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).
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).
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.
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.
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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