Reduced Match Exposure in the Previous 2 Matches Accounts for Hamstring Muscle Injury Incidence in Professional Football Players.

BACKGROUND: Hamstring strain injuries are one of the most prevalent injuries in football (soccer). We examined the influence of accumulated match-play exposure on the occurrence of hamstring strain injury in professional football from 2 teams (Spanish 1st Division, LaLiga) over 3 seasons, and determined specific cut-off points as indicators of injury risk. HYPOTHESIS: Overloaded players would be more likely to sustain a hamstring injury. STUDY DESIGN: Prospective, controlled, observational study. LEVEL OF EVIDENCE: Level 2b. METHODS: Playing time, total running distance, and high-speed running (>24 km/h) distance during official matches of players that sustained a hamstring injury were compared with uninjured, paired controls. Cumulative playing time and running performance of 4 matches before the injury was computed. Relative risk (RR) of injury occurrence was estimated by generalized estimating equations. Diagnostic accuracy was determined by receiver operating characteristics and the area under the curve. RESULTS: Thirty-seven hamstring strain injuries occurred, representing 23 ± 18 absence days per injury. Thirty-seven controls (uninjured players) were used as comparators. Low match-play exposures during 1 and 2 matches before injury were likely to explain injury occurrence (RR: 14-53%; P < 0.01). Metrics from the match before the hamstring muscle strain demonstrated the best accuracy to predict injury occurrence: high-speed running distance ≤328 m (sensitivity, 64%; specificity, 84%), playing time ≤64 min (sensitivity, 36%; specificity, 97%), and running distance ≤5.8 km (sensitivity, 39%; specificity, 97%). CONCLUSION: Relatively reduced competitive exposure in the previous 2 matches was associated with higher hamstring injury risk in professional football players. CLINICAL RELEVANCE: Screening simple metrics such as the accumulated match exposure during official matches and considering specific cut-off points for some running variables may be good indicators of injury risk and may assist in better individual injury management in professional soccer players.

Effects of Direction-Specific Training Interventions on Physical Performance and Inter-Limb Asymmetries.

This study analyzed the effects of two different training programs on functional performance and inter-limb asymmetries in basketball players. Twenty-four elite youth basketball players were randomly assigned to a training program including variable unilateral horizontal movements (VUH, n = 12) or unilateral lateral movements (VUL, n = 12). Eccentric-overload training (EOT) was performed twice a week for a six-week period. Functional performance assessment included a countermovement jump test, unilateral multidirectional jumping tests (i.e., lateral, horizontal, and vertical), a rebound jump test, a limb symmetry index, a 25 m linear sprint test, and several change of direction (COD) tests. Within-group analysis showed substantial improvements in almost all functional tests in both groups (ES = 0.35-0.89). Furthermore, almost all jumping asymmetries were improved in both groups (ES = 0.38-0.69) except for vertical jumping asymmetry in VUL (ES = -0.04). Between-group analyses showed a substantial and possibly better performance in vertical jumping asymmetry and 5 m in VUH compared to that of VUL, respectively. In contrast, lateral jumping with left (ES = 1.22) and right leg (ES = 0.49) were substantially greater in VUL than in VUH. Specific force-vector training programs induced substantial improvements in both functional performance tests and inter-limb asymmetries, although greater improvements of lateral and horizontal variables may depend on the specific force vector targeted.

Knee Flexor Eccentric Strength, Hamstring Muscle Volume and Sprinting in Elite Professional Soccer Players with a Prior Strained Hamstring.

The aim was to determine if players with a prior hamstring strain injury (HSI) exhibit bilateral deficits in knee flexor eccentric strength and hamstring muscle volume and differences in sprinting performance compared with players without a history of HSIs. Forty-six male professional soccer players participated in this study. Eccentric knee flexor strength, hamstring muscle volume (MRI), and a 20-m running sprint test (5- and 10-m split time) were assessed at the start of the preseason. Eccentric knee strength of the previously injured limbs of injured players was greater (ES: 1.18-1.36) than the uninjured limbs in uninjured players. Previously injured limbs showed possibly larger biceps femoris short heads (BFSh) and likely semitendinosus (ST) muscle volumes than the contralateral uninjured limbs among the injured players (ES: 0.36) and the limbs of the uninjured players (ES: 0.56), respectively. Players who had experienced a previous HSI were possibly slower in the 5-m (small ES: 0.46), while unclear differences were found in both the 10-m and 20-m times. Players with a prior HSI displayed greater eccentric knee flexor strength, possibly relatively hypertrophied ST and BFSh muscles, and possibly reduced 5-m sprinting performances than previously uninjured players. This can have implication for the design of secondary hamstring muscle injury prevention strategies.

Under-exposure to official matches is associated with muscle injury incidence in professional footballers.

External workload from matches is considered one of the most important muscle injury risk factors for football teams. However, there is scarce published evidence to support this belief. This study examined whether a particular profile of external match workload existed prior to a muscle injury. A total of 144 professional football players belonging to 2 teams were monitored over three seasons. For each muscle injury, a profile of external workload variables was determined for 5 to 8 games and expressed as: time playing exposure, total distance (TD) covered and high-speed running (HSR) covered. In addition, acute:chronic workload ratio (ACWR) was calculated. Sixty players (41.6%) reported a total of 86 muscle injuries during the three seasons. Muscle injuries occurred principally in matches (79.1%), the hamstring being the most affected muscle (44.1%). Injured players displayed substantially lower accumulated exposure time (ES = 0.45), TD (ES = 0.45) and HSR (ES = 0.39) in comparison with uninjured players in the last 5 games prior to injury. Compared to the uninjured players, ACWR for exposure (ES = -0.29/0.02) and running load (ES = -0.24/0.00) did not differ between match 5 and 2 prior to the injury, although uninjured players displayed a substantially greater ACWR in all 3 variables (ES = 0.31/0.35) than injured players in match 1 prior to the injury. Lower playing exposure (minutes played) and associated reduced running distances (TD and HSR) were observed in injured football players. Being under-loaded in official games could be a mediator for muscle injury in this cohort of elite football players.

Optimal Pretaper Phase on Physical Match Performance in Professional Soccer.

PURPOSE: To investigate the optimal pretaper duration on match running performance in a professional soccer team. METHODS: The training load was monitored during daily training sessions and matches during 2 seasons according to different periodization strategies. Matches' running distances were collected using match analysis system. The data were analyzed in 3 types of mesocycle blocks of 5 (M5), 4 (M4), and 3 weeks (M3), concludes all of them by 1 taper week. RESULTS: Significant decreases in the training load during the taper weeks compared to standard weeks were observed in 3 types of mesocycle blocks (d ≥ 5; P < .01). An increase in overall match running performance was observed in matches played after the taper weeks compared to matches played after the standard weeks during M4 for all speed ranges (d ≥ 1.3; P < .05). The increase was only observed in low-intensity running (d = 1.3; P < .04) and total distance, low-intensity running, and intense running (d ≥ 1.3; P < .05) in M5 and M3, respectively. Match running performance following the taper weeks between the 3 different mesocycle durations was significantly higher in M4 for the number of high-speed running, sprinting, and high-intensity running (P < .05). The greatest enhancement of match running performance was observed at M4 when the training load was decreased by approximately 18% during the tapering period. CONCLUSION: This study suggests that a period of 3 standard weeks of training followed by 1 taper week is the optimal taper strategy when compared to different pretaper durations.

Applying a holistic hamstring injury prevention approach in elite football: 12 seasons, single club study.

The aim was to investigate the preventive effect of a complex training program based on holistic hamstring health understanding in elite professional soccer players. This study involved an elite club in Europe and was conducted over 12 seasons. The last 2 seasons were the intervention period, and the others were the control seasons. During the intervention period, players performed a complex program organized into different interventions throughout the week having as a priority the player health. Hamstring injuries, absenteeism, injury rates, and injury burden between the control and intervention seasons were compared using a rate ratio (RR) with 95% CI. Players had a mean exposure of 333.5 ± 18.6 hours per season with no significant differences between the intervention and control seasons. The overall injury rate was 3 times lower during the two intervention seasons than during the previous seasons (P < .01); the match injury rate was 2.7 times lower (P < .01) and the training rate 4.3 times (P < .01). Injury burden was almost 4 times lower during the two intervention seasons than during the previous seasons (P < .01), and recurrences in the control group were 10% vs 0% in the intervention group. Hamstring injuries were reduced ~3 times during the seasons in which elite football players were exposed to multicomponent, complex prevention training with individual approaches based on player needs, management of training load, individualized physiotherapy treatment, and planned staff communication, in comparison to the control seasons without a clearly defined and structured injury prevention intervention.

Training-induced changes in anterior pelvic tilt: potential implications for hamstring strain injuries management.

An association has been reported between dynamic anterior pelvic tilt (APT) and hamstring injuries; however, no research has examined if a training-based preventive intervention could alter APT. Therefore, the aim of the present study was to examine if a specific 6-week multimodal intervention, based on the theoretical influence of neighbouring joints and biomechanical interactions between muscles that are inserted to the pelvis, induced changes in APT, during walking gait, hamstring flexibility and trunk endurance. Thirty-five active healthy males volunteered for this single-blind controlled trial and were split into two groups based on baseline data: a control group (CG, n = 20, continued their normal physical activities), and an intervention group (IG, n = 15, performed the intervention programme for 18 sessions over 6 weeks). A significant (p = 0.001) decrease in the APT kinematics during gait, significant increase in the Active Knee Extension Test (p = 0.001), and a significant increase in trunk endurance performance for flexion (p = 0.001), extension (p = 0.001) and side bridge (p = 0.001) were observed, in IG after the 6-week programme, compared to CG.

Inertial flywheel knee- and hip-dominant hamstring strength exercises in professional soccer players: Muscle use and velocity-based (mechanical) eccentric overload.

The primary aim of the present study was to analyze mechanical responses during inertial knee- and hip-dominant hamstring strengthening exercises (flywheel leg-curl and hip-extension in conic-pulley), and the secondary aim was to measure and compare regional muscle use using functional magnetic resonance imaging. Mean power, peak power, mean velocity, peak velocity and time in the concentric (CON) and eccentric (ECC) phases were measured. The transverse relaxation time (T2) shift from pre- to post-exercise were calculated for the biceps femoris long (BFl) and short (BFs) heads, semitendinosus (ST) and semimembranosus (SM) muscles at proximal, middle and distal areas of the muscle length. Peak and mean power in flywheel leg-curl were higher during the CON than the ECC phase (p<0.01). ECC peak power was higher than CON phase (p<0.01) in conic-pulley hip-extension exercise, while mean power was higher during the CON than ECC phase (p<0.01). Flywheel leg-curl showed a higher T2 values in ST and BFs and BFl (p<0.05), while the conic-pulley hip-extension had a higher T2 values in the proximal region of the ST and BFl (p<0.05). In conclusion, ECC overload was only observed in peak power during the conic-pulley hip-extension exercise. Flywheel leg-curl involved a greater overall use of the 4 muscle bellies, more specifically in the ST and BFs, with a selective augmented activity (compared with the conic-pulley) in the 3 regions of the BFs, while conic-pulley hip-extension exercise selectively targeted the proximal and medial regions of the BFl. Physiotherapists and strength and conditioning coaches should consider this when optimizing the training and recovery process for hamstring muscles, especially after injury.

No relationship between the nordic hamstring and two different isometric strength tests to assess hamstring muscle strength in professional soccer players.

OBJECTIVES: To investigate the relationship between one eccentric and two isometric tests commonly used to assess hamstring strength in professional soccer. SETTINGS: Professional soccer club. DESIGN: Cross-sectional study. PARTICIPANTS: Twenty male professional soccer players. MAIN OUTCOME MEASURES: Hamstring force was quantified during 3 tests (Nordic hamstring eccentric, 90:20 isometric posterior-chain strength and isometric 15° knee flexion) using a load cell, a handheld dynamometer and a force platform, respectively. RESULTS: Poor relationships and low concordance were observed between isometric and eccentric tests, showing different ability to discriminate hamstring weakness and asymmetries. The Nordic hamstring test identified between-limb asymmetry >15% in 30% of the players, 25% in the 15° knee flexion and 5% in the 90:20 test. All players obtained different results in the three tests with only one showing asymmetry >15% in two tests. CONCLUSION: Results obtained in each test cannot inform the others. Mechanical outputs of these tests must be used for particular purposes during the prevention, rehabilitation and monitoring process of hamstring injury. Isometric testing might be preferable during the initial phases of the recovery process of a hamstring injury while the Nordic hamstring test might be used in the last stages or detect muscle weakness/asymmetries in non-injured players.

Bilateral Deficit and Bilateral Performance: Relationship with Sprinting and Change of Direction in Elite Youth Soccer Players.

The purpose of the study was to examine the differences in bilateral deficit (BLD) at different loadings during the half-squat jump (SJ) and horizontal countermovement jump (HCMJ) to determine if there is a relationship with linear sprint or change of direction (COD). The second goal was to check if fast players were more powerful in SJ and HCMJ than slow players in bilateral performance (BP). Twenty-seven male youth soccer players participated in the study. Players were divided in two groups, faster and slower, according to their sprint performance (10 and 40 m). BLD average power with body weight (BW) and 25%BW were significantly higher than 50%BW (p < 0.01). BLD during HCMJ was significantly higher than BLD during SJ with BW, 25%BW and 50%BW (p < 0.01). There were no statistical relationships between BLD and sprint or COD performance (p > 0.05). Fast players showed significantly higher SJ power with all the different loads and HCMJ than slow players (p < 0.01), and fast players lost more time executing COD-90° than slow players (p < 0.01). There were no statistical differences between fast and slow players in BLD. BLD seems to be dependent on motor task, contraction type and load and could not be a proper measure to estimate sprint and COD performance. Faster players are confirmed to be more powerful players than slow players, and decrements in COD could be a key benchmark to identify deficit between linear and COD performance.

Physical Demands of Ball Possession Games in Relation to the Most Demanding Passages of a Competitive Match.

The main aim of this study was to determine the physical demands of different small-sided ball possession games (SSBPGs) according to player field position and compare these demands in relation to the most demanding passages of play (MDP) in competitive matches. Global positioning system data were obtained from 25 football players (20.4 ± 2.1 yrs, 1.78 ± 0.66 m, 69.7 ± 6.1 kg) belonging to the reserve squad of a Spanish La Liga Club. Players were categorized according to positional groups; full back (FB), central defender (CD), midfielder (MF), wide midfielder (WMF) and forwards (FW). The variables analyzed were relativized to metres per minute (m·min-1): total distance covered (TD), TD at high speed (HSR; >19.8 km·h-1), TD at sprint (SPR; >25.2 km·h-1), the number of accelerations (ACC) and decelerations (DEC) at high intensity (> +/-3 m·s-2), the average metabolic power (AMP; W·kg-1) and the high metabolic load distance (HMLD; >25.5 W·kg-1). The MDP were analyzed using a rolling average method, where maximal values were calculated for 3 and 5 minutes to compare with SSBPGs using AMP as a criterion variable. The results were obtained from the SSBPGs relative to the MDP (expressed in %) for each player position. FB showed the greatest magnitude of overload in ACC/DEC according to the MDP in the two smaller SSBPGs formats (201-217%), whereas MF showed lower values (105-140%). The load expressed in relation to the MDP can be different depending on the format of the SSBPGs and the characteristics of playing position. These factors should be considered by the coaches when planning training.

The effects of detraining and retraining periods on fat-mass and fat-free mass in elite male soccer players.

The aim of the study was to examine the effects of a detraining period (DTP) (i.e., off-season) with an individually prescribed training program, and a retraining period (RTP) (i.e., pre-season) combining soccer and flywheel-based strength training on fat-free mass (FFM) and fat-mass (FM) in 10 elite professional male soccer players. The present study used a controlled repeated-measures research design to investigate the changes in FFM and FM using dual-energy X-ray absorptiometry. Whole body %FM increased (effect size (ES) = 0.87 ± 0.46) and FFM reduced after DTP (ES = -0.30 ± 0.19), returning to values comparable to the end of the previous season after RTP. At regional levels, arms, legs, and trunk %FM increased (ES = from 0.42 to 1.29) while trunk-FFM was reduced (ES = -0.40 ± 0.26) after DTP, returning to the values observed at the end of the previous season after RTP. Legs-FFM did not change after DTP, with a substantial increase after RTP in comparison with pre-season values (ES = 0.34 ± 0.29 and 0.53 ± 0.36 for the right and left leg, respectively). Despite the small sample size of the present study, the findings indicate that elite soccer players can be allowed 2 weeks of rest during a five-week DTP, since the changes in %FM and FFM were relatively small, and FM and FFM returned to the optimal initial values for competition after the proposed RTP during the pre-season.

Effects of Strength Training on Body Composition in Young Male Professional Soccer Players.

The present prospective cohort study investigated changes in body composition (BC) in young male football players (n = 18, 16.1 ± 0.8 years; 181.0 ± 0.1 cm; 71.3 ± 4.9 kg) after combined football and strength training (ST) during a whole in-season period (26 weeks). BC was measured at whole-body absolute and regional levels by dual-energy X-ray absorptiometry in eighteen players at the beginning and at the end of the competitive period. The ST was organized into three different session types: ST in the gym, specific ST on the field, and individual ST (weak points). The results of the present study indicated that fat-free mass (FFM) was substantially higher following the competitive period (5.1% ± 1.2%), while percentage of fat showed no changes during the competitive period. At the regional level, arms' and legs' FFM increased at the end of the season, and bone mineral content (BMC) and density (BMD) increased in arms, legs, pelvis, thoracic spine, and lumbar spine. In conclusion, within the limitation of the potential positive impact of growth and/or maturation, present results seem to indicate that an ST program that supplements football-related training sessions could be an effective option to increase FFM, BMC, and BMD at both whole-body and regional level across the competitive season in young male professional football players.

Dissociation between changes in sprinting performance and Nordic hamstring strength in professional male football players.

The aim of the present study was to evaluate the consequence of implementing a Nordic Hamstring exercise (NHE) protocol during the first 15 to 17 weeks of the season to assess the effect on sprinting and NHE strength (NHEs) in professional football players. The study examined 50 healthy male professional football players (age 18.8±0.8yr; height 176.8±6.9cm; weight 71.3±5.7kg) belonging to 3 of the reserve squads of three Spanish La-Liga clubs divided in 2 intervention teams [Nordic-Group1 (NG-1) and Nordic-Group2 (NG-2, extensive experience in NHE)] and 1 team as a control-group (CG). NHEs and linear sprint (T5, T10, T20-m) were evaluated at the beginning of the season and at the end of an intervention period of conditioning and football training, supplemented with a NHE protocol (24 sessions for NG-1 and 22 sessions for NG-2) or without using the NHE at all (CG). Sprint times were substantially improved in all groups (ES from -2.24±0.75 to -0.60±0.37). NHEs was enhanced absolute and relative to body-mass only in NG-1 after the training period (ES from 0.84±0.32 to 0.74±0.26), while in the NG-2 there were only improvements in average NHEs relative to body-mass (ES = 0.39±0.36). The improvements in T20-m were substantially greater in NG-2 vs. NG-1, and there were no differences in sprint performance changes between NG-1 and CG. Changes in sprinting performance and NHEs were unrelated. NHEs was largely correlated with the body-mass of the players. Results indicate that the improvements in sprint are not dependent on the NHEs changes, with no relationships between NHEs and sprint performance, and between sprint changes and changes in NHEs.

A Repeated Power Training Enhances Fatigue Resistance While Reducing Intraset Fluctuations.

Oliver, GS, Julio, TF, Moras, G, José Luis, AS, and Alberto, MV. A repeated power training enhances fatigue resistance while reducing intraset fluctuations. J Strength Cond Res 33(10): 2711-2721, 2019-The present study analyzed the effects of adding upper-body repeated power ability (RPA) training to habitual strength training sessions. Twenty young elite male basketball players were randomly allocated into a control group (CON, n = 10) or repeated power group (RPG, n = 10) and evaluated by 1 repetition maximum (1RM), incremental load, and RPA tests in the bench press exercise before and after a 7-week period and a 4-week cessation period. Repeated power group performed 1-3 blocks of 5 sets of 5 repetitions using the load that maximized power output with 30 seconds and 3 minute of passive recovery between sets and blocks, respectively. Between-group analysis showed substantial greater improvements in RPG compared with CON in: best set (APB), last set (APL), mean power over 5 sets (APM), percentage of decrement, fluctuation decrease during APL and RPA index (APLpost/APBpre) during the RPA test (effect size [ES] = 0.64-1.86), and 1RM (ES = 0.48) and average power at 80% of 1RM (ES = 1.11) in the incremental load test. The improvements of APB and APM were almost perfectly correlated. In conclusion, RPA training represents an effective method to mainly improve fatigue resistance together with the novel finding of a better consistency in performance (measured as reduced intraset power fluctuations) at the end of a dynamic repeated effort.

Estimating fat-free mass in elite youth male soccer players: cross-validation of different field methods and development of prediction equation.

This study determined the most effective field method for quantifying fat-free mass (FFM) in elite youth male soccer players compared to dual X-ray absorptiometry (DXA) values and to develop prediction equations for FFM based on anthropometric variables. Forty-one male elite-standard youth soccer players, ages 16.2-18.0 years, undertook FFM assessments including bioelectrical impedance analysis, and different skinfold-based prediction equations. DXA provided a criterion measure of FFM. Correlation coefficients, bias, limits of agreement, and differences were used as validity measures, and regression analyses to develop soccer-specific prediction equations. Slaughter et al (1988), Durnin and Wormersley (1974), and Sarria et al (1998) equations showed the lowest biases, and no significant, standardized, and substantial differences against DXA. The new youth soccer-specific anthropometric equation explained 91% of the DXA-derived FFM variance using three circumferences, eight skinfolds, and one bone breadth. All field methods compared in this study may not be adequate for estimating FFM in elite youth male soccer players, except the equations of Slaughter et al (1988), Durnin and Wormersley (1974), and Sarria et al (1998). We recommend the use of the new soccer-specific equation proposed in this study as a valid alternative to DXA to quantify FFM among elite youth male players.

Association of Monocarboxylate Transporter-1 (MCT1) A1470T Polymorphism (rs1049434) with Forward Football Player Status.

The aim of this study was to investigate the association between the MCT1 (monocarboxylate transporter 1) A1470T polymorphism and positional roles in a large cohort of professional football players from five different countries. We compared genotype distributions of the MCT1 A1470T polymorphism between football players (n=694) and non-athlete controls (n=781) from Italy, Poland, Lithuania, Ukraine and Malta, and we analyzed the MCT1 genotype distributions with respect to the players' positions in the field (e. g. forwards, midfielders, defenders and goalkeepers). Genomic DNA was extracted from either buccal epithelium or peripheral blood using a standard protocol. In the pooled cohort of Italian, Polish, Lithuanian and Ukrainian football players, forwards (n=148) were more likely than controls (n=781) to possess the A allele (χ2=7.067, p=0.029, FDR q value 0.116), with a greater likelihood of having the AA genotype compared with the TT genotype (OR=1.97; C.I.=1.07-3.64; p=0.021, FDR q value 0.086). The MCT1 AA genotype was significantly more frequent in forwards then in controls. Further studies are needed to confirm these findings in other professional football player cohorts.

In-season eccentric-overload training in elite soccer players: Effects on body composition, strength and sprint performance.

The aim of this study was to describe the changes in body composition, strength and sprint performance in response to an entire competitive season of football training supplemented with 2 inertial eccentric-overload training sessions a week in young male professional soccer players. Whole body and regional composition (assessed using dual-energy X-ray absorptiometry), power output in half-squat and 40-m sprinting performance were evaluated in fourteen players. The eccentric-overload training consisted of training sessions a week of 1-2 sets of 10 exercises of upper-body and core (Day 1) and lower-body (Day 2), during the entire competitive season (27 weeks). Whole body fat mass decreased (-6.3 ± 3.6%, ES = -0.99 ± 0.54) substantially while lean mass increased (2.5 ± 0.8%, ES = 0.25 ± 0.09), with some regional differences. There was a substantial increase in half-squat power output (from 3% to 14%, ES from 0.45 to 1.73) and sprint performance (from 1.1% to 1.8%, ES from -0.33 to -0.44), however performance changes were not correlated with changes in body composition. A combined soccer and eccentric-overload training program was able to promote positive changes in body composition and physical factors relevant to both on-field performance and injury prevention in elite soccer players.

Validation of Field Methods to Assess Body Fat Percentage in Elite Youth Soccer Players.

This study determined the most effective field method for quantifying body fat percentage in male elite youth soccer players and developed prediction equations based on anthropometric variables. Forty-four male elite-standard youth soccer players aged 16.3-18.0 years underwent body fat percentage assessments, including bioelectrical impedance analysis and the calculation of various skinfold-based prediction equations. Dual X-ray absorptiometry provided a criterion measure of body fat percentage. Correlation coefficients, bias, limits of agreement, and differences were used as validity measures, and regression analyses were used to develop soccer-specific prediction equations. The equations from Sarria et al. (1998) and Durnin & Rahaman (1967) reached very large correlations and the lowest biases, and they reached neither the practically worthwhile difference nor the substantial difference between methods. The new youth soccer-specific skinfold equation included a combination of triceps and supraspinale skinfolds. None of the practical methods compared in this study are adequate for estimating body fat percentage in male elite youth soccer players, except for the equations from Sarria et al. (1998) and Durnin & Rahaman (1967). The new youth soccer-specific equation calculated in this investigation is the only field method specifically developed and validated in elite male players, and it shows potentially good predictive power.

Variability of GPS-derived running performance during official matches in elite professional soccer players.

BACKGROUND: The aim of this study was to assess the match-to-match variability obtained using GPS devices, collected during official games in professional soccer players. METHODS: Global positioning system (GPS)-derived data from nineteen elite soccer players were collected over two consecutive seasons. Time-motion data for players with more than five full-match were analyzed (N.=202). Total distance covered (TD), TD>13-18 km/h, TD>18-21 km/h, TD>21 km/h, number of acceleration >2.5-4 m.s-2 and >4 m.s-2 were calculated. The match-to-match variation in running activity was assessed by the typical error expressed as a coefficient of variation (CV%) and the magnitude of the CV was calculated (effect size). RESULTS: When all players were pooled together, CVs ranged from 5% to 77% (first half) and from 5% to 90% (second half), for TD and number of acceleration >4 m.s-2, and the magnitude of the CVs were rated from small to moderate (effect size=0.57-0.98). The CVs were likely to increase with running/acceleration intensity, and were likely to differ between playing positions (e.g., TD>13-18 km/h 3.4% for second strikers vs. 14.2% for strikers and 14.9% for wide-defenders vs. 9.7% for wide-midfielders). CONCLUSIONS: Present findings indicate that variability in players' running performance is high in some variables and likely position-dependent. Such variability should be taken into account when using these variables to prescribe and/or monitor training intensity/load.