The development of sport performance testing assessing an athlete's physical abilities presents a unique challenge for the sports scientist, strength and conditioning coach, athletic trainer, and sport-specific coach. This challenge increases with sport complexity, where perhaps the greatest challenge is presented by highly dynamic, field-based sports, such as soccer, requiring a diverse mixture of skill, athleticism, and fitness. In particular, the need to gather sufficient information to effectively characterize physical performance with the amount of time allotted for testing is inherently challenging. A key component to achieving this balance requires limiting the redundancy of information between tests. Additionally, physical testing has been proposed to serve multiple purposes (5); these are as follows:
- Examine training effects
- Athlete motivation
- Objective feedback for the athlete
- Increase an athlete's awareness of training goals
- Evaluation of an athlete's ability or readiness to compete
- Evaluation of an athlete during and returning from rehabilitation
- Development and planning of programs
- Exercise prescription and identification of potential weaknesses.
Observationally, the above purposes are prioritized differently according to an individual's role within a team (e.g., strength and conditioning coach, sport-specific coach, athletic trainer, physical therapist, and sports scientist). To thoroughly address the aforementioned, a systematic approach to testing may be proposed regardless of the sport: (a) test selection, (b) test administration, (c) use of testing information, (d) retesting for the assessment of performance increases resulting from training, (e) testing for return-to-play assessment, and (f) data presentation (Table). The purpose of this article is to use this approach to develop a soccer-specific player profiling system that allows for a more complete assessment, comparison, and diagnosis of players' physical attributes.
PERFORMANCE TEST SELECTION: GENERALIZABILITY AND VALIDITY
Test selection begins with an assessment of sport-specific demands (physiological and biomechanical), followed by selection of tests. Matching of demands with testing begins with an understanding of the available tests and their associated validity and reliability. Specifically, how well the test reflects component(s) related to actual sport performance (20), how accurately the test assesses the physical attribute it is intended to measure (20), and finally, the reliability, or reproducibility, of the test must be considered (26). Ultimately, test reliability is highly dependent on the consistency of test administration. A short list of the primary factors that must be controlled for are as follows: environment (e.g., ambient temperature, testing surface, and motivation), time of day, warm-up, exercise sequence, and athletes' condition at the time of testing (e.g., maximal recovery from the previous training, hydration, and nutritional status) (14).
Based on its metabolic demands, soccer has been characterized as an intermittent endurance sport (3) where fatigue is observed in primarily 2 manners. The first is in the middle of a match after bouts of sprint and high-intensity work. This is reflected in the observation that sprints greater than 30 m result in greater recovery time than those of shorter duration (4) and that the 5-minute interval containing the greatest amount of high-intensity work was followed by a 5-minute segment at a lower than match average intensity (4,42). The second is as a function of soccer match duration. This is reflected in the observation that players cover less total distance and perform less sprinting in the second half compared with the first half (42) and that both running distance and intensity are decreased in the final 15 minutes relative to all previous intervals (43). The result is that physiological fitness testing must integrate 2 components. The first is the intermittent anaerobic endurance targeting the assessment of the ability to resist and recover from transient fatigue. The second is the aerobic endurance targeting the assessment of the ability to resist fatigue as a function of match duration.
Soccer's multidirectional and dynamic nature, incorporating as many as 1,346 changes (43), speculatively requires an individual to possess diverse characteristics of athleticism. For this reason, multiple characteristics are considered in test selection. Foremost, focus is placed on the physical attributes: sprinting, agility, power, and balance that characterize the actions most frequently contributing to noncontact injury in soccer (i.e., running, twisting or turning, jumping, and landing (24)). Because of moderate-to-high correlations between tests of strength, power, and sprint speed (52), limiting test redundancy is an important consideration. As these relationships have been linked to both movement specificity (37) and the duration of an action (52), great emphasis is placed on selecting ecologically valid tasks. Finally, unilateral testing is prioritized for the following reasons: (a) the highly unilateral nature in which soccer is played (12); (b) the dominant versus nondominant limb imbalances observed to increase with soccer playing experience (27,29,51); (c) the observation that limb asymmetry greater than 15% is associated with lower extremity injury (33,44); (d) the majority of injuries in sport affect a single limb; (e) finally, unilateral testing allows for the more effective development of return-to-play criteria via either comparison of injured and noninjured limbs, and/or comparison of preinjury to postrehabilitative performance.
Combined the outlined components of fitness and athleticism result in priority being placed on the following physical characteristics: intermittent endurance capacity, and where applicable unilateral testing of lower extremity strength, power, agility, and sprint speed.
PERFORMANCE TESTING: FITNESS
There are a number of tests that characterize anaerobic and aerobic capacity or power. The inclusion of an intermittent component simultaneously limits the available tests and is problematic because of the interplay between the aerobic and anaerobic systems. Specifically, the large contribution of the aerobic system to recovery from intermittent high-intensity bouts (21) makes isolation of aerobic and anaerobic characteristics difficult. The result is that emphasis is placed on selecting tests related to match performance where emphasis is placed on repeated high-intensity and/or sprint performance.
A primary means for analyzing the intermittent high-intensity performance has been tests of repeated sprint ability. These tests are appealing because they provide 2 quantitative indicators of performance: (a) total sprint time (i.e., the cumulative time required to complete all sprints) and (b) fatigue index (i.e., the decrement in performance from the first to last sprint) (2). However, these tests are accompanied by practical difficulties that make their use problematic. First, the decrement in repeated sprint performance is seldom linear with an increase or plateau observed with the increasing sprint trials (53). Second, reliability and validity of the test are at times challenged by the player. Specifically, where a maximal effort during each subsequent sprint interval is required, players also understand that they are being assessed on their performance decrement. This may result in greater inconsistency as players pace themselves. Third, regardless of the test selection (dependent on the test selected, either running distance or sprint time is measured), test administration may be logistically challenging in a team setting. Thus, where distance is the primary measure, a large number of testers are required to score performance, and when sprint time is measured, the number of players who may be effectively tested at one time is limited.
The majority of these shortcomings and practical problems are circumvented by the Yo-Yo intermittent recovery level 1 (YYIR1) and level 2 (YYIR2) tests. These tests consist of 20-m shuttle running performed at progressive running speeds, integrating a 10-second active recovery period between each consecutive shuttle run, with the pace controlled by a digital metronome (34). This allows for the effective application of testing in a team setting with relative ease and time efficiency. Additionally, research has confirmed the strength of the YYIR tests' reliability and validity. Specifically, the distance run in the YYIR1 and YYIR2 tests have demonstrated a coefficient of variation of 4.9% (34) and 9.6% (36), respectively. Compared with maximal graded treadmill tests, they have been shown to elicit maximal heart rates (HR) of 99 ± 1% in YYIR1 (34) and 98 ± 1% in YYIR2 (36).
This is a key element in the development and use of HR as a means for quantifying training load and exercise prescription. The YYIR1 has been correlated with physical match performance. In male professionals, it highly correlated with high-intensity running distance (r = 0.71), moderately correlated with combined high-intensity and sprint running distance (r = 0.58), and total distance run (r = 0.53) (34). In female professionals, it highly correlated with high-intensity running distance (r = 0.76) and high-intensity running distance during the final 15 minutes of each half (r = 0.83), and moderately correlated with total distance run (r = 0.56) (35). Finally, both tests are sensitive enough to detect differences between professional players of different levels and positions (42), as well as seasonal changes in fitness level corresponding to physical match performance (36). As the YYIR2 is formatted in a manner identical to the YYIR1, but uses a more rapid increase in running speeds, it is speculated that strong correlations to match performance would persist. Additionally, this difference allows for a maximal effort to be achieved by highly fit players more rapidly during the YYIR2, in line with the suggestions for maximal testing (7). For these reasons, the YYIR1 and YYIR2 tests are currently the most effective tests for assessing soccer-specific metabolic performance.
PERFORMANCE TESTING: ATHLETICISM
Prioritization of testing in soccer is placed on the lower extremity, with emphasis on characterizing the aforementioned components of athleticism: strength, power, agility, and sprint speed. Refinement of testing focuses on the following: (a) ecological validity, (b) limited redundancy among tests, (c) ability to assess return to play after injury or time off, and (d) ability to detect lower limb performance asymmetry.
Beginning with lower extremity strength, greatest emphasis is placed on selecting a test that is both relevant to soccer and unilateral in nature. The star excursion balance test (SEBT) is proposed for the following reasons. First, a large number of the technical movements in soccer involve multiplaner movements performed in a single-limb stance (i.e., passing, receiving, shooting). The finding that different SEBT reach directions result in differences in movement (47) and muscle activation of the hip and thigh (17) increases its appeal. Second, the SEBT has been shown to be sensitive to previous ankle injury (25), which is an important attribute as injury to the ankle in soccer contributes to 17–24% (1,24) of time loss injuries. Third, it is capable of detecting asymmetries that may contribute to future injury (44). Finally, the SEBT has demonstrated an intraclass correlation coefficient (ICC) as high as 0.95 when scoring is averaged across the 3 best performances (32).
The SEBT consists of the player standing in a single-leg stance at the center of an 8-pointed star (see Figure 1) with performance measured by the reach distance of the contralateral limb in the respective directions (32). Analyses of the SEBT have determined that performance is most effectively quantified by measuring reach distance in 3 directions: (a) anteromedial, (b) posteromedial, and (c) medial, with the hands placed on the hips, and following 4 practice trials in each direction (46). Although the SEBT may be categorized as a test of dynamic balance, observationally, the test is an amalgamation of balance, unilateral strength, coordination, and flexibility.
Selection of power tests is similarly focused, with emphasis placed on unilateral high-velocity movements. This leads to the possible selection of single-leg hop tests grouped as follows: (a) single-leg hop for distance, (b) timed single-leg hops, and (c) timed multiplaner single-leg hops. Although it may be suggested that vertical tests of power are more applicable in a sport involving jumping, this suggestion may be countered by findings that the number of jumping actions during a match is relatively small (3,6). Additionally, given the moderate-high correlation between tests of horizontal and vertical power (r2 = 0.695) (23), the tests appear to examine similar characteristics of lower extremity power. Power may be seen as being expressed specific to muscle action: (a) those incorporating stretch-shortening cycle (SSC) work (often termed reactive power), and (b) those incorporating concentric work (18). The ability to test both types of power provides insight into an athlete's ability to express power (39) and assists in training design by allowing for a more tailored approach (18).
Two unilateral tests of power are proposed. For reactive power, the triple hop for horizontal distance (Figure 2) is proposed because of its high test-retest reliability (ICC = 0.97; standard error of the mean [SEM] = 11.17 cm) relative to crossover hopping for distance (ICC = 0.93; SEM = 17.74) and 6-m hopping for time (ICC = 0.92; SEM = 0.06 seconds) (47). For concentric power, a single-leg hop for distance performed in a concentric manner, as outlined by Booher et al. (9), is proposed (ICC = 0.97; SEM = 5.93 cm). Specifically, the athlete begins in a static single-leg squat position and performs a single maximal horizontal hop.
Sprinting comprises a relatively small amount of a soccer match. This is reflected by a number of observations that sprinting contributes to less than 2.5% of the total distance run (15). Less than 1% of the total match duration is spent sprinting (6). The total number of sprints is relatively small and ranges from 3 to 40 bouts (15). Although sprinting reflects a relatively small amount of the work performed, observationally, sprinting is frequently incorporated in the most decisive action of a match and is therefore of paramount importance. As the majority of sprints in soccer are relatively short in duration and distance (1.7 to 2.1 seconds (6) and 19.3 ±3.2 m (15), respectively), this should be reflected in testing. Additionally, given the aforementioned number of changes that occur during a match, emphasis is placed on the capacity for testing to discriminate between the different qualities of speed. Thus, if possible, testing should allow for the assessment of (a) starting speed (0–10 m), (b) acceleration speed (10–20 m), and (c) composite speed (0–20 m) (Figure 3) (22).
The role of agility in soccer is evident in the 608 to 822 changes in direction that are observed in a match (8). Recently defined as “a rapid whole-body movement with change of velocity or direction in response to a stimulus (48),” the addition of cognitive components may be problematic when trying to assess the purely physical components. For the purpose of the current article, agility will be defined exclusive of the proposed cognitive components. The existence of agility as a discrete athletic attribute is evident in consistent low correlations with performance tests of speed, strength, and power (11,19,38,48). Examination of multiple agility tests resulted in the observation that agility persists as a “complex motor ability (49).” Furthermore, the incorporation of complex changes in movement is demonstrated by the finding that a “general agility factor (49)” is characterized most highly in tests requiring the largest changes in direction. The isolation of agility performance is suggested to decrease when a large number of changes in direction are included (16). This is demonstrated through increasing correlation among tests incorporating multiple cutting actions with sprint performance: 1) the Illinois agility test, which incorporates 7 cutting actions and covers a total distance of 36.6 m (9.1 m: r = 0.61; 18.3 m: r = 0.68; 27.4 m: r = 0.71; 36.6 m: r = 0.59), 2) the proagility test, which incorporates 2 cutting movements and covers a total distance of 18.3 m (9.1 m: r = 0.59; 18.3 m: r = 0.65; 27.4 m: r = 0.66; 36.6 m: r = 0.59) (50), and 3) L-run, which incorporates 3 cutting actions and covers a distance of 20 m (5 m: r = 0.57; 10 m: r = 0.64; 20 m: r = 0.73) (19).
In contrast, the 505 test, which incorporates a single unilateral cutting action performed in 15-m shuttle run fashion (Figure 4), has demonstrated no correlation (16) to low correlations with sprint speed (5 m: r = 0.52; 10 m = 0.57; 20 m = 0.58) (19). Consistent with tests of strength and power, the unilateral nature of the 505 allows for the more effective assessment of return to play and existence of limb asymmetry. This is of particular importance given the aforementioned contribution of turning and twisting actions to noncontact injury in soccer (24).
The final component of the testing battery is the use of qualitative assessments of movement. Bridging the gap between subjective (qualitative) and objective (quantitative) testing is the functional movement screen (FMS). Proposed as an assessment of fundamental movement abilities (41), the FMS consists of 7 tests: 1) deep squat, 2) hurdle step, 3) in-line lunge, 4) shoulder mobility, 5) active straight leg raise, 6) push-up, and 7) rotary stability; it uses a 3-point scoring system (41). Although discussion of the testing protocol and corresponding basis for the point system is beyond the scope of the current article, it is clear that the scoring criteria allows for the development of an objective measure of movement ability. In this regard, FMS scoring has shown strong interrater reliability, with agreement between “novice” and “expert” raters excellent in 14 of the 17 criteria examined during testing (41).
The FMS has demonstrated an ability to predict potential injury in football, where a preseason score of less than 14 was associated with an 11-fold increase in the likelihood of injury (31), as well as its sensitivity to training intervention (30). Although research is limited to the deep squat assessment, the FMS scoring system appears to effectively capture the biomechanical differences that exist between scoring groups in comparison with 3-dimensional analyses (10). The unilateral nature of 5 of the 7 tests and subsequent ability to capture limb asymmetry further enhances the appeal of the FMS. An additional strength of the testing is the potential subjective analyses that, when combined with an understanding of the anatomical relationships to movement, may allow for potential tailoring of exercise prescription.
DATA ANALYSIS AND PRESENTATION
This section will address maximizing the use of data to better inform potential decisions of the sports scientist, strength and conditioning coach, athletic trainer, and sport-specific coach. The primary purpose is to discuss how to use and display data to characterize the individual player within the team and by position, to identify potential limb asymmetry, and to monitor changes over time. For this reason theoretical data was developed to illustrate the different manners that information may be displayed to exemplify potential differences within an individual, group of players, and a team.
Analysis of team performance begins with the tabling of results, general analysis, and, if possible, comparison of test results with research. Thereafter, analysis within the team continues via calculation of standardized scores both within the team and/or positional groups. This allows for the following group comparisons to be made: (a) overall performance or ranking of the individual within the team (Figure 5) and (b) comparison of individual players by position (Figure 6). In the first, a player's position relative to their teammates is immediately evident. In the second, comparison by position may highlight physical characteristics between players, which may otherwise go unnoticed.
A theoretical comparison of central midfielders available on a team (see Figure 6) shows that player 13 is the most highly fit but has diminished speed, SSC power, and agility relative to players 9 and 10. A comparison of players 9 and 10 reveals that player 9 performed more highly in acceleration and total sprint performance (sprint and maximal), and SSC power. However, player 10 demonstrated the greatest starting speed, agility, and unilateral strength and balance. In this manner, analysis of the individual by team or position may highlight a player's weaknesses and strengths relative to their teammates. Although these interpretations must be made in light of a player's other soccer-specific qualities, they may have implications for personnel selection and potentially team tactics. Ultimately, the use of test results in this manner provides an objective measure to what may otherwise remain subjective observation(s).
Analysis of the individual player focuses on 1) examining limb asymmetry, 2) information to more effectively prescribe training, and 3) examining longitudinal change. Calculation of limb asymmetry may be done via the following equation:
followed by the assignment of a negative sign to the value if the player's nondominant limb is stronger and a positive sign if the dominant limb is stronger (28). The result is that limb asymmetries in strength, power, agility, and FMS score are expressed as percentage difference and readily expressed graphically (see Figure 7). The diagnosis of differences in this manner allows for a more discriminating look at a player's physical attributes. For instance, specific to Figure 7, the player demonstrates a consistent, low-level, dominant limb asymmetry, except for in triple-hop performance (SSC_POWER_asymm) where the nondominant limb performed to a higher level. This type of discrepancy may highlight the need for further examination via medical personnel to determine if a potential problem exists.
Several tests allow for more discriminative analyses that potentially enhance exercise prescription and more effective monitoring of training effects. Regarding the metabolic testing, YYIR1/YYIR2 may be used to establish maximal HR via the use of HR monitors, allowing for refined monitoring and prescription of training load. Additionally, maximal running speed may be effectively used to prescribe running intensities relative to soccer match performance (13). Two tests of unilateral power were proposed, each focusing on the expression of power relative to specific muscle actions (SSC versus concentric). Although the field basis of these tests does not allow for the direct calculation of reactive strength index (39), the difference between SSC and concentric power may be estimated as follows:
Calculations of reactive strength index testing may be compared with previous research, where the mean difference observed is 12.1% (40). Additionally, comparison over time may provide insight into training adaptation(s) and enhance training prescription abilities and diagnosis. Finally, although discussion of the FMS has thus far pertained mainly to its use as a scoring system, the subjective observations made when testing may be used to further tailor movement training. The effective use of the FMS in this manner is reflected by the effectiveness of a “standardized” program in increasing FMS score (30).
A discriminative approach to return to play after injury is inherent in the proposed player profiling system. Specifically, the following traits are addressed to allow for the progressive assessment of recovery during the latter stages of injury rehabilitation: 1) muscle action, 2) movement plane, and 3) movement velocity. A phasic approach to the assessment of player recovery may thus be proposed. Initial application of testing is performed as a player recovers and consists of movement (FMS) and balance (SEBT) tests. Similarly, each of these tests may be progressed across the lower extremity; FMS testing progressing from bilateral (deep squat) to split (in-line lunge) to single-leg (hurdle step), and SEBT by movement plane (injury-dependent) from anteromedial to medial and posteromedial. Increasing recovery is accompanied by increasing test demand. This progression may begin with the integration of the single-leg hop test for distance, followed by sprint and triple-hop testing, and finally 505 agility testing as it incorporates multiplanar movement performed at maximal velocity. A theoretical model of this progressive approach may be seen in Figure 8, where tests of increasing demand are incorporated as the individual recovers. Ultimately, the progression and timing of testing relative to injury is dependent on the individual injury and must be addressed by and in cooperation with the medical staff.
The ability to capture and analyze longitudinal change at the individual level and team level is of paramount importance: first, to assess return to play after injury and second, to assess the effects of training. Longitudinal change may be expressed as the percent change relative to any testing interval (e.g., baseline, peak). Applying this to the development of return-to-play criteria, physical performance during the rehabilitation phase may be quantified relative to a player's performance at any previous testing interval. The theoretical model presented in Figure 8 allows for a quantitative and progressive assessment of the player's recovery from injury. For instance, there is a progressive increase in the performance in selected measures from weeks 6 through 10, with the final phase of testing (week 11) being characterized by performance values in excess of 90% of baseline. Ultimately, what is an acceptable level of performance for return to play is unknown and likely needs to be judged on an individual basis. A benefit of the proposed testing profile is the ability to track a spectrum of physical characteristics of increasing demand (i.e., muscle action, movement plane, movement velocity) allowing for a more educated assessment of player function and therein the likelihood of reinjury. Finally, as a key element of testing is to assess training effects, longitudinal performance may be displayed as percentage change between the testing periods:
Alternately, longitudinal change may be expressed graphically in the same manner, as previously discussed in Figure 6 or Figure 8.
The primary purpose of this article was to develop a comprehensive testing system for soccer. Strong emphasis has been placed on selecting discriminative and progressive tests focusing on physical performance, exercise prescription, limb asymmetry, and return-to-play criteria. A secondary focus has been on maximizing testing information and subsequent use of information to effectively integrate the disciplines within sports science. Specifically, the varying roles of a team's staff (coach, strength and conditioning coach, athletic trainer, and physical therapist) have been addressed. The level of integration proposed addressed the following key factors relative to each person's role: (a) player comparison(s), (b) time-related changes in performance, (c) exercise prescription, (d) potentially problematic limb asymmetries, and (e) return-to-play criteria after injury.
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Keywords:© 2012 National Strength and Conditioning Association
performance testing; lower extremity rehabilitation; limb asymmetry; return from injury; return to play; return to competition; field-based testing