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 From The Desk of Clarence Bass



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“The bottom line seems to be that in the many studies that have compared training with heavy versus moderate resistance as long as sets end with a high degree of effort there are no differences in strength outcome.” Richard A. Winett, PhD

“Some people may have a fear of injury—that need not exist.” Ralph N. Carpinelli, PhD

Forget Heavy, Think Effort

Muscle Fiber Activation and Rep Range

The health establishment struggles mightily to persuade people to exercise regularly with mixed results at best. The latest scientific findings and government guidelines say that strength training should be part of the mix at least twice a week http://www.cbass.com/GetMoving.htm . Many people, including those that need it most, are turned off by weight training. They imagine themselves having to lift heavy weights, and that turns them away.

Is that true? Do they have to lift very heavy weights? An eye-opening new study says "NO." The study has the potential to change how strength training is perceived--and get many more people, perhaps millions more, pumping iron.

The study could revolutionize strength training for everyone, from pencil necks to muscle heads.

I'm eager to tell you about it. Pour yourself a cup of coffee and pull up a chair.

We have been told that heavier resistance produces greater gains in size and strength. “Only the heaviest possible weight will bring the maximum number of muscle fibers into action,” I wrote in Ripped 2. The underlying idea is correct—but there's more to the story. It turns out that many experts in the field have made the same error.

Dr Ralph N. Carpinelli, Human Performance Laboratory at Adelphi University in Garden City, New York, has made an exhaustive review of the scientific literature on this issue and reported his findings in the Journal of Exercise Science and Fitness, volume 6, number 2, 2008. His report is important, exciting, and complicated. I’m going to summarize and, where necessary, explain the results. Please bear with me. You’ll be glad you did.

Carpinelli’s analysis turns on the size principle, which governs how muscle fibers are recruited. Carpinelli says the size principle is “perhaps the most supported principle in neurophysiology.” So let’s start there. What is it?

The Size Principle

The size principle is a law that explains the order in which muscle fibers contract. In a nut shell, it says small fibers contract before large fibers.

The small fibers are slow-twitch, and the large fibers are fast-twitch. The slow-twitch fibers are the endurance fibers, which predominate in marathon runners and other endurance athletes. Like the Energizer Bunny, they don’t give out, they keep on contracting. They don’t generate much force, however. Fast-twitch fibers are the strength fibers, which rule the roost in sprinters, weight lifters, and other strength athletes. They are strong, but fatigue rapidly. Most of us are born with a roughly equal balance of slow/small and fast/large fibers. (Some fast fibers, which we’ll come to later, are intermediate in strength and endurance.)

Muscle fiber activation begins with a signal from the brain to motor units, which include a nerve and a companion group of muscle fibers. Fast-twitch and slow-twitch motor units operate separately; motor units are either slow or fast. Identical-twitch fibers are serviced by a single motor nerve that comes down into the muscle like an electrical wire. Slow-twitch units have approximately 100 fibers. Fast-twitch units may have as many as 10,000 fibers. Slow-twitch units, therefore, take up less space and have many more connecting wires or nerve branches than do fast-twitch units. Consequentially, many more slow-twitch motor units are likely to be triggered than larger fast-twitch units. You might have 1,000 slow units activated compared to only 50 to 100 fast units.

Importantly, motor units operate on an all-or-none basis. For a unit to be recruited, the nerve impulse must be strong enough to activate all of the muscle fibers in the unit maximally. If that threshold is not met, no fibers in the unit contract. Motor units contract for all they’re worth or not at all.

The bottom line is that it’s much harder to trigger fast motor units than it is slow units; it takes a lot more current or stimulus, more intensity.

With that background, we’re ready to sum up the size principle, which Carpinelli expresses as follows: “The size principle states that when the central nervous system recruits motor units for a specific activity, it begins with the smallest, more easily excited, least powerful motor units and progresses to the larger, more difficult to excite, more powerful motor units to maintain or increase force.”

In summarizing orderly motor unit recruitment, Jack Wilmore and David Costill drop the reference to size and say it more directly in the third edition of their highly regarded textbook Physiology of Sports and Exercise: “In low-intensity activity, most muscle force is generated by slow-twitch fibers. As the intensity increases, fast-twitch fibers are recruited, and at the higher intensities, the fast-twitch fibers are activated.”

Strange as it may seem, speed makes no difference. Motor units are recruited in an orderly sequence, slow to fast, no matter what the speed of the movement. Speed of action does, however, affect the amount of force developed. Slow movements generate more force. “The closer you get to zero velocity, the more force can be generated,” say Wilmore and Costill. Slow motion dampens momentum; at zero speed force is maximized.

Force, however, is not the variable that triggers muscle fiber contractions. I'll say that again, because it's very important. Force is not the kick off factor. 

As Carpinelli writes in his report, “Force [is] not the prerequisite for recruitment; force [is] the result of a more intense stimulus.” He continues, “The level of effort…determines the degree of motor unit activity.” Effort, of course, begins in the brain.

Keep the distinction between force and effort in mind, because we’ll be coming back to it over and over. Effort generates force, not the reverse. 

That brings us to the big question that every weight trainer wants answered: What’s the best and safest way to stimulate and build the maximum number of muscle fibers?

Is heavier better?

The studies that Dr. Carpinelli reviewed attempt to answer that question. But do they succeed? Do the findings support the conclusions? Where do the well designed studies come down? Read on and find out.

Some studies have misapplied the size principle, according to Carpinelli. We’ll look at those studies first.

Misunderstanding the Size Principle

Dr. Carpinelli analyzes more than 30 specific studies and, in some cases, books in this section of his report. I will summarize representative studies and explain how Carpinelli says the size principle was misapplied or bypassed.

Here’s the problem, as Carpinelli sees it: “Although the size principle is described reasonably accurately, it is often followed by a misunderstanding of the underlying neurophysiological concept and its practical application.” For example, many authors conclude that maximum or near maximum force—very heavy resistance—is necessary in order to recruit the large motor units and maximize strength gains. In other words, they decide that heavier is better.

“[That] is an invalid reverse inference of the size principle,” says Carpinelli. As noted above, force or resistance is not the controlling factor.

For example, the authors claim, citing the size principle, that heavier resistance (3 to 5 rep max) recruits higher-threshold motor units than lighter resistance (12 to15 rep max). Force or resistance, they assert, is the factor that determines whether high- or low-threshold motor units are recruited.

That’s demonstrably wrong, according to Carpinelli. Resistance (poundage) makes little difference, says Carpinelli, as long as the last few reps are at or near maximum. Effort, not force, is the controlling factor.

The simplest example, says Carpinelli, is an isometric muscle action. “If a person is holding a 20 kg [about 45 lbs] dumbbell at an elbow angle of 90 degrees…the first 10 seconds may feel relatively easy. After about 60 seconds [however] the person will no longer be able to hold the 20 kg mass.”

What changed? The force, the weight, remained the same, so force was not the controlling factor. It was the effort that changed, the required effort, wasn’t it? The weight felt heavier and heavier as time passed, until the person was no longer able to hold it at a right angle.

“Despite the increasing effort throughout the 60 seconds duration, the muscular force remained constant until it decreased at 60 seconds when the individual was no longer capable of producing [the] muscular force [necessary to hold the weight],” Carpinelli explained. “At the point of maximal effort (~60 seconds), all the motor units in the pool were recruited [including the large/fast motor units] for that specific isometric muscle action.”

He’s right, isn’t he?

Other studies claim that heavier resistance produces greater strength gains, but provide no credible supporting evidence. Often citations are provided which offer no actual support. Some references allude to the size principle and others make claims or recommendations without supporting evidence.

Carpinelli methodically dissects study after study showing specifically how each author’s citations failed to support their claims or recommendations. This is “important,” he maintains. “It is not sufficient simply to cite the reference without noting exactly what the authors of those studies and reviews report.”

Several studies claim that advanced weightlifters may be able to override the orderly recruitment of the size principle because they “can inhibit the lower-threshold motor units and preferentially activate the higher-threshold motor units.” In other words, they are somehow able to recruit the large motor units first. No citation or other evidence is offered in support of the assertion that the size principle can be violated. This is unsubstantiated opinion.

One author claimed that a 10 RM (repetition maximum) builds strength slower than a 5 RM. The reference cited, however, was a training study which compared 6-8 RM, 30-40 RM, and 100-150 RM. The study did not include 5 or 10 RM protocols. (We’ll discuss the drawbacks of very high reps below.)

Other authors claimed that “high-velocity movements” skip over the smaller motor units so that the larger units can be recruited first. Again, no supporting evidence was offered. By the same token, another author claimed that slow movements cannot generate enough force to trigger the larger motor units. As before, no training studies or other evidence was offered in support.

Still another author claimed that 5 RM is more effective for the bench press and squat, while 10-15 RM is most productive for other exercises such as the thigh curl. “No rationale was presented for this apparent incorrect interpretation of the size principle,” Carpinelli states, “or why the hamstrings and quadriceps would require a different range of repetitions.”

Here’s another recommendation guaranteed to make you scratch your head: RM loads 6-8 are best for building maximum strength, while 10-12 RM are better for muscular hypertrophy. “He cited no references to support his recommendations or his erroneous interpretation of the size principle,” Carpinelli writes. “In fact, there is very little evidence to suggest that his recommended differences in the range of repetitions elicit different outcomes.”

There’s more, but you get the idea loud and clear. Let’s move on to studies supported by solid evidence. Happily, there are plenty of studies that pass the test.

Let’s start with a study of motor unit activation.

Effort and Motor Unit Activation

Again, Carpinelli maintains effort drives motor unit activation. Fortunately, scientists have devised a test to measure the effect of effort on activation level.

Motor unit activation level (AL) can be measured by comparing voluntary and induced response. “During an MVC [maximal voluntary contraction], a supramaximal [greater than maximum] electrical stimulus is superimposed with surface electrodes onto a muscle or its nerve,” Carpinelli explains. “When the superimposed twitch technique is applied properly, the electrical stimulus fully activates all the motor units in the pool. If all the motor units have been recruited [voluntarily] and are firing at optimal frequencies, no additional force will be detected [as a result of the electrical stimulus].”

AL is expressed as a percentage of the evoked response. If the voluntary response matches the electrical response, AL is 100%. If the voluntary response is less, the shortfall will be expressed as a percentage of the induced response. 

Like body fat measurement, AL testing is indirect and not perfect. “Although there are some questions,” says Carpinelli, this type of testing “is capable of detecting decrements in voluntary activation of less than 1%.”

AL studies provide an objective measurement and are clearly more credible than unsupported claims or recommendations.

Motor unit activation studies, writes Carpinelli, “strongly support” the size principle. “It is the intensity of effort that determines the AL of motor units and the resultant force output. A greater effort produces greater motor unit activation. Maximal effort produces maximal, or near maximal, activation of motor units. The resultant force, which is the dependent variable—not the independent variable—is a maximal force produced in a specific individual for a specific exercise. It is entirely dependent on the intensity of effort. However, it is important to recognize that none of the [AL] studies speculate on a minimal recruitment threshold for strength gains…A maximal effort only insures maximal voluntary motor unit activation.”

Carpinelli describes what he considers to be the “most relevant [AL] study with the greatest practical application to resistance training.”

Researchers measured voluntary and evoked motor unit recruitment in 14 resistance trained males (age ~ 21) before and after 5, 10 and 20 RM (repetition maximum) dumbbell curls. They found was no significant difference in voluntary motor unit AL after 5 RM (95.5%), 10 RM (93.5%), and 20 RM (95.1%).

They concluded: “The commonly repeated suggestion that maximal strength methods (resistance heavier than a 6 RM) produce greater neural adaptations or increases in neural drive was not substantiated in this study.”

“In fact,” Carpinelli adds, “their study unequivocally demonstrates the direct relationship between intensity of effort—not the amount of resistance or time under tension—and voluntary motor unit activation.”

Now, let’s look at studies that measured strength gains using different reps and resistance.

Support for Heavier-is-Better

Although not cited in any of the studies we’ve been discussing, Dr. Carpinelli did find one study that reported some strength advantages for low reps and heavy resistance.

The 2002 study had previously untrained males (age ~22 years) perform the leg press, squat, and knee extension for 4 sets of 3-5 RM or 3 sets of 9-11 RM for 8 weeks. The subjects doing 3-5 reps increased strength (1 RM) significantly more than those doing 9-11 reps in the squat (61% compared to 31%) and leg press (100% vs. 81%), but not in the knee extension (67% and 56%, respectively).

Interestingly, muscle hypertrophy gains (slow-twitch and fast-twitch fibers) were similar for both rep ranges. Muscle size increased significantly in both groups, with no significant difference between groups.

The authors concluded: “It has often been accepted that improved strength/power results from high intensity/low volume training, whereas low intensity/high volume training maximizes muscle hypertrophy. Based on data from the present investigation, this may not be entirely true. Indeed, data from the present investigation suggest low and intermediate RM training induces similar muscular adaptations, at least after short-term training in previously untrained subjects.” 

They did not attempt to explain why there was no significant difference in knee-extension strength. No mention was made of the size principle.

Perhaps it should come as no surprise that supporters of heavier-is-better training do not cite this inconclusive study.

Importantly, Carpinelli found many studies showing no advantage for heavier-is-better training.

No Support for Heavier-is-Better

“Studies that report the effects of training with different amounts of resistance…strongly support the [size principle],” states Carpinelli.

He lists 20 resistance training studies that reported no significant difference in strength gains for 2 to 20 RM. 

Carpinelli singles out a study of 10 pairs of identical twins as especially “noteworthy.” Identical twins are ideal subjects for study because they have exactly the same genetic make-up. They take the “nature” out of the “nature-or-nurture” question. They lay bare the difference between training protocols. If heavier is better, comparison of identical twins should show it.

Carpinelli relates the results of the study: “After training two times a week for 10 weeks, increases in isometric strength (averaged for the eight positions tested) were significant in both groups. However, there was no significant difference in the strength gain as a result of training with 7-10 RM (13.2%), or 15-20 RM (12.8%).”

Why Moderate Weights and Reps Are Best

I promised that we’d talk about the drawbacks of very high reps.  Let’s do that before we discuss Dr. Carpinelli’s conclusions.

Both high and low reps are problematic. Moderate reps, 6 to 20, are probably best—for practical and scientific reasons.

From the practical standpoint, very high reps are unpleasant. For most people (me included), they’re mind-numbing, a drag. Low reps, on the other hand, are cumbersome and potentially dangerous. Except for competitive power or Olympic lifters, as we’ve seen, there’s little or no reason to do low reps.

“Very high RMs (loads lighter than 20 RM),” says Carpinelli, “may involve mechanisms of fatigue that are not conducive to stimulate optimal increases in muscular strength.”

Body by Science, an important book by Doug McGuff, MD, and John Little (2009), explains the problem: “If you use a weight that is too light….you will recruit the slow-twitch fibers into service, but because they fatigue so slowly, by the time you have started to recruit the intermediate fibers, some of the same slow-twitch motor units will have started to recover. They will then recycle back into the contraction process, thus preventing you from ever engaging the higher-order muscle fibers.”

McGuff and Little say the problem is similar with a weight that allows only one or two reps: All motor units (slow and fast) are activated, but the fast-twitch units fatigue so fast that “the set will terminate before you’ve had the opportunity to thoroughly involve and stimulate the bulk of your slow- and intermediate-twitch fibers.”

Dr. Carpinelli might say it’s supposition, but McGuff and Little argue that a moderately heavy weight allows you recruit the full range of motor units, “but not so quickly that only the fast-twitch fibers receive the bulk of the stimulation, and not so slowly that the slow- and/or intermediate-twitch motor units can recover and you end up cycling through the same lower-order motor units again.” 

Makes sense, doesn’t it? Moderation in all things.

Carpinelli’s Conclusions—and Mine

“Recommendations to train with very heavy resistance (loads heavier than 6 RM), because they purportedly result in superior strength gains, are based on a faulty [understanding of the size principle] and have very little supporting evidence,” Carpinelli concluded.

Resistance is largely a matter of “personal preference,” says Dr. Carpinelli. “If a maximal—or near maximal—effort is applied at the end of a set of repetitions, the evidence strongly suggests that the different external forces produced with different amounts of resistance elicit similar outcomes.”

That’s it. So simple, yet so meaningful—and potentially influential.

“If the size principle was correctly applied, effective resistance training may appeal to a larger proportion of the population,” Carpinelli opines. “This would include competitive and recreational athletes as well as those in the general population who perceive resistance exercise as the lifting of very heavy weights and therefore potentially dangerous.” He continues, “Because some people may have a fear of injury—that need not exist—the heavier-is-better perception may actually be a deterrent to resistance training, which deprives those most in need of health-related benefits.”

Reducing resistance by even a little can be the difference between satisfaction and aversion. I know from my own experience.

The dumbbell bench press is one of my favorite exercises, but getting the bells in position is a big problem. I have to psyche up more to get the dumbbells in position than to do the presses. If I miss the groove and fail to get the weights in position, it jams my shoulder and hurts like hell. While writing this article I decided to drop each dumbbell by five pounds, and do slower more controlled reps. Wow, it made all the difference. Getting the dumbbells in position was no longer an ordeal--and the muscle response was much better. I could feel every muscle fiber in my chest working. The movement was a joy again.

If ever there was a landmark review study in the resistance training field, this is it. Dr. Carpinelli’s impressive—and bold—effort has the potential to open the door to the health and fitness benefits of resistance training for millions of additional people. Let’s hope the powers that be are listening—and that the many who can benefit get the message.

BRAVO, Ralph.

Editor’s Note: Checkout Dr. Carpinelli’s earlier review of studies on the volume issue: One Set or Many? http://cbass.com/ONESET.HTM  See also Challenge Yourself on volume versus high-intensity training (HIT):  http://www.cbass.com/CHALLENG.HTM

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