THE PRINCIPLE OF “ORDERLY RECRUITMENT” OF MUSCLE FIBERS – AND ITS RELEVANCE TO BODYBUILDERS

The following is adapted from John Little’s new book Max Contraction: The Scientifically Proven Program For Building Muscle Mass In Minimum Time.

The mind and body being interrelated, it should come as no surprise that it is our brain that recruits the body’s muscle fibers solely as it perceives the need for them. This is accomplished via the Central Nervous System through the motor nerves, which, in keeping with the dictates of the brain, follow a relatively fixed order in the recruitment process. The process involves only the precise amount of electrical current necessary to activate the precise amount of muscle fibers required to generate a precise amount of force.

THE NATURE OF MUSCLE FIBER RECRUITMENT

Anyone who has read my articles, courses and books should now be aware that a muscle must be in a position of maximum contraction to recruit all of its available muscle fibers – but this is only one half of the equation. The other half is the amount of weight or the “load” the muscle fibers are made to contract against. If the load is maximal and the contraction is maximal, then the fiber recruitment will be, in turn, maximal. And conversely, if the load is minimal and the contraction is minimal, then the fiber recruitment will be, in turn, minimal. Moving a given muscle group into a position of full or maximum contraction is crucially important for two reasons; first, since muscles contract by shortening or reducing their length, a muscle has to be in a fully contracted, or Max Contraction position if all of its fibers are to be contracted at the same time (and to induce maximum levels of growth stimulation, as many as possible of the available fibers must be made to contract); second, as a result of this fact, the position of Max Contraction is the only position where it’s even possible that a maximum load (or overloading) of the muscle can occur. Any other position -- or even a full range of motion -- results in sub maximal loading owing to the fact that not all of the fibers are brought together (i.e., the muscle is either not contracting at all -- as in the position of full extension -- or is both technically and by definition out of a position of “maximum contraction” and hence unable to perform a maximum contraction) and disadvantageous leverage factors serve to reduce the resistance that the muscle is potentially capable of contracting against. When these two factors are properly aligned, it is – for the first time in the history of exercise – potentially possible to stimulate all of a given muscle’s fibers thoroughly. These two factors – point of contraction and optimum overload – are, in fact, the only relevant factors in the calculus of inducing maximum size and strength increases. The second half of this calculus, optimum overload, is worthy of a more in depth study as understanding the role of overload in the muscle fiber recruitment process will of itself dispel a lot of mythology and dogma within the world of bodybuilding and strength training.

Human anatomy and physiology studies have revealed that there exist four distinct muscle fiber types within our species. Talk to most would-be experts or personal trainers and you’ll hear a very simplified (and scientifically incorrect) synopsis that there exist only “fast-twitch” and “slow-twitch” muscles. However, physiologists have actually identified not one, not two, but three types of fast-twitch muscle alone. In addition, the classification schemes for muscle fibers have differed over the years, resulting in three classifications receiving prominence in classifying the same thing. The classification of the four fiber types under these three classification schemes are as follows:

CLASSIFICATION OF THE FOUR FIBER TYPES

Fast twitch muscle fibers differ from their slower cousins in many ways, endurance capacity being one of them. In fact, it’s in the endurance realm rather than in the velocity or speed department that their differences become most apparent. The Fast-Oxidative (FO) (Type II A) fibers have relatively good endurance (the term “oxidative” refers solely to the aerobic machinery within the Fast-Oxidative fiber itself). Another fast-twitch fiber is the Fast-Glycolytic (FG) (Type II B) which are very fast in contracting, and very powerful but have nothing to offer in the way of endurance (the term “Glycolytic” refers to the anaerobic machinery within the Fast-Glycolytic fiber itself). As an example, the huge deltoids and massive arms of Mr. Olympia caliber bodybuilders are comprised almost entirely of FG fibers.

An intermediate in speed, endurance and power are the Fast-Oxidative-Glycolytic (FOG) (Type II AB) fibers, which contain both the anaerobic and aerobic machinery within their cellular makeup. On the other side of the coin, Slow muscle (S) (Type I), so called because in comparison with, say, FG fibers, they appear thus, is an endurance fiber used primarily by those who engage in distance activities. It is very powerful aerobically with lots of aerobic enzymes, blood vessels and myoglobin (an oxygen-storing endurance compound). On the down side, however, the S fibers aren’t capable of creating much force and, consequently, don’t possess the inherent mass potential of their quicker cousins.

An individual’s fiber type and distribution appear to be genetically predetermined -- a product of breeding as opposed to environmental influences – which means that training in a certain fashion will not “create” a higher complement of a particular fiber type than one is born with. Still, most of us are brought into the world with a more or less even distribution of all types of fibers -- both fast and slow twitch. This, obviously, is not good news if you want to be a powerlifter, as it would stand to reason that a higher complement of FG fibers would be of greater benefit for this sport -- but then some of us were born to be marathoners, not sprinters. As a result, premiere powerlifters have a high FG fiber percentage, while distance runners would have a greater complement of type-S fibers.

Of the four fiber types, the to “S” or Slow fibers are the easiest engage owing to the fact that they doncurrent. ’t require a lot of Slightly more energy is required to engage the FO fibers and more still for the FOGs. The ones that require the highest electrical output to engage are the FGs. And here is why it is of the utmost importance to have your muscles contract against the heaviest weights possible if you wish to activate the FGs, as the brain is in no hurry to hit the switch for those FG fibers -- the ones you want to stimulate for size and strength increases. The brain would rather engage the least amount of muscle fibers necessary to accomplish a given task. After all, the brain is an organ of survival and it knows how to conserve energy as the conservation of energy has proven, over millions of years, to be an asset for survival.

The brain will first attempt via the Central Nervous System to contract against a heavy resistance by recruiting only the “S” fibers, however these will prove inadequate for the task. The brain will then recruit the FOs and shortly thereafter the FOG fibers to assist with the task of contraction. If the weight is light or moderate, then these are all the fibers that will be recruited. However, if the weight is truly heavy enough; i.e., so heavy that you can only contract against it for 1-to-6 seconds, the brain will have realized that it needs more fire power than it’s been providing and only then will it send out the signal to engage the elusive FG fibers. This process is known in physiology circles as “Orderly Recruitment,” for the brain does not engage in the firing of muscle fibers randomly. When recruiting muscle fibers for the purpose of contraction the brain doesn’t concern itself with issues of speed but force. It has no concern with how fast you want to lift a weight or how quick you wish to run – again, it cannot randomly recruit muscle fibers. Instead, the brain ascertains the exact amount of force your muscles require to move a precise resistance and, accordingly, recruits the precise amount of muscle fibers required to do the job. ¹

An interesting aspect of this phenomenon is that when the brain sends sufficient current to activate the FG fibers in a Max Contraction set, we automatically know that the “FOs” and “FOG” fibers -- that is, ALL of the available muscle fibers -- have also been recruited and engaged, thereby ensuring the greatest possible growth stimulation.

NOTES ON TEXT:

1.) Physiologists H.S. Milner-Brown and colleagues empirically validated the fact that the load imposed upon muscle during contraction is the major factor dictating the type and volume of muscle fiber recruitment; the results of their research were published in the Journal of Physiology, 230, 350; 1973. See also J Neurophysiol. 1986 May;55(5):1017-29 and J Neurophysiol. 1987 Jan;57(1):311-24.

Article copyright © 2003, John Little and Northern River Productions, Inc. Any reproduction in whole or in part without the expressed written permission of the author is strictly prohibited.