Muscles are made up of muscle fibers: it’s not revolutionary, but the way your fibers work has been massively overlooked by many trainers and experts.
Stick with us if you’re looking to make the most of your training and learn the simple approach to training that your competition is probably ignoring.
Muscle Fibers 101: What are muscle fibers?
You need to know a little bit about how muscles work if you’re going to make the most of your training. This shouldn’t be a problem, because the science behind sports performance is awesome.
What are the different types of muscle fibers?
Muscle fibers are in a continuum between fast-twitch and slow-twitch. Each fiber is somewhere on this spectrum but, for the sake of simplicity, they’re split into 3 major groups (1):
- Type 1 (slow-twitch)
- Type 2A (Glycolytic fast-twitch)
- Type 2X (Phoshpergenic fast-twitch)
The difference between these muscle fibers is all about how fast they contract (key for things like strength and power) and how quickly they fatigue (which determines their use in endurance activities).
As with everything in life, it’s not perfect and there’s a trade-off: the stronger a muscle fiber is, and the more power it exerts, the easier it fatigues. To over-simplify a bit, slow-twitch fibers are built for endurance while fast-twitch fibers are for putting in muscular work or exerting maximal force (2).
The important thing to remember is that different muscle fiber types depend on different energy systems for their energy (3).
There’s even a table that’s widely-used to discuss the way that these fibers work:
The important part, however, is what we do with this information – that’s what this article is about. We’re going to cover the basic science, how it applies to your training and how you can employ it to maximize your results.
Why the Science Matters to your Training
The science is, as always, catching up with the training methods. It provides enlightening insight on why the best in the world are doing what they do – and why they’re getting better!
Putting the science to work in your own training is a big deal and gives you the competitive edge. You don’t need to be Mr Olympia or an actual Olympian to gain the benefits.
The speed with which muscles contract is key to whether you’re powerful and explosive or have huge endurance.
Slower twitch fibers are great at repeating the same low-intensity movements again and again – they’re all about endurance energy pathways and are designed to handle work from around 8+ minutes (4).
Fast twitch fibers are stronger and produce more force (5) but have a relatively hard limit on what you can achieve before they’re totally fried and need rest. These produce large amounts of lactic acid and are probably the reason you want to throw up after sled pushes (6)!
Your goals and the way that you train are going to have specific fiber-speed demands. For example, the fibers used for sprinting are totally different to those used in marathon-running. The sprinter wants more fast-twitch fibers and the runner will benefit more from slow-twitch fibers.
Mitochondria: Powerhouse of the Cell!
Mitochondria are the powerhouse of the cell – probably one of the most popular and memorable lines from your time in school. They are also important for your training and how you push yourself to achieve your potential.
Mitochondria a key part of the Krebs cycle – a complicated energy transfer system that kicks in when you’re all out of ATP and glycogen in the muscles (7). It’s used to break down fuel from the bloodstream (energy from food and body-fat) and turn it into energy for the muscles.
Slow-twitch fibers are packed with mitochondria, while fast-twitch fibers are far scarcer (8). This is what makes slow-twitch muscle fibers so slow to fatigue: they replenish their energy supplies with the 1000’s of calories of energy stored in body-fat and other slow-burning sources (9).
You can increase mitochondrial density by performing endurance exercise, and it positively impacts your ability for extended bouts of exercise (10), so it’s clearly important!
Changes in Muscle Fibers
The important question should be obvious: can you change muscle fibers and the way that they work?
The science looks promising, but a whole bunch of studies have simply missed the point: clinical research tends to be around 6-12 weeks of training (11), but the kind of adaptations that you’re looking for with your training is years in the making!
If you were to look at the adaptations you can make in muscle size over 6-12 weeks, you might also conclude that the changes are likely to be small forever and that you can’t make much difference.
However, training age is one of the most important variables in success: whether that success is size, strength, power, endurance or anything else you care to mention.
Scientific articles have begun to suggest that you can make changes to muscle fibers (12). While they (arguably) underestimate the potential for adaptation and change, many have come out in support of the idea that training can change the way that your muscle fibers develop.
What Does All this Mean?
The changing status of muscle fibers in the body is a key way that you can adapt your training, benefit from adaptation, and start an upward cycle of maximizing your potential for a certain type of training (such as strength, power, hypertrophy or endurance).
The changes that occur in muscle fibers – from innervation to power to endurance – play a key role in developing athletic characteristics and your relationship with movement.
For example, the adaptation of muscle fibers and nerves to fast-twitch training increases the amount of control and proprioception you have in certain muscles and joints (13). This is how you make your pecs dance – it’s the ‘mind-muscle connection’ that plays a huge part in bodybuilding.
Increasing your fast-twitch muscle fibers improves your control over muscles, improves stability during loaded or powerful exercise, and even reduces your risk of injury – as fast-twitch muscle fibers are the primary shock absorbers for jerky movements (14, 15). A list of things you should want!
How the different muscle fibers make up your body
Everyone has a mix of muscle fibers that is based on their athletic history, their hormones, and the way that they have lived.
To make things even more mysterious, your muscle fiber distribution depends on the muscle itself and where in your body it’s located: different muscles have different fiber types.
You might be wondering what your fiber types are and whether you’re built to out-run a horse or blaze the 100m. We have 2 bits of sobering news for you:
- You’re probably somewhere in the middle: most people aren’t at either extreme
- You can’t tell without having some of your muscle cut out and examined in a lab
Sorry about that.
We’ll tell you a little bit more, though, which might soften the blow.
Being Average: What Determines Fiber Type?
There are a bunch of factors for why you have a certain ratio of different types of muscle fibers.
The first one is simple: humans evolved to be a versatile, endurance-based hunter. As a species, we’re more comfortable running for so long that our prey simply dies of starvation. You can’t get past the fact that slow-twitch fibers are necessary for life and they’re a big part of why we evolved in the first place (instead of being eaten).
Second, you’ve probably (hopefully) done a variety of exercises in your life. While you might not be a lifelong athlete, your life requires more types of activity than sprinting or marathon-running. Even as a child, you jumped and ran, examples of fast- and slow-twitch exercise.
When you’re this age, your muscle fibers change and adapt more easily. This variety of exercises during your youth have caused you to adapt (16).
If you’re training children to be great at weightlifting or sprinting, they’ll do more jumping and explosive-power exercises to make sure that they adapt to it. However, if you’re looking to train young athletes in athletics they’ll change muscle fiber types to get better at this.
This is why, if you weren’t a power or endurance athlete in your younger years, you’re probably a well-rounded person with a variety of interests but have less capacity for adaptation now (17). Which is probably a bad thing if you’re looking to become Olympic-level in anything – buy what you do from now counts.
Muscle Fiber Types are Guesswork
It’s simple: muscle biopsies are the only way to find out what your muscle fibers are like.
This involves paying for the privilege of having a tiny portion of your muscle removed and examined. It’s the only accurate way to check your fiber types, but the cost and process are probably too much for anyone but elite athletes.
The alternatives are pretty hit and miss. To start with, there are some general trends in the way fibers work:
- Your lower body has slower-twitch fibers than your upper body (probably because there’s more non-optional endurance exercise like walking)
- Women have less slow-twitch fibers, though they tend to be lower in power output due to innervation and contractile strength
One example of self-testing is performing a 1RM test and then comparing it with how many reps you can perform at 80%.
This gives a general idea of how well you are equipped to deal with maximum force output: more repetitions probably mean you have more slow-twitch fibers and better endurance.
The problem with this method is that you’re going to be limited on your repetitions by things like technique and postural strength. Even on a machine like the leg press, technique and the strength of surrounding muscles can mess up your outcome.
Another way of gauging your personal abilities and fiber distribution is this: if you’re really good at powerful exercises (throwing, running, jumping, Olympic lifts) relative to your overall strength then you’re probably a fast-twitch-dominant human.
This is relative to the performance of others, but it’s better than a rep-test and less likely to involve being stabbed with needles in a full biopsy.
Workouts for the different types of muscle fibers
Because the different types of muscle fibers respond to different loads, there are different exercises you can do to target each type specifically. Why would you want to do this? Well, many of us are training for a specific reason such as a sport or even for a certain appearance. Endurance and power sports require Type 1 and Type 2X, while when shooting for looks you’ll be focusing on growing Type 2A.
When training for general fitness you’ll want to experiment, so you can figure out what the makeup of your individual muscle groups are, so you’ll know what is most beneficial for you. When working on general fitness, you don’t want to focus on one type of fitness routine and possibly neglect a large part of your muscle makeup.
Below are training and diet tips for each muscle fiber type. They them all and see what benefits you most.
Training your Fibers: Type 1
We’ve mentioned it a lot, but type-1 fibers are all about extended periods of endurance exercise.
Type-1 fibers use oxygen as part of their fuel – they work for those gassy endurance sessions and take over when you’re performing low-moderate intensity exercise for more than around 8-15 minutes.
These fibers are great because they’re tied to muscular endurance. They’re a big part of cardio performance because they contain huge amounts of mitochondria (18) – they pull from fat stores and digesting food to make sure that you can keep going when you’re out of stored ATP and muscular glycogen.
The problem with slow-twitch fibers is that they’re not well innervated: you don’t have the level of control over them seen with faster fibers and they don’t have the same force output.
For type-1 fibers, it is possible to become larger or increase mitochondrial density, and capillary density (19). These are both great outcomes, but they’re very different: larger fibers is great for size while more mitochondria or capillary function boosts endurance.
It’s been suggested, however, that they’re great for hypertrophy because of their large size/thickness (20).
The problem with this idea is that they start large, but there aren’t really any studies on how much each type of fiber grows through training. Just because they’re big, it doesn’t mean you’re going to make them much bigger and running doesn’t seem to make your legs jacked!
Type-1 fibers probably aren’t going to be your main driver of muscle gains, but If you want to boost your type-1 focus its probably to become an endurance monster rather than getting bigger.
Boosting Type-1 Fiber Gains
Here’s how to improve your training results for endurance and Type-1 fibers.
Muscular endurance is the name of the game. You’re going to want to get into high rep ranges with moderate-low intensity. However, you can also rush to the point of fatigue to increase your type-1 response and boost hypertrophy and mitochondrial density in these fibers (21).
Occlusion training is one way to do this: it involves restricting blood flow to the area by tying a band, rope, bandage or other substance around the limb. This method has been popular with bodybuilders for years and the science is starting to show benefits for Type-1 training (22).
This shouldn’t be a surprise: type-1 muscles are used for when there’s nothing left in the tank, so it’s to be expected that artificial fatigue boosts results. The condition of Hypoxia (lack of oxygen) has been shown to increase the efficiency and function of these fibers and occlusion is all about making this harder (23).
Example Workout for Maximum Endurance (Type 1)
This simple lower body session provides a balance of muscular endurance and cardio-respiratory exercise to improve endurance output. Occlusion exercises on the Bulgarian split squats provide pre-fatigue which will increase the difficulty and benefits of everything afterwards.
While this session only has a few exercises, they’re going to be massively effective and you’re definitely going to feel it the next day. Make sure you’re doing your recovery work and eating plenty of calories or you’re going to waste away and feel sore as hell!
Back Squat: 3 sets of 15 reps (50-60% of 1RM)
Bulgarian Split Squat (with Occlusion): 3 sets of 10 reps (each side)
Superset: Dips and DB Rows: 4 sets to failure
Row: 5 – 10km (dependent on proficiency)
The primary fuel for endurance is triglycerides: fats in the bloodstream that are broken down from dietary fats or created in the liver. This fat-driven system has actually been shown to respond well to ketogenic and high-fat diets, though there is a lot of science on the benefits of slow-digesting carbs for performance.
We’d also strongly recommend taking a beta-alanine supplement. This has been shown to have positive effects on endurance performance (24), meaning better performance and better results.
Boosting Type-2A Fiber Gains
These are probably the most numerous and well-trained fibers in your body – simply because they can be used for most types of exercise.
They’re the ones that handle exercise between the super-fast-twitch and slow-twitch fibers. They cover everything from about 10 seconds to just under 10 minutes, and they’re the most useful for regular size or strength training.
They’re powered by glycogen – the carbohydrate-fuel that your body stores in the muscles and a big contributor to size.
This not only means that they’ve got better endurance than the 2X fast-twitch fibers but using them increases the amount of glycogen you store in your muscles (25). This is one reason why bodybuilders train in this range and why their muscles look so ‘full’.
These fibers are pretty middle-of-the-road for everything from fatigue to nervous control and power output. They’re the workhorse for most of your activity and overlap with both type-1 and 2X fibers.
They’re more changeable than other types because they’re not entirely specialized for power or endurance.
Training for 2A fibers is interesting because it doesn’t represent an extreme in either direction. It benefits most from a balance of intensity and volume: not just heavy weights or maximum repetitions, but maximum repetitions at a considerable weight.
The traditional approach to muscular hypertrophy in general, and 2A, is 70% for 10+ repetitions. This makes sense, as the time necessary for 10 repetitions is likely to be over the 8 second mark. With that said, we do have a recommended system for improving your performance in 2A fibers specifically.
Begin by taking 70% of your 1RM and performing maximum repetitions with this weight. From here, increasing the load is the best bet. Increasing your ability to perform repetition-maximums at moderate intensity is the key: you need to perform more reps at heavier weights. Aim at increasing the overall volume of your sets one way or another.
Forthcoming research is also playing a major part in how you should be training for size and type-2A exercise: because they overlap heavily with strength and endurance (26), mixing up your rep scheme is one of the best ways to increase 2A hypertrophy. Research at the University of Londrina shows that more set-by-set variety increases muscle growth.
Example Workout for Maximum Hypertrophy (Type 2A)
An example of upper-body hypertrophy, this is a simple approach to hypertrophy based on the balance of intensity and volume we discussed above.
Dumbbell Flyes: 2 sets of 8 reps (80% of 8RM)
Decline Bench Press: 10RM + 3 sets of 8 reps (90% of 10RM)
Incline DB Press: 6RM + 2 sets of 12 (70% of 6RM) + 1 set of 20 reps (50-60% of 6RM)
Pendlay Row: 4 sets of 12 reps (70% 1RM)
Superset: Lu Raises + Cable Row: 4 sets to failure (aim for 12-15 reps)
Obviously, a high-carb diet is a great way to fuel these muscle fibers and keep yourself progressing in the 65-80% intensity on resistance training.
If you’re aiming to improve your muscular size, 2A fibers are a great choice because they’re plentiful, adaptable, they overlap with the other two types and are closely linked to glycogen storage.
If you’re trying to get full and grow, this is the kind of region you want to look at – there’s a reason it’s the preserve of the bodybuilder.
Get your carbs in and ensure that you’re putting them to work – this will increase your chance to improve glycolytic super-compensation. This is the fancy name for increasing the muscles’ storage of glycogen, which is important when your muscle volume (size) is mostly constituted by fluids.
Boosting Type-2X Fiber Gains
2X fibers are the type that make sprinters and Olympic weightlifters so impressive: huge strength and amazing power. These muscle fibers produce the greatest amount of force in the shortest time possible: they’re the powerhouse for lifting in the 1-3 rep range, jumping, and throwing heavy objects.
Increasing power is one of the best-researched and most sought-after adaptations in strength and conditioning: it’s been a huge part of the way that elite athletes have trained for years. You can use this to improve your own strength and power – shift big weight and develop amazing athletic prowess!
There’s a lot of research on increasing your 2X fibers, compared to the other fiber types, because their effects are so impressive!
Start by training for the functions that these fibers perform: training should focus on a variety of short-term, high-output exercises in strength and power (27). 2X fibers have 2 main uses:
- Maximum force output (on 1-3 rep sets), and
- Rapid force output under time constraints.
Training for these high-power fibers means hitting at least one of these: move maximum weights or move sub-maximal weights as quickly as possible (this includes body-weight exercises like jumps).
The important part is training at the maximum of your force output while under time restraints: moving slowly against light weight isn’t going to result in much 2X recruitment or growth (28). Heavy weights, or anything where you can work on blistering speed (between 30% and 65% of 1RM) is going to be a better choice.
One of the best ways to improve your 2X fiber growth and power output, as well as a wide variety of other health and fitness benefits, is to focus on eccentric training. Eccentric training is all about focusing on the lengthening or lowering phase of a lift – the bit where you’re moving with gravity.
Eccentric training preferentially targets 2X fibers and induces greater training effect and growth in these powerful tissues (29, 30, 31, 32).
The eccentric part of any exercise is the main contributor to muscle growth and boosts your ability to produce power.
Working eccentric training into power exercises is about lowering/lengthening slowly and then rapidly contracting. Focus on moving as slowly as possible on the way down and as quickly as possible on the way up: this is a great way to combine eccentric training and moving as powerfully as possible.
Example Workout for Power and Strength (2X)
This session is a great example of a lower-body power session. It focuses on all the components mentioned above, combining rate of force development (ROFD) exercises (like the box jump) as well as heavy eccentric loading on the squat and RDL.
Movements like slam ball throws are a great way of developing other force characteristics like nervous system efficiency and building throwing mechanics. All the exercises in this session focus on slow eccentrics and explosive concentric movements – basics of putting together huge power!
Box Jump: 5 sets of 3 (80% height)
Power Clean from Blocks (or hex bar jump): 5 sets of 3 (65% of 1RM)
Slow-Eccentric Speed Squat: 5 sets of 2 (70% of 1RM squat)
Slow-Eccentric Speed RDL: 4 sets of 4 (70% of heaviest RDL)
Slam Ball (Sideways): 3 sets of 10 (each side)
Dieting for 2X fiber development isn’t an easy one to figure out: it is a matter of increasing your ATP stores in the muscles themselves. This isn’t an easy thing to achieve – you can only change it a small amount and the range of stores is somewhere between 5 and 8 seconds worth of ATP – not a lot!
If you’re looking to train for 2X fiber growth, you’re going to want to increase your creatine intake to make sure you can keep power output high and get the best from your workout.
You can get this from things like red meat, but with the saturated fat content and digestive load, it’s probably easier to get a high-quality creatine supplement into your diet. It’s a cheap and easy way to increase your strength- and power-endurance (34) which means a more effective workout.
Closing Remarks: How much Can we Change?
The scientific literature on changing muscle fiber types is an amazingly-interesting area of emerging research and it definitely deserves attention for anyone who cares about muscle and performance.
However, you’re here for the practical applications. If you’re looking to maximize your results it matters – the only problem is that it’s a decade away from being definitive!
The science suggests that you can adjust your long-term muscle fiber type distribution: the studies have just been too short-term to record these changes properly. They also get obscured easily by short-term changes – as Andy Galpin discusses in this article, there are a lot of dietary and exercise-induced short-term changes in fiber type.
The fact is that the science is there to support training-based changes if you approach your workouts in the right way and focus on your long-term adaptation and lifetime goals.
You’re not going to become Usain Bolt if you’ve spent 30 years running marathons, but you’re going to be able to change your fibers enough to target your needs and goals.
Your training should be specific: even if you’re not aiming directly at muscle fiber changes, you’re going to need to focus on the energy system that you’re training or you’re going to receive sub-optimal results!
Training specific to your muscle fiber goals and energy systems will mean maximal results and the workouts and approaches we’ve discussed above are a great place to start.
Don’t miss out on these benefits just because the science isn’t 100% yet: as Chris Sommers says –
if you’re only ever working within the science, you’re already behind the curve!
The Cliffs: Type – Use – Training Table
Earlier in this piece we shared a table on the characteristics of the different muscle fibers. Well this time we’ve put together one of our own to provide you with the practical information for how you can improve your training by targeting muscle fibers.
Training by Targeting Muscle Fibers
|Type of Fiber||Key Characteristics||Training|
|1||• Aerobic / Endurance Exercise|
• Slow fatiguing, slow recovery
• Fueled by triglycerides (Fats)
|• High-repetition resistance training
• Medium-Intensity Steady-State Cardio
• Occlusion training and pre-fatigue training
• Diet: High fat-intake + intra-workout carbs
• Supplement: Beta-Alanine
|2A||• Middle-duration exercise|
• Middle of the road on fatigue, recovery and size
|• High-volume, moderate-intensity resistance training (70-80%)
• Autoregulate rep-maxes to achieve a balance of intensity and volume
• Diet: Complex carbs + intra-workout sugars
• Supplement: Beta-Alanine
|2X||• Short-duration exercise|
• Maximum strength and power
• Fuelled by muscular stores of ATP
|• Slow-eccentric, explosive concentric training
• Low-reps, maximum output: either weight or speed
• Add jumps and plyometrics to combine eccentric and ROFD gains!
• Supplement: Creatine Monohydrate
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