Power, velocity, and agility are fundamental aspects of a soccer player's performance because they are the basis for performing different actions such as high-velocity and short-duration movements (1–7s), sudden jumps, and changes in direction.1,2 Mara et al.3 observed the accelerations and decelerations that occur during an official game involving elite female soccer players (24.3 years of age), reporting that the players performed 423 (±126) accelerations and 430 (±125) decelerations at different velocities per game. The average time of accelerations and decelerations was±5s and±4s, respectively, and the mean and maximum distances traveled per effort were 1–4m and 2–8m, respectively, differing between each intensity category (p<0.001, partial η2=0.753–0.908). It was also identified that high-velocity movements constituted 1–11% of the total distance traveled during a match. The crucial events in soccer are mainly performed at high intensity, including linear sprints, vertical jumps, changes in direction, passes, tackles and many other actions that require increased strength and power generation by the muscles of the lower limbs.2 An increase in muscle contraction force of the lower limbs allows for better acceleration and velocity, which are the basis for dribbling and other fundamental skills in soccer.4 In addition, greater power can be produced by increasing the execution velocity of the force, which will be manifested in higher velocity at the moment of execution of a sports gesture.4

In this respect, several studies have demonstrated correlations between maximal squat strength and jump height, 10-m sprint, and 30-m sprint performances. For example, Wisloff et al.5 observed correlations (r=0.78 and 0.94) between the one-repetition maximum (1RM) in squats and in 10-m and 30-m sprint velocities in professional soccer players, and Chelly et al.6 identified significant correlations between the 1RM in squats and velocity over a distance of 5m (r=0.66) in young soccer players (17–19 years). Additionally, Peterson et al.7 identified a positive correlation in school athletes between 1RM in squats and sprint velocity over 36m (r=−0.60) as well as between the maximal strength in the extensor muscles of the leg and velocity over 18m (r=0.82) and 36m (r=0.85). Conversely, other studies8 did not observe correlations between maximal strength and velocity.

The positive effects of strength training (ST) on jumping and sprinting abilities in adult soccer players have been widely studied. ST has shown to have beneficial effects on the muscular power and motor skill performance of adolescent athletes. However, little information is available in the literature concerning children who play soccer. Most of the studies have been conducted with adolescent and adult soccer players.9

Although the importance of ST has a long history in sports training and a number of studies confirm this, the most efficient and appropriate method for its development remains controversial. This is due to the fact that the set of factors comprising the ST load are diverse (e.g., % 1RM, number of sets and repetitions, type and order of exercises, duration of breaks between sets and repetitions, and velocity of execution) and that neuromuscular and structural adaptations will differ according to the way each of them is handled.10

In this regard, there is contradiction with the velocity and %1RM at which the movement must be executed.10–16 Moreover, the adaptation mechanisms that occur at a certain velocity of training and their transfer to other types of movements are unclear. Several studies have indicated that ST at high velocities and low loads (30–60% 1RM)9–11,17,18 generates better performance results in vertical jump, sprint ability, maximal aerobic speed than does training at low velocities and high loads (70–95% 1RM).6,19,20

Therefore, the basis of the problem is associated with the gaps that exist in knowledge about the effects of training velocity13 and the need for studies that analyze the effects of ST on different physical abilities in children who play soccer. The present study aims to analyze and compare the effects of two types of ST (high velocity-low load vs low velocity-high load) on muscle structure and athletic performance in jumping and velocity of movement in children who play soccer.

Despite the importance of strength and speed in soccer, as far as we know, no study has compared the types of strength training and the changes that these produce in different fitness indicators girls’ soccer players

The purpose of this study was to compare the effects of two types of strength training, one based on the VMP and the other in the traditional method with loads of 80% 1RM, on the strength and maximum power of lower limbs, sprint ability, the vertical jump, pedaling power and muscle mass, in a group of girls soccer players.

Table 1 shows the descriptive characteristics of the different variables evaluated in each of the study groups before and after the twelve weeks of training. The training generated significant positive changes in the three groups. In VG, these changes were between <0.000 and <0.008 in all muscle strength and power tests and also in lower limb lean mass (LLLM). The RMG showed significant changes between <0.000 and <0.007 in the variables PMaxC, SJ, 1RMS, and LLLM, and the CG exhibitedchanges between <0.000 and <0.003 in PMaxC, PMaxRC, CMJ, SJ, PMaxS, and 1RMS.

According to the Tanner scale of physical development, there were no significant differences between the groups in either of the two components (development of breasts or pubic hair), which places all the girls at a similar stage of sexual maturation. Fig. 1 shows the results for each group and the consolidated results.

Regarding intergroup differences in the post-test results, statistically significant differences were observed between the VG and RMG (p<0.008) in CMJ (Fig. 2) and squat power (p<0.01) tests (Fig. 3), between the VG and CG (p<0.01) in CMJ (Fig. 2), squat power (p<0.000) (Fig. 3), and maximal squat strength (p<0.000), and between the RMG and CG in maximal squat strength (p<0.000) only (Fig. 4).

Figure 2.

Differences between the VG and RMG in CMJ, pre- and post-test after training in each of de groups.

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Figure 3.

Differences between the VG and RMG in squat power, pre- and post-test after training in each of de groups.

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Figure 4.

Differences between the VG and RMG in maximal squat strength pre- and post-test after training in each of de groups.

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An important element that stands out in this research is the efficiency of the training. Each participant of the VG group on average developed a total training volume of 15,921.3kg vs. 27,648kg of the RMG group, indicating 42% less in the total volume training, of equal and achievement and greater increases in the set of variables observed in the study, than the RMG group, which is of the utmost importance in the context of ST.

Several studies have shown a clear relationship between absolute and relative strength and performance in sprint velocity and jumping in adult athletes. However, this relationship has not been well studied in younger athletes.25 According to a literature review of this topic in the SPORTDiscus, PubMed, EBSCOhost, JSTOR, Ovid, ProQuest, ScienceDirect, Taylor & Francis, and Wiley Online Library databases, this is the first study to analyze the effect of two training programs on improving muscle power in children who play soccer, and it is also the first to include high-intensity sprint training in cycle ergometer sets.

The main finding of this study is that participants of the VG who performed strength and pedal training at high speed showed better results in all physical performance variables analyzed (lower limb strength, jumping, running) as well as in muscle mass. The results from this study suggest that only 12 weeks of training combined with typical tactical-technical soccer training could provide greater gains in performance than could ST with high loads or typical soccer training.

The VG and RMG demonstrated increases in LLLM. Training with loads of 80% of the maximal strength led to a slightly greater increase in muscle volume than did velocity training (5.7% vs 5.4%). Nonetheless, it is important to note that the VG performed 45% fewer repetitions than the RMG and obtained similar increases in the total muscular volume of the lower limbs. The greater number of repetitions performed by the RMG during the entire training period resulted in different metabolic and neural stresses,26 which may have a greater impact on the processes of muscular hypertrophy.

The results of this study are even more impressive if we consider that deep squat and bicycle sprint training was only performed for 12 weeks (36 sessions).

Although there is a consensus that ST leads to muscle hypertrophy, controversy remains with regard to which type of training generates a better response.27 Gains in muscle mass have been reported in several studies in which increases in muscle mass were indicated by measuring the circumference of the upper arm12 or by bioimpedance.27–29 Nimphius et al.30 implemented strength, power, and movement velocity training for 20 weeks in female softball players aged 18.1 years and observed changes in the characteristics of muscle architecture through magnetic resonance, which correlated with the percentage of variation in maximal strength, movement velocity, and jumping. In contrast, another study indicated no significant gains in muscle mass.6

Overall, changes produced by ST in preadolescent populations are a topic of debate. Despite observing significant improvements in muscle strength, Ramsay et al.31 did not identify muscle hypertrophy (as measured by computed tomography) in prepubertal boys. (9–11 years) after a 20-week high-intensity ST program. This would indicate that observed gains in peak power are due to neuromuscular adaptations, such as selective activation of the motor units, enhanced recruitment of motor units, and better synchronization.

Sufficient evidence exists of acute responses of anabolic hormones in men and women, regardless of age. Although any type of resistance exercise appears to stimulate secretion of anabolic hormones, a substantial hormonal response has not been shown, especially for GH, after high-intensity exercises with few repetitions.32 Goto et al.33 evaluated GH responses at different exercise levels (50–90% of 1RM). The results indicated that the exercises performed at 50% of 1RM caused marked increases in lactate and GH concentrations in the blood. These results support the observation that the VG achieved a greater increase in muscle mass than the other groups.

Although the three groups showed increases in maximal squat strength after 12 weeks of training at high speed, a maximum loss of speed of 20% resulted in a gain of 60.7% in maximum squat strength, which is significantly higher than the gains obtained by the other two groups (54% for the RMG and 25% for the CG), despite the fact that as indicated previously, the VG had a lower total volume per work session. These results are consistent with the increase in muscle mass of the lower limbs in the VG and RMG.

The gains in maximal strength obtained in the present study are consistent with those reported in the literature.12 Regardless, the conclusions are not convincing. Some studies reported greater strength gains when training was performed at high velocities,10–14 whereas others did not observe differences in strength gains between groups training at low or high velocities.16

Several studies have compared the effects of training at different velocities (low and high) for strength development but do not reach a consensus about the effect of movement velocity on strength increases associated with ST10–12 due to the number of variables (weight, sets, repetitions, execution velocity) that interact in the response to training; thus, the results are heterogeneous among studies.34 It is also likely that this discrepancy is influenced by methodological differences among studies. A plausible explanation of why several studies did not find greater strength gains with high-velocity training may be because repetitions were performed until muscle failure, which leads to a progressive and involuntary decrease in the velocity of the final repetitions17 thus, the velocity of the final repetitions becomes very similar between high- and low-velocity groups, with a tendency to equal the average training velocity.13

It is also important to note that although the three groups showed improvements in all tests, the VG presented the best results in power tests compared to the other two groups. In vertical jump height (CMJ), we observed an increase of 14.8% in the VG vs 1% in the RMG and 6.4% in the CG, and in squat power, we observed an increase of 30.4% in the VG vs 5.1% in the RMG and 14.1% in the CG.

In the tests of sprinting over 30m, and power on a cycle ergometer, despite not identifying significant differences among the three groups, the VG obtained better results for velocity over 30m (the VG exhibited an increase of 2.96% vs 0.49% in the RMG and 1.13% in the CG) and in power on the cycle ergometer (the VG exhibited an increase of 22.3% vs 16.32% in the RMG and 15.03% in the CG).

A possible explanation for this result may be that the higher number of repetitions performed by the RMG group at low speed (MPV<0.40m/s) could be responsible for a significant reduction in faster fibers.35 Strength and speed decreased between repetitions of the same set, which is concomitant with an increase in the degradation of purine nucleotides as individuals approach muscle failure, leading to a loss of strength.24

These results coincide with those of other studies, such as that of Chelly et al.,6 who reported similar results with a significant increase in the peak power obtained on a bicycle, in CMJ height, and in the velocity of movement, in a group of young soccer players. Pareja-Blanco et al.10 also indicated that ST performed at high velocities in a group of men (23 years) lead to improved stimuli that induce neuromuscular adaptations compared to ST with high loads and at low velocity. In a group of soccer players (17 years), Kotzamanidis et al.19 observed the effects of combined strength and speed training for 13 weeks, with a frequency of three times a week, and reported that combined strength and velocity training results in significant positive effects in strength, velocity, squat jump, and CMJ.

The mechanisms by which training velocity influences strength adaptations were not investigated in this study, but other authors have indicated that such mechanisms may include changes in the type of muscle fiber,36 the expression of Types IIX to IIA myosin heavy chains,15 or a greater ability to selectively recruit fast-twitch motor units. Fast-velocity training leads to higher rates of discharge in motor units and to an increase in the frequency of stimulation37; these improved responses may even be due to neuromuscular adaptations in response to the specificity of the tests used in training. These phenomena might be the basis of the greater response to training.

Studies with young soccer players have commonly used high-load repetitions (70–95% 1RM) to improve strength and power.6,20,38 However, ST with high loads can induce excessive fatigue, which does not allow players to have effective ball practice immediately after this type of training. Moreover, training with high loads is not transferable to sprint velocity performance because the nervous system cannot learn and control the levels of strength or muscle mass acquired in very fast movements

The recent review by Silva et al.,39 in which 22 studies on ST in soccer players of different ages(mostly older than 20 years) were considered, indicates that strength/power training independent of the players’ normal training led to increases, ranging from 11% to 52%,in the 1RM of well-trained players during the squat exercise.

They also noted that the average increase of 24.4% of 1RM in squats leads to an increase in the CMJ and SJ of approximately 6.8% and that increases of 23–26% in the 1RM resulted in average improvements of 1.9% in 40-m running speed, which indicates that the large increases in 1RM cannot be translated into superior improvements in sprint performance in high level players.

This series of movements has important practical implications for explosive sports such as soccer. A meta-analysis of 38 studies conducted by Harries et al.40 aimed to determine the effectiveness of endurance training programs on muscle power and athletic performance in adolescent athletes. The results indicated that ST improves jumping performance and sprint velocity.

The findings of the present study suggest that when no maximum losses of 20% of the MPV are allowed in each series, the applied force and speed will be greater in each repetition, leading to an increase in the average speed of the training35 and resulting in neuromuscular adaptations necessary for sports that involve power actions such as soccer.

This study is one of the first to examine the effects of two ST programs, one based on VMP and the other at 80% of 1RM, on the neuromuscular performance and muscle composition of girls who play soccer. The findings contribute to consolidate the evidence observed by several authors in relation to the positive effects of the speed of execution in a ST program,22,35 which distorts the hypothesis that intensity is determined solely by the magnitude of% 1RM. The results indicate, on the contrary, that the execution speed is a fundamental component of the ST intensity.

In conclusion, the results of the present study suggest that strength training performed at high speed (0.70ms−1) compared to one performed at low speed (0.47ms−1) is more efficient and produces excellent stimuli that would lead to induce better neuromuscular adaptations. They also seem to indicate that high loads and many repetitions are not required to significantly improve maximum strength and, more importantly, sports performance. High-speed training generated more effective gains than low-speed training, at maximum squat strength, CMJ, short-distance sprint and muscle mass.

This study was funded with resources from the National Pedagogic University of Colombia (Universidad Pedagógica Nacional de Colombia) and Saint Thomas University of Colombia (Universidad Santo Tomas de Colombia).

The manuscript was prepared and reviewed with the participation of all of the authors, who declare that there are no conflict of interests that jeopardize the validity of the results presented.