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The following should be enough (even for a total moron) and speak for itself…

Power Factor Training: Precision or Confusion?

Andrew M. Baye

"By definition, work requires movement… no movement means no work; and while this is undoubtedly true in regard to mechanical work, it certainly is not true in relation to metabolic work.

Muscles produce force, and it is easily possible for a muscle to produce a high level of force without producing movement; logically, it appears that the metabolic cost of muscular force production would be related to the level of force produced and the time that the force is maintained… rather than the amount of mechanical work performed.

If, for example, a 100-pound barbell is held motionless at the halfway position of a curling exercise, then the muscles will be required to produce a certain level of force to prevent the downward movement of the barbell. Providing that force will certainly entail metabolic work… yet no mechanical work is involved.

Slowly curling a 100-pound barbell also requires a greater metabolic cost than curling the same barbell at a more rapid pace; even though the amount of mechanical work involved is exactly the same in both cases.

Many other examples could be given to illustrate the same point, but it should now be obvious that attempts to relate metabolic cost to mechanical work are doomed to failure… there is no meaningful relationship."

– Arthur Jones, The Metabolic Cost of Negative Work, Athletic Journal, January 1976

In their book Power Factor Training, Peter Sisco and John Little make the mistake of attempting to measure muscular force output, or metabolic work, using a formula based on measurements of mechanical work and power. This Power Factor is determined by multiplying the amount of weight used during an exercise by how many repetitions are performed, and dividing the result by the duration of the exercise in minutes. For example, if you were to perform 10 repetitions of the bench press with 300 pounds in 2 minutes, your Power Factor would be 1,500 pounds per minute. Sisco and Little claim this allows for a "precise numerical measurement of muscular output," and that it "…represents a revolution in strength training." (p. 16) The truth is, the Power Factor measurement represents nothing more than a tangled mess of assumptions based on misunderstandings of various basic principles of mechanical physics and exercise.

As Arthur Jones discovered decades ago while developing testing machines for Nautilus research, mechanical definitions of work and power do not apply to metabolic work. As he explains in the above quote from The Metabolic Cost of Negative Work, any attempts to accurately measure exercise intensity or muscular force output based on measurements of mechanical work and power are futile. This alone is reason enough to completely disregard the Power Factor as a "…precise numerical measurement of muscular output" without further discussion. However, there are numerous other flaws in the Power Factor Training theory that I feel deserve mention, as they illustrate several important points.

Measuring Mechanical Work and Power

For the sake of argument, let’s assume that measurements of mechanical work and power can be used to quantify muscular force output, or intensity of exercise. The Power Factor still does not qualify as a "precise mathematical measurement" of this. The Power Factor, supposedly a measure of power output, ignores the fact that work, a factor of power, is the product of weight and distance, not just weight. To determine the amount of mechanical work performed, it is necessary to also factor in the vertical distance the weight is moved during an exercise.

According to the Sisco and Little, the reason for ignoring the factor of distance is because "…it is difficult to precisely measure the travel of the bar when lifting, especially in movements that involve an arc of motion, which require computations using pi (3.14159). Secondly, the length of your arms and legs isn’t going to change over time, so all those distance measurements would just factor out of any comparisons that are made, leaving only differences in the weight lifted and the time." (p. 16). While such may be the case if one can be certain of performing each repetition of each exercise over the exact same distance every single time they work out, things don’t usually work out that way. Also, work equals the amount of weight multiplied by the vertical distance the weight travels, not the total distance, so no computations using pi would be necessary.

Since Sisco and Little recommend performing "strong-range partial" repetitions, this factor becomes even more important. It is highly improbable that a person performing partial repetitions would consistently perform each repetition of an exercise over the exact same distance, much less maintain this consistency between workouts. While the difference might only be a matter of one or two inches, this is significant. To return to our example of bench pressing 300 pounds ten times, if you were to move the weight 10 inches each repetition, you would perform 30,000 inch-pounds of work during the set. If you were to increase the distance you raise the weight by only as little as one inch, the amount of work you perform during the set would increase to 33,000 inch-pounds, a 10% increase in work. Assuming an error of only one inch plus or minus on an exercise with an average range of motion of 10 inches, this amounts to an error of 20%, hardly what I would consider "precision."

The words Power Factor imply, incorrectly, a measurement of power output. Power is a derivative of work (power = work/time). Without first accurately measuring work, one can not calculate power. Since the Power Factor does not take into account the distance the weight is lifted, and therefore the actual amount of work performed, it is not a measure of power. So much for mathematical precision.

Part of the reason I suspect Sisco and Little ignore the factor of distance in calculating one’s Power Factor is because in most cases an equal or greater amount of mechanical work is performed during full range exercise than during "strong-range" partials with a heavier weight. For example, if you can perform full range bench presses with 300 pounds, lifting the bar a vertical distance of 2 feet, it is unlikely that you would be able to perform partials over the second half of the movement with anything near twice that amount. For the sake of this example though, suppose you could. Whether you move 300 pounds 2 feet, or 600 pounds 1 foot, the mechanical work performed is the same.

If one were to use a correct measure of mechanical work, it would be obvious that most full-range exercises would yield higher power outputs than strong-range partials with heavier weight, and thus higher Power Factors.

Weight vs. Resistance During Exercise

Remember that it is not the amount of mechanical work performed, but the amount of force the muscle is required to produce which determines the intensity of an exercise. This is where Sisco and Little’s "strong-range" partial theory fails most miserably, in failing to distinguish between weight and resistance.

Weight is simply a measure of the amount of pounds used. The resistance is the amount of force the muscles must produce to lift that weight, and is the product of weight and lever. Depending on leverage factors, it is possible to lift a tremendous amount of weight without encountering a significant resistance, or to produce a tremendous amount of resistance using a very small amount of weight.

The fact that you are capable of using more resistance in some positions during an exercise than others is due to changes in leverage, and does not mean that your muscles are producing more force or working more intensely if you perform partials in those positions with a heavier weight. For example, the reason a person can perform "strong-range" partials with more weight during an exercise like the squat is because the closer one is to a position of full extension of the hips and knees, the greater their lever advantage. If your bones could withstand the force, you could literally support several tons of weight in the fully extended position of the squat, although the muscles of the legs would do very little except a small amount of work to balance the weight.

Sisco and Little also completely ignore the fact that there is no "strong-range" in a properly designed machine, since the resistance varies in proportion with the strength curve of the involved muscular structures.

The Issue of Momentum: Power vs. Muscular Force Production

Sisco and Little’s claim that exercise intensity is directly related to mechanical power output: that the more work a person performs in a given unit of time, the greater the intensity of exercise, is also mistaken. Here they are confusing power production (work/time) with exercise intensity (inroad/time), erroneously assuming a direct relationship between mechanical power output and the amount of metabolic work a muscle performs. As I pointed out earlier, no such relationship exists. In some cases, exercise intensity is actually lower despite a higher mechanical power output.

For example: If a person performs 10 reps with 300 pounds in the bench press using the traditional Nautilus 2/4 protocol, the set will take approximately one minute. Assuming the weight is lifted a vertical distance of 2 feet, the mechanical power output would be 6000 ft. lbs./min. The Power Factor would be 3000.

Actual Power Output 300 pounds x 2 feet x 10 reps / 1 minute = 6000 ft. lbs./min.

Power Factor 300 pounds x 10 reps / 1 minute = 3000

All other factors being equal, if a person performs 10 reps with 300 pounds in the bench press using the SuperSlow 10/5 protocol, the set will take approximately two and a half minutes. Assuming the weight is lifted a vertical distance of 2 feet, the mechanical power output would be 2,400 ft. lbs./min. The Power Factor would be 1,200.

Actual Power Output 300 pounds x 2 feet x 10 reps / 2.5 minutes = 2,400 ft. lbs./min.

Power Factor 300 pounds x 10 reps / 2.5 minutes = 1,200

According to Sisco and Little, the SuperSlow set would only be 40% as intense as the set using the traditional Nautilus protocol. It will become immediately obvious to anyone who performs the above experiment that despite a lower mechanical power output and Power Factor, the SuperSlow set is far more intense. The reason is that less momentum is produced, which means that a greater amount of muscular force output is required to move the weight.

The Power Factor measurement ignores the fact that an increase in mechanical power output during an exercise does not necessarily mean that the muscles have produced more force, or that the exercise is more intense. Depending on changes in weight and movement speed, muscular force output can either increase or decrease relative to mechanical power output. To increase mechanical power output during exercise requires either an increase in weight, movement speed, or both. A set performed with a heavier weight at the same movement speed would require a greater muscular force output. A set performed with the same weight at a faster speed would produce a higher amount of momentum, and therefore require less muscular force output. The greater the momentum, the less force the muscle is require to produce to lift a particular weight a given distance, resulting in a lower intensity level.

Another example: If a person performs one repetition using a 15/15 protocol with the maximal amount of weight they are capable of lifting, or performs three 5/5 repetitions with only a third of that amount of weight, the Power Factor for either set will be the same.

300 pounds x 1 repetition / .5 minutes = 600

100 pounds x 3 repetitions / .5 minutes = 600

To take it a step further, imagine using only 1/15th the weight used in the 15/15 one-rep maximum, for 15 repetitions at a 1/1 cadence. Although this would seem almost ridiculously easy compared to the intense maximum attempt, the two Power Factors would be equal.

300 pounds x 1 repetition / .5 minutes = 600

20 pounds x 15 repetitions / .5 minutes = 600

Since the Power Factor does not take into account the effects of momentum and several other factors that affect muscular loading during exercise, attempts to use it as a measure of progress are meaningless. Also, Power Factor Training’s emphasis on performing more work per time encourages the use of faster movement speeds, which are not only less productive but also expose the body to a greater amount of force and thus greater risk of injury.

The Power Index: More Confusion

If a person performs 4 repetitions with 300 pounds in the bench press using the SuperSlow 10/5 protocol during one workout, then performs 6 repetitions with 300 pounds using the SuperSlow protocol during the next workout, the Power Factor remains exactly the same, despite the obvious fact that an increase in strength has occurred.

300 pounds x 4 reps / 1 minute = 1,200

300 pounds x 6 reps / 1.5 minutes = 1,200

Sisco and Little’s answer to this problem is the Power Index, a measure of what they call Volumetric Intensity, or the duration for which a person is capable of maintaining a particular level of intensity. One’s Power Index for a particular exercise is determined by multiplying the Power Factor by the amount of weight lifted times the repetition count then dividing by 1,000,000. For example:

(300 pounds x 4 reps) squared / 1 minute) / 1,000,000 = a Power Index of 1.44

(300 pounds x 6 reps) squared / 1.5 minutes) / 1,000,000 = a Power Index of 2.16

Although this may appear to solve the problem, realize that the Power Index can be considered a meaningful measurement only if all of the other factors remain constant, such as repetition speed, range of motion, etc. For example, if a person performs 4 repetitions of an exercise with 300 pounds using a SuperSlow 10/5 protocol, or if they perform 10 repetitions of an exercise with 120 pounds using the traditional Nautilus 2/4 protocol, they would have the same Power Index, although the SuperSlow set would be considerably more intense.

(300 pounds x 4 reps) squared / 1 minute) / 1,000,000 = a Power Index of 1.44

(120 pounds x 10 reps) squared / 1 minute) / 1,000,000 = a Power Index of 1.44

Without standardization of repetition speed and other relevant factors, no meaningful comparison between workouts is possible. Even if all other factors are relatively constant, as Sisco and Little mention the importance of briefly on page 27, the Power Index is unnecessarily complicated and actually less precise than using weight x repetitions or Time Under Load as a measure of progress.

Sisco and Little further suggest that the Power Factor and Power Index should be calculated for the entire workout and that it will provide an accurate measure of the intensity of the workout and progress. That this is pointless should be obvious.

Exercise Intensity: Work/Time (Power) vs. Inroad/Time

One of the major flaws in the Power Factor Training theory is the confusion of work for inroad in defining intensity. On page 36, the authors define momentary intensity as "…Im = W/t, where W is the total weight lifted in pounds and t is the total time in minutes." First, if you are defining something which is momentary, which means that it occurs at a particular point in time, and not over a period of time, then time is not a factor in it’s measurement. This leaves us with only weight, and as I’ve already explained, the amount of weight lifted is only one of several interrelated factors affecting the intensity of exercise.

Starting out with Sisco and Little’s formula for momentary intensity (Im = W/t), suppose we then consider the distance that the weight is moved. We will then have a measure of the work performed, but not of the amount of force the muscle has to produce to perform the work, since that is also affected by momentum, which is related to the speed of movement.

Suppose we then consider acceleration and factor out the effects of momentum. We then know how much force is required to move the weight over a particular distance, but we still don’t know the intensity of effort at any particular moment during the exercise, since it is relative to the maximal amount of force the muscle is capable of producing at that particular moment.

Suppose we then somehow measure the muscle’s maximal force producing capacity when fully rested. We will then be able to estimate the percentage of effort required to begin lifting a particular amount of weight at the start of an exercise, and the amount of muscular force being produced at the point of momentary muscular failure, when positive movement ceases. However, we still won’t know the "momentary intensity" of muscular contraction at any moment in between, unless we can determine the exact rate at which the muscle fatigues. Unless someone invents a machine capable of accurately measuring muscular force output during dynamic exercise, we never will.

What we can estimate accurately enough for our purposes though, is the difference between a muscles maximal force producing capacity when rested, and the amount of force it is capable of producing at the moment of muscular failure using a particular amount of weight. The difference, which is the percentage to which the fresh strength of the muscle has been reduced during the exercise, is inroad. For example, if the maximal amount of force the muscles involved in a particular exercise are capable of producing when fresh is 100 pounds, and they are worked to failure using 80 pounds, at the point of momentary muscular failure the inroad is 20%.

[It is important to mention that this is grossly oversimplified for the sake of this example. There are numerous other interrelated factors affecting the resistance encountered during exercise and muscular force production which must be considered, such as intra-muscular friction, the effects of stored energy and various sources of torque, motor skill, etc.]

The intensity of exercise is more appropriately expressed as Inroad/Time, or the average degree to which a muscle has been fatigued per unit of time. The higher the intensity of exercise, the more force the muscle is called upon to produce, the greater the degree to which the muscle will be fatigued within a particular time frame. Inroad/Time is directly related to one’s effort during exercise.

Intensity is not a measure of mechanical power output, or the amount of work performed in a particular unit of time. It is a measure of the percentage of one’s momentary ability at which they are working at a particular moment; how hard, not how much. Inroad/Time, while indirect, is still the most practical measure of this.

For example, if the maximum amount of force you can produce in the bench press is 300 pounds, and you fail after two minutes using 210 pounds, you have produced a 30% inroad.

Suppose that you perform 8 repetitions during that time, using the SuperSlow 10/5 protocol. Your Power Factor would be 840.

210 pounds x 8 reps / 2 minutes = 840

Suppose that your training partner can produce a maximum of 400 pounds in the bench press, and he fails after two minutes using 280 pounds for 8 SuperSlow 10/5 reps. He would have produced the same degree of inroad, 30%, but his Power Factor would be 1,200.

280 pounds x 8 reps / 2 minutes = 1,120

Does his higher Power Factor indicate that his set was more intense? Absolutely not. Since the repetition speed and the time under load were the same for both of you, the fact that he was able to use more weight would indicate that he is stronger. However, the intensity of effort during an exercise using a particular amount of weight is relative to the momentary muscular strength of the person, and not an absolute.

Conclusion

Precise record keeping is essential for accurate assessment of progress, and therefore proper regulation of the volume and frequency of one’s training. However, the Power Factor Measurement is not the precise measure of intensity or muscular output Sisco and Little claim, much less a measure of anything else relevant to your training since measurements of mechanical work and power do not apply to muscular work. If repetition speed, range of movement, and all other relevant factors are consistent from workout to workout, weight x repetitions or Time Under Load are simpler and more appropriate measures of progress.

There is no scientific basis for Sisco and Little’s notions regarding "strong-range" partials, or their assertion that they are superior to full-range exercise.

Now, if you didn′t get it that way, I′m not advocating PFT 🙂

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