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Review: The first part of this article showed that the resistance you can use in any given exercise is dependent upon many different factors. These include the number of repetitions you perform, the duration for each repetition, the range of motion, and where the exercise is performed within your routine. The amount of resistance you can use for an exercise also depends upon the brand and generation of machine you may be using and, of course, is dependent upon genetic factors such as ‘good’ or ‘bad’ leverage for different movements and your ability to recruit motor units. The main points of the article were that given these dependencies, becoming fixated on using a certain resistance did not make sense and that as long as a muscle was fatigued within about 30-90 seconds, it did not matter how much resistance was used. A wide range of resistance on a given movement can increase strength and muscular hypertrophy. Realizing resistance is relative should lead to thinking more about the attributes of an effective stimulus in resistance training and how to productively train. The Size Principle: Within basic neuromuscular research, these issues were studied many years ago. The studies led to what is considered the single most important and universal principle governing neuromuscular activity,
the size principle. The principle states that motor units are recruited for a given activity based on their size. The process starts with the smallest, easiest to excite, less powerful motor units and progresses in an orderly way to the largest, harder to excite, more powerful motor units. The recruitment process is efficient and effective. The number and type of motor units that are recruited and, hence, the internal force that is generated, fit the task at hand. In the original research and writings, some of it classic1, what was stated was that the stimulus generating internal force on motor units was the degree of effort involved, that is, the intensity of the stimulus. If the degree of effort was low, the larger motor units would not need to be recruited. If the degree of effort was high, the larger motor units would need to be recruited to be able to continue the task. The key training variable as far as maximizing motor unit recruitment according to the size principle is intensity as defined by degree of effort. However, beginning more than 40 years ago, there was confusion about the size principle. In some publications, the principle was stated but then the interpretation and application were incorrect. Intensity, the degree of effort, was confused with the amount of external force, i.e., the amount of weight on the bar. Internal force was confused with external force. The basic idea was that to effectively recruit larger motor units to increase strength and maximize hypertrophy you had to lift very heavy weights. You had to use high external force. For example, you had to lift weights that were 90% of your one repetition maximum (1 RM). Historically, it is easy to see how this mistake was made. Most earlier resistance training was simply a derivative of weightlifting where the idea was to lift heavier and heavier weights. So, it seemed logical that the path to success was to lift heavy weights. But, it is actually possible to lift a heavy weight, such as 90% of 1 RM, while exerting lower effort, i.e., lower intensity. For example, let’s say a person is able to perform three repetitions with 90% of 1 RM. However, the person only performs one repetition. This represents high force (90% 1 RM) but lower effort (one rep where three could be performed). The size principle suggests that performing one repetition with 90% of 1 RM, in this case, would not be an effective training stimulus. If intensity, the degree of effort, and not force, is the determinant of motor unit recruitment, then a wide range of external force can constitute an effective stimulus. How effective the stimulus is will depend upon the number of repetitions, repetition duration, and how much momentum was involved. However, seemingly, the most important factor is that within about 30 to 90 seconds, at the end of the set, there’s a great deal of effort involved in completing the last repetition. This is a simple axiom of effective training that you doubtless have read and thought about many times. Seen in a new light, however, the simple axiom is profound. Here are reasons why it is profound. Reasons: One reason is that when you understand the size principle, it’s apparent that certain widely promoted training models are not scientifically correct. Do not follow training models that misinterpret the size principle. Consider the training models promoted by the National Strength and Conditioning Association (NSCA) and the leading lights in that organization. Their programs revolve around weightlifting and powerlifting for athletes, bodybuilders, and the general population. Everything is prescribed by the amount of external force as represented by percentages of 1 RM’s. For example, at some phase in a program you are supposed to use heavier weights such as 5 RM (a resistance where you can only perform five repetitions) so that you activate the larger motor units and increase strength. In another phase, you are supposed to use 8-12 RM to enhance both strength and muscular hypertrophy. The physiological mechanisms for differences in the effects of these different RM’s have not been explained. These training prescriptions continue even though there is virtually no evidence that there is any difference in strength or muscular hypertrophy outcomes associated with using a specific RM in training2. For example, there’s little evidence that training with 70% of 1 RM loads and ‘failing’ on the 12th repetition leads to any better or worse strength or muscular hypertrophy outcomes than training with 85% of 1 RM and failing on the 6th repetition. “The size principle indicates that intensity defined as degree of effort is the key.”
If there are no differences in outcomes attributable to training, probably something else common to either load matters. The common factor is likely that with either load it’s possible to reach a point where a great deal of effort is required to complete the set. The size principle suggests why there should in theory be no real differences in outcome between these two protocols. The size principle also correctly predicts why such different prescriptions generally have not produced different outcomes in strength training studies. Publications in NSCA’s journals and related journals have incorrectly interpreted the size principle. These journals also have published articles that appear to support their beliefs about the importance of specific RM’s. When the studies are critically analyzed, it has been found that they do not support the NSCA’s training models and their interpretation of the size principle2. The interpretation fits the lifting background and ‘persona’ of the NSCA and has led to programs that involve trying to lift heavier and heavier weights. Because a number of leaders of the NSCA have good academic credentials and are at prestigious professional settings, a possibility is that some of them realize their interpretations of their studies and the size principle are wrong. So, why don’t they ‘come clean’? A great deal would be lost by recognizing that intensity as defined by the degree of effort, and not external force, is the generative stimulus. What would be understood is that the whole NSCA approach based on weightlifting at best isn’t optimal and at worst is incorrect and potentially injurious for the majority of trainees. Understanding the size principle also helps us analyze other training models or claims made about how certain ‘sensational new’ training models work. Here are two examples. You can undoubtedly come up with other examples. One prominent Olympic caliber track star from England has developed a special resistance training approach for track athletes. Her contentions (based on interviews on CNN) are that because track athletes have to move very fast, they have to lift weights at blinding speeds but not train to failure. She also said that raising and lowering
weights very quickly activates ‘fast twitch fibers’ (larger motor units). These points were the basis of her resistance training program for fast running. There are few convincing demonstrations that lifting weights very quickly transfers to any other activity other than lifting weights very quickly. Using lighter weights, moving quickly with a great deal of momentum, and not training to failure almost assures, following the size principle, that larger motor units will not be activated. Everything about her resistance training system was incorrect. Because she could run fast, no one seemed to doubt her training wisdom. Interviewing exercise scientists to assess the viability of her approach was not part of the story. Other training programs that have been advertised claim either through specific training approaches or the use of certain techniques or devices that you can ‘bypass’ the slow twitch fibers (smaller motor units) and directly activate the fast twitch fibers (larger motor units). These claims, never really substantiated, violate the size principle that defines an orderly recruit process from smaller to larger motor units based on the intensity of the stimulus. Where does this lead us? Here’s another reason why understanding the size principle has profound implications for training. We’ve read many times before and likely thought about it too that it’s intensity that matters in training. We may have even explained this axiom to other people. But, did we really apply this axiom to our own training? For many of us, the search has always been for methods that will enable us to lift more weight. In our own way, we equated a larger and larger external force with a better and better stimulus. We likely have realized that paying more attention to form, range of motion, and repetition duration mattered and we may have also seen that such training tactics actually reduced external force. That is, when you perform longer duration repetitions with very controlled turnarounds within a good range of motion, you likely are using less resistance than training within conventional repetition patterns. But, then within this better training methodology, the goal often was still to lift heavier weights, to use more force. If intensity, the degree of effort, is the key, we should be looking at our training in a different way. We should consider for every movement how we can better target particular muscle groups through better cognitive and affective focus and control and through better physical execution of each movement. How can each movement be made harder and a set made more intense without increasing (and often decreasing) resistance, the external force? Again, we’ve all heard this before, but have we really put this into practice? Isn’t it possible that for an exercise where we are using 100 lbs, we could use 80 or even 70 lbs and make the exercise more precise, harder, and the overall set more intense? Given the size principle, wouldn’t that constitute more effective training? And, wouldn’t following the size principle in this way and not trying to lift heavier and heavier weights – an impossibility in any case - be a much more effective age appropriate way to train? Note: Every reader knows the name Arthur Jones and knows that he developed the original Nautilus machines and high intensity training. We also recognize that by advocating high intensity training, the training was briefer and less frequent than conventional training. The past and current controversies and any hyperboles about outcomes aside, we know that his writings, work, and exercise machines were a departure from conventionality. Think more about what Jones said and what he created. A better description of his work is ‘paradigm shift’. The entire purpose of the approach and each machine was to deliver a high intensity stimulus to muscle groups. Arthur Jones knew that intensity and not external force per se was the critical factor. No machine was designed, for example, to enable you to lift a ton in the squat, although he presumably could have created such a machine. Even with training with free weights and not his machines, Jones stressed intensity. For example, with the squat, Jones advocated very strict form and a large range of motion (albeit, beyond the capability of most people) that would reduce the resistance for most trainees and increase the intensity of the exercise. Jones always emphasized how demanding squatting was when performed correctly but never recommended using a great deal of resistance in “How can you make your taining more precise and intense? this movement. He also advocated using higher repetitions in squatting because of the very high compression forces when heavy resistance was used. For example, he noted (actually, hypothesized in the absence of data) that when a very large man could perform 20 repetitions in the squat with 400 lbs, then he would have as much leg strength and development as could be acquired. For more average size men, an estimate was 20 repetitions with 300 lbs. The paradigm shift was dropping the central idea of weight lifting and bringing to life intensity of training and targeting muscle groups, an approach that best fits the size principle.
?References1. Henneman E. Relation between size of neurons and their susceptibility to discharge. Science. 1957; 126: 1345-1347.2. Carpinelli RN, Otto RM, Winett RA. A critical analysis of the ACSM position stand on resistance training: insufficient evidence to support recommended training protocols. JEPonline. 2004; 7: 1-64.
the size principle. The principle states that motor units are recruited for a given activity based on their size. The process starts with the smallest, easiest to excite, less powerful motor units and progresses in an orderly way to the largest, harder to excite, more powerful motor units. The recruitment process is efficient and effective. The number and type of motor units that are recruited and, hence, the internal force that is generated, fit the task at hand. In the original research and writings, some of it classic1, what was stated was that the stimulus generating internal force on motor units was the degree of effort involved, that is, the intensity of the stimulus. If the degree of effort was low, the larger motor units would not need to be recruited. If the degree of effort was high, the larger motor units would need to be recruited to be able to continue the task. The key training variable as far as maximizing motor unit recruitment according to the size principle is intensity as defined by degree of effort. However, beginning more than 40 years ago, there was confusion about the size principle. In some publications, the principle was stated but then the interpretation and application were incorrect. Intensity, the degree of effort, was confused with the amount of external force, i.e., the amount of weight on the bar. Internal force was confused with external force. The basic idea was that to effectively recruit larger motor units to increase strength and maximize hypertrophy you had to lift very heavy weights. You had to use high external force. For example, you had to lift weights that were 90% of your one repetition maximum (1 RM). Historically, it is easy to see how this mistake was made. Most earlier resistance training was simply a derivative of weightlifting where the idea was to lift heavier and heavier weights. So, it seemed logical that the path to success was to lift heavy weights. But, it is actually possible to lift a heavy weight, such as 90% of 1 RM, while exerting lower effort, i.e., lower intensity. For example, let’s say a person is able to perform three repetitions with 90% of 1 RM. However, the person only performs one repetition. This represents high force (90% 1 RM) but lower effort (one rep where three could be performed). The size principle suggests that performing one repetition with 90% of 1 RM, in this case, would not be an effective training stimulus. If intensity, the degree of effort, and not force, is the determinant of motor unit recruitment, then a wide range of external force can constitute an effective stimulus. How effective the stimulus is will depend upon the number of repetitions, repetition duration, and how much momentum was involved. However, seemingly, the most important factor is that within about 30 to 90 seconds, at the end of the set, there’s a great deal of effort involved in completing the last repetition. This is a simple axiom of effective training that you doubtless have read and thought about many times. Seen in a new light, however, the simple axiom is profound. Here are reasons why it is profound. Reasons: One reason is that when you understand the size principle, it’s apparent that certain widely promoted training models are not scientifically correct. Do not follow training models that misinterpret the size principle. Consider the training models promoted by the National Strength and Conditioning Association (NSCA) and the leading lights in that organization. Their programs revolve around weightlifting and powerlifting for athletes, bodybuilders, and the general population. Everything is prescribed by the amount of external force as represented by percentages of 1 RM’s. For example, at some phase in a program you are supposed to use heavier weights such as 5 RM (a resistance where you can only perform five repetitions) so that you activate the larger motor units and increase strength. In another phase, you are supposed to use 8-12 RM to enhance both strength and muscular hypertrophy. The physiological mechanisms for differences in the effects of these different RM’s have not been explained. These training prescriptions continue even though there is virtually no evidence that there is any difference in strength or muscular hypertrophy outcomes associated with using a specific RM in training2. For example, there’s little evidence that training with 70% of 1 RM loads and ‘failing’ on the 12th repetition leads to any better or worse strength or muscular hypertrophy outcomes than training with 85% of 1 RM and failing on the 6th repetition. “The size principle indicates that intensity defined as degree of effort is the key.”
If there are no differences in outcomes attributable to training, probably something else common to either load matters. The common factor is likely that with either load it’s possible to reach a point where a great deal of effort is required to complete the set. The size principle suggests why there should in theory be no real differences in outcome between these two protocols. The size principle also correctly predicts why such different prescriptions generally have not produced different outcomes in strength training studies. Publications in NSCA’s journals and related journals have incorrectly interpreted the size principle. These journals also have published articles that appear to support their beliefs about the importance of specific RM’s. When the studies are critically analyzed, it has been found that they do not support the NSCA’s training models and their interpretation of the size principle2. The interpretation fits the lifting background and ‘persona’ of the NSCA and has led to programs that involve trying to lift heavier and heavier weights. Because a number of leaders of the NSCA have good academic credentials and are at prestigious professional settings, a possibility is that some of them realize their interpretations of their studies and the size principle are wrong. So, why don’t they ‘come clean’? A great deal would be lost by recognizing that intensity as defined by the degree of effort, and not external force, is the generative stimulus. What would be understood is that the whole NSCA approach based on weightlifting at best isn’t optimal and at worst is incorrect and potentially injurious for the majority of trainees. Understanding the size principle also helps us analyze other training models or claims made about how certain ‘sensational new’ training models work. Here are two examples. You can undoubtedly come up with other examples. One prominent Olympic caliber track star from England has developed a special resistance training approach for track athletes. Her contentions (based on interviews on CNN) are that because track athletes have to move very fast, they have to lift weights at blinding speeds but not train to failure. She also said that raising and lowering
weights very quickly activates ‘fast twitch fibers’ (larger motor units). These points were the basis of her resistance training program for fast running. There are few convincing demonstrations that lifting weights very quickly transfers to any other activity other than lifting weights very quickly. Using lighter weights, moving quickly with a great deal of momentum, and not training to failure almost assures, following the size principle, that larger motor units will not be activated. Everything about her resistance training system was incorrect. Because she could run fast, no one seemed to doubt her training wisdom. Interviewing exercise scientists to assess the viability of her approach was not part of the story. Other training programs that have been advertised claim either through specific training approaches or the use of certain techniques or devices that you can ‘bypass’ the slow twitch fibers (smaller motor units) and directly activate the fast twitch fibers (larger motor units). These claims, never really substantiated, violate the size principle that defines an orderly recruit process from smaller to larger motor units based on the intensity of the stimulus. Where does this lead us? Here’s another reason why understanding the size principle has profound implications for training. We’ve read many times before and likely thought about it too that it’s intensity that matters in training. We may have even explained this axiom to other people. But, did we really apply this axiom to our own training? For many of us, the search has always been for methods that will enable us to lift more weight. In our own way, we equated a larger and larger external force with a better and better stimulus. We likely have realized that paying more attention to form, range of motion, and repetition duration mattered and we may have also seen that such training tactics actually reduced external force. That is, when you perform longer duration repetitions with very controlled turnarounds within a good range of motion, you likely are using less resistance than training within conventional repetition patterns. But, then within this better training methodology, the goal often was still to lift heavier weights, to use more force. If intensity, the degree of effort, is the key, we should be looking at our training in a different way. We should consider for every movement how we can better target particular muscle groups through better cognitive and affective focus and control and through better physical execution of each movement. How can each movement be made harder and a set made more intense without increasing (and often decreasing) resistance, the external force? Again, we’ve all heard this before, but have we really put this into practice? Isn’t it possible that for an exercise where we are using 100 lbs, we could use 80 or even 70 lbs and make the exercise more precise, harder, and the overall set more intense? Given the size principle, wouldn’t that constitute more effective training? And, wouldn’t following the size principle in this way and not trying to lift heavier and heavier weights – an impossibility in any case - be a much more effective age appropriate way to train? Note: Every reader knows the name Arthur Jones and knows that he developed the original Nautilus machines and high intensity training. We also recognize that by advocating high intensity training, the training was briefer and less frequent than conventional training. The past and current controversies and any hyperboles about outcomes aside, we know that his writings, work, and exercise machines were a departure from conventionality. Think more about what Jones said and what he created. A better description of his work is ‘paradigm shift’. The entire purpose of the approach and each machine was to deliver a high intensity stimulus to muscle groups. Arthur Jones knew that intensity and not external force per se was the critical factor. No machine was designed, for example, to enable you to lift a ton in the squat, although he presumably could have created such a machine. Even with training with free weights and not his machines, Jones stressed intensity. For example, with the squat, Jones advocated very strict form and a large range of motion (albeit, beyond the capability of most people) that would reduce the resistance for most trainees and increase the intensity of the exercise. Jones always emphasized how demanding squatting was when performed correctly but never recommended using a great deal of resistance in “How can you make your taining more precise and intense? this movement. He also advocated using higher repetitions in squatting because of the very high compression forces when heavy resistance was used. For example, he noted (actually, hypothesized in the absence of data) that when a very large man could perform 20 repetitions in the squat with 400 lbs, then he would have as much leg strength and development as could be acquired. For more average size men, an estimate was 20 repetitions with 300 lbs. The paradigm shift was dropping the central idea of weight lifting and bringing to life intensity of training and targeting muscle groups, an approach that best fits the size principle.
?References1. Henneman E. Relation between size of neurons and their susceptibility to discharge. Science. 1957; 126: 1345-1347.2. Carpinelli RN, Otto RM, Winett RA. A critical analysis of the ACSM position stand on resistance training: insufficient evidence to support recommended training protocols. JEPonline. 2004; 7: 1-64.
