Power Factor Training: Precision or Confusion?

I wrote this article in the early 2000s for an issue of The Exercise Standard at the suggestion of Ken Hutchins, and it used to be available on this site but was left out during a redesign several years ago. Although Power Factor Training came out way back in 1997 and Sisco and his methods currently appear to have only a miniscule following, I’ve decided to update and repost my review since the topics covered have come up in discussion recently. In the original article I criticized both authors, however since John Little is no longer using or recommending the method and has moved on to better things I have left him out of this, focusing instead of Pete Sisco who continues to make the same mistakes covered here, mistakes which have the potential to seriously injure people who follow his training recommendations.

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 his book Power Factor Training, Peter Sisco makes 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 ten repetitions of the bench press with three hundred pounds in two minutes, your power factor would be one thousand five hundred pounds per minute. Sisco claims 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 that 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 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; you simply compare the height of the weight at the beginning and end of the repetition.

Since Sisco recommends 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 three hundred pounds ten times, if you were to move the weight ten inches each repetition, you would perform thirty thousand 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 thirty three thousand inch-pounds, a ten percent increase in work. Assuming an error of only one inch plus or minus on an exercise with an average range of motion of ten inches, this can amount to an error of up to twenty percent, which is 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 ignores the factor of distance in calculating the 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 three hundred pounds, lifting the bar a vertical distance of two 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 three hundred pounds two feet, or six hundred pounds one 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 Versus Resistance

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’s “strong-range” partial theory fails most miserably, in failing to distinguish between weight and resistance.

The resistance force your muscles must overcome to lift a weight is the product of weight and lever. Weight alone tells you nothing about the difficulty of an exercise. Depending on leverage factors, it is possible to lift a tremendous amount of weight without encountering significant resistance, or to produce a tremendous amount of resistance using very little weight.

The fact you are capable of using more resistance in some positions during an exercise than others is due largely 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 also ignores the fact there is no “strong-range” in most properly designed machines, since the resistance varies in proportion with the strength curve of the involved muscular structures.

Power Versus Muscular Force Production

Sisco’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 ten reps to failure with three hundred 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 two feet, the mechanical power output would be six thousand foot-pounds/minute. The power factor would be three thousand:

Actual Power Output: 300 pounds x 2 feet x 10 reps / 1 minute = 6,000 ft. lbs./min.

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

All other factors being equal, if a person performs ten reps to failure with three hundred pounds in the bench press at half that speed using a 4/8 protocol, the set will take two minutes. Since the set would take twice as long the mechanical power output would be reduced by half to three thousand foot-pounds/minute. The power factor would be one thousand five hundred:

Actual Power Output 300 pounds x 2 feet x 10 reps / 2 minutes = 3,000 ft. lbs./min.

Power Factor 300 pounds x 10 reps / 2 minutes = 1,500

According to Sisco the slower set would be only half as intense as the set using the traditional Nautilus protocol, however as long as both sets are done to momentary muscular failure the maximum intensity of effort would be the same for each. You could argue the first set was harder because of the faster inroading, or the second set was harder because of the increased time under tension and metabolic demand, but they would both involve equal intensity of effort.

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: Whether a person performs one very slow repetition using a 30/30 cadence with three hundred pounds, or performs ten repetitions at a 3/3 cadence with only thirty pounds, or performs thirty repetitions at a 1/1 cadence with only ten pounds, the power factors will be the same despite the average force over time being much higher for the very heavy, very slow rep:

300 pounds x 1 repetition / 1 minute = 300

30 pounds x 10 repetitions / 1 minutes = 300

10 pounds x 30 repetitions / 1 minutes = 300

Obviously, it is much harder to perform a single extremely slow repetition with three hundred pounds than to perform a higher number of repetitions with a much lighter weight.

Another problem with the power factor is it encourages the use of faster movement speeds. This, combined with the ability to use much heavier weights due to the smaller lever you work against in the “strong range” partials, increases your risk of injury.

The Power Index: More Confusion

If a person performs ten repetitions at a 3/3 cadence with three hundred pounds in the bench press protocol during one workout, then performs twelve repetitions with the same cadence and weight 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 10 reps / 1 minute = 3000

300 pounds x 12 reps / 1.2 minutes = 3000

Sisco’s answer to this problem is the “power index”, a measure of what he calls “volumetric intensity”, or the duration for which a person is capable of maintaining a particular level of “intensity”. Your power index for a particular exercise is determined by squaring the product of the weight and repetitions, dividing it by the time in minutes, then dividing by 1,000,000. For example:

(300 pounds x 10 reps) squared / 1 minute) / 1,000,000 = a power index of 9

(300 pounds x 12 reps) squared / 1.2 minutes) / 1,000,000 = a power index of 10.8

Although this may appear to solve the problem, it would be simpler to just count repetitions or time under load, and since it is based on the power factor the power index is still a poor way to measure exercise performance.

Precise record keeping is essential for accurate and objective evaluation of progress, which is necessary for regulation of the volume and frequency of your training. However, the Power Factor Measurement is not the precise measure of intensity or muscular output Sisco claims, or a measure of anything relevant to your training since measurements of mechanical work and power alone do not accurately describe what is happening during muscular work. If repetition speed, range of movement, and all other relevant factors are consistent from workout to workout,  the repetition count or time under load is a simpler and more appropriate measure of progress.