Report by George Chen
INTRODUCTION
Power factor training is a resistance training system that claims to take the guess work out of training by effectively quantifying muscular intensity and the muscle-stimulating benefits of any workout. The developers claim that two indices, the power factor and power index, can be used by athletes to determine the selection of exercises, weights, sets, and reps that will produce maximum results for them (8). If these claims are true, the many existing styles of resistance training can be compared head-to-head in an objective manner. It can also lead to innovations in designing new training systems which may be even more effective than those existing.
PHYSIOLOGY
Power and work output are believed to be important parameters that can stimulate muscular hypertrophy. The developers of Power Factor training defined two parameters based on the mechanical definition of power to aid athletes in designing their workouts (8):
Power factor (PF): A measurement of the intensity of muscular overload during an exercise
PF = W / T
W = Total amount weight lifted in lbs (from multiple reps)
T = Total time in minutes
Power index (PI): A measurement of the duration of a given power factor
PI = (W^2) / T * 10^(-6)
= W * PF * 10^(-6)
Power Factor training is based on the concept of progressive overload. Athletes record these power indices for each workout and strive to increase these indices on subsequent workouts. For example, to increase their power factor, they can attempt to lift more total weight in the same period of time or the same amount of weight in a shorter period of time. To increase their power index, they need to sustain a given power factor for a longer period of time by possibly doing additional sets.
From these power indices, the developers noted that certain types of training and exercises tend to produce higher power factors and indices for most trainers. These styles of training form the cornerstone of their system. Partial reps are recommended because they increase the number of reps that can be performed in a given period of time. Furthermore, these partials are performed in their strongest range of motion to allow the heaviest weights to be used. Compound exercises (exercises that involve multi-joint movements) are preferred over isolation exercises because more weight can be used. However, the developers encourage the athletes to experiment for themselves to fine tune the best plan for them since each person is different (8).
Expectation: I don't expect Power Factor training to be effective because it over simplifies certain biomechanical factors both in the definition of the power indices and in their use in tailoring workouts for maximum effectiveness.
EVALUATION
Their definition of the power factor oversimplifies the definition of mechanical power by neglecting the distance the weight is lifted. Mechanical power is defined to be the force (F) exerted on an object multiplied by the distance (D) the object traveled divided by the time (T) of force application (5).
Power = F * D / T
By neglecting distance (D) in their definition of the power factor, the developers have overestimated the value of partial reps in their training scheme since the weight moves over a shorter distance per rep.
Furthermore, their definition of the power factor only takes into account the forces exerted on the bar. Higher forces exerted on the bar does not automatically translate to higher muscular tension. Neglecting this fact, the developers have overestimated the value of compound exercises and strongest-range-of-motion exercises. By definition, compound exercises work multiple muscle groups simultaneously, so the power generated is derived from multiple muscles. Therefore, the external power generated in a compound exercise should not be compared to that generated in an isolation exercise in which the power is derived primarily from a single muscle group. For example, the power generated from a squatting exercise contains strong contributions from the vastus, gluteus, and hamstring muscles. One cannot reasonably compare the power generated from a squat to that generated from a leg extension, derived mainly from the vastus.
The definition of the power factor also overestimates the value of strongest-range-of-motion exercises since muscular tension is often at a minimum in these ranges while most of the weight is supported by intersegmental joint loads. Therefore, more weight can be lifted in these ranges with the same or reduced muscular tension. For example, Power Factor training recommends athletes to perform partial reps in the final four inches of the squat movement near the lock out position. In this position, the legs are almost straight and relative less muscular force is needed to support any given amount of weight.
Beyond the problematic definition of the power factor and its application is the rationale of the definition itself. The power factor is defined to be the total weight lifted divided by the total time of lifting over multiple sets -- including rest time between sets and time spent lowering the weight where negative work is performed. In the realm of resistance training where high-powered anearobic exercise is the goal, this definition of power is less meaningful than peak instantaneous power during the concentric phase or even sustained power during a single set, before recovery time becomes a factor. If their definition of power factor was valid as an indicator of training effectiveness, athletes should decrease their rest periods as much as possible without greatly affecting the power output during the set. However, in a study where two groups were trained for isokinetic strength, one with short (40 seconds) and one with long rest intervals (160 seconds), longer rest periods resulted in greater improvement in hamstring muscle strength (7). Without a doubt, the group resting only 40 seconds between sets generated a much higher power factor. However, training with shorter rest intervals was less effective probably because subjects fatigued on subsequent sets and were less capable of generating high instantaneous power.
A rationale for maximizing instantaneous power during the concentric phase can be made from the force-velocity property of muscle. At close to maximum isometric force, very little power is generated since the muscle can barely produce the force necessary to maintain its length. Similarly, near the muscle's maximum shortening velocity, very little power is generated because the force exerted is minimal. Typically, maximum power is generated at an intermediate load and speed between a quarter and a third of their maximal values (4). Indeed, a training strategy of lifting relative light loads (approximately 30% of maximum) in a weighted squat jump at high speeds achieved the best overall enhancement in dynamic athletic performance when compared with plyometric and traditional weight training (9). However, traditional weight training with heavy loads (80-90% of maximum) still produced the largest isometric strength gains. It is not known which training modality would have produced greater muscular hypertrophy since the study was only five weeks long, a period in which most strength gains are probably due to neural factors (1). Nonetheless, the relatively light power-maximizing load (about 30% of maximum) found effective for enhancing dynamic athletic performance is considerably lighter than the loads recommended to maximize the power factor.
However, one of the problems with attempting to maximize instantaneous power in traditional weight training exercises is that it cannot be achieved with a light weight without throwing the weight up in the air at the end of the motion. Note that the aforementioned study employed a squat jump as their maximum power stimulus. If the subjects attempted to use the same weight and were required to keep themselves on the ground, they could not have generated maximum power since they need to decelerate the bar to zero velocity at the end of each rep. For this reason, the weight that maximizes power during the concentric phases of most traditional weight training exercises is biased towards weights somewhat heavier than the theoretical 30% of maximum. There is evidence that maximizing the power generated during the concentric phase of a weight training exercise is an effective training stimulus even though the theoretical maximum power cannot be reached. Using a shoulder press exercise, it was determined that the weight which generated maximum power was somewhat higher than the theoretical 30% based on the force-velocity property of muscle. However, training with this weight at maximum speed still produced greater increases in strength and power output than with heavier weights at lower speeds (3).
More evidence for the maximum-instantaneous-power stimulus is found in studies where the experimental variable was the speed of lifting while lifting load was maintained. In these studies, more power is clearly generated at higher speeds, and a greater training effect would be expected at higher speeds. Indeed, this has been found to be the case in two studies. In the first study, one group performed barbell squats using a 2 second up, 2 second down tempo, while another group performed squats using a 1 second up, 1 second down tempo. The fast training group demonstrated greater training effects when tested with vertical jumps, long jumps, maximum squats, and isometric and isokinetic knee extensions (6). In another study in which one group trained with knee extensions at 60 deg/s (slow) and another at 300 deg/s (fast), the slow training group improved peak torque only at the trained velocity while the fast training group improved at both velocities. Furthermore, only the fast training group demonstrated a significant enlargement of type II muscle fibers (2). These results suggest that the improvements in the slow group were primarily from neural factors while the improvements in the fast group can at least partially be attributed to muscular hypertrophy. These studies suggest that power output is an important training stimulus.
The power index, which is given proportionally less attention in the Power Factor training system, appears to be the developers' attempt to reconcile the fact that power factors decrease as workout length is increased. If power factor was the only index in the system, athletes would conclude that they should be performing only one set of each exercise. Thus, the developers defined the power index to be the power factor multiplied by the total weight lifted divided by a million to keep the large number manageable. This index biases the athlete to train with more volume since it gives more emphasis to the total weight lifted. The system never discussed how exactly to find the optimal combination of power factor and power index, which is a major shortcoming since improvement in one tends to work against the other. Furthermore, the definition of the power index has no physiological or mechanical basis.
CONCLUSION
Power factor training is just another system which has touted itself to be the "key" to effective training. Because the developers use mathematical equations, they have fooled some trainers to believe that they are employing real science in evaluating their workouts. However, their definition of the power factor ignores the distance the weight moves, leading to a false rationale for partial-rep training. Furthermore, by only taking into account the weight on the bar and not muscular tension, the system produced a false rationale for the superiority of compound and strongest-range-of-motion exercises. This is not to say that these types of training have no value in a weight training program since there may be other reasons for employing them. The important thing is that the power factor cannot be used as claimed -- as an objective way of determining the muscle stimulating benefits of any workout. Beyond the mechanical oversimplification in their definition of the power factor, is the problematic definition itself. All the literature suggest that instantaneous power during the concentric phase of an exercise is an important training stimulus, not the average power generated over a workout.
REFERENCES
1. Brooks, G. A., T. D. Fahey, T. P. White. Muscle strength, power, and
flexibility. In: Exercise Physiology: Human Bioenergetics and Its Applications, Second
Edition. Mountain View: Mayfield Publishing Company, 1996.
2. Coyle, E. F., D. C. Feiring, T. C. Rotkis. Specificity of power improvements
through slow and fast isokinetic training. Journal of Applied Physiology. 51: 1437-42,
1981.
3. Mastropaulo, J. A. A test of the maximum-power stimulus theory for strength.
European Journal of Applied Physiology. 65: 415-420, 1992.
4. McMahon, T. A. Muscles, Reflexes, and Locomotion. Princeton: Princeton
University Press, 1984.
5. Meriam, J. L., L. G. Kraige. Engineering Mechanics (Volume Two): Dynamics,
Third Edition. New York: John Wiley & Sons, 1992.
6. Morrissey, M. C., E. A. Harman, P. N. Frykman. Early phase differential
effects of slow and fast barbell squat training. American Journal of Sports Medicine.
26: 221-30, 1998.
7. Pincivero, D. M., S. M. Lephart, R. G. Karunakara. Effects of rest interval
on isokinetic strength and functional performance after short-term high intensity
training. British Journal of Sports Medicine. 31: 229-34, 1997.
8. Sisco, P., J. Little. Power Factor Training. Chicago: Contemporary Books, 1997.
9. Wilson, G. J., R. U. Newton, A. J. Murphy. The optimal training load for the
development of dynamic athletic performance. Medicine and Science in Sports and
Exercise. 25: 1279-86, 1993.
Email: gchen@stanford.edu
Homepage: http://www.stanford.edu/~gchen/