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Abstract

BACKGROUND:

Supervised strength training has been shown to promote physiological adaptations in children and youth that can be beneficial to the process of physical growth and developed, and general health status.

METHODS:

Sixty-three children (9.2 $\pm$ 0.5 years old) were randomly assigned to training groups: multi jumps (MJ) ( $n=$ 20), sled towing (ST) ( $n=$ 21), or uphill running (UR) ( $n=$ 22). The following tests were applied before and after each intervention: vertical jump (ABK, CMJ, SJ), standing broad jump (SBJ), velocity/agility 4 $\times$ 10 m, and 20 m sprint test (ST ${}_{\text{20m}}$ ).

RESULTS:

After 8 weeks of training, statistically significant changes were found in all study variables for all groups, but with different effect sizes. In the ABK jump, the largest effect size was observed in UR (1.40, 0.97–1.85), while in ST and MJ it was medium (0.67, 0.44–0.98 and 0.48, 0.17–0.82, respectively). Similarly, the effect size in the SJ jump was large for the UR and ST groups (1.10, 0.78–1.51 and 1.30, 0.98–1.64, respectively) and medium in MJ (0.56, 0.24–0.97). However, the magnitude of the effect registered in SBJ was large in MJ (0.80, 0.55–1.15) and medium in UR and ST (0.56, 0.32–0.86 and 0.64, 0.42–1.013, respectively).

CONCLUSIONS:

All three training programs improve jumping and velocity/agility performance but based on clinical significance, UR and ST methods can be considered more efficient to improve physical performance in children.

Keywords

Exercise training muscle strength plyometric exercise physical education and training

1. Introduction

Differentiation along the lines of physical function specialization reflects the changes that occur at the morphological and neural levels during growth and development, which have an influence over motor performance. The scientific evidence in the last years has shown that properly prescribed and supervised strength training is an optimal tool to promote positive adaptations in children and youth during the development and maturation process [1]. Similar to other populations, strength training may not only optimize the expected increase in strength but also improve power, speed and motor performance in children [2]. These effects on physical performance are mainly attributed to neural adaptation mechanisms, such as improvements in unit motor recruitment and synchronization, stimuli frequency or neural myelination [3, 4].

Knowledge of mechanical and physiological aspects related to the training load, in addition to applied training stimuli during child development, are essential factors for effective training programming and for improving neuromuscular performance in school age. Thus, neurophysiological variables as hormonal parameters, neuroplasticity, inter and intramuscular coordination, and the maturation status play a central role in the ability to adapt to a specific training stimulus [5]. In this sense, the neuromuscular adaptation depends on the characteristics of the stimuli and the speed of growth before or after the peak height velocity (PHV) [6] which constitute a critical period for training in these ages dominated by “accelerated adaptation windows for training” or “training capacity windows” [7]. These are associated with the accelerated natural development of specific motor skill [7] and can also be benefited and enhanced with the application of strength training [1]. All these arguments demonstrate the importance of strength training in childhood.

Strength training favors motor performance, a fact that implies benefits in physical fitness and functionality in scholar children. In this way, the stimulation of motor performance at school age encompasses a dynamical and complex process that integrates the central nervous system and the muscular system, which benefits the quality in the execution of basic and complex motor skills that are reflected in the physical fitness. In fact, it has been shown that different strength training methods lead to improvements in various strength components such as strength performance [3], muscular power production [8], sprint [9], agility [10], and general motor performance [3], which are directly associated with regular physical activity at school age. However, current scientific literature presents new training methods [11, 12, 13] that have not been yet explored in school populations and that are potentially applicable in this population due to the practicality of their implementation.

Methods such as multi jumps (MJ), sled towing (ST) and uphill running (UR) have been shown to be efficient in improving strength in the athletic population. In fact, MS has been a widely studied method in the sports context and in children’s populations [14, 15], proving to be efficient for improving motor performance in variables such as travel speed [16] and jump height [17]. Likewise, the ST method has demonstrated its effectiveness for the stimulation of running speed in sprint athletes [11]. On the other hand, UR is a potentially effective intervention strategy to improve muscle power [18]. However, these exercise interventions have not been explored in the school population. In addition, it is not clear which strength training method turns out to be more efficient in order to improve motor performance, highlighting the need for further studies [1]. Thus, the aim of the present study is to identify the effects of three methods of strength training (MJ, ST and UR) during an eight-week training program on speed, vertical jump height, and velocity/agility variables in prepubertal schoolchildren.

2. Methods

2.1 Participants

Sixty-three boys in prepubertal stage (9.2 $\pm$ 0.5 years old; 135.4 $\pm$ 8.9 cm; 32.8 $\pm$ 8.3 kg; 17.7 $\pm$ 3.7 kg $\cdot$ m ${}^{-2}$ ) were recruited to participate in this study and were randomly assigned by lottery into three groups: UR ( $n=$ 22); MJ ( $n=$ 20) or ST ( $n=$ 21). The inclusion criteria were minimum participation of 80% in the intervention programs, absence of musculoskeletal injuries and/or bone malformations, and the written authorization from the parents to participate in this study. The protocol was in accordance with the latest revision of the Declaration of Helsinki and the current Colombian laws in relation to clinical research in humans (Resolution 008430/1993 Health Ministry). In addition, the research protocol was approved by the Ethics and Environmental Impact Committee of the University of Pamplona (Minutes No. 004 of the session of May 21, 2018).

2.2 Procedure

All tests were performed during school hours. The participants and their parents received the following instructions: a) to have a good night’s sleep before each test day, b) to have a good breakfast and being well hydrated before evaluations, and c) to wear the same athletic shoes during pre- and post-intervention tests. For the measurement of performance variables, the participants attended four practice sessions (familiarization) of the test procedures. Researchers spent twenty minutes of technical training for each test. Evaluations were applied at baseline and at the end of the intervention (pre- and posttest, respectively).

The motor performance tests started with the jumps and were followed by evaluations of speed and agility. The execution of the jumps was performed in the following order: first vertical jumps (Abalakov (ABK)), Counter Movement Jump (CMJ) and Squat Jump (SJ) and, secondly, the standing Long Jump Test or Broad Jump (SBJ). On the other hand, the assessment of velocity/agility was carried out through the 4 $\times$ 10 m shuttle run test (4 $\times$ 10 m) and the 20-meter test (ST ${}_{\text{20m}}$ ).

2.2.1 ABK jump height (ABK)

The ABK test was performed using an electronic contact platform system (Axón Jump, Bioingeniería Argentina). For the ABK jump, all schoolchildren started from an upright position with the weight evenly distributed on both feet and performed a vertical jump (arms swung back and forth in this boost phase). During the flight phase, the legs were fully extended [19]. Three attempts were completed with a 10–15 seconds rest in-between. Jump height was recorded in cm and the best performance attempt was used for the subsequent statistical analysis.

2.2.2 CMJ jump height (CMJ)

The CMJ test was performed using an electronic contact platform system (Axón Jump, Bioingeniería Argentina). All participants started from an upright position with their hands on the hips and performed a vertical jump. During the flight phase, the legs were fully extended [19]. Three attempts were completed, with a 10–15 seconds rest between them. Jump height was recorded in cm and the best performance attempt was used for the subsequent statistical analysis.

2.2.3 SJ jump height (SJ)

The SJ test was performed using an electronic contact platform system (Axón Jump, Bioingeniería Argentina). The participants were instructed to rest their hands on the hips, to put their feet shoulder-width apart, and to adopt a flexed knee position (90 degrees) for 3 seconds. After this, with the verbal command “set … Go!”, the subjects performed a maximum vertical jump without making any additional movement and without releasing their hands from the hips. Researchers ensured that legs were fully extended during the flight phase [19]. Three attempts were completed with a 10–15 seconds rest between them. Jump height was recorded in cm and the best performance attempt was used for the subsequent statistical analysis.

2.2.4 Standing broad jump (SBJ)

All participants stood behind the jump line, with their feet parallel and shoulder-width apart. They were instructed to bend their knees with arms in front of the body and parallel to the ground. From that starting position, they had to swing their arms, push hard and jump as far as possible. The children were explained to land with both feet and remain upright. Jump distance was measured from the take-off line to the point where the back of the heel closest to the take-off line landed [19]. Three attempts were completed with a 10–15 seconds rest between them. The length was recorded in cm and the best performance attempt was used for the subsequent statistical analysis.

2.2.5 4 $\times$ 10 m shuttle run test (4 $\times$ 10 m)

Two parallel lines separated by 10 meters were prepared. Three foams were placed (one at the start line [Sponge B, EB] and two in the second line [Sponges A, EA, and C, EC]). All subjects stood behind the starting line and the test began after a sound instruction. In the first displacement, the participant ran to the other line. In order to catch EA, the subjects had to pass both feet over the lines marked on the ground to start the next journey. Then they moved to the start line and changed EA for EB, then ran back to the opposite line, where they changed the EB sponge for the EC. Finally, the subjects went back to the starting line without slowing down until they crossed it [20]. An evaluator registered time in seconds with a chronometer.

2.2.6 20 m sprint test

A flat and straight field of 28 meters long was chosen. The running time was measured with an accuracy of 0.01 seconds using red infrared photoelectric cells (Globus Ergo System ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ , Codognè, Italy) [21]. The starting position was standardized with the preferred toe behind the starting line. The test began after a sound instruction. The photoelectric signal was placed at 20 m and was set at approximately 0.7 m in height (at hip level) to capture trunk movement and prevent false triggering of the limb [21].

Table 1
Characteristic of interventions

TW	Sled towing training			Multi jump training		Up hill running training
	% BW	S $\times$ R	Distance (m)	Exercise	S $\times$ R	Exercise	S $\times$ R	Distance (m)
1	2.5	3 $\times$ 10	5	Bounding	3 $\times$ 8
		2 $\times$ 5	10	Forward hop	3 $\times$ 10	Sprint	1 $\times$ 8	12
				Hurdle jump	2 $\times$ 10	Strides	1 $\times$ 2	12
2	2.5	2 $\times$ 3	10	BJ (20 cm)	2 $\times$ 10
	3.5	2 $\times$ 3	5	Bounding	2 $\times$ 10	Sprint	2 $\times$ 3	6
	2.5	1 $\times$ 2	15	Forward hop	2 $\times$ 10	Sprint	2 $\times$ 3	12
				Hurdle jump	2 $\times$ 10	Strides	2 $\times$ 3	6
3				BJ (30 cm)	3 $\times$ 10
	5	3 $\times$ 4	10	Bounding	3 $\times$ 10	Sprint	3 $\times$ 5	12
	5	3 $\times$ 4	15	Forward hop	4 $\times$ 10	Sprint	1 $\times$ 2	26
				Hurdle jump	3 $\times$ 10
4				BJ (30 cm)	3 $\times$ 10
	5	3 $\times$ 4	15	Bounding	3 $\times$ 10	Sprint	3 $\times$ 5	12
	5	3 $\times$ 4	10	Forward hop	4 $\times$ 10	Sprint	1 $\times$ 2	26
				Hurdle jump	3 $\times$ 10
5				BJ (40 cm)	3 $\times$ 15
	5.5	3 $\times$ 6	10	Bounding	3 $\times$ 20	Sprint	3 $\times$ 6	12
	5.5	3 $\times$ 5	15	Forward hop	4 $\times$ 15	Sprint	2 $\times$ 2	26
				Hurdle jump	3 $\times$ 15
6				BJ (40 cm)	4 $\times$ 15
	6.5	3 $\times$ 6	10	Bounding	4 $\times$ 20	Sprint	4 $\times$ 6	26
	6.5	3 $\times$ 5	15	Forward hop	5 $\times$ 15
				Hurdle jump	3 $\times$ 15
7				BJ (40 cm)	3 $\times$ 15
	8	3 $\times$ 6	10	Bounding	4 $\times$ 20	Sprint	4 $\times$ 6	26
	8	3 $\times$ 5	15	Forward hop	5 $\times$ 15
				Hurdle jump	3 $\times$ 15
8				BJ (40 cm)	3 $\times$ 15
	10	3 $\times$ 4	10	Bounding	4 $\times$ 20	Sprint	4 $\times$ 6	26
	10	2 $\times$ 2	20	Forward hop	4 $\times$ 15
				Hurdle jump	3 $\times$ 15

BJ, Box jump; %BW, body weight; R, Repetitions; S, sets; TW, training week.

2.3 Intervention

The eight-week intervention for the three groups had a frequency of three training sessions per week with a recovery period between sessions of 36 hours. All training sessions were held at the facilities of the Emilio Valenzuela School (Bogotá D.C., Colombia) during the physical education classes (one hour each class). The methodology of the MJ intervention was based on previous studies [22, 23]. The intensity of the training load was monitored using the session perceived effort rate (session-RPE) [24], for which the children were previously familiarized. The ST group towed a load between 2% to 8% of the body mass, the number of sprints ranged from 15 to 33 repetitions and the distance traveled varied between 5 to 15 meters per session, according to Rumpf et al. [25]. The UR group performed the uphill running on a route with two continuous ramps, 11 ${}^{\circ}$ and 12 ${}^{\circ}$ of inclination, respectively. The training volume ranged from 10 to 24 repetitions with distances between 12 meters (half stroke) and 26 meters (full stroke) according to the procedures proposed by Figueira et al. [26]. In addition, all progressions were linear (weakly increases) throughout the duration of the intervention. The specific training protocols for each of the groups are described in Table 1.

2.4 Statistical analysis

The change from baseline ( $\Delta=$ post-test – pre-test) for all variables was calculated and the results expressed as mean, standard deviation, and 95% confidence interval. The normality and homoscedasticity of the data were verified with the Shapiro-Wilk and Levene tests, respectively. The comparison of the baseline variables was performed with a one-way analysis of variance (ANOVA), and the post-hoc comparison with the Bonferroni test. The comparison between the groups was made with the analysis of covariance (ANCOVA), considering the measurement at the pre-test as a covariate, the Group (UR, MJ and ST) as a fixed factor, and the $\Delta$ as the dependent variable. In addition, the partial eta squared ( $\eta_{p}^{2}$ ) was presented as the effect size in this model. These procedures were performed with the statistical package SPSS version 25 (IBM Corp, Armonk, NY: IBM Corp) and a significance level of 0.05 was assumed for all tests. On the other hand, under the estimation approach, the 95% confidence intervals of Hedges’ $g$ were calculated with the package Data Analysis using Bootstrap-Coupled Estimation (Dabestr) 0.3.0 [27] within the R statistical computing environment version 4.0.0 [28].

Figure 1.

Consort flow diagram of the three study groups.

3. Results

All sixty-three schoolchildren completed the intervention and met the minimum attendance at supervised training sessions (80%) (Fig. 1).

3.1 Baseline data

The comparison of the variables at the baseline showed differences in ABK between the groups, specifically between ST and UR ( $P=$ 0.004), and MJ and UR ( $P=$ 0.022). Likewise, differences were found between ST and MJ ( $P=$ 0.037) for SBJ (Table 2).

Table 2
Baseline of study participants

	ST ( $n=$ 21)		MJ ( $n=$ 20)		UR ( $n=$ 22)		$P$
Age (y)	9.2	$\pm$ 0.5	9.2	$\pm$ 0.5	9.1	$\pm$ 0.5	0.773
Height (cm)	133.3	$\pm$ 12.8	136.2	$\pm$ 6.2	136.8	$\pm$ 5.9	0.391
BW (kg)	31.9	$\pm$ 8.3	34.0	$\pm$ 10.7	32.5	$\pm$ 5.5	0.704
BMI (kg $\cdot$ m ${}^{-2}$ )	17.9	$\pm$ 3.8	18.1	$\pm$ 4.8	17.3	$\pm$ 2.3	0.767
ABK (cm)	25.9	$\pm$ 4.5	25.1	$\pm$ 4.7	21.4	$\pm$ 3.7	0.003 ${}^{\text{a,b}}$
CMJ (cm)	22.0	$\pm$ 3.4	21.9	$\pm$ 4.8	19.6	$\pm$ 3.1	0.071
SJ (cm)	21.3	$\pm$ 3.6	22.1	$\pm$ 5.8	19.6	$\pm$ 3.3	0.173
SBJ (cm)	114.2	$\pm$ 23.4	95.8	$\pm$ 22.7	103.5	$\pm$ 22.6	0.041 ${}^{\text{c}}$
4 $\times$ 10 m (s)	14.9	$\pm$ 1.9	16.0	$\pm$ 3.5	15.6	$\pm$ 1.8	0.322
ST ${}_{\text{20m}}$ (s)	4.4	$\pm$ 0.4	4.7	$\pm$ 0.7	4.5	$\pm$ 0.4	0.208

BMI, body mass index; ABK, Abalakov; CMJ, countermovement jump; SJ, squat jump; SBJ, standing broad jump; 4 $\times$ 10 m, 4 $\times$ 10 m shuttle run test; ST ${}_{\text{20m}}$ , 20 meters sprint test; ${}^{\text{a}}$ , difference between ST and UR ( $P=$ 0.004); ${}^{\text{b}}$ , difference between MJ and UR ( $P=$ 0.022); ${}^{\text{c}}$ , difference between ST and MJ ( $P=$ 0.037).

Table 3

Results of the study variables after 8 weeks of intervention

	ST ( $n=$ 21)		MJ ( $n=$ 20)		UR ( $n=$ 22)		Ancova
	X $\pm$ SD (95% CI)	$P$	X $\pm$ SD (95% CI)	$P$	X $\pm$ SD (95% CI)	$P$	$P$ ( $\eta_{p}^{2}$ )
ABK (cm)	3.1 $\pm$ 3.3 (1.6, 4.6)	$<$ 0.05	2.4 $\pm$ 2.7 (1.1, 3.7)	0.004	5.9 $\pm$ 3.7 (4.3, 7.6)	$<$ 0.05	0.530 (0.02)
CMJ (cm)	4.4 $\pm$ 5.2 (2.1, 6.8)	$<$ 0.05	3.6 $\pm$ 2.8 (2.2, 4.9)	$<$ 0.05	3.5 $\pm$ 3.0 (2.2, 4.9)	$<$ 0.05	0.610 (0.02)
SJ (cm)	6.0 $\pm$ 3.8 (4.3, 7.7)	$<$ 0.05	3.3 $\pm$ 4.3 (1.3, 5.3)	0.002	4.5 $\pm$ 4.3 (2.6, 6.4)	$<$ 0.05	0.807 (0.01)
SBJ (cm)	14.4 $\pm$ 14.1 (8.0, 20.8)	$<$ 0.05	17.8 $\pm$ 10.2 (13.0, 22.6)	$<$ 0.05	11.5 $\pm$ 10.7 (6.8, 16.3)	$<$ 0.05	0.703 (0.01)
4 $\times$ 10 m (s)	$-$ 1.9 $\pm$ 1.6 ( $-$ 2.6, $-$ 1.2)	$<$ 0.05	$-$ 2.7 $\pm$ 2.4 ( $-$ 3.8, $-$ 1.6)	$<$ 0.05	$-$ 2.0 $\pm$ 1.2 ( $-$ 2.5, $-$ 1.5)	$<$ 0.05	0.637 (0.02)
ST ${}_{\text{20m}}$ (s)	$-$ 0.4 $\pm$ 0.3 ( $-$ 0.6, $-$ 0.3)	$<$ 0.05	$-$ 0.5 $\pm$ 0.4 ( $-$ 0.7, $-$ 0.3)	$<$ 0.05	$-$ 0.4 $\pm$ 0.3 ( $-$ 0.6, $-$ 0.3)	$<$ 0.05	0.779 (0.01)

BMI, body mass index; ABK, Abalakov; CMJ, countermovement jump; SJ, squat jump; SBJ, standing broad jump; 4 $\times$ 10 m, 4 $\times$ 10, 4 $\times$ 10 m shuttle run test; ST ${}_{\text{20m}}$ , 20 meters sprint test.

Figure 2.

The paired Hedges’g for the three groups is shown on the Cumming estimation plot. ABK (A), CMJ (B), SJ (C), SBJ (D), 4 $\times$ 10 m (E) y ST20m (F). The raw data is plotted on the upper axes; each paired set of observations is connected by a line. On the lower axes, each paired mean difference is plotted as a bootstrap sampling distribution. Mean differences are depicted as dots; 95% confidence intervals are indicated by the ends of the vertical error bars (27).

3.2 Outcomes

After 8 weeks of training, statistically significant changes were found in all study variables for all groups (Table 3), but with different effect sizes (Fig. 2). In the ABK jump, the largest effect size was observed in UR (1.40, 0.97–1.85), while in ST and MJ it was medium (0.67, 0.44–0.98 and 0.48, 0.17–0.82, respectively). Similarly, the effect size in the SJ jump was large for the UR and ST groups (1.10, 0.78–1.51 and 1.30, 0.98–1.64, respectively) and medium in MJ (0.56, 0.24–0.97). However, the magnitude of the effect registered in SBJ was large in MJ (0.80, 0.55–1.15) and medium in UR and ST (0.56, 0.32–0.86 and 0.64, 0.42–1.013, respectively).

In relation to speed/agility capacity, the changes observed in the 4 $\times$ 10 m presented a large effect size in all groups, namely UR ( $-$ 1.29, $-$ 1.73– $-$ 0.89), ST ( $-$ 1.21, $-$ 1.67– $-$ 0.80) and MJ ( $-$ 0.97, $-$ 1.49– $-$ 0.63). In the case of ST ${}_{\text{20m}}$ , the magnitude of the effect in the UR and ST group was large ( $-$ 1.22, $-$ 1.78– $-$ 0.78 and $-$ 1.14, $-$ 1.59– $-$ 0.65, respectively), while in MJ the magnitude was medium ( $-$ 0.746, $-$ 1.05– $-$ 0.48). On the other hand, the analysis of covariance determined that there was no difference between the groups for any of the study variables ( $p>$ 0.05).

4. Discussion

The aim of this investigation was to evaluate the effects of three methods of strength training (MJ, ST and UR) on variables of performance jump and velocity/agility in prepubescent schoolchildren. The results indicate that all three strength training methods were effective for developing improvements in physical performance ( $P<$ 0.05). In terms of effect size, UR was the more effective method for the stimulation of muscle power. These results could be related to the neuromuscular requirements associated with this kind of training [29]. Thus, the results of Roberts and Belliveau [30] suggest a change in the mechanism of muscular mechanics vantage, establishing that when running over a flat surface there is a low contribution of the hip joint moment. This happens as the movement is developed with a favorable mechanical vantage, which decreases the joint moment, while in the uphill running the mechanical advantage is less favorable and joint moment are necessarily high, which requires a higher muscular activation for performing the movements. Additionally, we can speculate that repetitive stimuli of the UR method could provoke chronic adaptation in the capacity for generating joint moments at the hip. This might be used in the vertical jump, which reinforces the importance of hip kinematic in this movement [31]. Therefore, it has been evidenced that uphill running increases electromyography activity in hip and knee flexors and extensor muscles, mainly in concentric phases for these muscles [29]. Hence, all that evidence seems to justify the positive effect of this training method over the jumping capacity.

On the other hand, our results also indicated that ST was a very effective method to improve jump performance, The potential explanations can be extrapolated from the evidence presented in studies developed in the adult population which has demonstrated an enhancement in the neuromuscular adaptations related to intramuscular coordination [32, 33]. In fact, our results are in line with the evidence presented by Harrison et al. [33], who established significant improvements in SJ due to the increase of muscular strength in the initial phase after a six-week intervention of ST training in rugby players. Moreover, the results presented by Gil et al. [32] support our assumption, since the evidence showed by these authors revealed significant improvements in SJ after a six-week intervention of ST in elite soccer players (percentage of change in SJ $=$ 5.16%). Authors argue that those improvements in jump performance were supported in the subject’s increase in mean power and propulsive power. In addition, Pareja-Blanco et al. [34] evidence significative improvements in ABK after an eight-week intervention with a combined training (ST with light load and squats) (percentage of change in ABK $=$ 6.5%). This could be explained by the fact that extensor knee muscle power traditionally has been related to jumping height and acceleration in the sprint [35]. In consequence, ST is a very efficient method for improving jump performance due to the increase in muscle strength, maximum force and the rate of force development [36, 37]. It could be established that the main effect of the ST method is the improvement of muscle power by the increase in the activation and synchronization of fast motor units. Likewise, the ST method seems to increase motor performance through the increase of muscle force in the horizontal or vertical direction [38, 39], depending on the direction of application of the load during the exercises.

In relation to MJ, improvements were also found in the study variables. These results coincide with the findings of other studies that have reported that plyometric training, combined with strength training or as a complement to the training session, improves strength indicators such as jumping performance [40, 41, 42, 43]. Other studies have also shown that plyometric training improves strength in lower limbs, such as the long jump, in prepubertal schoolchildren [44]. The results evidenced in this study can also be related to the fact that MJ training improves jump performance by increasing the stretch-shortening cycle [45], relegating neuromuscular effects to the background. However, these results must be analyzed under two considerations that have important practical implications. First, it should be considered that most of these studies are plyometric interventions that do not include only jumping, but a combination of plyometric exercises or are complements to a training session; second, the effects the level of physical fitness of the children must be taken into account, as reported by Kotzamanidis [46], who demonstrated a minor plyometric training effect in children without previous training, in comparison to other studies developed with athletic children [35, 43, 47].

After the post-intervention assessment, our study revealed that the sprint speed was benefited no matter the training method. One possible reason for this was evidenced by Moran et al. [5], who reported greater neuronal and motor development and better quality and coordination of movement. Hence, our data is in line with the observations of Philippaerts et al. [6], where it was concluded that the sensitive phase for the development of velocity ( $-$ 0.4 s $\cdot$ year ${}^{-1}$ ) is around the PHV. It has been documented that maximum sprint speed appears to develop around and after PHV, and this is related to stride frequency and contact time [48]; however, speed can be improved at different stages of a child’s maturation. In this sense, the accumulated evidence shows that velocity improves with plyometric training [49, 50], strength or combined training [51] before PHV or prepubescent in children. This is consistent with our results since we found improvements in speed with the different training protocols (ST, UR, and MJ).

5. Conclusion

Our results suggest that the three training methods improved jump performance and velocity/agility in prepubescent schoolchildren, without finding significant differences between the groups. However, in terms of the magnitude of the effect, the greatest changes were found in the order UR, ST, and UM. Based on this, we recommend these training strategies under professional supervision to improve motor performance in prepubertal schoolchildren.

Author contributions

CONCEPTION: Diego A.R. Jaimes and Dennis Contreras.

PERFORMANCE OF WORK: Johanny G. Cárdenas.

INTERPRETATION OR ANALYSIS OF DATA: Diego A.R. Jaimes, Jorge L. Petro, Diego A. Bonilla and Dennis Contreras.

PREPARATION OF THE MANUSCRIPT: Diego A.R. Jaimes, Jorge L. Petro, Diego A. Bonilla, Ailin O. Duarte and Dennis Contreras.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Diego A.R. Jaimes, Jorge L. Petro, Diego A. Bonilla and Dennis Contreras.

SUPERVISION: All authors.

Ethical considerations

The research protocol was approved by the Ethics and Environmental Impact Committee of the University of Pamplona (Minutes No. 004 of the session of May 21, 2018). Parents of all participants provided written informed consent.

Funding

The authors report no funding.

Footnotes

Acknowledgments

The authors would like to thank the Manuel Samper Alum (principal) and Juan Carlos Bello (vice-principal) at the Fundación Colegio Emilio Valenzuela (Bogotá, Colombia) for their support on this research by providing the instruments of intervention, measurement and infrastructure. We also express our sincere thanks to all the children who participated in this study and we thank the teachers who assist in the intervention.

Conflict of interest

The authors declare that they have no competing interests.

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Effects of three 8-week strength training programs on jump,speed and agility performance in prepubertal children

Abstract

BACKGROUND:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Procedure

2.2.1 ABK jump height (ABK)

2.2.2 CMJ jump height (CMJ)

2.2.3 SJ jump height (SJ)

2.2.4 Standing broad jump (SBJ)

2.2.5 4 × 10 m shuttle run test (4 × 10 m)

2.2.6 20 m sprint test

Table 1 Characteristic of interventions

2.4 Statistical analysis

3.1 Baseline data

Table 2 Baseline of study participants

4. Discussion

5. Conclusion

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

2.2.5 4 $\times$ 10 m shuttle run test (4 $\times$ 10 m)

Table 1
Characteristic of interventions

Table 2
Baseline of study participants