Muscle Factor Model
How muscles function during and adapt to training
In the early 1970s a man named Arthur Jones introduced a revolutionary strength training method to the bodybuilding and strength training world. Jones had been studying muscle physiology for about 30 years and had long understood that the standard training methods of the day were not completely consistent with what was known about how muscles function during exercise or how they adapt to exercise. Many of the training practices of the day were rooted in tradition and contradictory to known physiological facts. Jones, a lifetime strength trainee himself, believed that training would be more effective if it were modified so that it worked in accordance with what was then known about muscles. He figured that a training program based on how the body really functioned would produce much better results than those training methods that ignored, denied, or were ignorant of the true workings of the body.
Utilizing his understanding of muscle physiology Jones spent many years testing and experimenting with different training methods, constantly seeking to discover training methods that produced the best results. Being independently wealthy afforded Arthur both the time and money required to test his ideas and he ultimately spent 20+ years and millions of dollars in his quest. The end result of all his work was a revolutionary training method – High Intensity Training – and a completely new type of exercise machine – Nautilus Training Equipment.
However, there was a problem; Arthur’s high intensity training method was not just revolutionary; it was contradictory to the conventional training wisdom of the day. Humans, being only human, are usually reluctant to abandon long-held beliefs and so many were resistant to Arthur’s methods. Controversy broke out about Arthur’s high intensity training method and two opposing camps formed – one group supporting high intensity training and one supporting conventional (high volume) training. These two groups spent lots of time and effort defending their methods and attacking those of the opposing camp. Even today, more than 35 years after Arthur first introduced high intensity training, the two camps still exist and the debate still rages. In fact, one of the the primary debates in the bodybuilding world is still centered around which method – high intensity or high volume – is best.
Of significance is that Arthur’s high intensity training method was basically the first time that exercise physiology was used as the foundation of a training program. Before Arthur, training was mostly based on tradition and what the top champions of the day were doing. Arthur completely ignored tradition and the training of the top champions of the day and focused on designing training based completely on the functioning of muscles. The fact that his methods continue to be widely used today is a testament to the effectiveness of his physiology-based training method.
The Problem of Two Opposing Theories
All this is not to say that the entire world has embraced high intensity training. As noted above, today the strength training and bodybuilding world basically consists of two opposing training methods – high volume and high intensity. Both methods are currently used and promoted as the best training method by their respective proponents.
The reason both training methods still exist is because both are known to work, at least for some number of people. And therein lies the problem. In science, anytime a theory is shown to be contradicted by even a single observation, then, by definition, that theory is inaccurate. When a theory is shown to be inaccurate it must be abandoned or modified. The high volume training theory and high intensity training theory are, in essence, opposing theories as to how the body works. Since these two theories contradict each other it means that both theories are wrong, at least to some degree.
The body works in one way, not in two contradictory ways. Or, said another way, there is one set of principles/laws by which the body functions, not two contradictory set of principles/laws. We know that both training methods produce results for some people. We also know that, by definition, both theories are wrong to some degree since they contradict each other. What all this tells us is that we are missing some important information as to how muscles function during and adapt to training. Once this missing physiological information is filled in, both of the competing theories will be assimilated and replaced by a new training theory. The missing physiological information is what has allowed the two competing training theories to continue to exist for more than 35 years and has prevented further advances in training methods.
Enter the Muscle Factor Model
In 2006, while conducting background research for an article on strength training for endurance runners, I came across a strength training study whose results were quite startling. The study compared a non-traditional training method to a standard periodized training program and found that the non-traditional method produced 50% greater increases in strength than did the periodized program. The researchers themselves were unable to explain why the non-traditional program produced the best results and noted that the results were contradictory to both current beliefs about the functioning of muscles and classical training methodology.
That particular study caused me to rethink some of what physiology currently teaches about muscle activation during exercise and its adaptation following exercise. In turn, this led to a breakthrough in muscle physiology; a breakthrough I have termed the Muscle Factor Model. I suggest that this new model more accurately explains how muscles function during and adapt to exercise. Furthermore, this new model suggests some significant modifications in training methods for any sport in which strength, power, or endurance is important. I believe the muscle factor model is a key piece of the missing physiological information and will ultimately result in the integration of high volume and high intensity training. The muscle factor model may lead to the most significant changes and refinements in training since the introduction of periodization in the United States back in the 1980s. I realize those are bold claims so let’s have a look at this new model. We begin with a discussion of muscle contractile properties.
Muscle Fiber Contractile Properties
Physiologists generally divide muscle fibers into three basic types – Slow Twitch, Fast Twitch A, Fast Twitch B – each with its own distinct contractile properties.
Slow twitch fibers are the weakest of fibers, contract relatively slow, and have very high levels of endurance.
Fast Twitch A fibers are stronger than Slow Twitch fibers, contract relatively fast, and have high levels of endurance.
Fast Twitch B fibers are the strongest of fibers and have the fastest contraction speed but have the least amount of endurance.
The above description of the contractile properties of each muscle fiber type might lead you to believe that each type of fiber has distinct contractile properties. Nothing could be further from the truth. Muscle fibers of any type are not all alike; they don’t all contract the same; they are not homogenous. Instead there is a broad continuum of contractile properties in all the muscle fibers of any type. Physiologists have measured up to a 129x range of contractile properties in muscle fibers of the same type. What this means is that in any specific fiber type you will find fibers that contract much slower or faster than other fibers of the same type; fibers that contract much more or much less forcefully than other fibers of the same type; fibers that possess much more or much less endurance than other fibers of the same type. For example, physiologists measured the time to exhaustion in a group of fast twitch fibers and found some of the fast twitch fibers fatigued in as little as 16 seconds while other fast twitch fibers were able to contract for 34 minutes before reaching fatigue. The contractile properties discussed earlier tell us what the average contractile properties are for each type of muscle fiber. The average Slow Twitch fiber is slower, weaker, and has greater endurance than any of the Fast Twitch fibers. The average Fast Twitch B fiber is stronger and faster but less enduring than other fiber types. But the broad range of contractile properties across all muscle fibers means that fibers of the same type do not all have the same level of strength, endurance, or speed.
A very important point about muscle fiber contractile properties is that there is a strong inverse relationship between a muscle’s strength and its endurance. The stronger a muscle fiber the less endurance it has and vice versa. Weaker fibers possess much greater endurance than do strong fibers. Stronger fibers possess much less endurance than weaker fibers. This point is critical to understand.
Muscle Activation During Exercise
Not all muscle fibers are activated during exercise because the body only activates the minimum number of fibers required in order to get the job done. Muscle fibers are activated in a very specific order, from weakest to strongest. Physiologists have termed this the size principle of activation. Basically, muscle fibers are recruited based on the amount of force required to complete the task at hand. Recall that there is a wide variation in the strength of muscle fibers; every whole muscle has fibers with different levels of strength, from very weak all the way up to very strong. The weaker fibers are recruited first with the strongest of fibers only being recruited during the heaviest of tasks. Fibers are generally recruited in the following order based on the level of force required to perform the task:
Slow twitch – Fast Twitch A – Fast Twitch B
There are 2 important points to understand about muscle fiber activation – 1) it is a team sport and 2) total force is the sum of the force of all the active fibers.
1. It’s a team sport: Muscle fiber work together. Activation proceeds from Slow Twitch – Fast A – Fast B. It is NOT the case that Slow Twitch fibers exclusively handle the easy tasks, Fast Twitch A exclusively handle the moderate tasks and Fast Twitch B exclusively handle the heavy tasks. Instead, as the load increases from easy to moderate to heavy an increasing number of fibers are activated and all are working together to complete the task.
2. The total force produced by a whole muscle during a task is the sum of the force of all the individual fibers. All active fibers, whether Slow Twitch, Fast A, or Fast B, contribute force during movement and the total amount of force generated by a muscle is the sum of the force of every active fiber. During a really heavy lift, even though the Fast A and Fast B fibers are activated and doing the bulk of the work, active Slow Twitch fibers are producing force and helping lift the weight.
In practical terms this is what it means:
If you pick up a light weight, then only Slow Twitch fibers will be activated because little force is needed to pick up the weight.
If you pick up a heavy weight then both Slow Twitch + Fast Twitch A fibers will be activated because more force is required to lift the weight. Note that the Slow Twitch fibers are still active during this exercise, but since they are unable to generate enough force to get the job done by themselves, some Fast Twitch A fibers are also required to help out.
Pick up an even heavier weight and now you are using Slow Twitch + Fast Twitch A + Fast Twitch B fibers to lift the weight. The Slow Twitch and Fast Twitch A fibers did not possess enough strength to lift the weight by themselves, so the strongest of fibers, the Fast Twitch B fibers, were activated.
The same thing applies to any activity. For example, running at a slow pace activates only Slow Twitch fibers because the force required to run slowly is small enough that the Slow Twitch fibers are strong enough to handle the job themselves. Running at a faster pace activates Slow Twitch + Fast Twitch A fibers because running faster requires more force to be generated. Very fast running (i.e. intervals and sprints) and fast or steep uphill running activate the Slow Twitch + Fast Twitch A + Fast Twitch B fibers due to the high level of force required to run at very fast paces.
Muscle Fiber Activation at Exhaustion
As an exercise proceeds it becomes increasingly difficult to maintain a set amount of force production because of fatigue. The first repetition of an exercise might be reasonably easy but repetition 20 with that same weight might be an all-out effort. Are all fibers activated during the hard to all-out effort that athletes routinely reach during intense workouts? Only in some cases; in most cases not all fibers are activated.
During exercise as a person’s active muscle fibers fatigue some inactive muscle fibers are recruited to assist those active fibers that have fatigued. However, there is a limit to the amount of additional fibers that are recruited. Not every muscle fiber is activated during exhaustive exercise. Instead, the person reaches exhaustion or terminates the exercise. About the only time that all fibers are active is during the heaviest of tasks, such as during very heavy weight lifting (i.e. about 6 or less reps). Less forceful tasks, such as high rep strength training or distance running, do not result in 100% activation of all available muscle fibers, even at the end of the exercise when the trainee is working as hard as they can in that particular exercise. For example, one study found a little less than 70% leg muscle fiber activation while running to exhaustion on a level treadmill and a bit more than 70% activation during exhaustive running up an inclined treadmill.
Overload and Intensity
One of the primary principles of training is the overload principle. Exercise physiology generally describes overload like this – the application of an activity specific overload in order to cause physiologic improvement and bring about a training response. What this means is that muscles must be trained with a sufficient level of intensity in order to cause adaptation to occur. There is nothing earthshaking in the concept of overload as it has been a principle of training for more than a century.
However, we need to carry the concept of overload a bit further and apply it to individual muscle fibers; what is true for a whole muscle is also true for individual muscle fibers. In order to cause a training response in any individual muscle fiber that muscle fiber must be trained with a sufficient level of overload, with a sufficient level of intensity. This is accomplished by training a fiber reasonably close to its maximum capacity. Or said another way you must sufficiently fatigue a fiber in order for it to adapt and improve. This point is critical in understanding how muscles fibers work and adapt to training.
Let’s examine this principle in training terms.
You put weights on a bar so that you are only able to lift the bar a maximum of 10 times. Since the bar is very heavy you will activate Slow Twitch + Fast A + Fast B fibers while lifting it. After 10 reps (about 30 seconds of lifting) you are no longer strong enough to lift the weight an additional repetition so you set the bar down, ending the exercise. Which fibers did you overload?
You only overloaded some of your Fast B fibers. Specifically, you overloaded those Fast B fibers that fatigued in 30 seconds or less.
There were a whole bunch of fibers that you didn’t overload. Which ones? Those fibers that take longer than 30 seconds to fatigue were not fully overloaded when the set ended.
At the end of the set some of your Fast B fibers were exhausted and couldn’t continue to contract. But a lot of your Fast B and all your Fast A and Slow Twitch fibers were not exhausted at rep 10 because they posses more endurance than the strongest of the Fast B fibers (remember, it has been shown that it can take several minutes to exhaust all the Fast B fibers). The reason you terminated the exercise at rep 10 is because the whole muscle lacked the strength to lift the weight, but only some of the Fast B fibers were fatigued.
This set fatigued, and therefore overloaded, some of the Fast B fibers and those are the fibers that will get stronger. But the remainder of your Fast B and all your Fast A and Slow Twitch fibers were not particularly overloaded and will adapt little to none.
When those few Fast B fibers adapt you will be stronger but you will not be as strong as you could get. Why? Because lifting a heavy weight is a team effort and all your Fast B, all your Fast A and all your Slow Twitch fibers contribute to the total strength of the muscle but you didn’t adequately train all your Fast B or your Fast A and Slow Twitch fibers to get stronger. Only when you train all your fibers to overload will you get as strong as you are genetically capable of getting.
Putting it All Together = Muscle Factor Model
When we put all the above facts together, we arrive at the Muscle Factor Model. In order to cause an adaptive response in a muscle fiber, that muscle fiber must 1) be active and 2) be overloaded; failure to accomplish both of these results in little to no adaptation in that muscle fiber.
Recall the inverse relationship between a muscle fiber’s level of strength and its endurance capacity – the higher the strength the less the endurance, the lower the strength the greater the endurance. If you are going to overload a muscle fiber you must work it to a reasonable level of fatigue. Considering that muscle fibers posses widely varying levels of endurance, this means that only a relatively few muscle fibers are fatigued at the end of any normally conducted exercise session.
In training terms this means:
In order to overload weak muscle fibers with abundant endurance requires long training sessions conducted at low levels of force production.
In order to overload stronger muscle fibers with moderate levels of endurance requires moderate duration training sessions conducted at moderate levels of force production.
In order to overload the strongest of muscle fibers with poor endurance requires short duration training sessions conducted at high levels of force production.
If you want to maximize your performance, then you have to train all the muscle fibers that contribute to force production during your chosen activity. You have to train your weak fibers, your moderate fibers, your strong fibers, and your strongest fibers. Since force production is a team effort any untrained fibers detract from the overall performance of the team (in this case the team is the whole muscle).
The muscle factor model provides a more complete explanation for how muscle fibers work during and adapt to exercise. Only muscle fibers that are active and overloaded during exercise will adapt and grow. The only way to overload a muscle fiber is to train it to a sufficient level of fatigue. Normally performed exercise programs usually do not train all or most of the fibers in a whole muscle due to the way muscle fibers are activated during exercise and because muscle fibers have widely varying levels of endurance. The only way to maximize performance is to train all the muscle fibers that are active during the event; any untrained muscle fibers prevent the athlete from reaching his/her maximum potential.