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Testing Strength and Power in Soccer Players: The Application of Conventional and Traditional Methods of Assessment

Paul, Darren J.1; Nassis, George P.1,2

Journal of Strength and Conditioning Research: June 2015 - Volume 29 - Issue 6 - p 1748–1758
doi: 10.1519/JSC.0000000000000807
Brief Review
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Paul, DJ and Nassis, GP. Testing strength and power in soccer players: The application of conventional and traditional methods of assessment. J Strength Cond Res 29(6): 1748–1758, 2015—Soccer is a highly complex sport influenced by many physical, psychological, tactical, and technical factors. In terms of basic physical components, strength and power are considered requisites for many important actions such as tackling, jumping, and shooting. Hence, assessment of strength and power is commonly performed within a soccer club's test battery. The objective is to use valid, reliable, and sensitive measures that allow for trustworthy analysis of the physical characteristics of players. Before any credence can be placed in test results, test's validity, reliability, and sensitivity needs to be established. This will allow practitioners to make informed decisions about test selection. This review examines the reliability, validity, and sensitivity of different strength and power assessments in soccer. The suitability of conventional and functional tests is detailed and the strengths and weaknesses of isokinetic dynamometry, hand-held dynamometry (HHD), repetition maximum, and power testing are also addressed. Generally, the tests considered in this review provide moderate to high reliability in soccer players of different training level. Similarly, the consensus demonstrates test methods to be sensitive to training interventions. In comparison, test validity seems less established. Isokinetic dynamometry has often been recognized as a gold standard measure of testing strength. Other methods of assessment are emerging as viable options (e.g., HHD), likely due to functionality and suitability of testing. Given the demands within a soccer club setting, practitioners should endeavor to use testing procedures that are informative yet not time consuming or labor intensive. By providing this, practitioners may have the option to perform more regular monitoring throughout the season rather than a limited number of specific time periods.

1Excellence in Football Project, National Sports Medicine Programme, Aspetar—Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar; and

2School of Physical Education and Sport Science, National and Kapodistrian University of Athens, Athens, Greece

Address correspondence to Darren J. Paul, darren.paul@aspetar.com.

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Introduction

Soccer is a highly complex sport influenced by many physical, psychological, tactical, and technical factors (72). The physical demands typically require the majority of play to be performed at a low intensity. The most significant part (scoring goals) of the game, however, is often influenced by high-intensity explosive efforts (27). As a result, in terms of basic physical components, strength and power are considered highly important (40). For that reason, assessment of strength and power is commonly performed within a soccer club's test battery.

Tests of isokinetic dynamometry, hand-held dynamometry (HHD), repetition maximum (RM), and power have all been cited in the literature as methods of assessment. Despite being commonly used, the suitability and usefulness of these tests has not been fully elucidated. Before any credence can be placed in test results and regularly implemented, the validity, reliability, and sensitivity of test methods and procedures have to be established (2). Validity, defined as the degree to which a test measures what it purports to measure, can take different forms. This includes the test's validity in relation to a criterion (predictive or concurrent); a construct (convergent and divergent) or the characteristics of the test (content and logical) (6). When deciding whether or not a test is useful, the user should always be satisfied that the tests are shown to be valid (12). Reliability can be defined as the consistency or reproducibility of test results when carried out on more than 1 occasion (41,43). It can be a constituent of the following: instrumental reliability (the reliability of the measurement device), rater reliability (the reliability of the researcher administering the measurement device), or response reliability (the reliability of the variable measured) (12). The reliability of a test may be influenced by its duration, mode and type as well as the gender, athletic status, and age of the individual tested (22,43). Finally, the usefulness of a test can be determined by its sensitivity, defined as the ability of an instrument to measure changes in a state (42). Essentially, the test must be adequately sensitive to detect systematic and meaningful changes at individual and group level (41). This measurement characteristic can be evaluated by comparing test random changes (i.e., typical error of measurement representing test noise) with the smallest worthwhile change (SWC) expected from a training intervention (43).

Issues surrounding the validity, reliability, and sensitivity are important to determine the integrity of a test. Intuitively, for the practitioner to provide meaningful results to the players and coaching staff, it is their prerogative to use testing methods regarded as possessing high levels of these traits (validity, reliability, and sensitivity). It is apparent (Tables 1 and 2) that several different tests and test variations are available. However, this is unlikely to account for the vast number of other methods used in the real world but not present in the literature. The practitioner therefore, may have a multitude of different methods at their disposal. Unfortunately, there exists not 1 test method that encapsulates all parameters of strength and power. The reality rather, being that each possesses their own innate strengths and weaknesses (Table 3). It is likely to prove challenging, thus, for the practitioner to be fully informed regarding the usefulness and appropriateness of the different tests available and deemed appropriate for the players, team, or sport they are working with. Therefore, the aim of this review is to detail the reliability and validity of strength and power methods of assessment in soccer players.

Table 1

Table 1

Table 2-a

Table 2-a

Table 2-b

Table 2-b

Table 3

Table 3

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Dynamometry

Isokinetic dynamometry testing has long been regarded as a principal method for assessing muscle function and imbalances in clinical, research, and sports environment (33). Often considered as the gold standard criterion, assessment can provide quantification of a variety of muscle function indices (e.g., peak and average torque, joint angle of peak torque, work, and power) (24). Isokinetic dynamometry is well regarded because it has shown to detect injury (48) as well as discriminate between playing level/training status (16) and positional differences (18,79) in soccer players.

Although a number of muscles and muscle groups can be measured, the main application of dynamometry is in assessing hamstring to quadriceps ratio. This seems pertinent for soccer given that studies have shown hamstring injuries are among the most common (81,85). It is proclaimed that this may stem from strength imbalances, as identified by isokinetic dynamometry (19). In an attempt to elicit practical significance, research has examined the relationship between indices of isokinetic strength and measures of functional performance, including sprint (18), repeated sprint ability (62), and vertical jump (20,48,58). Unfortunately, the low to moderate shared variance denotes these tests to be widely independent methods, likely due to different movements performed (58). A possible discrepancy in the knee joint power development and muscle activation when using isokinetic tests to predict jumping performance is likely 1 explanatory factor (48). Also, whereas current practice generally tends to test in a seated position to assess muscle function of the knee, a prone position (∼10° hip flexion) may show greater association with sprinting. First, the prone position is functionally relevant as it most closely simulates hip joint angle in a running/sprinting position. Second, the same position (prone) simulates better the knee flexor and extensor muscle length-tension relationships, which occurs in late and early contact phase of sprinting (86,87). Nevertheless, the literature is dominated by traditional procedures that are often not based on robust scientific rationale. This is evident by the fact 60 and 300o·s−1 seem to be among the most commonly applied measures for isokinetic dynamometry (Table 1). However, it is questionable as to how informative this is given that the speed of movements is far from that of movements within the game.

Soccer players may be prone to strength imbalances between the left and right limbs as they seldom use both with equal emphasis (88). This is likely to cause muscle asymmetry and subsequently increase the susceptibility to injury (28). Strength asymmetry has shown an association with important exogenous factors of soccer performance such as playing position (23), training age/background (4,35), and training level (16) (Table 1). Such data, however, should be interpreted with caution, particularly when the causative factors are unknown. For example, it is unclear whether positional differences in muscular strength are due to selection of a specific type of player for a position, or whether characteristics of a certain position predispose a player to injury (67).

Although no clear differences are seen between different levels (Table 1), the reliability, validity, and sensitivity of testing in youth soccer players, compared with senior level, requires greater consideration. The rationale being that any training may be masked by the natural strength gained purely by growth and maturation. The rates of anatomical growth and maturation can vary independently and their effects on strength do not simply correlate with chronological age (23). The complexity can be accentuated by possible differences between muscles. Age has shown to have a greater effect on peak torque/body weight (PT/BW) during concentric compared with eccentric hamstring strength in young soccer players (29). Therefore, while concentric strength may build in a fairly linear fashion, eccentric hamstring PT/BW may not be as highly influenced by pubertal development stages. Lehence et al. (52) has found a higher percentage of lower limb muscular imbalances within junior (<17 years—61%; <21 years—58%) than adult soccer players (50%). Given the supposed link between eccentric strength and injury risk in soccer, this may have significant practical implications regarding the early implementation of strength testing and injury prevention.

Despite favorable results regarding test reliability (Table 1), several issues need to be considered when interpreting and using isokinetic dynamometry. For example, although considered to more closely mimic the fast actions in soccer, it seems higher speed assessments are also more likely prone to error (88). Test reliability is also likely to be reduced if high-speed movements precede those of lower speeds, and the reliability for 1 joint should not be considered characteristic of other untested joints (2). Also, Forbes and Siegler (28) cautioned that strength and muscle balance should not be considered as stable variables across the course of a competitive season in youth soccer players. Such findings are somewhat comparable with observations in senior level players (25). This questions the long-term stability of performance during isokinetic dynamometry testing in soccer players. This notion is particularly relevant given recent research showing varying levels of long-term stability in measures of youth soccer players (10).

Isokinetic dynamometry is relatively well established; however, discrepancies may exist between the timewise nature of peak force hamstring/quadriceps (H/Q) strength ratio and the potential to stabilize the knee joint during rapid match play situations. This reduced capacity may possibly diminish its relevance to rapid soccer movements (89). Peak force during a maximal voluntary contraction is typically reached in approximately 500 milliseconds from the onset of contraction. However, it is suggested that 50 milliseconds is the time frame whereby many rapid ground contact situations occur (51). Intuitively, the ability to rapidly activate the muscle is considered an important characteristic; hence, the rate of force development (RFD) during maximal voluntary static contraction may prove a useful method of testing (34). The RFD is the slope of the force (moment) and time curve obtained under isometric conditions, and its maximal values are attained between 80 and 120 milliseconds (1). Although the hamstring:quadriceps concentric (Hcon:Qcon) and hamstring:quadriceps rate of force development (HRFD: QRFD) may be considered similar, it seems the physiological and clinical meanings of indices in soccer players are likely different (34). Table 1 illustrates that this form of testing (isometric) is far less common than more traditional eccentric and concentric actions. Also, it is apparent that despite several studies reporting it to be a reliable method of testing, few studies have actually identified the effect of a training intervention using this approach (Table 1). It is plausible that the real-life setting does not provide the opportunity to perform regular time points to use such a device. To establish its usefulness within soccer, it is clear that further research is needed to identify the reliability and validity of isometric strength testing.

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Portable and Hand-Held Dynamometry

Hand-held dynamometry has become an increasingly popular, cost-effective, and easy-to-use method that can overcome the subjective nature of a manual muscle testing and labor intensive and time-consuming traits of the traditional isokinetic dynamometry (77). Its appeal is accentuated by studies reporting HHD to show comparatively high reliability in a variety of populations (55,77).

Compared with isokinetic dynamometry, HHD has shown to produce moderate (r = 0.32) to high (r = 0.61) correlation for hamstring and quadriceps torque in a group of professional soccer players (82) (Table 1). In a group of young soccer and Australian Rules Football players, acceptable intrarater test-retest reliability for right hip abduction (intraclass correlation coefficient [ICC] = 0.81) and left hip abduction (ICC = 0.84) have been shown (55). Lower values were observed for inter-rater right hip abduction (ICC = 0.73) and left hip abduction (ICC = 0.58) (55). Furthermore, a group of male adult semiprofessional soccer players reported, with the exception of inter-rater dominant hip flexion (ICC = 0.66), HHD reliability to be “fair” to “excellent” for intra-rater (ICC range = 0.70–0.89) and inter-rater (ICC range = 0.71–0.87) (30). Interestingly, intrarater reliability was lowest for measures of dominant hip flexor strength (ICC = 0.66) and highest for adduction at 90° hip flexion (ICC = 0.87), representing the strongest and weakest muscle groups, respectively. This partly supports the notion that greater strength of muscle groups may adversely affect the reliability of HHD testing (3). It seems the abductors and adductors are the most commonly measured using HHD (Table 2). Given the evidence, above it seems questionable as to its reliability for larger and stronger muscle groups, this warrants further investigation.

Generally, studies investigating the reproducibility of hip strength measurement using HHD have typically reported its relative reliability (15,70). However, relative reliability does not provide a cutoff score for delineating a true change from the measurement variation, which is necessary for making valid clinical decisions (77). By establishing strength changes, Thorborg et al. (77) proposed that above 10% could be considered to be “real” changes in healthy individuals. In young soccer players, Paul et al. (63) reported that in some instances, changes in strength of more than 12.5% are necessary to be considered meaningful and not due to measurement variation. The practical significance of establishing Minimal Detectable Changes can be contextualized by previous research, albeit not in soccer. In this particular study, hip abductor strength in a group of young elite Australian football players was shown to significantly decrease in the week before (∼6%) and during (∼12%) the onset of groin injury (21). Although an increasingly popular method of assessment, there are, however, issues surrounding HHD that require careful consideration, for example, inconsistency of rate of force applied by tester (Table 3).

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Repetition Maximum

Support for strength testing using RM is based on the notion that tests using free barbells will reflect the functional strength of the soccer player more accurately than assessments such as isokinetic dynamometry. Evaluating the effects of training within a specific context is considered to provide a more sensitive measure, thus representing a more accurate evaluation of strength gain (2). Despite the increasing prevalence of strength and conditioning coaches in soccer clubs, however, the available literature advocating 1RM testing is lagging, as evident in Table 2. One possible explanation for the lack of research could be the time constraints, logistical issues, implications of weightlifting skill, experience of the individual, and the overall apprehension practitioners may have in trying to test a player's 1RM within the season (2). Hence, it is recommended that soccer players only engage in RM testing if they are familiar with the exercise, so not to negate the test reliability (8), or more importantly cause injury (37). Although shown to be safe when performed in a supervised situation, there is no denying that such a demanding multijoint movement does expose the player to an increased likelihood of injury.

In a group of semiprofessional players (training daily), a moderate relationship has been reported between 1RM and countermovement jump (CMJ) (r = 0.50) and 15-m sprint time (r = −0.47), whereas only isometric knee extensor maximal force (performed in a seated custom made dynamometer) was correlated with CMJ and isokinetic peak torque knee extensor, albeit accounting for 22% of the common variance (68). The introduction of beginning strength training from an early age is considered to be important to the long-term development of a player (49). Consequently, practitioners should be aware of issues surrounding reliability and validity when using RM testing. For example, to provide a valid indicator of true strength, practitioners should endeavor to achieve 1RM within 4 consecutive attempts. This will reduce the confounding effect of fatigue (2). Also, as individuals increase their strength, they would have to be regularly re-evaluated to maintain the specific percent of the 1RM. Similarly, if the training modality changes (switching from machines to free weights or using another training facility), new equations would have to be applied for each exercise (75).

To overcome some of the issues surrounding 1RM testing, alternative methods that use calculations from a number of submaximal repetitions have been used as predictive estimates. The prediction of 1RM from submaximal tests seems to be good in principle (in general, correlation coefficients >0.90) (64), although these have not been pertinent to soccer players and studies have mostly failed to cross-validate prediction equations. LeSuer et al. (53) found that formulas most accurately predicted the bench press and squat but significantly underestimated the deadlift, despite high correlations. It must be accepted, therefore, that there will likely be a degree of error associated with estimates and that care should be taken when using such equations (64), particularly for soccer players who may not have the strength training experience.

There also seems a dearth of literature regarding other strength-power indicators (i.e., clean and jerk, snatch and derivatives) often used in the applied setting. The notion being that using these exercises can increase an athlete's performance by imitating sport-specific movements (73). Unlike traditional resistance exercises such as the back squat, performed at a relatively low movement speed, Olympic weightlifting offers a more explosive maximal movement (26). Although it is suggested that Olympic weightlifting exercises require more time for the learning of specific skills, it may be that such a greater skill complexity, compared with, for example, traditional jump training, facilitates the development of a broader physical abilities spectrum, which seems to be better transferred to performance (80). Research has shown the hang power clean to be better correlated to sprint performance than the squat in a group of semiprofessional Australian Rules Football players (44). Recently, Comfort et al., (17) reported the power clean to be reliable within (r ≥ 0.96) and between (r ≥ 0.98) sessions and established the SWC necessary to represent an adaptive response to training in rugby players proficient at performing the power clean. Although the power clean has also shown to be a reliable (ICC = 0.98) method of testing in youth (∼15 years) (26) and collegiate football players (ICC = 0.98) (55), to date, there seems no equivalent data for soccer players.

It is evident that further research is needed regarding the usefulness and application of Olympic weightlifting testing in soccer performance. Part of this should attempt to establish the cost-benefit ratio of performing these exercises, given the time demands of learning such complex exercises in high-level sport (80). Derivatives or alternatives to Olympic weightlifting (e.g., isometric midthigh hang pull) also warrant investigation. For example, some claim that isometric tests bear little resemblance to the dynamic nature of most sporting activities, whereas others highlight the relevance during such movements (36,74). It seems the understanding of the relationships between isometric and strength measures with functional performance tests in certain sports, such as soccer, remains rudimentary and may possibly underestimate its importance (75).

Anecdotally, practitioners often proclaim 1RM testing as being valuable to better understand the multijoint strength qualities of method of athletes. Yet, the literature is not representative of this with only 2 studies using this method in soccer players (14,69). Interestingly, these have been performed using young players (age range: 13–17 years) (Table 2). Clubs unwilling to make available to their competitors through research publication may explain the absence of information regarding elite senior level. Alternatively, the practitioners may simply avoid performing such testing because it may not be appropriate within the congested fixtures. Regardless of the circumstances, it seems, this method of testing is offers reliability values comparable with the more common CMJ (Table 2).

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Power

Testing power seems to be more commonly applied in real-life setting compared with that of strength. Countermovement vertical jump, in particular, can be considered one of the most featured tests used within soccer clubs; its importance is construed from its prevalence during a game (83). This is also seen within the literature (Table 2). The frequency of jumping within training and actual match play makes testing for this component slightly easier to sanction, compared with other tests such as dynamometry and RM. Being familiar with jumping, gained through training and match play, is sure to be a contributing factor to the favorable reliability results observed in research studies (Table 2). However, when performing a vertical jump test, and interpreting the test results, a number of factors need to be taken into consideration regarding its reliability (43). For example, studies (Table 2) have tended to report intraday reliability, the results of which have shown test performance to remain stable across a single day. However, it seems the coefficient of variation between trials conducted on consecutive days is likely greater than those on the same day (43). This can be seen by the favorable reliability results in Table 2. Therefore, it is possible that same day test-retest correlation may not account for both errors of measurement and temporal instability (39). Immediate test-retest measure performed on the same day may denote the time period lapsed as insufficient. Consequently, the second assessment may not actually be independent of the first (39).

To establish criterion validity, several studies have attempted to examine a relationship between vertical jump and isokinetic dynamometry. The results, however, are far from conclusive. For example, Augustsson and Thomee (7) showed a moderate correlation between vertical jump and isokinetic dynamometry (r = 0.51) and 1RM (r = 0.57). In contrast, research has shown that such tests are independent methods for the assessment of bilateral differences, meaning that single-joint, open-chain movement leads to different results from multijoint closed chain movements (59). Bilateral asymmetry, as identified by a vertical jump test, can be a consequence of either bilateral strength differences of correspondent muscle groups of the different limbs, strength differences of different muscles within the limb, or differences in movement technique. It seems likely that variability of movement and performance is a natural biological phenomenon, and bilateral asymmetries are a consequence of this variability (38). From this perspective, the methodological issue is that of distinguishing between natural and dysfunctional bilateral asymmetries. For instance, bilateral strength differences of the same muscle measured by isokinetic test might not be verified by a functional test. Other muscle groups might compensate these differences and highlight the relatively small relationship between scores on tests of muscle function and dynamic performance. Although such leg deficits may be considered statistically significant, they may not be clinically meaningful or relevant for practitioners (9). Jumping and propulsive forces (e.g., running and agility) are often generated in a unilateral fashion and may not correspond with acyclic movements in the vertical plane (62). Rather, it would seem of interest to identify a dynamic cyclic analysis separating the vertical and horizontal components of force as being of diagnostic value to the strength and conditioning practitioner or clinician (9). For example, Brughelli et al. (9) reported that athletes (Australian Rules football) with previous hamstring injuries had contralateral leg deficits in horizontal but not vertical force during running at submaximal velocities. This outcome highlights the need to gain an overall insight into the different patterns of human movement as valid indices of performance. Similar to this notion is the work of Impellizzeri et al. (47) who used a double-force platform to establish preliminary normative data useful for the diagnosis of abnormal bilateral strength asymmetry in athletes. A reference interval was calculated, of which values falling outside this interval −15% (left stronger) and 15% (right stronger) can be considered abnormal. However, it is unclear as to how threshold values were derived, and it remains questionable whether all individual players' injuries have the same threshold.

Although commonly performed within a test battery, it is not clear as to the effect of improving vertical jump may have on match performance. For example, some research has shown a significant relationship between team success (final league standing) and average jumping height (5). However, this seems somewhat simplistic and should not be considered causal. Within the lay community, one of the underlying assumptions is a linear relationship between the training-induced changes in the test scores and gains in exercise performance (61). Although an attractive proposition, this is unlikely to be the case, rather, it would seem that vertical jump performance is unable to discriminate players of different match performance (65). This tenet is advocated by the small to moderate correlation that vertical jump shares with very high-intensity activity (11).

Although power is considered a highly important factor for performance, further understanding regarding optimal efficiency and the possible transfer to performance, such as motor skills, is needed (57). That is, assessments may highlight a greater jump performance during a CMJ test, but players may not be able to “learn” how to use their increased strength and power during an open environment task. Thus, although it could be suggested that transference does occur, it is also reasonable to assume large increases in strength manifest in only minor, if any, changes in motor performance (57). Even if transference does occur, it cannot be expected that this instantaneously transfers to superior performance (75). Finally, it is evident (Table 2) that various derivatives of jump testing have been used. Although CMJ is the most commonly used, multiple jumps tests, reactive and static jumps, and CMJ with different weights are also prevalent among the literature. It is unrealistic to suggest that all these different approaches are feasible or indeed necessary. In some instances, they are likely to assess generic, rather than specific, qualities. Therefore, although all have shown to be reliable, it is the coaches' prerogative as to the most important parameters that will aid practice. Essentially, this should be reviewed in this regard as part of a multidisciplinary approach to testing.

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Conclusions

Testing strength and power can be performed using several different methods, each possessing their own index of performance as well as their own strengths and weaknesses. This review provides some practical guidelines and considerations for general and specific strength and power testing. As with any test, it is the practitioner's prerogative to ensure that the information collected can enhance practice and prove beneficial to the coach and player. There seems an appreciation that performing more frequent measurements may likely be more reflective of individual's response to a given playing and training load. It is always important to remember that strength and power characteristics constitute only 1 facet of performance and should be viewed in this regard and as part of a multidisciplinary approach; hence, testing should reflect this.

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Acknowledgments

The authors have received no funding for the preparation of this article and declare no conflicts of interest that are directly relevant to the content of this review.

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        Keywords:

        dynamometry; test; measurement; maximum; performance

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