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The aim of this study was to compare the effects of two velocity-based resistance training (RT) programs using moderate loads (45-60% 1RM) but different magnitudes of velocity loss (VL) limits (10% vs. 30%) on the changes in physical performance in young soccer players. Twenty young soccer players were randomly allocated into two groups: VL10% (n = 10) and VL30% (n = 10). All participants were assessed before and after the 8-week RT program (twice a week) involving the following tests: 20 m running sprint (T20), countermovement jump (CMJ), kicking a ball (KB), and progressive loading test in the full squat (SQ) exercise. The RT program was conducted using only the SQ exercise and movement velocity was monitored in all repetitions. Significant 'time × group' interaction (p < 0.05) was observed for sprint performance, KB and 1RM in the SQ exercise in favor of VL10%. No significant changes between groups at post-test were observed. The VL10% resulted in significant (p < 0.05-0.001) intra-group changes in all variables analyzed, except for KB, whereas VL30% only showed significant (p < 0.05) performance increments in a sprint test and 1RM in the SQ exercise. The percentage of change and the intra-group's effect size were of greater magnitude for VL10% in all variables analyzed compared to VL30%. In conclusion, our results suggest that, for non-trained young soccer players, squat training with low to moderate relative loads and 10%VL is sufficient to elicit significant increases in muscle strength and sport-specific actions compared to 30%VL in the set.

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The upper body and trunk muscles are crucial to perform soccer kicks. Resistance training targeting these muscles may modify the pattern adopted during kicking. This study aimed to investigate the effect of resistance training of the arm and anterior trunk muscles on instep kicking kinematics. Twenty-six male participants were randomly allocated into a training group or control group. The training group underwent resistance training of arm and trunk muscles and practiced the instep kick for 8 weeks. The control group only practiced kicking during the same period. The trunk, hip, and knee kinematics were assessed during the instep kick before and after the intervention. Kinematics were analyzed according to their data distribution with statistical parametric or nonparametric mapping. The effect of the training on the 1-repetition maximum test was analyzed using a repeated-measures multivariate analysis of variance. The training group showed greater hip extension after the training during the backswing phase (Hedge g effect size of 0.316-0.321) and increased 1-repetition maximum for all exercises. There were no other differences. The present study documented the nonlocal effect of strengthening training in which arm and trunk muscle training resulted in changes in hip kinematics during the backswing phase of the instep kick.

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El objetivo de este estudio fue comparar la aptitud física entre jóvenes futbolistas colombianos y describirlas según el pico de velocidad de crecimiento. Participaron 58 jugadores de edades comprendidas entre 13,46 y 15,31 años. Se evaluó una antropometría básica y se determinó el pico de velocidad de crecimiento PVC, también se aplicaron seis pruebas físicas relacionadas con fuerza, velocidad y agilidad. Se realizaron gráficas de dispersión de las pruebas físicas según el PVC, múltiples correlaciones entre variables y, finalmente, la población se dividió en tres grupos según el club al que pertenecen los jugadores haciendo una comparación ANOVA entre club A, club B y club C. Aparentemente, el PVC describe las pruebas de fuerza-potencia en mayor medida que las pruebas de agilidad según la apreciación de las gráficas de dispersión. En las múltiples correlaciones se evidencia una gran asociación entre el CMJ con las otras pruebas; en la comparación de clubes, el club A tuvo jugadores más altos, más pesados y con un PVC más adelantado, así mismo presentó mejor rendimiento en las pruebas, principalmente en CMJ F=13,813 P<0,01, Lanzamiento Balón Medicinal F = 11,053 P<0,01, Velocidad de Remate F = 18,916 P<0,01, y Repeated Sprint Ability F=12,950 P<0,01. Se concluye que la influencia de las condiciones antropométricas favorables se relaciona con un mejor desempeño en las pruebas físicas en esta población de jóvenes futbolistas que se encuentran en la época del salto de crecimiento.

The purpose of this study was to compare soccer players' physical fitness and describe them according to Peak Height Velocity (PHV). The participants were 58 young soccer players between 13.46 and 15.31 years old. Basic anthropometry was evaluated, and Peak Height Velocity was determined. Also, six physical tests related to strength, speed, and agility were measured. The physical test was plotted according to PHV, with multiple Pearson correlations among variables. Finally, a sample was compared in an ANOVA, considering the club to which each player belonged to (club A, club B, and club C). It seems that PHV describes strength and speed better than the capacity for agility. The CMJ test had strong correlations with other physical tests, and in ANOVA comparisons, club A had taller, stronger players, with an advanced PHV in comparison to clubs B and C. Likewise, Club A had a better performance in physical tests, mainly in the CMJ F=13.813 P<0.01 medicine ball throw F= 11.053 P<0.01, kicking speed F= 18.916 P<0.01, and repeated sprint Ability F=12.950 P<0.01. In conclusion, favorable anthropometric conditions influence better performance in physical tests in this population of young soccer players in their growth spurt period.

O objetivo deste estudo foi comparar a aptidão física entre jovens futebolistas colombianos e descrever a aptidão de acordo com o pico de velocidade de crescimento. Participaram 58 jogadores com idades entre 13,46-15,31 anos. Foi avaliada uma antropometria básica e determinado o pico de velocidade de crescimento PVC, também foram aplicados seis testes físicos relacionados à força, velocidade e agilidade. Foi usado um gráfico de dispersão para os testes físicos de acordo com o PVC e também múltiplas correlações entre as variáveis, finalmente, os participantes foram divididos em três grupos de acordo com o clube que jogavam, fazendo uma comparação ANOVA entre clube A, B e C. Aparentemente o PVC descreve os testes de força-potência melhor que os testes de agilidade. Nas correlações múltiplas fica evidente uma grande associação entre o CMJ e os demais testes; e na comparação de clubes, o clube A teve jogadores mais altos, mais pesados e com PVC mais avançado, e também apresentou melhor desempenho nos testes, principalmente em CMJ F=13,813 P<0,01 lançamento de bola medicinal F= 11,053 P<0,01, velocidade de remate F= 18,916 P<0,01, e nos sprints repetido F=12,950 P<0,01. Conclui-se que a influência das condições antropométricas está relacionada ao melhor desempenho em testes físicos nesta população de jovens futebolistas colombianos que estão no período de velocidade do crescimento.

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The purpose of this study was to investigate the angular kinetic energy transfers and expenditure among the trunk (bisegmented), the pelvis and the kick limb during maximal soccer instep kicking, and to characterize kicking kinetics and kinematics. Eighteen adult male amateur soccer players (24.0 ± 4.1 years old) were assessed. Three-dimensional kinematics and ground reaction force were measured. A 6-degrees-of-freedom model was assumed, comprising the upper trunk, lower trunk, pelvis, thigh, shank and foot, and the thoraco-lumbar, lumbo-pelvic, hip, knee, and ankle joints. Angular kinematics and joint moments were computed. Power flow analysis was done by calculating the joint powers (to describe joint-to-segments energy transfers) and the proximal and distal segment powers (to describe segment-to-segment transfers). Power, kinematic and kinetic time series were presented to describe the energy flows' directions. The total mechanical energy expenditure (TMEE) at each joint was also calculated. The TMEEs pointed to substantial energy expenditure at the trunk (27% of the summed work produced by the analyzed joints). In the initial phases of kicking, the trunk generates downward energy flows from the upper to the lower trunk and from the lower trunk to the pelvis, and then to the lower limb, sequentially, which favors angular motions for ball contact. There is a formation and release of a tension arc only at the hip joint, and deceleration of the segments slightly sooner than ball contact, differently from theoretical accounts. There are energy flows, hitherto unknown, among the trunk, pelvis and kick limb, revealing mechanical strategies of kicking.

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PURPOSE: The aim of the present study was to investigate the effects of muscular fatigue on the interaction between posture and movement during a lower limb pointing task. METHODS: Participants (n = 16), aged 18-30 years, kicked a ball toward a target in four conditions of fatigue: No muscular fatigue (NF), fatigue in the kicking (FM) or postural limb (FP) alone, and fatigue in both limbs (FMP). The mean amplitude and speed of the centre of foot pressure (CoP) and centre of mass (CoM) displacements were estimated through a force platform and an optoelectronic system, respectively. In addition, surface electromyography (EMG) of the biceps femoris, rectus femoris, medial gastrocnemius, and peroneus longus was recorded to investigate the anticipatory postural adjustments (APAs). RESULTS: Muscular fatigue yielded a decreased kicking accuracy (p < 0.001) and an increased time to perform the movement (p < 0.001), mainly during the backswing motion. In addition, significant increases in the mean amplitude and speed of the CoP and CoM displacement were found in the anteroposterior (AP) and mediolateral (ML) axes (ps < 0.001), especially when both limbs were fatigued. The EMG analysis confirmed that fatigue modified the way APAs were generated. During fatigue, postural muscle activity increased, but was delayed with respect to movement onset (ps < 0.001). This pattern of response was more consistent when both limbs were fatigued (p < 0.001). CONCLUSION: The present results suggested an additive effect of fatigue and a functional adaptation and subsequent decrease in the overall variability of APAs, indicating that postural and motor processes are interdependent.

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O presente artigo teve por objetivo investigar os efeitos do reforçamento condicionado sobre a precisão do comportamento do "chute no gol" em jogadores de futebol em formação. A pesquisa foi realizada com dois participantes e compreendeu duas fases: 1) Linha de Base (LB) e 2) Fase de Reforçamento. Foram colocados quatro alvos no gol (inferior direito, inferior esquerdo, superior direito e superior esquerdo) para indicar aos participantes onde deveriam chutar. Na LB apenas foram registrados os acertos ou erros dos "chutes no gol" de cada participante. Na Fase de Reforçamento cada acerto fazia com que o participante ganhasse um ponto que, ao final das sessões, era trocado por dinheiro. Para um dos participantes aumentou-se a quantidade de reforços para a precisão num dos alvos (Ref.+ bônus). Com o objetivo de verificar se a porcentagem de acertos retornaria aos níveis basais foi possível repetir o procedimento empregado na LB1, suspendendo o reforçamento condicionado aos acertos nos alvos IE e SE para o participante P1 e nos alvos SE, ID e SD para o participante P2. Os resultados mostraram que ambos os participantes elevaram seus índices de acerto na Fase de Reforçamento em comparação aos dados da Linha de Base.

The aim of this article was to investigate the effects of conditioned reinforcement on the accuracy of the "kicking the ball into the goal" behavior in soccer players in training. The research was carried out with two participants and comprised two phases: 1)Base Line (LB) and 2) Reinforcement Phase. Four targets were placed on the goal (lower right, lower left, upper right and upper left) to indicate to the participants where they should kick. In LB only the hits or errors of the "kicking the ball into the goal" of each participant were recorded. In the Reinforcement Phase each hit scored one point that at the end of the sessions was exchanged for money. For one of the participants, the amount of reinforcement for the precision in one of the targets was increased (Ref. + Bonus). With the objective of verifying if the percentage of correct "kicks that made it into the goal" would return to Base Line levels it was possible to repeat the procedure used in LB1 for the participant P1 withdrawing the reinforcement to the hits in IE and SE targets and, for P2, to SE, ID and SD targets The results showed that both participants increased their hit rates in the Reinforcement Phase compared to the Base Line data.

El presente artículo tuvo por objetivo investigar los efectos del refuerzo condicionado sobre la precisión del comportamiento del "tiro hacia la portería" en jugadores de fútbol en formación. La investigación fue realizada con dos participantes y comprendió dos fases: 1) Línea de Base (LB) y 2) Fase de Refuerzo. Se colocaron cuatro blancos (inferior derecha, inferior izquierda, superior derecha y superior izquierda) en la portería para indicar a los participantes donde debían patear. En la LB sólo se registraron los aciertos o errores de los "tiros en la portería" de cada participante. En la Fase de refuerzo, cada acierto hacía que el participante ganará un punto, que al final de las sesiones, era intercambiado por dinero. Para uno de los participantes se aumentó la cantidad de refuerzos para la precisión en uno de los blancos (Ref. + Bonificación). Con el objetivo de verificar si el porcentaje de aciertos retornaría a los niveles basales, fue posible repetir el procedimiento realizado en la LB1, suspendiendo el reforzamiento condicionado en los aciertos en los blancos Inferior izquierda y superior izquierda para el participante P1, y en los blancos superior izquierda, inferior derecha y superior derecha para el participante P2. Los resultados mostraron que ambos participantes aumentaron sus índices de acierto en la fase de reforzamiento en comparación a los dados de la Línea de Base.

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BACKGROUND: Kicking performance is the most studied technical action in soccer and lower limbs kinematics is closely related to success in kicking, mainly because they are essential in imparting high velocity to the ball. Previous studies demonstrated that soccer leagues in different countries exhibit different physical demands and technical requirements during the matches. However, evidencewhether nationality has any influence in the kinematics of soccer-related skills has not yet been reported. The nationality of the players is an aspect that might be also relevant to the performance in kicking. OBJECTIVES: The aim of this study was to compare the lower limbs kinematic patterns during kicking, between Brazilian and Japanese young top soccer players. PATIENTS AND METHODS: Seven Brazilian (GA) and seven Japanese (GB) U-17 players performed 15 side-foot kicks each, with a distance of 20 m away from the goal, aiming a target of 1 × 1 m in upper corner, constrained by a defensive wall (1.8 × 2 m). Four digital video cameras (120 Hz) recorded the performance for further 3D reconstruction of thigh, shank and foot segments of both kicking and support limbs. The selected kicking cycle was characterized by the toe-off of the kicking limb to the end of the kicking foot when it came in contact with the ball. Stereographical projection of each segment was applied to obtain the representative curves of kicking as function of time for each participant in each trial. Cluster analysis was performed to identify the mean GA and GB curves for each segment. Silhouette coefficient (SC) was calculated, in order to determine the degree of separation between the two groups' curves. RESULTS: Comparison between the median confidence intervals of the SC showed no differences between groups as regards lower limb patterns of movements. Task accuracy was determined by the relative frequency that the ball reached the target for all attempts and no differences were found (GA: 10.48 ± 14.33%; GB: 9.52 ± 6.51%; P = 0.88). CONCLUSIONS: We conclude that lower limb kinematic patterns, in support and ball contact phases, are similar in young Brazilian and Japanese soccer players during free kicks when adopting the side-foot kick style.

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OBJETIVO: Caracterizar o padrão de chutes espontâneos de lactentes nas idades de um e dois meses, bem como verificar se o peso externo modifica o padrão dos chutes nessas idades. MÉTODOS: Oito lactentes foram filmados nas idades de um e dois meses, estando em supino em uma cadeira infantil reclinada a 0º, na qual havia um móbile na extremidade superior e um painel na extremidade inferior. O experimento teve a duração de seis minutos e 20 segundos, durante os quais foram observados os movimentos de chutes nas situações sem e com peso de 1/10 e 1/3 da massa do membro inferior do lactente. Os pesos externos foram adicionados nos tornozelos do lactente. Pela análise das imagens coletadas por filmadoras digitais, foram verificados a freqüência de chutes e de contatos dos pés em um painel, os movimentos uni e bipodais, a preferência podal e o padrão de coordenação intramembro. RESULTADOS: O teste qui-quadrado revelou aumento significativo da freqüência de chutes nas idades de dois meses e nas situações de peso de 1/10 e pós-peso. Na situação de peso de 1/3 da massa do membro, verificaram-se diminuição do contato dos pés no painel e, na idade de dois meses, aumento da freqüência de contatos. Em todas as situações e idades, houve predomínio por chutes unipodais, não havendo preferência por um dos membros. Além disso, o padrão de coordenação intramembro dos chutes foi caracterizado como em-fase em ambas as idades. CONCLUSÕES: Fatores intrínsecos como a idade e extrínsecos como o peso externo referente a 1/10 da massa do membro inferior promoveram o aumento da freqüência dos chutes espontâneos em lactentes nas idades de um e dois meses.

OBJECTIVE: To characterize the spontaneous kicking patterns among one and two-month-old infants, and find whether an external load can modify such patterns at this age. METHODS: Eight infants were filmed at the ages of one and two months, while in the supine position in a baby chair reclined at 0°. There was a mobile above the infants' arms and a board above their legs. The experiment lasted for six minutes and 20 seconds and, during this time, the infants' kicking movements with and without external loads of 1/10 and 1/3 of their leg mass were observed. The external loads were added to the infants' ankles. The analysis of the images collected using digital video recorders were used to verify the frequency of kicking, the frequency of foot contact with the board, the frequency of one or two-foot movements, foot preference and intra-limb coordination pattern. RESULTS: The chi-square test showed that the frequency of kicking was significantly increased at the age of two months and under the conditions of 1/10-load and post-load. Foot contact with the board was less under the condition of 1/3-load but increased at the age of two months. One-foot kicking predominated under all the conditions and at all ages, and the infants showed no preference between the legs. In addition, the intra-limb coordination pattern of the kicking was characterized as in-phase at both ages. CONCLUSIONS: Intrinsic factors such as the infants' age and extrinsic factors such as external loads of 1/10 of their leg mass promoted increased frequency of spontaneous kicking in one and two-month-old infants.