There have been numerous research studies on CORRECTING THE USE OF THE TERM POWER IN THE STRENGTH AND CONDITIONING LITERATURE DUANE V.

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1 BRIEF REVIEW CORRECTING THE USE OF THE TERM POWER IN THE STRENGTH AND CONDITIONING LITERATURE DUANE V. KNUDSON Department of Health and Human Performance, Texas State University, San Marcos, Texas ABSTRACT Knudson, DV. Correcting use of term power in strength and conditioning literature. J Strength Cond Res 23(6): , 2009 Many strength and conditioning papers have incorrectly adopted colloquial use of 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 literature. This paper summarizes scientific evidence on external mechanical power as a shortterm, high-intensity neuromuscular (anaerobic) performance or training variable. Many problems in measurement and use of power in strength and conditioning research were identified, as well as problems in use of vertical jump as a field test of power. A critical review of biomechanics, measurement, and training research does not support this colloquial use of term power. More research is needed that improves our understanding of domains of muscular strength or neuromuscular performance, as well as partial correlation and multiple regression analyses to document unique associations between se domains, biomechanical variables, training effects, and sport performance. Strength and conditioning research should limit use of term power to true mechanical definition and provide several specific and measurement details on this measurement. KEY WORDS dynamometer, fitness, muscle, performance, testing, work INTRODUCTION There have been numerous research studies on strength training since early 20th century that informed development and practice of rehabilitation (physical medicine and physical rapy) and strength training professions (7). In 1978, National Strength and Conditioning Association (NSCA) Address correspondence to Dr. Duane V. Knudson, dknudson@txstate. edu. 23(6)/ Ó 2009 National Strength and Conditioning Association was founded in United States to promote development of strength and conditioning professions and scientific knowledge on strength training. A major tenet of 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 field proposed standards for strength and training terminology (29,44,45,49). Despite se efforts and about 100 years of research on strength training in humans, re remains inconsistency in definition and use of muscular performance variables like power in literature. This article focuses on one example of this terminology problem: large number of papers referring nebulous concepts of power, power events, or power training as synonyms for a unique short-term, highintensity (anaerobic) neuromuscular performance characteristic. These papers also assume, with little scientific data in support, that estimation of peak mechanical powers directly correspond to a meaningful neuromuscular performance characteristic for se short, high-intensity movements. This is unfortunate since re is a long history of research and laws of physics arguing against this interpretation. Several authors have already noted confusion, calculation errors, lack of consistent terminology, and methodology in strength training performance variables in exercise science literature (20,29,44,45,49,63,68). The paper extends this work and is organized in three parts summarizing problems in terminology and definitions of muscular power, linking of jump scores to this power construct, and or biomechanical factors limiting 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 large bodies of research on definition of mechanical power, principle of specificity, and domains of muscular strength/neuromuscular performance that challenge 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 y also 1902

2 represent an opportunity for researchers to make meaningful contributions to field. Problems Of The Colloquial Meaning Of Power Numerous papers have been published addressing power in human movement. Just over last 10 years ( ) SportDiscus and Google Scholar indexed over 500 and 21,000 citations, respectively, for a search on muscular and power. Many of se papers suffer from problems including an unclear definition of muscular power, a lack of specificity about data and model used to calculate power flow, and when this power flow occurs in movement. A larger problem is 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 true mechanical definition. This section will explore se problems in strength and conditioning literature. What Power? The mechanical definition of power is 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 refore 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 body and certain sport movements, so colloquial use of term power as a unique performance neuromuscular performance characteristic is not consistent with true definition of power. Exercise physiologists have studied external (usually ignoring work and power in moving body segments) mechanical power in steady-state, cyclic movements using ergometers that measure overall external power flow from body. This is classic steady-state conditions of work and power measurement in physics that can provide relevant information about se continuous movements. Peak powers measured in se steady-state conditions still may not correspond to true peak external power output (72). Unfortunately, it has become common to extend 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 creation of custom dynamometers for measuring external power flow in shortterm dynamic movements of 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 whole body center of mass or from body to an external object like 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 strength and conditioning literature suffer from a lack of specificity, standardization of methodology, and details of biomechanical model/system used in calculating mechanical power (15,19,24,33). Most power variables reported in strength and conditioning literature focus on gross, muscular external power flow to anor object from 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. Anor distinction that is important to specify is if power reported is eir 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 movement and or conditions, so it is not unique, limit-defining muscular performance characteristic that it is commonly assumed to be in colloquial usage of word. Without reporting all specifics about what power is being measured, re can be widely disparate power values. A power flow using a model of 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 whole bar/athlete system (15,24,35). For example, peak power flow in ankle plantar flexion in jumping can be 1000 watts because of transfer of energy within lower extremity, while maximum peak ankle power in isolated strength testing is about 400 watts (12). Without all important specifics of biomechanical model used and how power is calculated, readers will not know what muscular power variable is being discussed or how to compare results to previous studies. Power Where? For many decades, biomechanics scholars have worked to develop techniques to calculate mechanical work and power flows through joints of body during movement. The complexity of musculoskeletal system creates quite a few problems in documenting source of power transfers in biomechanical systems (5,73) and linking joint powers to translational motions like jumping (54). The heart of problem is scalar nature of work and power. Currently, kinetic biomechanical models can calculate net internal joint power flows in many human movements, but scalar nature of work and power and complexity of body (internal joint forces, biarticular muscles, ligaments, etc.) make exact anatomical sources of se power flows impossible to determine. This limitation is a problem for rehabilitation/conditioning professionals who would like to know specific muscles or muscle groups that are VOLUME 23 NUMBER 6 SEPTEMBER

3 Use of Term Power producing or absorbing mechanical work in a movement, but is also a problem for people wanting to assume a power measurement is representative of 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 true definition of mechanical power and would normally be external muscular power for a specified movement. The details of biomechanical system/model used in calculation ( where) needs to be specified along with temporal relationship of this specific power variable to movement. The average and peak power calculated will vary widely based on biomechanical model used for calculation (73). Garhammer (26) provides a nice review of 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 mechanical power in human movements. The next two sections will review validity of equations for estimating external lower extremity muscular power flow from vertical jump field tests and lack of evidence for this as a unique, meaningful muscular performance or training variable. 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 vertical jump was high early in 20th century as scholars explored possibility of a general athletic ability. Out of this rich body of literature on jumping and or movements emerged a strong set of evidence against this general ability hyposis and for specific muscular fitness and performance factors. Once Sargent jump test was claimed to be an estimate external muscular power output in athletes, many populations of subjects were tested using se equations (48). A debate has raged since as to importance and accuracy of this new estimate of athletic power. Adamson and Whitney (4) rejected this use of variable power on oretical grounds that impulsive actions of jumping are entirely different from 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 ( ) 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, re 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 combination of se variables used in jump power estimates account for a minority of 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 temporal difference between peak force and peak velocity means that peak and average powers measured may not be as meaningful in describing dynamics of jumping as or biomechanical variables. The main argument that seems to be overlooked by recent research on vertical jump power equations (77) is that net vertical impulse exactly determines vertical jump height (Newton s Second Law of Motion) while power flow to ground is a more variable curve that just happens to be correlated with net vertical impulse. In or words, why focus on power when 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 first 40 ms) is an important factor that helps maximize impulse and is variable that is more strongly associated with jumping or sprint acceleration than power (21,38,54,65,74,79). While se associations are strongly influenced by homogeneity of sample (22) and more data are needed on highly trained athletes, re is also no oretical 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 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 arms and trunk also argues against 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 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 oretical/mechanistic link to performance or training is problematic. The strength of association and accuracy concerns has not stopped many papers proposing new regression equations to estimate primarily lower extremity external power flow to ground in jumping 1904

4 (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 se jump equation studies do not acknowledge evidence reviewed in this section arguing against importance of mechanical power or poor accuracy of se estimates of external power output. In short, lower extremity external power estimates from jump height are inaccurate because of impulsive nature movement, variation due to technique, muscle actions, body size, and weak unique association between jump height measures and true external power flow. The next section will summarize or variables related to muscular power output that compromise colloquial meaning of power in strength and conditioning. Or Challenges to Application of Power in Training Numerous scholars have been interested in 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 weak associations with performance noted earlier as well as several or factors. Some of se factors include primary type of muscle action in event (concentric, eccentric, isometric), coordination/skill, neuromuscular activation, and or 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 domains of muscular strength performance do not support use of muscle power as a meaningful training or performance variable. Areas of future research are also identified. 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 force and power output for all three muscle actions (eccentric, isometric, concentric). Several areas of muscle mechanics evidence argue against importance of peak muscular power as a universal and meaningful muscle performance variable. First, right compromise of force and velocity that creates peak muscular power (Pmax) depends on many factors, including level of analysis (fiber, muscle, segment, limb, body), movement, technique, and external load (20,33,42,43,61). Second, 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, re is little evidence that maximizing muscular power output is meaningful or related to performance in most human movement activities (20). Obtaining 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 or sport skills. Success in most sports involves maximizing or performance variables like speed, force, technique, or some combination of se 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 complexity of body and various movement goals precludes an influential and unique link between maximal muscular power flow and success in shortterm, high-intensity events like jumping or sprinting. 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 ÔstrengthÕ performance have been called static, dynamic, and ballistic strength. These strength variables and power are correlated with each or (70), but size of associations (r, 0.61) do not support common notion of generality or a single domain of muscular strength (10), so it is clear that re 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 or biomechanical variables, such as impulse and rate of force development (20,33) that are logically consistent with se 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 eir combinations of high-force/low-speed or high-speed/low-force regions in concentric region of force-velocity relationship. It is refore likely that research on this important strength/neuromuscular performance parameter focus on impulse and rate of force development, rar 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 se kinds of studies supports three domains of muscular strength performance and does not support colloquial use of power as a unique and meaningful neuromuscular performance domain. More research is needed to improve our understanding of domains of muscular strength performance and improve accuracy of strength performance terminology. Colloquial jargon like power and explosive strength may be useful coaching cues, but y have little place in strength and conditioning research and professional VOLUME 23 NUMBER 6 SEPTEMBER

5 Use of Term Power literature. Use of term power should be reserved to true measures of mechanical power with specific methodology and qualifiers noted in earlier in this paper. Opportunities for Future Research An important area of muscle performance research is biomechanics of various domains of muscle performance and ir responses to training. Applied biomechanical research like that reviewed by Schmidtbleicher (65) will help furr refine definitions of se strength variables. For example, it would be useful to know range of movement durations (e.g., ms) and external masses moved that most closely correspond to 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 se 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 correlational and training studies of tenuous link between power training and athletic performance. They concluded that preoccupation with Pmax training is problematic and or strength variables could be of greater association with improved athletic performance. Future research on associations between strength variables and training responses, refore, should address several limitations of previous correlational studies. The intercorrelation of many strength and athlete variables in training studies, for example, often masks meaningful, unique associations that are interest. Unfortunately, most strength and conditioning studies present only zero-order (Pearson) correlations, which do not show unique associations between strength variables and performance. Partial correlations, regression, and multivariate analyses like factor analysis are statistical tools that help researchers tease out se complex relationships between many variables. A recent study that did utilize partial correlations confirmed that dynamic strength accounted for significant variance in or performance measures like power, but dynamic strength was only intercorrelation examined (60). Or limitations of previous research that needs to be addressed in future studies are use of small samples of subjects (often less than 20) that do not allow for study of many variables or provide generalizability of results to similar subjects and more research on highlytrained athletes. More research is also need looking at differential results of training on 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 large body of research on specificity of performance of motor tasks (58) and specificity of strength training effects (18,28,32,41,42,56,76). This research on specificity also supports hyposis that functional or sport-specific performance tests should be used to monitor training rar than dynamometer-based tests (2,51,56,58). Gains from training assessed with testing depend strongly on tests being similar to training mode (50). The use of functional rar than dynamometer tests in evaluating training programs also avoids problems of how stature and body mass influence strength and power measurements (71). In or words, height of an athlete s vertical or running jump often times is 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. SUMMARY Basic research on calculating power in biomechanical systems, muscle mechanics, as well as more applied research on training specificity and domains of strength/ neuromuscular performance do not support colloquial interpretation of power as a meaningful short-term, highintensity muscular performance or training variable. Strength and conditioning research and application articles should usually avoid this use of term power, unless research is specifically measuring power with true mechanical definition of power (rate of doing mechanical work) and explicitly defines model, methodology, and or 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 principle of specificity and support three domains of strength or neuromuscular performance. 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