An investigation into the capabilities and affecting factors of isometric mid-thigh pull force production in elite youth soccer players

Elite youth soccer player’s performance depends on a multitude of factors(Stølen et al. 2005). Muscular force production capability is of likelyimportance due to the frequency of powerful actions that occur within matchplay (Faude et al. 2012). Little is known though of the training undertaken atelite youth soccer academies to increase muscular force productioncapabilities particularly across maturation groups. There is also little evidencearound the nature, or genetic influence on this variable in these populations.Greater knowledge on these areas would be beneficial to aid anunderstanding of performance and to plan practical interventions. 
Eight weeks of training for players in the under 9 (U9) through to under 21(U21) age groups (Chapter 3) were recorded to investigate the duration ofeach training type completed. The total training duration increased from U9 toU14 before reducing at U15 and then remaining unchanged through to, andincluding, the U21 age group. Soccer training accounted for 97 ± 4 % ofsession time in the U9 to U14 groups and 74 ± 3 % in the U15 to U21 groups.The remainder of training was made up of work that was not soccer based.The data in this case study suggest that training time is focussed on thetechnical/tactical development throughout the academy, particularly in theyounger age groups. 
Study 2, part A (Chapter 4) provided baseline isometric maximal voluntaryforce (MVF) data for players and an maturation-matched non-elite controlgroup. MVF was slightly higher in the elite compared to control cohorts during an isometric mid-thigh pull (IMTP, 118.29 ± 13.47 N compared to 109.69 ±17.00 N). Such data may indicate that ability to produce force, specificallyisometric MVF, may not be a crucial determinant of performance in elite youthsoccer based on this sample. 
The purpose of study 2, part B (Chapter 4) was to establish the effect of 8weeks typical training on elite youth soccer players’ IMTP MVF. This was alsocompared to a non-elite control cohort matched for maturation status and notundertaking training. Isometric MVF did not change in either group over the 8week period (P = 0.386). These data suggest that this elite youth soccertraining simply maintained current strength levels and was not sufficient toelicit increases in isometric MVF.
Study 3 (Chapter 5) examined variations in four separate genes, all identifiedas potentially having an influence on muscular force production capabilities:PPARA rs4253778, NOS3 rs2070744, COLIA1 rs2249492 and VDRrs2228570. Allele and genotype frequency was determined along with theinfluence of those single nucleotide polymorphisms (SNPs) on isometric MVF.Only NOS3 was different in genotype distribution between cohorts with TTgenotype showing 45.0 % and 30.4 % frequency in elite and control cohortsrespectively (P < 0.001). Furthermore, no difference was seen betweencohorts for isometric MVF data when comparing influence of any genotypes ofany gene. These data provide novel information around genotype frequencyin this population and would suggest that the gene variations examined heremight not play a role in force production in these populations.
Overall, these findings suggest that muscular force production may beimportant for elite youth soccer performance, but that current trainingpractices to improve this characteristic should be revised. This seemsespecially relevant given the lack of genetic association with force productionin the gene variations examined here, i.e. elite youth soccer players do notappear to be selected based on a genetic predisposition for greater isometricforce production capabilities.