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Correcting the Use of the Term “Power” in the Strength and Conditioning Literature

Knudson, Duane V

Journal of Strength and Conditioning Research: September 2009 - Volume 23 - Issue 6 - p 1902-1908
doi: 10.1519/JSC.0b013e3181b7f5e5
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Knudson, DV. Correcting the use of the term “power” in the strength and conditioning literature. J Strength Cond Res 23(6): 1902-1908, 2009-Many strength and conditioning papers have incorrectly adopted the colloquial use of the term “power” as a measure of short-term, high-intensity muscular performance despite a long history of research and editorials critical of this practice. This has lead to confusion, incorrect interpretations, and conflicting results in the literature. This paper summarizes the scientific evidence on external mechanical power as a short-term, high-intensity neuromuscular (anaerobic) performance or training variable. Many problems in the measurement and use of power in strength and conditioning research were identified, as well as problems in the use of the vertical jump as a field test of power. A critical review of the biomechanics, measurement, and training research does not support this colloquial use of the term “power.” More research is needed that improves our understanding of the domains of muscular strength or neuromuscular performance, as well as partial correlation and multiple regression analyses to document the unique associations between these domains, biomechanical variables, training effects, and sport performance. Strength and conditioning research should limit the use of the term power to the true mechanical definition and provide several specific and measurement details on this measurement.

Department of Health and Human Performance, Texas State University, San Marcos, Texas

Address correspondence to Dr. Duane V. Knudson, dknudson@txstate.edu.

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Introduction

There have been numerous research studies on strength training since the early 20th century that informed the development and practice of the rehabilitation (physical medicine and physical therapy) and strength training professions (7). In 1978, the National Strength and Conditioning Association (NSCA) was founded in the United States to promote the development of the strength and conditioning professions and the scientific knowledge on strength training. A major tenet of the NSCA has always been to promote scientific research on strength training in order to provide a body of knowledge to provide evidence-based strength and conditioning programs.

Pioneering leaders in the field proposed standards for strength and training terminology (29,44,45,49). Despite these efforts and about 100 years of research on strength training in humans, there remains inconsistency in the definition and use of muscular performance variables like “power” in the literature. This article focuses on one example of this terminology problem: the large number of papers referring nebulous concepts of “power,” “power events,” or “power training” as synonyms for a unique short-term, high-intensity (anaerobic) neuromuscular performance characteristic. These papers also assume, with little scientific data in support, that the estimation of peak mechanical powers directly correspond to a meaningful neuromuscular performance characteristic for these short, high-intensity movements. This is unfortunate since there is a long history of research and laws of physics arguing against this interpretation. Several authors have already noted the confusion, calculation errors, lack of consistent terminology, and methodology in strength training performance variables in the exercise science literature (20,29,44,45,49,63,68). The paper extends this work and is organized in three parts summarizing the problems in terminology and definitions of muscular power, the linking of jump scores to this power construct, and other biomechanical factors limiting the application of this colloquial meaning of power in training.

The recent rush of interest in vague estimates of peak powers in many short, dynamic human movements has ignored the large bodies of research on the definition of mechanical power, the principle of specificity, and the domains of muscular strength/neuromuscular performance that challenge the importance of this colloquial meaning. This misplaced emphasis, incorrect use of terminology, and lack of attention to previous strength and conditioning research has contributed to inconsistent results and misrepresented findings (20). These problems present a barrier to both scientific advancement and professional application of knowledge in strength and conditioning, but they also represent an opportunity for researchers to make meaningful contributions to the field.

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Problems Of The Colloquial Meaning Of Power

Numerous papers have been published addressing “power” in human movement. Just over the last 10 years (1998-2008) SportDiscus and Google Scholar indexed over 500 and 21,000 citations, respectively, for a search on “muscular and power.” Many of these papers suffer from problems including an unclear definition of muscular power, a lack of specificity about the data and model used to calculate power flow, and when this power flow occurs in the movement. A larger problem is the colloquial meaning and assumed relevance of this peak “power” to human muscular performance and training. Many articles refer to “power” as if it were a clearly defined, generic neuromuscular or athletic performance characteristic, and not the true mechanical definition. This section will explore these problems in the strength and conditioning literature.

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What Power?

The mechanical definition of power is the rate of doing work. Because forces only do mechanical work when movement is present, mechanical power flow is present in most human movements. It is therefore nearly useless to refer to “power events” or “power athletes” because all movements, except for stabilized postures created by isometric muscle actions, involve muscular power flow. There is no one-to-one correspondence between maximizing mechanical power output of the body and certain sport movements, so the colloquial use of the term power as a unique performance neuromuscular performance characteristic is not consistent with the true definition of power.

Exercise physiologists have studied external (usually ignoring the work and power in moving the body segments) mechanical power in steady-state, cyclic movements using ergometers that measure overall external power flow from the body. This is the classic steady-state conditions of work and power measurement in physics that can provide relevant information about these continuous movements. Peak powers measured in these steady-state conditions still may not correspond to the true peak external power output (72). Unfortunately, it has become common to extend the measurement of power to short, dynamic or impulsive movements like a maximal effort jump. A later section will discuss why maximal power is not uniquely associated with neuromuscular performance in dynamic activities like jumping, throwing and striking.

Some studies have reported the creation of custom dynamometers for measuring external power flow in short-term dynamic movements of the whole-body, individual joint, and lower and upper extremity (1,9,25,76). Biomechanical measurements from force platforms and kinematic sensors can be used to make calculations of external power flow to the whole body center of mass or from the body to an external object like the floor, a sledge, or an Olympic bar (26). This combination of kinematic and kinetic data and appropriate biomechanical modeling are necessary for accurate measures of power flows in human movements (16,17). Unfortunately, many papers in the strength and conditioning literature suffer from a lack of specificity, standardization of methodology, and details of the biomechanical model/system used in calculating mechanical power (15,19,24,33). Most power variables reported in the strength and conditioning literature focus on the gross, muscular external power flow to another object from the person. The lack of specificity and citations in many papers, however, often does not allow readers to know exactly if mechanical power is being accurately measured.

Another distinction that is important to specify is if the power reported is either an instantaneous, often a peak value, or an average power flow over a specified time or event. We will see in this paper that peak or average mechanical powers are not strongly and uniquely related to jumping and performance in many sports. The peak muscular power observed will also vary based on the movement and other conditions, so it is not the unique, limit-defining muscular performance characteristic that it is commonly assumed to be in the colloquial usage of the word.

Without reporting all the specifics about what power is being measured, there can be widely disparate power values. A power flow using a model of the whole body or Olympic bar as a point mass will clearly result in a different power value than a model that is based on a multisegment rigid body model of the whole bar/athlete system (15,24,35). For example, the peak power flow in ankle plantar flexion in jumping can be 1000 watts because of transfer of energy within the lower extremity, while the maximum peak ankle power in isolated strength testing is about 400 watts (12). Without all the important specifics of the biomechanical model used and how power is calculated, readers will not know what muscular power variable is being discussed or how to compare the results to previous studies.

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Power Where?

For many decades, biomechanics scholars have worked to develop techniques to calculate the mechanical work and power flows through the joints of the body during movement. The complexity of the musculoskeletal system creates quite a few problems in documenting the source of power transfers in biomechanical systems (5,73) and linking joint powers to translational motions like jumping (54). The heart of the problem is the scalar nature of work and power. Currently, kinetic biomechanical models can calculate net internal joint power flows in many human movements, but the scalar nature of work and power and the complexity of the body (internal joint forces, biarticular muscles, ligaments, etc.) make the exact anatomical sources of these power flows impossible to determine. This limitation is a problem for rehabilitation/conditioning professionals who would like to know the specific muscles or muscle groups that are producing or absorbing mechanical work in a movement, but is also a problem for people wanting to assume a power measurement is representative of the causes of motion of a biomechanical system.

This location and interpretation problem means that if muscular power is to be used as a variable for studying sport and exercise movements, it should be limited to the true definition of mechanical power and would normally be external muscular power for a specified movement. The details of the biomechanical system/model used in the calculation (the where) needs to be specified along with the temporal relationship of this specific power variable to the movement. The average and peak power calculated will vary widely based on the biomechanical model used for the calculation (73). Garhammer (26) provides a nice review of the typical errors in various power calculations in different models and temporal phases of various human movements. Recent studies have also reported significant differences in power calculations based on different methodologies (15,16,24). Many specific qualifiers (internal/external, average/peak, movement phase) and measurement (model, instruments and calculations) details must be explicitly documented to talk meaningfully about the mechanical power in human movements.

The next two sections will review the validity of equations for estimating external lower extremity muscular power flow from vertical jump field tests and the lack of evidence for this as a unique, meaningful muscular performance or training variable.

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Power in Jumping?

Jumping is a relatively fast, fundamental movement pattern common to many sports. The vertical jump has been a surrogate skill within jumping that has been of measurement interest for over a hundred years. Interest in the vertical jump was high early in the 20th century as scholars explored the possibility of a general athletic ability. Out of this rich body of literature on jumping and other movements emerged a strong set of evidence against this general ability hypothesis and for specific muscular fitness and performance factors.

Once the Sargent jump test was claimed to be an estimate the external muscular power output in athletes, many populations of subjects were tested using these equations (48). A debate has raged since as to the importance and accuracy of this new estimate of athletic “power.” Adamson and Whitney (4) rejected this use of the variable power on theoretical grounds that the impulsive actions of jumping are “entirely different” from the steady-state rate of doing work usage of mechanical power in engineering and exercise physiology. Their view is supported by several biomechanical studies reporting weak or nonsignificant correlations (0.04-0.51) between force platform and cinematographic measures of mechanical power output and vertical jump height (11,14,67). Once associations with body mass are factored out, there is little meaningful association between mechanical power and vertical jump height (11). Differences in push-off distance (rise in center of mass from countermovement to take-off) also complicate power estimates from vertical jump height (62). Studies that performed multiple regression with body mass and jump height report that the combination of these variables used in jump power estimates account for a minority of the variance (41%-45%) in external power flow in vertical jumps (14,67). The very short duration of many dynamic jumping events (like running one-leg jumps) and the temporal difference between peak force and peak velocity means that peak and average powers measured may not be as meaningful in describing the dynamics of jumping as other biomechanical variables.

The main argument that seems to be overlooked by the recent research on vertical jump power equations (77) is that the net vertical impulse exactly determines vertical jump height (Newton's Second Law of Motion) while the power flow to the ground is a more variable curve that just happens to be correlated with net vertical impulse. In other words, why focus on power when the impulse-momentum relationship (Newton's Second Law) completely links kinetics to (r = 1.0) movement kinematics? This rapid increase in force (rate of force development, especially in the first 40 ms) is an important factor that helps maximize impulse and is the variable that is more strongly associated with jumping or sprint acceleration than power (21,38,54,65,74,79). While these associations are strongly influenced by the homogeneity of the sample (22) and more data are needed on highly trained athletes, there is also no theoretical reason to believe that mechanical power (scalar) represents a more meaningful biomechanical or performance variable than vectors like resultant force or impulse that uniquely determine the movement. Jumping research should focus on factors that maximize propulsive impulse in a short amount of time since many sport jumps have temporal restrictions on performance.

The influence of coordination of the arms and trunk also argues against the use of vertical jump as a movement for assessing lower extremity power (78). Recent studies that mass normalize power measures report low associations between power and jump or sprint performance, and stronger associations with impulse (32) and jump technique (66). In addition, a recent principal components analysis of a large sample of male jumpers showed that the association between strength and power measurements is related to body size and technique factors like jump type (static, countermovement, hop) and muscle action (47).

Using a regression equation combining several weak predictors like jump height and body mass to estimate a muscle power variable that has a weak association and a poor theoretical/mechanistic link to performance or training is problematic. The strength of association and accuracy concerns has not stopped the many papers proposing new regression equations to estimate primarily lower extremity external power flow to the ground in jumping (13,31,39,46,59). Some of this work is loosely based on studies reporting significant correlations between external power flows measures and jump height in a vertical jump (6,23,30). The problem is that these jump equation studies do not acknowledge the evidence reviewed in this section arguing against the importance of mechanical power or the poor accuracy of these estimates of external power output. In short, lower extremity external power estimates from jump height are inaccurate because of the impulsive nature the movement, variation due to technique, muscle actions, body size, and the weak unique association between jump height measures and true external power flow. The next section will summarize other variables related to muscular power output that compromise the colloquial meaning of power in strength and conditioning.

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Other Challenges to the Application of Power in Training

Numerous scholars have been interested in the mechanical power flow in muscles and from muscular contractions. Important texts summarizing this research are Jones et al. (40) and Komi (43). The practical application of short-term muscular power measurements, however, is complicated by both the weak associations with performance noted earlier as well as several other factors. Some of these factors include the primary type of muscle action in the event (concentric, eccentric, isometric), coordination/skill, neuromuscular activation, and other motor performance parameters (3). This section will build on some recent review papers in this area (3,20,33,70) to note how research on muscle mechanics, specificity, and the domains of muscular strength performance do not support the use of muscle power as a meaningful training or performance variable. Areas of future research are also identified.

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Muscle Mechanics

The force generated by active muscle varies widely relative to muscle velocity. This force-velocity relationship of muscle has been well known and studied for decades (27). This relationship defines the force and power output for all three muscle actions (eccentric, isometric, concentric). Several areas of muscle mechanics evidence argue against the importance of peak muscular power as a universal and meaningful muscle performance variable.

First, the right compromise of force and velocity that creates peak muscular power (Pmax) depends on many factors, including the level of analysis (fiber, muscle, segment, limb, body), the movement, technique, and external load (20,33,42,43,61). Second, the external load (mass) that maximizes external power (Pmax) varies across individuals and may not be as meaningful a variable as is commonly believed (20,33). Third, there is little evidence that maximizing muscular power output is meaningful or related to performance in most human movement activities (20). Obtaining the highest rate of doing work (greatest mechanical effect from both force and velocity) is not logically or uniquely associated with success in jumping, sprinting, throwing, or other sport skills. Success in most sports involves maximizing other performance variables like speed, force, technique, or some combination of these variables. Some steady-state ergometer sports like cycling and rowing might have a logical association between maximizing sustained work rate and performance in some conditions, but the complexity of the body and various movement goals precludes an influential and unique link between maximal muscular power flow and success in short-term, high-intensity events like jumping or sprinting.

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Specificity and Domains of Muscular Strength/Neuromuscular Performance

Early factor analyses of numerous muscular performance tests have consistently shown three main muscular strength or neuromuscular performance parameters (36,37,53). These three domains of muscular ‘strength’ performance have been called static, dynamic, and ballistic strength. These strength variables and power are correlated with each other (70), but the size of the associations (r < 0.61) do not support the common notion of generality or a single domain of muscular strength (10), so it is clear that there is a spectrum of muscular performance with, at least, three major kinds of strength expression.

Several studies have reported that strength measures combining velocity and force correlate more strongly with jumping than static-strength measures (52,57,65,80,81). Recent reviews have concluded that other biomechanical variables, such as impulse and rate of force development (20,33) that are logically consistent with these strength measures combining speed and force, may be more strongly related to athletic performance than peak mechanical power. The numerous biomechanical factors that influence human movement means that peak external power output could occur in either the combinations of high-force/low-speed or high-speed/low-force regions in the concentric region of the force-velocity relationship. It is therefore likely that research on this important strength/neuromuscular performance parameter focus on impulse and rate of force development, rather than peak power.

The strength and conditioning field needs to be a leader in promoting a consistent use of terminology for muscular strength performance and rehabilitation variables. This terminology should be based on scientific support through correlation studies, research on potential mechanisms of effect, and training studies. The consensus of all these kinds of studies supports the three domains of muscular strength performance and does not support the colloquial use of power as a unique and meaningful neuromuscular performance domain. More research is needed to improve our understanding of the domains of muscular strength performance and improve the accuracy of strength performance terminology. Colloquial jargon like “power” and “explosive strength” may be useful coaching cues, but they have little place in strength and conditioning research and professional literature. Use of the term “power” should be reserved to true measures of mechanical power with the specific methodology and qualifiers noted in earlier in this paper.

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Opportunities for Future Research

An important area of muscle performance research is the biomechanics of the various domains of muscle performance and their responses to training. Applied biomechanical research like that reviewed by Schmidtbleicher (65) will help further refine the definitions of these strength variables. For example, it would be useful to know the range of movement durations (e.g., 40-250 ms) and external masses moved that most closely correspond to the various domains of strength. More research is also needed on variables like impulse, time intervals and improving reliability for rate of force development variables. All these studies need to be performed in both single-joint and multijoint movements using athletes with a wide range of training experience.

Cronin and Sleivert (20) reported an excellent review of the correlational and training studies of the tenuous link between “power training” and athletic performance. They concluded that the preoccupation with Pmax training is problematic and other strength variables could be of greater association with improved athletic performance. Future research on associations between strength variables and training responses, therefore, should address several limitations of previous correlational studies. The intercorrelation of the many strength and athlete variables in training studies, for example, often masks the meaningful, unique associations that are interest.

Unfortunately, most strength and conditioning studies present only zero-order (Pearson) correlations, which do not show the unique associations between strength variables and performance. Partial correlations, regression, and multivariate analyses like factor analysis are the statistical tools that help researchers tease out these complex relationships between many variables. A recent study that did utilize partial correlations confirmed that dynamic strength accounted for significant variance in other performance measures like power, but dynamic strength was the only intercorrelation examined (60). Other limitations of previous research that needs to be addressed in future studies are the use of small samples of subjects (often less than 20) that do not allow for study of many variables or provide generalizability of the results to similar subjects and more research on highly-trained athletes.

More research is also need looking at differential results of training on the domains of muscular performance. This diagnostic strength assessment (2,10,51,55,69,75) provides important information on exercise specificity and tests that are associated with training effects. These observations are consistent with the large body of research on the specificity of performance of motor tasks (58) and the specificity of strength training effects (18,28,32,41,42,56,76). This research on specificity also supports the hypothesis that functional or sport-specific performance tests should be used to monitor training rather than dynamometer-based tests (2,51,56,58). Gains from training assessed with testing depend strongly on the tests being similar to the training mode (50).

The use of functional rather than dynamometer tests in evaluating training programs also avoids the problems of how stature and body mass influence strength and power measurements (71). In other words, the height of an athlete's vertical or running jump often times is the best measure for monitoring jumping performance and training effects. It makes little sense to transform jump heights and athlete masses into an inaccurate and likely meaningless estimate of external muscular power.

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Summary

Basic research on calculating power in biomechanical systems, muscle mechanics, as well as more applied research on training specificity and the domains of strength/neuromuscular performance do not support the colloquial interpretation of power as a meaningful short-term, high-intensity muscular performance or training variable. Strength and conditioning research and application articles should usually avoid this use of the term power, unless the research is specifically measuring power with the true mechanical definition of power (rate of doing mechanical work) and explicitly defines the model, methodology, and other specific qualifiers of power calculation. The use of height of vertical jump to estimate power is especially problematic. Factor analysis and training studies are consistent with the principle of specificity and support three domains of strength or neuromuscular performance. Future research should use consistent neuromuscular performance terminology, refine our knowledge of these domains of neuromuscular performance, and use multivariate statistical analyses to determine the unique associations between muscle performance variables, training effects, and sport performance.

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    Keywords:

    dynamometer; fitness; muscle; performance; testing; work

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