Latest research in football - week 36 - 2019

As previous literature updates, I have performed a PubCrawler search looking for football articles in NCBI Medline (PubMed) and GenBank databases.

Following studies were retrieved for this week:

#1 Combined Resistance and Plyometric Training Is More Effective Than Plyometric Training Alone for Improving Physical Fitness of Pubertal Soccer Players
Reference: Front Physiol. 2019 Aug 7;10:1026. doi: 10.3389/fphys.2019.01026. eCollection 2019.
Authors: Zghal F, Colson SS, Blain G, Behm DG, Granacher U, Chaouachi A
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Summary: The purpose of this study was to compare the effects of combined resistance and plyometric/sprint training with plyometric/sprint training or typical soccer training alone on muscle strength and power, speed, change-of-direction ability in young soccer players. Thirty-one young (14.5 ± 0.52 years; tanner stage 3-4) soccer players were randomly assigned to either a combined- (COMB, n = 14), plyometric-training (PLYO, n = 9) or an active control group (CONT, n = 8). Two training sessions were added to the regular soccer training consisting of one session of light-load high-velocity resistance exercises combined with one session of plyometric/sprint training (COMB), two sessions of plyometric/sprint training (PLYO) or two soccer training sessions (CONT). Training volume was similar between the experimental groups. Before and after 7-weeks of training, peak torque, as well as absolute and relative (normalized to torque; RTD r ) rate of torque development (RTD) during maximal voluntary isometric contraction of the knee extensors (KE) were monitored at time intervals from the onset of contraction to 200 ms. Jump height, sprinting speed at 5, 10, 20-m and change-of-direction ability performances were also assessed. There were no significant between-group baseline differences. Both COMB and PLYO significantly increased their jump height (Δ14.3%; ES = 0.94; Δ12.1%; ES = 0.54, respectively) and RTD at mid to late phases but with greater within effect sizes in COMB in comparison with PLYO. However, significant increases in peak torque (Δ16.9%; p < 0.001; ES = 0.58), RTD (Δ44.3%; ES = 0.71), RTD r (Δ27.3%; ES = 0.62) and sprint performance at 5-m (Δ-4.7%; p < 0.001; ES = 0.73) were found in COMB without any significant pre-to-post change in PLYO and CONT groups. Our results suggest that COMB is more effective than PLYO or CONT for enhancing strength, sprint and jump performances.

#2 Training/Match External Load Ratios in Professional Soccer Players: A Full-Season Study
Reference: Int J Environ Res Public Health. 2019 Aug 23;16(17). pii: E3057. doi: 10.3390/ijerph16173057.
Authors: Clemente FM, Rabbani A, Conte D, Castillo D, Afonso J, Truman Clark CC, Nikolaidis PT, Rosemann T, Knechtle B
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Summary: The aim of this study was two-fold: (i) to describe the training/match ratios of different external load measures during a full professional soccer season while analyzing the variations between different types of weeks (three, four and five training sessions/week) and (ii) to investigate the relationship between weekly accumulated training loads and the match demands of the same week. Twenty-seven professional soccer players (24.9 ± 3.5 years old) were monitored daily using a 10-Hz global positioning system with a 100-Hz accelerometer. Total distance (TD), running distance (RD), high-speed running (HSR), sprinting distance (SD), player load (PL), number of high accelerations (ACC), and number of high decelerations (DEC) were recorded during training sessions and matches. An individual training/match ratio (TMr) was calculated for each external load measure. Weeks with five training sessions (5dW) presented meaningfully greater TMr than weeks with four (4dW) or three (3dW) training sessions. Additionally, TDratio (TDr) was significantly greater in 5dW than in 3dW (mean differences dif: 1.23 arbitray units A.U.) and 4dW (dif: 0.80 A.U.); HSRr was significantly greater in 5dW than in 3dW (dif: 0.90 A.U.) and 4dW (dif: 0.68 A.U.); and SDr was significantly greater in 5dW than in 3dW (dif: 0.77 A.U.) and 4dW (dif: 0.90 A.U.). Correlations between the weekly training loads and the match demands of the same week were small for PL (r = 0.250 [0.13;0.36]), ACC (r = 0.292 [0.17;0.40]) and DEC (r = 0.236 [0.11;0.35]). This study reveals that ratios of above 1 were observed for specific measures (e.g., HSR, SD). It was also observed that training sessions are not adjusted according to weekly variations in match demands.

#3 Variations of Internal and External Load Variables between Intermittent Small-Sided Soccer Game Training Regimens
Reference: Int J Environ Res Public Health. 2019 Aug 15;16(16). pii: E2923. doi: 10.3390/ijerph16162923.
Authors: Clemente FM, Nikolaidis PT, Rosemann T, Knechtle B
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Summary: The purpose of this study was twofold: (i) analyze the variations of internal and external load between intermittent regimens (6 × 3' and 3 × 6') during a small-sided game (SSG); and (ii) analyze the variations of internal and external load within-intermittent regimens (between sets). Ten male amateur soccer players (age: 21.7 ± 2.1 years) participated in this study. Almost certain large decreases in total distance (-8.6%, [-12.3; -4.8], Effect Size (ES): -1.51, [-2.20; -0.82]) and running distance (-34.0%, [47.0; -17.8], ES: -2.23, [-3.40; -1.05]) were observed when comparing the 3 × 6' and 6 × 3'. Very likely moderate and large decreases in total accelerations (-24.0%, [-35.1; -10.9]; ES: -1.11, [-1.75; -0.47]) and total of decelerations (-26.7%, [-38.8; -12.1]; ES:-1.49, [-2.36; -0.62]), respectively, were found when comparing the 3 × 6' and 6 × 3'. Very likely increases in rated of perceived exertion in the set 3 in comparison to the 1st during the 3 × 6' SSG (34.5%, [12.4; 61.0], ES: 1.35, [0.53; 2.16]) and the 6 × 3' (29.9%, [11.6; 51.2]; ES: 1.17, [0.49; 1.85]). Longer sets increase the perception of effort and contribute to a large decrease in total and running distances, and total of accelerations and decelerations. Meaningful decreases in time-motion demands occur between sets 2 and 3 while perceived effort increases.

#4 Physical Performance Measures Correlate with Head Impact Exposure in Youth Football
Reference: Med Sci Sports Exerc. 2019 Aug 28. doi: 10.1249/MSS.0000000000002144. [Epub ahead of print]
Authors: Kelley ME, Jones DA, Espeland MA, Rosenberg ML, Miles CM, Whitlow CT, Maldjian JA, Stitzel JD, Urban JE
Summary: Head impact exposure (HIE) (i.e., magnitude and frequency of impacts) can vary considerably among individuals within a single football team. To better understand individual-specific factors that may explain variation in head impact biomechanics, this study aimed to evaluate the relationship between physical performance measures and HIE metrics in youth football players. Head impact data were collected from youth football players using the Head Impact Telemetry (HIT) System. HIE was quantified in terms of impact frequency, linear and rotational head acceleration, and risk-weighted cumulative exposure metrics (RWELinear, RWERotational, and RWECP). Study participants completed 4 physical performance tests: vertical jump, shuttle run, 3-cone, and 40 yard sprint. The relationships between performance measures and HIE metrics were evaluated using linear regression analyses. A total of 51 youth football athletes (ages: 9-13 years old) completed performance testing and received a combined 13,770 head impacts measured with the HIT System for a full season. All performance measures were significantly correlated with total number of impacts in a season, RWELinear-Season, and all RWE-Game metrics. The strongest relationships were between 40 yard sprint speed and all RWE-Game metrics (all p≤0.0001 and partial R>0.3). The only significant relationships among HIE metrics in practice were between shuttle run speed and total practice impacts and RWELinear-Practices, 40 yard sprint speed and total number of practice impacts, and 3-cone speed and 95th percentile number of impacts/practice. Generally, higher vertical jump height and faster times in speed and agility drills were associated with higher HIE, especially in games. Physical performance explained less variation in HIE in practices, where drills and other factors, such as coaching style, may have a larger influence on HIE.

#5 Proprioception is not associated with lower extremity injuries in U21 high-level football players
Reference: Eur J Sport Sci. 2019 Aug 30:1-20. doi: 10.1080/17461391.2019.1662492. [Epub ahead of print]
Authors: Namazi P, Zarei M, Abbasi H, Hovanloo F, Rommers N, Rössler R
Summary: Football is a contact sport with a significant risk of injury. Although proprioception is well studied in rehabilitation, little is known about the association between proprioception and the occurrence of sport injuries. The purpose of this study was to look into the association between ankle and knee proprioception and lower extremity injuries in young football players. 73 football players from the highest U-21 league in Iran volunteered to participate in this study. Before the start of the 2017-2018 competitive season, joint position sense was measured at 30°, 60° and 90° knee flexion and at 10° and 15° ankle dorsiflexion, and inversion using the Biodex Isokinetic pro 4 system. The teams' medical staff recorded football-related lower extremity injuries. We used mixed effects Cox regression models to calculate hazard ratios (HRs) with 95% CIs, acknowledging the clustered data structure. 22 players (30.1%) suffered one or more lower extremity injuries during the season. None of the proprioception measures examined was significantly associated with the risk of lower extremity injuries. Based on these results of our sample, joint position sense does not seem to be associated with lower extremity injuries in young male football players.

#6 Few training sessions between return to play and first match appearance are associated with an increased propensity for injury: a prospective cohort study of male professional football players during 16 consecutive seasons
Reference: Br J Sports Med. 2019 Aug 29. pii: bjsports-2019-100655. doi: 10.1136/bjsports-2019-100655. [Epub ahead of print]
Authors: Bengtsson H, Ekstrand J, Waldén M, Hägglund M
Summary: It has been hypothesised that injury risk after return to play following an injury absence is influenced by the amount of training completed before return to competition. The aim was to analyse if the number of completed training sessions between return to play and the first subsequent match appearance was associated with the odds of injury in men's professional football. From a cohort study, including 303 637 individual matches, 4805 first match appearances after return to play following moderate to severe injuries (≥8 days absence) were analysed. Rate ratios (RRs) were used to compare injury rates in the first match appearances with the average seasonal match injury rate. Odds ratios (ORs) were used to analyse associations between the number of completed training sessions and general (all injuries), muscle, and non-muscle injury odds. Injury rate in the first match after return to play was increased by 87% compared with the average seasonal match injury rate (46.9 vs 25.0/1000 hours, RR=1.87; 95% CI 1.64 to 2.14). The odds of injury dropped 7% with each training session before the first match (OR 0.93; 95% CI 0.87 to 0.98). The same association was found for muscle injuries (OR 0.87; 95% CI 0.79 to 0.95) but not for non-muscle injuries (OR 0.99; 95% CI 0.91 to 1.07). Injury rates in the first match after injury are higher than the average seasonal match injury rate, but the propensity for player injury is decreased when players complete more training sessions before their first match.

#7 C-Reactive Protein Serum Levels as an Internal Load Indicator of Sprints in Competitive Football Matches
Reference: Int J Sports Med. 2019 Aug 29. doi: 10.1055/a-0985-4464. [Epub ahead of print]
Authors: Jatene P, Dos Santos GS, Portella DL
Summary: This study compared internal load variable dynamics across three consecutive football matches and investigated its relationship with the number of sprints performed by players. Twenty-three male players had blood and salivary samples collected for hormonal concentration (testosterone, cortisol, and testosterone-cortisol ratio), and serum analysis (interleukin-6, interleukin-1-beta, and c-reactive-protein), respectively. Sprints were measured through Global Position System devices. Testosterone and testosterone-cortisol-ratio presented a decreasing behavior up to the second match, and all other indicators presented an increasing behavior during the same period, c-reactive-protein was the only indicator observed to significantly rise up to the third match as well (0.38±0.02 mg/L; 0.49±0.05 mg/L; 0.69±0.05 mg/L; 0.89±0.08 mg/L). C-reactive-protein showed strong correlations with sprints in the second and third matches (p<0.01, r=0.71 and 0.79), and weak-to-moderate in the first one (p<0.05, r=0.59). Interleukin-6 and interleukin-1-beta presented weak-to-moderate correlation in every match (p<0.05, r=0.48 to 0.51; r=0.51 to 0.55) while testosterone-cortisol ratio presented weak-to-moderate correlation only in the third one (p<0.05, r=0.42). Multilevel linear regression showed that c-reactive-protein had a higher R2 than other biomarker in any regression model (R2=0.624; p<0.001). Therefore, c-reactive-protein can be a valid and reliable indicator of sprinting in competitive football. Future research should explore longer periods of monitoring and/or others external load variables so that other behaviors may arise to knowledge.

#8 Radiographic analysis of lower limb alignment in professional football players
Reference: Arch Orthop Trauma Surg. 2019 Aug 28. doi: 10.1007/s00402-019-03266-9. [Epub ahead of print]
Authors: Krajnc Z, Drobnič M
Summary: The purpose was to radiographically analyze lower limb alignment in adult asymptomatic professional football players and to correlate these values to clinical measurements. Twenty-four asymptomatic players [24.2 (3.6) years] were enrolled. Standard bilateral lower limb anteroposterior weight-bearing radiographs were acquired and clinical measurement of intercondylar/intermalleolar (ICD/IMD) distance was performed. Coronal plane mechanical alignment was assessed by five angles: leg mechanical axis (LMA), lateral proximal femoral angle (LPFA), lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and lateral distal tibial angle (LDTA). Their values were compared to the reference values for adult population. An inter-individual comparison between right/left and dominant/non-dominant leg was added. The sum of bilateral LMA was correlated against ICD/IMD and against ICD/IMD adjusted for body height. Football players presented with ICD/IMD of 46.5 (19.8) mm. Two, out of five, lower leg coronal angles showed significant differences (p < 0.001) compared to reference data from literature: LMA 5.8 (3.0)º vs.1.2 (2.2)º and MPTA 83.5 (2.6)º vs. 87.2 (1.5)º. No significant differences between left/right leg and dominant/non-dominant leg were established. Summed up bilateral LMA showed a high correlation to IMD/ICD (r = 0.8395; R2 = 0.7048), and even higher to ICD/IMD adjusted for body height (r = 0.8543; R2 = 0.7298). This study was radiographically confirming increased varus of elite football players toward general population. Apex of the varus deformity was located in the proximal tibia. Clinical measurement of ICD/IMD adjusted for body height highly correlated with the radiographic values of coronal alignment; therefore, it may be used in population studies.

#9 Short term effects of a weight loss and healthy lifestyle programme for overweight and obese men delivered by German football clubs
Reference: Eur J Sport Sci. 2019 Aug 28:1-35. doi: 10.1080/17461391.2019.1660809. [Epub ahead of print]
Authors: Pietsch B, Weisser B, Hanewinkel R, Gray C, Hunt K, Wyke S, Morgenstern M
Summary: Numbers of obese and overweight people continue to grow in Germany as they do worldwide. Men are affected more often but do less about it and few weight loss services attract men in particular. To evaluate the effectiveness of a men-only weight loss program, Football Fans in Training (FFIT), delivered by football clubs in the German Bundesliga, we did a non-randomized trial with a waiting list control group. Participants' data were collected between January 2017 and July 2018. FFIT is a 12-week, group-based, weight loss program and was delivered in stadia and facilities of 15 professional German Bundesliga clubs. Inclusion criteria were age 35-65 years, BMI ≥ 28 and waist circumference ≥100 cm. Clubs recruited participants through Social Media, E-Mail and match day advertisement. 477 German male football fans were allocated to the intervention group by order of registration date at their respective clubs. 84 participants on waiting list were allocated to the control group. Primary outcome was mean difference in weight loss with treatment condition over time as independent variable. We performed a multilevel mixed-effects linear regression analysis. Results were based on Intention-to-treat (ITT) analysis with Multiple Imputation. After 12 weeks, the mean weight loss of the intervention group adjusted for club, course and participants' age was 6.24 kg (95% CI 5.82 to 6.66) against 0.50 kg (-0.47 to 1.49) in the comparison group (p < 0.001). The results indicate that Football Fans in Training effectively helped German men to reduce their weight and waist circumference.

#10 Use of functional performance tests in sports: Evaluation proposal for football players in the rehabilitation phase
Reference: Turk J Phys Med Rehabil. 2018 May 15;64(2):148-154. doi: 10.5606/tftrd.2018.1462. eCollection 2018 Jun.
Authors: Gomez-Piqueras P, Gonzalez-Rubio J, Sainz de Baranda P, Najera A
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Summary: Based on the criteria of a group of experts, this study aims to select a set of functional performance tests which can be applied to evaluate the functional status of a football player in the recovery process and make a decision in relation to their return to practice. A total of 16 experts were selected by the coordinator group to judge an initial list of functional performance tests and, thus, reach a consensus about the tests which are best suited to the needs of the injured player. Each of the experts had to evaluate each one of the tests in a scale from 1 to 5 in relation to their suitability. Delphi method was used to reach consensus in the expert group. From the initial list of 25, the tests which obtained the best evaluation were: Counter movement jump (4.3±0.9), Single hop test (4.1±0.8), Triple hop test (4.1±0.9), Crossover hop test (4.1±0.7), Yo-Yo intermittent recovery (4.2±0.6), Barrow test (4.1±0.6), Shuttle run 8¥5 m (4.1±0.8). Star excursion balance test (4±0.7) and Y balance test (4.1±0.7). In the opinion of the experts selected here, these tests are the ones which best respond to the needs involved in a complex decision such as RTP.

#11 Injury Incidence, Prevalence and Severity in High-Level Male Youth Football: A Systematic Review
Reference: Sports Med. 2019 Aug 26. doi: 10.1007/s40279-019-01169-8. [Epub ahead of print]
Authors: Jones S, Almousa S, Gibb A, Allamby N, Mullen R, Andersen TE, Williams M
Summary: At a young age, high-level youth footballers enter structured practice where they engage in regular training and matches. The academy system is considered fundamental to a young footballer's tactical, technical and physical development. Yet, with regular training and matches, high-level youth footballers may be exposed to the risk of injury. This systematic review analyses and summarises published scientific information on high-level youth football injury characteristics and calculates the risk of them sustaining an injury over the course of a typical season. The search was performed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Of the 1346 studies found, 23 fulfilled the inclusion criteria. Quality assurance scores for the selected research articles ranged between two and five out of eight. A high degree of heterogeneity between studies was observed. The probability of sustaining a time-loss injury during a high-level youth season ranged between < 1% and 96% for under 9- to under 16-year age groups and 50% and 91% for under 18- to under 21-year age groups. Pooled estimates for total (training and match) incidence per 1000 h was 5.8 for youth players aged under 9 to under 21 years, 7.9 for older players (under 17-under 21 years) and 3.7 for younger aged players (under 9-under 16 years). Training injury incidence rate ranged from 0.69 to 7.9 per 1000 h for all age groups in youth football. Match injury incidence rate for high-level youth players ranged from 0.4 to 80.0 per 1000 h. Close to one-fifth (18%) of all high-level youth football injuries were classified as severe and required > 28 days recovery time. Muscle strain injury accounted for 37% of all injuries reported in youth football. High probabilities (> 90%) of sustaining a time-loss injury over one typical high-level football season were found. High-level youth players lose large portions of the seasonal development to injury, with players seemingly suffering long absences from training and matches, consequently affecting health and well-being and possibly burdening club/parental finances and healthcare systems.

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