The Physiological and Genetic Factors Underpinning Powerful Actions in Elite Youth Soccer

There is no information available in the scientific literature that documents a specific assessment protocol for analysing a soccer player’s maximal power capabilities. As soccer-associated muscular power has not previously been investigated, it is not known how important power is in elite soccer and, if it is, the physiological and genetic determinants of soccer-associated power remain unknown. Such information could be used to optimise soccer-specific talent identification and development strategies. With this in mind, the overriding aim of our thesis was to investigate the physiological and genetic factors underpinning powerful actions in elite youth soccer. When devising an assessment of soccer-associated muscular power, there needs be a detailed analysis of the specific actions performed during elite competitive match-play that can be described as powerful. The aims of our first experimental study (Chapter Three) were to compare the frequency and durations of powerful actions during competitive English Premier League [under 18 (U18) and under 21 (U21)] elite youth soccer matches using a novel soccer specific powerful action (SSPA) notational analysis coding system. We found that while elite soccer match-play requires players to perform powerful actions in multiple directions [68 horizontal accelerations (in the horizontal-forward or mediolateral directions), eight sprints, and six vertical jumps (three bilateral and three unilateral)], horizontal accelerations of short duration (A (rs6265), COL5A1 C>T (rs12722), and COL2A1 C>T (rs2070739)] played a role in determining elite youth soccer player status, and speed and power capabilities, in ESP and CON at different stages of maturation. We found that ACTN3 R- and BDNF G-allele frequencies were more frequent in post-PHV compared to pre-PHV ESP. Moreover, while the COL2A1 CC genotype was associated with greater horizontal power and faster 20 m sprint performance, BDNF GG genotype appears to positively influence 20 m sprint performance during the pre-PHV period only. Overall, our findings illustrate that elite soccer may require different genetic profiles before and after maturation, and genetic screening could be included in talent identification criteria to help predict maximal power and sprint potential in ESP. In summary, we devised a muscular power assessment battery that measured independent power qualities and could discriminate between U18 and U21 ESP and NSP. Our subsequent analysis showed that the physiological factors underpinning unilateral CMJ performance were direction-specific, and UV and UH CMJ capabilities were underpinned by separate neuromuscular and tendon properties, and should be assessed and developed, independently in U18 and U21 ESP. We then recruited a larger cohort of ESP and CON, at different stages of maturation, and demonstrated that muscular power was important for elite soccer performance at mid and post-PHV, but not pre-PHV. Finally, we showed that genetic profiles of ESP differed between pre- and post-PHV, and that certain gene variants [COL2A1 C>T (rs2070739), BDNF G>A (rs6265)] were associated with specific power and speed capabilities in ESP. Overall, our studies provide novel information that could have significant implications on soccer-associated power related talent identification and training intervention strategies in elite youth soccer academies

COUNTERMOVEMENT JUMP PERFORMANCE IS NOT AFFECTED DURING AN IN-SEASON TRAINING MICROCYCLE IN ELITE YOUTH SOCCER PLAYERS

This study examined the change in countermovement jump (CMJ) performance across a microcycle of training in professional soccer players during the in-season period. Nine elite youth soccer players performed a CMJ test pre and post four consecutive soccer training sessions of an in-season weekly microcycle. Training load was quantified using global positioning systems (GPS), heart rate (HR) and rating of perceived exertion (RPE). Absolute change (pre to post training) in CMJ height across each training session was analysed using one-way repeated measures analysis of variance (ANOVA). Magnitude of effects was reported with the effect size (ES) statistic. Correlation analyses assessed the relationships between training load measures and the absolute change in CMJ height. Training load remained similar on all training days apart from a significant decrease in training load (all variables except high speed distance) on the last training session (P 0.05). This study revealed no significant change in CMJ performance across the in-season microcycle. This suggests that soccer players are able to maintain CMJ performance across an in-season training microcycle

The Importance of Speed and Power in Elite Youth Soccer Depends on Maturation Status

Maturation status is a confounding factor when identifying talent in elite youth soccer players (ESP). By comparing performance of ESP and control participants (CON) matched for maturation status, the aims of our study were to establish the importance of acceleration, sprint, horizontal-forward jump and vertical jump capabilities for determining elite soccer playing status at different stages of maturation. ESP (n=213; age, 14.0±3.5 yrs) and CON (n=113; age, 15.0±4.4 yrs) were grouped using years from/to predicted peak height velocity (PHV) to determine maturation status (ESP: pre-PHV, n=100; mid-PHV, n=25; post-PHV, n=88; CON: pre-PHV, n=44; mid-PHV, n=15; post-PHV, n=54). Participants performed three reps of: 10 m and 20 m sprint, bilateral vertical countermovement jump (BV CMJ) and bilateral horizontal-forward countermovement jump (BH CMJ). ESP demonstrated faster 10 m (P<0.001) and 20 m sprint (P<0.001) performance than CON at all stages of maturation. Mid-PHV and post-PHV ESP achieved greater BV CMJ height (P<0.001) and BH CMJ distance (ESP vs. CON; mid-PHV: 164.32±12.75 vs. 136.53±21.96 cm; post-PHV: 197.57±17.05 vs. 168.06±18.50 cm; P<0.001) compared to CON but there was no difference in BV or BH CMJ between pre-PHV ESP and CON. While 10 and 20 m and sprint performance may be determinants of elite soccer playing status at all stages of maturation, horizontal-forward and vertical jumping capabilities only discriminate ESP from CON participants at mid- and post-PHV. Our data therefore suggests that soccer talent identification protocols should include sprint, but not jump assessments in pre-PHV players

A coding system to quantify powerful actions in soccer match play: a pilot study

Purpose: The powerful activity profile of elite soccer match play has not been documented appropriately to inform specific maximal power assessment and development criteria. The aims of the current study were to develop a novel soccer specific powerful action (SSPA) notational analysis coding system that could be used to compare frequency and durations of powerful actions during elite youth soccer match play. Methods: Sixteen elite male English Premier League (EPL) Academy players (19 ± 1 yrs) were recorded by an individual camera during sixteen competitive EPL U18 and U21 games. Video footage was analyzed using performance analysis software and SSPAs were coded according to the following categories: initial acceleration, leading acceleration, sprint, unilateral jump and bilateral jump. Results: The novel SSPA coding system demonstrated very good inter- and intra-rater reliability (kappa coefficients ≥ 0.827). Moreover, elite youth EPL soccer players undertook significantly more initial (31 ± 9) and leading (37±12) accelerations than sprints (8±3; p=0.014 and p<0.001, respectively) and jumps (6±5; p=0.002 and p<0.001, respectively). Players performed a significantly greater number of initial and leading accelerations with action durations below 1.5 s compared to above 1.5 s (p=0.001 and p=0.002), respectively. Conclusions: Our novel SSPA coding system provides a highly reliable observational instrument for quantifying the frequency and duration of powerful actions performed during elite soccer match play. In our sample of elite youth soccer players, horizontal accelerations of short duration (<1.5 s) from different starting speeds appear the most dominant powerful action in elite youth soccer match play

The genetic profile of elite youth soccer players and its association with power and speed depends on maturity status

We investigated the association of multiple single nucleotide polymorphisms (SNPs) with athlete status and power/speed performance in elite male youth soccer players (ESP) and control participants (CON) at different stages of maturity. ESP (n = 535; aged 8–23 years) and CON (n = 151; aged 9–26 years) were genotyped for 10 SNPs and grouped according to years from predicted peak-height-velocity (PHV), i.e. pre- or post-PHV, to determine maturity status. Participants performed bilateral vertical countermovement jumps, bilateral horizontal-forward countermovement jumps, 20m sprints and modified 505-agility tests. Compared to CON, pre-PHV ESP demonstrated a higher ACTN3 (rs1815739) XX (‘endurance’) genotype frequency distribution, while post-PHV ESP revealed a higher frequency distribution of the PPARA (rs4253778) C-allele, AGT (rs699) GG genotype and NOS3 (rs2070744) T-allele (‘power’ genotypes/alleles). BDNF (rs6265) CC, COL5A1 (rs12722) CC and NOS3 TT homozygotes sprinted quicker than A-allele carriers, CT heterozygotes and CC homozygotes, respectively. COL2A1 (rs2070739) CC and AMPD1 (rs17602729) GG homozygotes sprinted faster than their respective minor allele carrier counterparts in CON and pre-PHV ESP, respectively. BDNF CC homozygotes jumped further than T-allele carriers, while ESP COL5A1 CC homozygotes jumped higher than TT homozygotes. To conclude, we have shown for the first time that pre- and post-PHV ESP have distinct genetic profiles, with pre-PHV ESP more suited for endurance, and post-PHV ESP for power and speed (the latter phenotypes being crucial attributes for post-PHV ESP). We have also demonstrated that power, acceleration and sprint performance were associated with five SNPs, both individually and in combination, possibly by influencing muscle size and neuromuscular activation

An injury audit in high-level male youth soccer players from English, Spanish, Uruguayan and Brazilian academies.

OBJECTIVES: To identify the most common injury types/locations in high-level male youth soccer players (YSP). DESIGN: Prospective cohort surveillance study. SETTING: Professional soccer club academies. PARTICIPANTS: Six hundred and twenty-four high-level male YSP [Under 9 (U9) to U23 year-old age groups] from academies in England, Spain, Uruguay and Brazil. MAIN OUTCOME MEASURES: Injury type, location and severity were recorded during one season. Injury severity was compared between age groups, while injury type and location were compared between nations. RESULTS: Four hundred and forty-three training or match injuries were recorded, giving an injury rate of 0.71 per player. Non-contact injuries were most common (58.5%), with most (44.2%) resolved between 8 and 28 days. Most injuries (75.4%) occurred in the lower limbs, with muscle (29.6%) the most commonly injured tissue. U14 and U16 suffered a greater number of severe injuries relative to U12 and U19/U20/U23/Reserves. Tendon injury rate was higher in Brazil vs. Spain (p < 0.05), with low back/sacrum/pelvis injury rate highest in Spain (p < 0.05). CONCLUSIONS: The proportion of severe injuries in U14 and U16 suggests YSP injury risk is maturation-dependent. Minimal differences in type and location between high-level YSP from four different countries suggest injury rates in this population are geographically similar

Patellar tendon properties distinguish elite from non-elite soccer players and are related to peak horizontal but not vertical power

Purpose: The aims of our study were to investigate differences in tendon properties between elite and non-elite soccer players, and to establish whether tendon properties were related to power assessed during unilateral jumps in different directions. Methods: Elite (n=16; age, 18.1 ± 1.0yrs) and non-elite (n=13; age, 22.3 ± 2.7yrs) soccer players performed three repetitions of each type (unilateral vertical, unilateral horizontal-forward and unilateral medial) of countermovement jump (CMJ) on a force plate. Patellar tendon (PT) cross-sectional area (CSA), elongation, stiffness and Young’s modulus (measured at the highest common force interval) were assessed with ultrasonography and isokinetic dynamometry. Results: Elite soccer players demonstrated greater PT elongation (6.83±1.87 vs. 4.92±1.88 mm, P=0.011) and strain (11.73±3.25 vs. 8.38±3.06 %, P=0.009) than non-elite. Projectile range and peak horizontal power during unilateral horizontal-forward CMJ correlated positively with tendon elongation (r=0.657 and 0.693, P<0.001) but inversely with Young’s modulus (r=-0.376 and -0.402, P=0.044 and 0.031). Peak medial power during unilateral medial CMJ correlated positively with tendon elongation (r=0.658, P=<0.001) but inversely with tendon stiffness (r=-0.368, P=0.050). No tendon property correlated with unilateral vertical CMJ performance (r≤0.168; P≥0.204). Conclusions: Patellar tendon strain was greater in elite vs. non-elite soccer players and can therefore be considered an indicator of elite soccer playing status. Moreover, a more compliant patellar tendon appears to facilitate unilateral horizontal-forward and medial, but not vertical CMJ performance in soccer players. These findings should be considered when prescribing the detail of talent selection and development protocols related to direction-specific power in elite soccer players

The genetic association with athlete status, physical performance and injury risk in soccer.

The aim of this review was to critically appraise the literature concerning the genetic association with player status, physical performance and injury risk in soccer. The objectives were to provide guidance on which genetic markers could potentially be used as part of future practice in soccer; and to provide direction for future research in this area. The most compelling evidence identified six genetic polymorphisms to be associated with soccer athlete status (ACE I/D; ACTN3 rs1815739; AGT rs699; MCT1 rs1049434; NOS3 rs2070744; PPARA rs4253778), six with physical performance (ACTN3 rs1815739; AMPD1 rs17602729; BDNF rs6265; COL2A1 rs2070739; COL5A1 rs12722; NOS3 rs2070744), and seven with injury risk (ACTN3 rs1815739; CCL2 rs2857656; COL1A1 rs1800012; COL5A1 rs12722; EMILIN1 rs2289360; IL6 rs1800795; MMP3 rs679620). As well as replication by independent groups, large-scale genome-wide association studies are required to identify new genetic markers. Future research should also investigate the physiological mechanisms associating these polymorphisms with specific phenotypes. Further, researchers should investigate the above associations in female and non-Caucasian soccer players, as almost all published studies have recruited male participants of European ancestry. Only after robust, independently replicated genetic data have been generated, can genetic testing be considered an additional tool to potentially inform future practice in soccer

Learning neural models for end-to-end clustering

We propose a novel end-to-end neural network architecture that, once trained, directly outputs a probabilistic clustering of a batch of input examples in one pass. It estimates a distribution over the number of clusters k, and for each 1≤k≤kmax, a distribution over the individual cluster assignment for each data point. The network is trained in advance in a supervised fashion on separate data to learn grouping by any perceptual similarity criterion based on pairwise labels (same/different group). It can then be applied to different data containing different groups. We demonstrate promising performance on high-dimensional data like images (COIL-100) and speech (TIMIT). We call this “learning to cluster” and show its conceptual difference to deep metric learning, semi-supervise clustering and other related approaches while having the advantage of performing learnable clustering fully end-to-end

Injury risk is greater in physically mature versus biologically younger male soccer players from academies in different countries

Objectives: To investigate if maturity status was associated with injury risk in male academy soccer players. Design: Prospective cohort surveillance study. Setting: Professional soccer academies. Participants: 501 players (aged 9-23 years) from eight academies in England, Spain, Uruguay and Brazil. Main outcome measures: Players were grouped by maturity offset as pre-peak height velocity (PHV), circa-PHV, post-PHV or adult. Injury prevalence proportion (IPP) and days missed were recorded for one season per player, with training/match exposure recorded in a sub-sample (n=166). Results: IPP for all injuries combined increased with advancing maturity, with circa-PHV (p=0.032), post-PHV (p<0.001) and adult (p<0.001) higher than pre-PHV. IPP was higher in post-PHV and adult than pre-PHV for non-contact (p=0.001 and p=0.012), soft-tissue (both p<0.001), non-contact soft-tissue (p<0.001 and p=0.005), muscle (both p<0.001), thigh (both p<0.001), ankle (p=0.035 and p=0.007) and hamstring injuries (p=0.041 and p=0.017). Ligament/tendon IPP was greater in adult versus pre-PHV (p=0.002). IPP for growth-related injuries was lower in post-PHV than pre-PHV (p=0.039). Injury incidence rates (n=166) exhibited similar patterns to IPP in the full cohort. Conclusions: Injury patterns were similar between post-PHV and adult academy players but, crucially, relatively more of these groups suffered injuries compared to pre- and circa-PHV (except growth-related injuries)