Soccer performance is a complex concept, with multiple factors interacting, including physiological, psychological, tactical, and technical aspects. Physical demands vary between playing positions [1
] and performance levels [2
], and assessments of physical characteristics have, therefore, been used for talent identification [3
], individual fitness profiling [4
], and to identify injury risk factors [5
]. Consequently, it is of importance that coaches and clinicians can assess and evaluate the physical status of a player or team based on comparable context and position-specific normative data. To date, several studies have investigated the physical characteristics of players in elite European leagues [4
]; however, data to describe elite soccer players in the Arabian Gulf states are sparse.
Equivocal evidence is available to show a reduced range of motion (ROM) or strength as injury risk factors [10
], while maximal strength and jump performance has been correlated to sprint performance [16
], which again is considered important in match decisive playing situations [17
]. Although recent studies have suggested that a single musculoskeletal screening test cannot predict which professional soccer players will go on to sustain a future injury [18
], these tests can provide value in terms of detecting current problems and establishing non-injured baseline values [20
]. Including these tests as part of a pre-participation evaluation could, therefore, provide practitioners with relevant performance indications that can guide training and rehabilitation programs.
Aerobic, anaerobic, and anthropometric characteristics have been reported in previous literature for professional players in the United Arab Emirates (UAE) [21
]. Although position-specific values were provided, the study was limited to a selection of physiological characteristics, while excluding strength, range of motion, and jump performance test variables. Physiological profiles have also been reported for national team players of Saudi Arabia [22
] and Kuwait [23
]; however, these studies do not report position-specific or pre-participation musculoskeletal screening data and were performed more than two decades ago. Information on anthropometric values can also be found for elite soccer players in Kuwait [24
] and Bahrain [25
], although the former study included a considerably small sample with measures used as baseline values to examine the effects of fluid loss following soccer match play and the latter is, again, more than two decades old and does not include measures of physical function.
In Qatar, normative values for professional soccer players have been reported for hip strength and range of motion [26
], and strength and flexibility have been examined in studies assessing injury risk factors [5
]. In these studies, the authors did not report or compare position-specific values, nor did they include measures of jump performance. Normative position-specific strength and range of motion values have important implications for injury management and rehabilitation. Ascertaining whether an athlete has regained muscular function or range of motion compared to the non-injured side is a commonly used outcome of rehabilitation; however, investigating whether the athlete has returned to baseline values is also an important component of rehabilitation outcomes. Measures of jump performance are routinely used to monitor physical performance and fatigue [28
] and have recently been included as part of pre-participation injury risk screening in elite youth soccer players [30
Establishing population and position-specific normative performance values will allow practitioners to more accurately identify deficiencies in performance that are relative to their athletes. Given the ever-growing popularity of soccer in the Arabian Gulf states, in conjunction with the upcoming FIFA World Cup in Qatar 2022, the need for a comprehensive profile of the physical characteristics displayed by players competing at the elite level within this particular region is highlighted. This study aimed to examine positional differences in commonly used clinical and performance tests in players participating in the Qatar Stars League (QSL).
2. Materials and Methods
2.1. Experimental Design
Professional soccer players from the QSL attended Aspetar Orthopaedic and Sports Medicine Hospital in Doha, Qatar, during the pre-season or early competition period of the 2017–2018 season, as part of a FIFA-compliant annual periodic health evaluation (PHE). The PHE involved laboratory blood analysis, dental and cardiac examination amongst others. In addition, players engaged in a musculoskeletal screening assessment which consisted of range of motion, strength, and dynamic screening tests.
One hundred and ninety-five professional soccer players registered at clubs competing in the QSL were recruited to participate in this study. The QSL is the highest level of professional soccer in Qatar, consisting of twelve teams, where the top three clubs qualify for either the group stage (winner) or play-off rounds (second and third) of the Asian Football Confederation (AFC) Champions League. Demographic information is displayed in Table 1
. Inclusion criteria required players to be registered as a professional soccer player in the QSL, free from current injury at the time of testing, and currently participating in full soccer training. All participants provided informed consent before data collection, and ethical approval was obtained from the Anti-Doping Lab Qatar Institutional Review Board (IRB: E2013000003). For players under 18 years, informed consent was obtained from parents or guardians.
Participants were requested to eat according to their normal diet in the day preceding the assessment and then refrain from eating and drinking substances other than water one hour before. Upon arrival, participants were provided with appropriate explanation and demonstration of all procedures. Additionally, athlete-reported playing position and leg dominance were recorded. Positional groupings were goalkeepers (GK), defenders (DEF), midfielders (MID) and strikers (STR), and the dominant leg was defined as their preferred kicking leg for a penalty kick. Anthropometric information was recorded before players completed a standardized warm-up consisting of 10 min of cycle ergometry. For all strength and jump tests, warm-up trials were provided to familiarize each participant and to ensure technical competence during the test. The tests were performed in a standardized order, with jump performance tests preceding strength measures.
2.3.1. Range of Motion
Hip range of motion was measured using the bent knee fall out (BKFO) test and internal rotation (IR) in a position of 90° of hip flexion as described previously [10
]. BKFO was measured as the distance between the most distal point on the head of the fibula and the surface of the plinth using a tape measure, while IR in 90° hip flexion was measured using a manual goniometer. A single test was used for the BKFO distance, and three repetitions were used for hip IR with the average score recorded.
To determine hamstring flexibility, a passive knee extension test (PKET) was performed as previously described [32
]. The athlete positioned their hip in maximal flexion by clutching the thigh to their chest, and the contralateral leg was fixed in place by the assessor. The maximal angle was measured using a handheld inclinometer (Digital Level Box JY-180ALB, Grand Index, Hong Kong) positioned on the tibia.
Ankle dorsiflexion was measured using the weight-bearing lunge test with the athlete facing a wall in a standing position [33
]. Range of motion was recorded using a tape measure and determined as the maximum distance away from the wall that the knee of the lead (testing leg) was able to touch during the forward lunge. Participants were instructed to maintain heel contact with the ground throughout the test. All testing was completed with the participants barefoot, and measures were recorded to the nearest 0.1 cm.
2.3.2. Jump Performance
Jump performance was assessed through countermovement jumps and a single-leg 10-s Hop test. For countermovement jumps, participants began either in a bilateral (CMJ) or a unilateral (SLCMJ) stance on a force plate (Force Decks v1.2.6109, Vald Performance, Albion, Australia) with their hands on hips and opposite hip flexed at 90°. Instructions were to perform a countermovement by dropping into a quarter squat and then immediately triple extending at the ankle, knee, and hip in an explosive concentric action. Bending of the knees while airborne was not permitted, and participants were required to repeat the test if this occurred. Jump height was calculated from the athlete’s flight time and recorded to the nearest 0.1 cm. All data were recorded at a sampling rate of 1000 Hz. Three trials were completed with a 30 s rest period between each repetition.
For the single-leg 10 s Hop test (10 s Hop), a series of repeated single-leg jumps were performed for a period of 10 s in accordance with previous guidelines [34
]. The test began with participants completing a rapid countermovement into a quarter squat followed by a maximal vertical jump. This action was replicated across the test capture period. Instructions were to jump as high as possible, minimize ground contact time while landing under control and try to maintain the same footprint, remaining facing forwards during the entirety of the test. One trial was completed on each leg. An optical measurement device (Optojump, Micrograte, Bolzano, Italy) was used to quantify average jump height and the reactive strength index (RSI) derived from flight time divided by ground contact time [35
Maximal strength was measured using isokinetic tests, the Nordic hamstring exercise and hand-held dynamometers. Quadriceps (knee extension) and hamstring (knee flexion) strength profiles were measured using an isokinetic dynamometer (Biodex Medical Systems, Shirley, New York, USA). Players were seated in a position with the hip flexed to 90° with all procedures replicating those of previous research [5
]. Assessment modes included five repetitions of concentric knee flexion and extension at 60 deg/s (QCon60 and HCon60) and five repetitions of eccentric knee extension at 60 deg/s (HEcc60) with the highest peak torque value recorded, in addition to the functional hamstring-to-quadriceps ratio (H:Q = HEcc60:QCon60). A minimum of 60 s of rest was provided between each contraction mode. Before each isokinetic test, participants were instructed as to the mode and procedure of the specific test and provided with appropriate practice repetitions. During testing, vigorous verbal encouragement was provided by the assessors.
Participants performed the Nordic hamstring exercise test on the NordBord (Vald Performance, Albion, Australia) in a kneeling position, chest and hips extended, arms across their chest and ankles secured by individual ankle braces which are attached to uniaxial load cells. Players were instructed to lower their body as slowly as possible to their maximum depth or until they achieved a prone position. After catching their fall, each player pushed themselves back to the start position to minimize concentric knee flexor activity. Three trials were completed with a 30 s rest period between each repetition. In addition to absolute and relative force values, the percentage of body mass-expected eccentric strength was calculated (Body mass-expected eccentric strength (Nm) = 4 × BM (kg) + 26.1), as described by Buchheit et al. [36
Eccentric hip adduction (ADD) and abduction (ABD) strength were evaluated with the athlete in a side-lying position using the break test method in accordance with previously published protocols [26
]. Strength values were quantified using a hand-held dynamometer (PowerTrack II Commander, JTECH Medical, Midvale, UT, USA). Three repetitions were performed on each side with a 30 s rest period between trials, and raw force values (N) were reported.
2.3.4. Statistical Analyses
Descriptive statistics (mean ± standard deviation (SD)) were calculated for each variable. Normality of the data was assessed using visual inspection and the Shapiro–Wilk test. A one-way analysis of variance (ANOVA) was used to examine for any positional differences in each physical quality, applying a Bonferroni post-hoc correction. Homogeneity of variance was tested via Levene’s test. All data were computed through Microsoft Excel®
2010. The ANOVA and tests for normality and variance were processed using SPSS®
V.22 (IBM, Chicago, IL, USA) with the level of statistical significance set at alpha level p
≤ 0.05. Cohen’s d
effect sizes were calculated to interpret the magnitude of between-group differences using the following classifications: standardized mean differences of 0.2, 0.5, and 0.8 for small, medium, and large effect sizes, respectively [38