Resistance Training Part 5

If the Henneman household 45 years ago was anything like mine is today, chances are when Elwood returned home after a productive day in the lab and relayed his newest findings to his wife, he received a congratulatory, “Oh, how exciting, honey!” before being asked to take out the trash! But that’s understandable because neurophysiology isn’t exactly everyone’s cup of tea and even Elwood himself probably didn’t realize how revolutionary the size principle he unveiled in 1965 was. Sure, it provided the first detailed account of how muscle fibers are called into play under different circumstances, but it would also stand the test of time and, despite numerous challenges, be widely recognized as a basic tenet of exercise physiology to this very day!

Last month, I explained how Henneman’s size principle indicates that the muscle fibers we want to activate during a resistance training session are those that reside at the upper end of the spectrum. Consequently, we have to force a drive through a significant portion of the recruitment hierarchy when training with weights. But this is where things get hazy. It is not unusual to see Henneman’s size principle referred to in strength training articles that are published in credible peer-reviewed research journals. However, if you scratch beneath the surface, you’ll find that application of the principle in many of these articles is flawed.

In an article published in the Journal of Exercise Science & Fitness (Vol. 6: No. 2, pp. 67-86, 2008), Long Island’s Dr. Ralph Carpinelli explains how Henneman’s size principle is frequently misunderstood and, therefore, misapplied by renowned authors in the resistance training realm. Consequently, some of the most frequently-cited recommendations regarding how much weight we should lift in the gym are misguided at best. Carpinelli explains that the size principle suggests there is orderly recruitment of motor units, which comprise nervous system communication pathways (motor neurons) and all of their associated muscle fibers. Development of muscle force for a task at hand is a consequence of this recruitment; specifically, activation of the appropriate number of motor units and also their rate of discharge (rate coding, which is how frequently signals travel the pathway from central nervous system to muscle). He then reviews important research that assesses motor unit activation and, importantly, what is required to maximally activate all available motor units. It is safe to say this is what quality resistance training should be all about.

The research Carpinelli cites is based upon the interpolated twitch technique (ITT), which involves the use of surface electrodes that are placed on the muscle or associated nerve being tested. An electrical stimulus passed through these electrodes generates a muscle twitch and when this is done with the subject already performing a maximal voluntary contraction (i.e., generating the most force their muscle can exert when contracting against an immovable object; a.k.a., an MVC), there are two possible outcomes. If all of the available motor units were already activated and firing at optimal frequency due to the voluntary effort of the subject, no additional muscle force will be detected when the additional external stimulus is applied. Conversely, if some motor units are quiescent or firing at a low frequency despite the fact that an MVC is being performed, the stimulus passed through the electrodes will produce a twitch on top of the force being generated. This would indicate that motor unit activation was incomplete during the MVC and the technique also allows any inherent shortfall to be quantified.

Carpinelli explains how ITT research indicates that maximal recruitment (a full drive from smallest to largest motor unit) can occur at anywhere from 30 to 90 percent of the MVC. This is a surprisingly large range because the precise point where your nervous system decides it must call all available motor units into play depends upon specific characteristics of the muscle being tested. Believe it or not, the human body comprises over 600 muscles and the wide range of MVC values at which maximal motor unit recruitment can occur belies the diversity that these muscles span. For example, smaller muscles that must typically operate with smaller gradations of force (e.g., those that control your eye lid or index finger) tend to have all of their motor units activated at relatively low levels of force (i.e., at a relatively low percentage of the MVC; for example, 30 percent) after which they rely on rate coding to further increase force output to maximal levels. On the other end of the spectrum, large muscles that we need for strength and power production (for example, the quadriceps muscles of the thigh) rely on rate coding to establish low levels of force and once this means for upping the ante is exhausted, recruitment plays the predominant role to increase force to the high levels that would characterize heavy resistance training or athletic endeavors. But regardless of this distinction, the ITT research also reveals what is required to drive through to this maximal level of recruitment and when you get to this take-home message delivered by Carpinelli, you and many of the ‘experts’ calling the shots in the field might be a bit surprised!

This article was originally published in New Living Magazine, which can be accessed on-line at





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