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What Principle has been Misinterpreted and Distorted to the Detriment of Resistance Trainees?
A Review of a Critical Analysis of the Size Principle

Carpinelli RN. The size principle and a critical analysis of the unsubstantiated heavier-is-better recommendation for resistance training. Journal of Exercise Science and Fitness. 2008; 6: 67-85.


Over the last several years, Carpinelli's incisive critical analyses1,2 of resistance training studies have shown that there is very little evidence to support the complex, higher volume training advocated by professional and commercial organizations. And, Carpinelli has shown that there is little evidence that the many variations of resistance training variables such as number of repetitions and percent of maximum (1RM) used for an exercise, also advocated by these same organizations to produce very specific outcomes, in fact, do produce such specific outcomes. Instead, Carpinelli has shown that simple, brief, resistance training with muscle groups trained twice per week produces good outcomes regardless of trainee status. Outcomes are largely attributable to genetic factors and far less to variations of routines.

Performing higher volume, complex routines will not particularly harm trainees. Such training, however, may become daunting and frustrating over time and perhaps lead people to lapse in their training or stop altogether.

One variation of a training program that is often advocated, however, can be harmful. The variation is based on the idea that to maximize strength and muscular hypertrophy, it is necessary to use very heavy resistance.

In this new critical analysis, Carpinelli decisively provided evidence about two central and related aspects of resistance training. The first is the size principle and the second is whether the use of heavy resistance provides superior outcomes.

Size Principle:

As described in a number of prior issues of Master Trainer, the size principle of motor unit recruitment is one of the standard neuromuscular principles with universal application. Universal means that the size principle applies to everyone and to every modality of training. There are no exceptions to the principle.

The principle states that motor units are recruited in the order of their size. The smaller motor units are recruited first and then the larger motor units are recruited. The percent of motor units recruited that are associated with specific muscles or muscle groups is dependent upon a dimension of the task performed. The dimension associated with the percent of motor units recruited is the degree of effort. The degree of effort is the intensity involved in performing the task. A high degree of effort, or maximal effort, that is a high degree of maximal intensity, will result in maximal motor unit recruitment.

In this critical analysis, Carpinelli showed that for about 30 years, most of the leading luminaries of resistance training researchers, many associated with a leading organization, have wittingly or unwittingly misinterpreted the size principle. The main fault is their equating external force, the resistance used in an exercise, to intensity. In the widely promulgated framework used by these researchers and their organization, intensity is based on the percent of 1 RM used in an exercise. For example, if 70% of 1 RM is used in an exercise, this is considered ‘moderate intensity’. If 90% of 1 RM is used, this is considered ‘high intensity’. Intensity in this framework is the amount of weight used relative to the 1 RM and not the degree of effort.

According to a proper interpretation of the size principle, if a set performed with 90% of 1 RM is concluded with a last repetition at maximal or close to maximal effort, this should maximize motor unit recruitment. Likewise, if a set is performed with 70% of 1 RM is concluded with a last repetition at maximal or close to maximal effort, this also should maximize motor unit recruitment. With the 90% of 1 RM, perhaps 5 reps are performed in the set. With 70% of 1 RM, perhaps 12 reps are performed in the set. But if both sets represent maximal or near maximal efforts, then the outcomes based on the size principle should be similar, or perhaps the same. This is because the effort - the degree of intensity - is the same.

Note that it is possible to have a high amount of external force such as the 90% of 1 RM, but only modest effort. This would occur, for example, if only one or two repetitions were performed with the 90% of 1 RM when four or five could be performed.


It also is likely that performing exercises with heavy resistance can increase the probability of injuries. However, because of their distortion of the interpretation of the size principle, organizations and commercial interests have advocated the use of heavy resistance to optimize outcomes.

Why have they advocated the use of heavy resistance? For some organizations, resistance training in its many forms is still construed as ‘weight lifting’. This presumably is the identification, area of expertise, framework, .and marketing points for these organizations. If resistance training really was not weightlifting and should be construed in a different way, then these organizations would not be viable and have little to offer to athletes and typical trainees.


But, this question remains. What do the results of studies actually show about the use of heavy resistance for optimizing outcomes?

In his critical analysis, Carpinelli reviewed 21 resistance training studies that compared within the same study training with lighter or moderate resistance typically performed with higher repetitions per set to training with heavy resistance with typically lower repetitions per set. In 20 of the 21 studies, there were no differences in strength outcomes.


If maximizing motor unit recruitment and strength depended upon the amount of resistance used in an exercise, then overwhelmingly the studies comparing training with different loads would show much better outcomes when heavy resistance was used. Instead, about 95% of such studies showed no difference in outcomes favoring the use of heavy resistance.

Proper interpretation of the size principle points toward the use of any reasonable, self-chosen resistance and time under tension for producing optimal outcomes when sets conclude with a maximal or near maximal effort. The use of more moderate resistance and somewhat longer time (60-90 seconds) under tension may also reduce the risk of injuries associated with heavy resistance.

This also makes resistance training more acceptable and a lot safer for many people, from top athletes, bodybuilders of all ages, and to a wide spectrum of the general public. Conversely, the belief and promotion of the use of heavy resistance by professional organizations and commercial concerns represent a weightlifting framework that may have discouraged many people – maybe even you - from engaging in resistance training and possibly resulted in many unnecessary injuries.

To read more about how to effectively use more moderate resistance based on a proper interpretation of the size principle, see our new training primer, Optimal Training.


  1. 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.
  2. Otto RM, Carpinelli RN. A critical analysis of the single versus multiple set debate. JEPonline. 2006; 9:32-48.