better in sport when playing to their strengths (and genetics).

It's therefore logical that athletes who are successful in endurance sport are those with naturally higher percentages of slow-twitch muscle fiber than athletes like power-lifters.

Marathoners seem to be able to 'cruise' forever.

They rely on muscle condition and energy pathways which differ drastically from athletes who train for strength and/or muscle-size (also known as hypertrophy).


Since low-intensity exercise doesn’t activate the fast-twitch muscle fibers, the response is mainly metabolic/cellular – and thus very little neurological strength adaptation will occur for strength-gains.

Low-intensity/higher rep-range affords muscular endurance of the mitochondria (cellular power plants) and capillaries, but only a small degree of strength/hypertrophy (increase in muscle-size).

As in the examples on the page dealing with weight training, if your goal is to develop strength, power, and/or muscle size, you need to ‘hit’ your fast-twitch muscle fibers, by lifting weights heavy enough – executing between 6 and 12 reps per set.

By training in this medium rep-range, the training effect (or adaptation) is neurological and metabolic/cellular. You get the best of both, i.e. good strength gains and muscle size. However, in order to tip the scale towards maximal strength, you'll need to focus on the lower rep-range (1-5). In this way, you'll be recruiting the maximum amount of muscle fiber.

That said, while affording optimal strength-gains, this low rep-range will not necessarily build muscle-size.

Ever wondered why strongmen and power-lifters consider themselves superior to bodybuilders?

Exactly! They feel that bodybuilders are in it purely for aesthetic reasons – and they're pretty much right.

Again, the adaptations which allow you to become stronger are primarily neurological. You develop the ability to recruit higher numbers of muscle fibers by stimulating those ‘higher-threshold fibers’ which aren’t activated in lighter workload training, which is why maximal strength-training improves inter and intramuscular coordination.

To get a visual impression of four different types of response and adaptation, take a look at the images on the page dealing with weight training.

The take-home message

Rep-range will affect your adaptive response, in that training loads influence neural or metabolic/cellular responses.

I’ve seen similar effects and outcomes in my own fitness programs. I find that focusing on the medium rep-range and occasionally doing some lower and higher-rep work, keeps me in 'my' desired shape. With the Sensible Fitness Program routines, my body remains strong, injury-resistant and well-toned. At the same time, I'm easily able to keep my body fat level low.

As mentioned above, you need to train your muscles according to the physiological and physical condition desired by targeting the respective muscle fiber types accordingly.

The muscular adaptation continuum below provides a graphic representation of the various types of training, and their effects.

So, having shown how and where slow and fast-twitch muscle fibers are relevant, you should now have a better idea of how to train in line with your desired goal. 

To ensure that the message is clear:

  1. If your weight-training comprises only light loads, the fast-twitch muscle fibers will not be significantly targeted. On the other hand, when you work with heavy loads (as you would in training for strength), you activate a higher number of motor units, thereby recruiting the maximum number of fast-twitch muscle fibers;
  2. If your aim is strength and power, then you should work primarily in the low, 1-5 rep-range. There will be neural adaptations and strength-gains, but without a significant gain in muscle size;
  3. If your aim is adding muscle mass (as in most bodybuilding and fitness programs), you need to focus mainly on the medium-low and medium, 6-12 rep-range. I've found though, that including the occasional high-rep set, can also aid hypertrophy. However, not everyone looking for mass-gains responds positively to high-rep training. You'll need to experiment a little;
  4. Remember, strength and hypertrophy are interrelated. Think of strength as the springboard from which to launch your muscle-building efforts. Strength-gains will allow you to progress in workload-poundage, the gains of which can be carried over to increased muscle-size and quality;
  5. If your aim is muscle endurance, then you know that the majority of your weight-training will be in the higher rep-range and most importantly, your focus will be actually doing endurance-related training (cardio) to fine-tune your slow-twitch muscle fibers;
  6. Regardless of whether your aim is muscular strength, size or endurance, for optimal conditioning you will be well-advised not to neglect cardiovascular exercise. I know, many of you have told me you can't bear to do cardio, and so omit it from your fitness programs;
  7. Consider this for a moment: aside from the fat-burning effect, which is a necessary part of getting into shape, cardio not only boosts the slow-twitch muscle fibers in generating aerobic energy – it also causes increases in capillary density which improves oxygen transportation. (See the benefits and importance of exercise for more on this.);
  8. Finally, cardio increases 'mitochondria' (tiny structures often called 'cellular power plants'), that generate most of the cell's adenosine triphosphate (ATP), which in turn powers muscles and tissue in the body.


While not wanting to refute any of the advice given on this page, I would again remind you to also consult the page on Somatotypes, as it discusses important aspects which are linked to muscle structure and fiber properties.

Each individual is born with unique genetic predispositions which make her/him more or less prone to losing or gaining weight, developing endurance strength, adding muscle or building strength, and so on.

Therefore, you're best-advised to approach your health and fitness activities, taking your inborn tendencies into consideration – so as to have realistic 'achievable' expectations of your fitness program goals.

In short, adopt an informed and sensible approach.


  1. Frederick C. Hatfield; “Fitness: The Complete Guide, Official Text for ISSA's Certified Fitness Trainer Program”, Edition 8.6.6. International Sports Sciences Association; 2004. Unit 3, Muscle Anatomy and Physiology; p. 105-119.
  2. Thomas R. Baechle, Roger W. Earle, editors; “Essentials of strength training and conditioning” / National Strength and Conditioning Association - 3rd. edition, 2008 Chapter 4; p. 143-151.
  3. Wilson, J.M. Loenneke, J.P. Jo, E. Wilson, G.J. Zourdos, M.C. Kim, J.S. (2012). The effects of endurance, strength, and power training on muscle fiber type shifting (2012). http://www.ncbi.nlm.nih.gov/pubmed/21912291 (accessed August 30, 2016).
  4. American Council on Exercise (ACE). Slow-Twitch vs Fast-Twitch Muscle Fibers. https://www.acefitness.org/blog/5714/slow-twitch-vs-fast-twitch-muscle-fibers (accessed August 30, 2016).

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