FAST TWITCH VERSUS SLOW TWITCH: DOES FIBER TYPE PREDICT ATHLETIC SUCCESS?

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Ever noticed how certain training protocols work wonders for some folks but leave others with more problems than solutions? While long distance, endurance-focused sessions, for example, may encourage positive training adaptations for one person, the same workload may compromise another’s progress, rendering them worse off than when they began. Though the variables which influence and ultimately determine athletic performance are many and varied, one important physiological determinant can be counted upon to predict the training success which underpins high level sporting accomplishment: muscle fiber composition. Indeed, the degree to which various training methods can be employed to reveal athletic potential is heavily influenced by the way in which we are able to maximize the recruitment of specific muscle fibers. Where and to what degree various types of muscle fiber are situated on our bodies can greatly influence how we may adapt to a specific training regimen. Furthermore, how these muscles respond to a given training stimulus is often contingent upon their inherent characteristics. Therefore, when planning our training to elicit maximum results we must have a broad understanding of what we are dealing with in the way of the different muscle fiber characteristics and which kinds of fibers are likely to advance, or restrict, our individual performance. By training the wrong way for our unique muscle fiber distribution we may negate our gym efforts while failing to achieve our training aims; however, by activating one system over the other we can both increase our chances of overcoming genetic limitations and achieve our training objectives. 

 

Furthermore, how these muscles respond to a given training stimulus is often contingent upon their inherent characteristics                

 

Fast twitch versus slow twitch

Most lifters with a modicum of training experience will have heard of fast and slow twitch muscle fibers and may even have a rough understanding of the distinctive qualities of each. What is not so clearly understood is how we may determine what percentage of each we possess and how we might best train to accommodate our unique composition. So what exactly are fast and slow twitch fibers and why might forming an understanding of their complexity be important in the first place? Because we are either fast or slow twitch dominant or, more rarely, possess a balanced combination of both types, it makes sense to know more precisely how we may design our training to target the further development of those that will enable us to achieve sporting success. Moreover, should we be deficient in one type yet aspire to achieve physical excellence in line with the unique functions of those we are short of we can also, through determining our distribution, decide upon which training methods to steer clear of and which to include.

 

Each fiber type determines how our muscles use energy to power physical performance

Though our muscle fibers are often thought to comprise two types (Fast Twitch and Slow Twitch) they are actually classified into three categories: Slow Twitch (ST or Type I), Fast Twitch Type IIA (or FT-A), and Fast Twitch Type IIB (or FT-B). Each fiber type determines how our muscles use energy to power physical performance, the kinds of activities an individual may best be suited to, the various ways in which muscles respond to training, how our muscles function, and, of particular relevance for bodybuilders, how large and strong our muscles may ultimately become.

 

SLOW TWITCH (ST or Type I)

Slow Twitch fiber activation is fully on display whenever we witness an ultra marathon runner pounding away for impossibly long periods of time. Slow Twitch fibers govern all low-impact activities of an aerobic nature that demand minimal force production. Identified by their slow contraction time and high resistance to fatigue, ST fibers are smaller in diameter than their FT counterparts and a smaller motor neuron facilitates the degree of contractility they possess. The larger mitochondrial and capillary density of ST fibers is largely involved in supplying these fibers with large amounts of oxygen and blood to ensure a steady supply of energy over long distances. These fibers’ high concentration of the iron and oxygen-binding protein myoglobin allows ST dominant individuals to work at high levels of exertion for long periods.  Low in glycogen and the high-energy substrate creatine phosphate, ST fibers are, instead, saturated with an abundance of triglycerides (stored fats) to provide a more sustained energy output. Finally, ST fibers include a preponderance of enzymes needed to facilitate the oxidative process – by carefully controlling enzyme activity our cells can control what reactions are supposed to take place; the types of reactions catalyzed by enzymes are unlikely to happen spontaneously – and few of those involved in glycolysis (the exclusive domain of the FT fibers).

 

the FT fibers are in stark contrast to the ST fibers, both in appearance – with fewer capillaries and less blood required to carry oxygen to them their coloring is white, whereas ST fibers, requiring much oxygen, are red – and function.

 

FAST TWITCH (FT: IIA and IIB)

While FT IIA and IIB fibers are similar in nature, the differences between them warrant their sub-categorization. As a grouping, the FT fibers are in stark contrast to the ST fibers, both in appearance – with fewer capillaries and less blood required to carry oxygen to them their coloring is white, whereas ST fibers, requiring much oxygen, are red – and function. The fast-twitch dominant athlete is not likely to excel in long distance running or activities of an otherwise aerobic nature. Rather, due to their fast contraction time and low resistance to fatigue, FT fibers tend to be more widely distributed in individuals who are better-suited to engaging in explosive activities such as sprinting and Olympic lifting. The quicker contractility of FT fibers can be explained, in part, by the activity of an enzyme called myosin ATPase – it is the role of Myosin ATPase to break down ATP inside the myosin head of a muscle’s contractile proteins. The faster ATP – the energy-source which powers muscular contraction – is liberated, the more rapidly we are supplied with the energy needed to complete short-burst activities. Because FT fibers are larger in diameter and have greater growth capabilities than ST fibers, a larger percentage of FT fibers are preferred by those looking to build more muscle.

 

FT IIA fibers feature characteristics of both ST and FT fibers, but at the lesser end of the two extremes. Though they are larger in size and have a large motor neuron, FT IIA fibers also contain higher mitochondrial activity and capillary density than do FT IIB fibers. Though high in glycogen and creatine phosphate they also contain triglycerides and moderate myoglobin levels, compounds found abundantly in ST fibers. FT IIA fibers kick in when we attempt activities of a predominantly aerobic nature that require a degree of explosiveness and relatively high force production: an ultra high rep set or a 400 meter race, for example. In contrast, FT IIB fibers are used for short-burst anaerobic activities requiring a high degree of force; pursuits that leave our lungs gasping for breath and our muscles packed to capacity with that metabolic by-product of anaerobic metabolism, lactic acid. Though FT IIB fibers contain an abundance of glycolytic enzymes and much creatine phosphate and glycogen, they are low on oxidative enzymes, triglycerides, capillary density, myoglobin and mitochondria. They are built for speed, not endurance.

 

Are you FT or ST?

So important has the distinction between FT and ST fibers when considering athletic performance become that many athletes are now regarded as being either FT or ST dominant. A lean and lanky MMA fighter may choose to grind on an opponent for three rounds, gradually sapping the strength from his adversary and turning the bout into a test of endurance. Another fighter, more compact and muscular, may prefer to blitz his opponent with a vicious onslaught of kicks and punches from the opening bell in an attempt to overpower his opponent early and gain the fast finish. This example may be used to illustrate – without attempting rigorous strength testing or, for a more thorough determination, performing a muscle biopsy – the differences between how a fast twitch (the latter) and slow twitch (the former) athlete is likely to perform and how their physical characteristics may differ. In a crude sense we can get a general idea of whether we are faster or slower twitch by the activities we are likely to excel in and how our body responds to various types of training. Packing on muscle fast and exploding up massive training poundages, yet lagging on long distance runs and suffering on the treadmill to nowhere might point to a greater distribution of FT fibers; being less muscular in build and better suited to triathlons than high intensity tri-sets may point to the physicality of one who is ST dominant.   

             

While invasively plunging the depths of our muscles and snipping a few fibers to be examined under a microscope may prove the most accurate way to directly determine FT/ST muscle fiber composition, most people are in no position to undergo such muscle-biopsy treatment. However, there is an effective, less complicated, method of testing for muscle fiber distribution: the good-old one repetition max (1RM) test. Used to gauge strength in order to determine the amount of weight required to complete certain rep-range protocols, the 1RM test determines the maximum poundage a person can lift once. Say we wish to assess the degree to which fast twitch fibers comprise the leg muscles, in particular the quads and hams (and such testing can only be done with muscle groups not individual muscles). To do this we would take a weight 80% of the 1RM for the squat and complete as many reps as possible. If the legs tire before seven reps we might conclude that they have a greater proportion (more than 50%) of FT fibers; if over 12 reps can be achieved then more than 50% ST fibers are likely to be responsible (less than 12 and more than seven reps suggests an equal number of FT and ST fibers due to excessive lactic acid accumulation, the FT fibers' inability to go the distance on this occasion is not down to their strength levels. On the other hand, the faster lactate clearance and more consistent energy production of the ST fibers enables the set to be continued in excess of 50% longer than that of the FT fibers. Along with such 1RM percentage testing, we may also closely monitor our own performance both in the gym and when playing sports. The aforementioned traits indicative of a greater degree of explosiveness combined with pronounced power output may suggest more FT fibers are at play; more staying power, but less overall size and strength, may suggest greater ST recruitment.  

 

 In a crude sense we can get a general idea of whether we are faster or slower twitch by the activities we are likely to excel in and how our body responds to various types of training.               

Training for your fiber type  

When training for a specific sport it is imperative to know what methods must be employed to recruit the maximum number of muscle fibers responsible for maximizing success. Though certain athletes will genetically be better suited to specific pursuits this does not mean those with fewer of the requisite muscle fibers cannot attain similar levels of achievement with the right training protocols in place. While an athlete with a greater percentage of FT fibers will excel in running short sprint intervals and in lifting maximum weights explosively this is not to say they cannot work to further build and improve the performance of their ST fibers. Thus, for an FT dominant athlete wanting to complete a marathon, for example, more specific long-distance aerobic-style training is in order and less anaerobic work should be performed to maximize the development of the ST fibers while promoting an atrophying of the FT fibers (a kind of selective-hypertrophy effect). In contrast, trainees with more ST fibers need not write off hopes of becoming a champion powerlifter, or bodybuilder. With more heavy, intense training such an athlete may change the composition of their muscles: whereas before they may have had a 70/30 percent distribution of ST to FT fibers this same ratio will remain largely unchanged but, instead, the cross sectional area of the muscle most activated is increased thus bringing the total cross-sectional area of both muscle types to a 50/50 ratio. While the endurance capabilities of the ST fibers will have declined due to their reduced size, the strength capabilities of the FT fibers (though smaller in number) will have increased. Vice versa for the power-oriented athlete who wishes to be a better triathlete.

 

FT fibers cannot be transformed into ST fibers no matter how many long-distance sessions we complete and ST fibers cannot take on the characteristics of FT fibers regardless of how big and strong our muscles become.

Though we cannot change our FT/ST muscle fiber composition we need not be limited by our present distribution. Indeed, even among predominantly FT or ST athletes there is greater disparity in the exact ratio of muscle fibers each possesses; thus, we will never realize our full potential in any sport until we train appropriately for that sport for a decent length of time. That said there can be no inter-conversion of fibers: FT fibers cannot be transformed into ST fibers no matter how many long-distance sessions we complete and ST fibers cannot take on the characteristics of FT fibers regardless of how big and strong our muscles become. What can however occur with the right training is a transformation whereby FTA fibers can adopt some of the strength and power qualities of the FTB fibers and FTB fibers can take on some of the endurance characteristics of FTA fibers. This modification along with the increased cross-sectional area of a specific muscle fiber type is the closest we may come to changing our genetic propensity to excel athletically.          

 

References        

Ingjer, F. Effects of endurance training on muscle fibre ATP-ase activity, capillary supply and mitochondrial content in man. J Physiol. 1979 Sep; 294: 419–432.

Karp, J. Muscle Fiber Types and Training. [Online] www.coachr.org/fiber.htm retrieved on 9.4.15      

Pipes, T.,V. (1994). Strength training and fiber types. Scholastic Coach, as referenced in Muscle Fiber Types and Training, by Jason R. Karp, Track Coach #155.

Thayer, R., Collins, J., Noble, E. G., Taylor, A. W., A Decade of Aerobic Endurance Training: Histological Evidence for Fibre Type Transformation. Journal of Sports Medicine & Phys Fitness. 2000 Dec; 40(4).

About David Robson

is a New Zeland-based professional freelance writer, book author, personal trainer and health and fitness expert. With his sound scientific and academic credentials he has helped thousands of people achieve their health and fitness goals both through his written works and guidance. As a staff writer, David’s articles can be read in leading health and fitness periodical Status Fitness Magazine. David can be contacted at: davidrobson19@hotmail.co.nz