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Effects of Vest and Sled Resisted Sprint Training on Sprint Performance in Young Soccer Players: A Systematic Review and Meta-analysis

Fernández-Galván, Luis Miguel1; Casado, Arturo2; García-Ramos, Amador3,4; Haff, Guy Gregory5,6

Author Information
Journal of Strength and Conditioning Research: July 2022 - Volume 36 - Issue 7 - p 2023-2034
doi: 10.1519/JSC.0000000000004255
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Abstract

Introduction

Modern soccer is characterized by a high playing speed which in turn is reflected by a great number of high-intensity actions and a high speed in ball circulation (7,14,79). Global positioning systems (GPS) and video analysis allow coaching staff to quantify the activities conducted by players during a match or training (74). As a result of using these systems, it is possible to determine the specific features of these activities and their evolution across different seasons. For example, high-intensity running (19.8–25.1 km·h−1) and sprinting (>25.1 km·h−1) actions increased by ∼30% between 2006–2007 and 2012–2013 seasons (7) in Premier League. In addition, players regularly initiate their all-out sprints from movements of moderate speeds (43,86). Most of these high-intensity actions, such as sprints or accelerations, jumps, rotations, and rapid change-of-direction (COD) maneuvers, are present in the decisive actions contained within a soccer match (29,73). Thus, sprint performance over distances of 10 m or less, and the speed attained during the first step are considered to be key indicators of performance potential in soccer players (17,18). Particularly, the ability to accelerate over very-short distances (e.g., ≤5 m) is considered to be a critical component of successful match performances (7,14,29). Therefore, coaches and researchers are constantly looking for better and more effective training methods that have the potential to improve and optimize the acceleration capability of professional soccer players.

In a 100-m race, there are several clearly defined phases, which can be simplified into 2 key phases. First, the acceleration phase is characterized by the start of the sprint where the athlete initiates the sprint from a semistatic position and then increases their speed over a short period of time. Second, the maximum-velocity phase where the athlete moves at high speed and attempts to maintain that speed across the remainder of the race (86). Performance during the maximum-velocity phase is typically evaluated as the time needed to complete 10 meters distance from the 30-m line to the 40-m line. In team sport athletes, acceleration capacity has been typically measured as the time needed to complete the initial 10 meters distance (0–10 m), whereas the maximum-velocity capacity has been identified as the time needed to complete the distance from the 30-m line until the 40-m line (30–40 m) (24,36). Strength training is considered to be crucial in the long-term development plan of soccer players (45,58). Contemporary models have proposed that strength training is important in preadolescence, highlighting a neural plasticity associated with prepubertal players that supports targeting muscular strength development during this time period to enhance neuromuscular adaptations, such as intramuscular and intermuscular coordination (44). Possessing high level of maximal strength has been positively linked to sprint and vertical jump performance (85), team success (5), and player performance (34). In their systematic review, Seitz et al. (75) reported significant correlation between lower body strength (i.e., squat strength) and the increase in sprint performance (r = −0.77; p = 0.0001). In addition, it was noted that the improvement in sprint time was modulated by the body mass (BM) (r = 0.35; p = 0.011), level of practice (p = 0.03), frequency of resistance training sessions per week (r = 0.50; p = 0.001), and rest interval between sets of resistance-training exercises (r = −0.47; p ≤ 0.001). Therefore, it is clear that there is a need to identify effective training methods aiming to improve maximal strength for the development of both single sprint performance at each phase and repeated sprint ability (RSA) in soccer players (8,31,75).

Given that sprint, acceleration, and power abilities are considered performance determinants in soccer, a great deal of research has focused on the impact of structured resistance training (12,17,25), plyometric training (12,18,67), strength training (21,26), COD (16,69), combined training methods (31), and resisted sprint training (RST) combined with traditional soccer training on soccer performance (4). Resisted sprint training with a loaded sled or a weighted vest has been reported to be effective at improving sprint performance during the early acceleration phase (4). These methods are generally used to increase the propulsive forces of lower-body muscles potentially increasing stride length during sprinting (46). These training stimuli promote the development of both the vertical and horizontal force components of sprinting (40). One of the main variables to consider in RST is the load applied (2). The added load will positively or negatively affect the kinetic (application of force) or kinematic (technical similarity) properties of the sprint (11). However, there is no clear evidence as to whether there is a specific load that allows for the maximization of performance improvements in the different phases of the sprint. Whereas performing RST with loads that are >15% BM or decreasing maximal velocity capacity more than 10% yielded changes in sprint kinematics (i.e., reduction of stride length and stride frequency) (1), this loading strategy also results in a performance improvement during the acceleration phase (1,46,69,77), no long-term negative effects on running technique have been observed (3,77). The training effects of sled towing with high loads (e.g., 30% BM) and concluded that this practice requires a higher horizontal force application have been reported in the literature (40,69,81). However, the same authors (40,69) performed an intervention with 2 experimental groups (low loads: 12.5–13% BM vs. high loads: 43–50% BM) without obtaining significant differences between the groups in the changes in sprint performance and horizontal rate of force development.

The effects of RST methodology on sprint performance have been previously reviewed in elite team sports adult athletes (1,2,4,33,49,65,77,83). Resisted sprint training was determined to be effective at improving performance during the acceleration phase (<10 m) (4,33,40,73,77,83), but was not more effective than unresisted sprint (URS) training (4,65,73,77,83). The determination of the load is a determinant aspect (2) influencing changes in sprint kinematics (23,39,46); however, there is not an optimal load for RST and it has been recommended that the resistive load should be selected according to the specific sport and physical condition of the athlete (4). Some authors indicated that the load should never exceed 20% of the subject's body mass (1,4,33,40,73,77,83) to not alter sprint kinematics. However, other authors reported that higher loads (≥20% of the body weight) are more effective as they increase the relative net production of horizontal momentum and propulsion (40,65). Of note is that the aim of RST is to increase force production capability and, therefore, acute changes in kinematics should not be an issue. Nonetheless, to our knowledge, no review article has focused on this specific topic in young soccer players despite the existing notable physiological and anatomical differences between children and adults in muscle architecture and size (61). Therefore, the objectives of this systematic review with meta-analysis are: (a) to determine the effect of vest and sled RST on the performance of the different sprint phases (initial [0–10 m], acceleration [0–30 m], and maximum-velocity [30–40 m]) in young (<20 years) soccer players, and (b) to elucidate whether the training equipment (sled or vest) and the magnitude of the load used in RST (above or below 20% of BM) influence the long-term training adaptations in sprint performance.

Methods

Experimental Approach to the Problem

This study was not evaluated by the Autónoma University of Madrid Institutional review board because it only uses descriptive data previously published in randomized control trials that have undergone institutional review. The descriptive data was obtained from the published articles and no raw data was reviewed or used in this study. Thus, this systematic review does not require institutional review as per Autónoma University of Madrid guidelines. This systematic review and meta-analysis adheres to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (64).

Literature Search

Electronic literature searches were conducted on the US National Library of Medicine (PubMed), Web of Science, and Google Scholar databases up to March 31, 2021. The systematic review included studies where at least one group received RST with sled or vest. The search was performed by 2 independent researchers (L.F. and A.C.) using the following keywords in English: “resisted sprint,” “resisted sled OR vest,” “sled OR vest training.” First, duplicate records were removed. Second, title and abstract of the articles were screened for potential eligibility. Third, a full-text read of potentially eligible studies was conducted. Authors were required to achieve a consensus on the included articles. In case of discrepancy between the 2 reviewers, a third author participated in the process until a consensus was reached. In addition, the reference list and citations of the studies that met the inclusion criteria were screened to find additional articles. Authors of the selected articles were contacted to request any relevant information that was missing from the article.

Subjects

Inclusion and Exclusion Criteria

The following inclusion criteria had to be met for a study to be considered in this review: (a) article written in English and published in a peer-reviewed scientific journal, (b) athletic population consisted of soccer players younger than 20 years (mean of the group), (c) sprint time (>5 m) was measured before and after training by an automated electronic machine (e.g., time gate or radar gun), (d) the experimental group performed sled or vest RST at maximum-velocity, (e) studies reported the load of the sled or vest and (f) the RST intervention lasted at least 4 weeks. Studies were excluded if the subjects did not perform all-out sprints during training or if the RST was performed with sled-push or running uphill. After critically analyzing the initial studies collected with the above criteria, a cohort of 12 studies was selected. Figure 2 shows the flow diagrams for the entire search process. The authors obtained/provided informed consents for the different studies.

Quality Assessment

Risk of bias and methodological quality of the included studies were independently assessed by 2 authors using the Cochrane Risk of Bias (RoB) assessment tool (37) and the Physiotherapy Evidence Database (PEDro) scale (50), respectively. The RoB tool includes the following items: selection bias (randomization sequence generation, allocation concealment), performance bias (blinding subjects, blinding therapists), detection bias (blinding outcome assessor), attrition bias (incomplete out-come data), reporting bias (source of funding bias/selecting outcome reporting), and other bias (sample size) and each item was classified as low-risk, high-risk, or unclear according to the Cochrane Collaboration's tool (37) (Figure 1).

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Figure 1.:
Risk of bias for included studies. Review authors' judgements about each risk of bias item presented as percentages across all included studies.

The PEDro score was used to evaluate the quality of the studies by assessing the following items: random allocation; concealed allocation; baseline between-groups similarity; subjects blinding; therapists blinding; assessors blinding; dropouts; intention-to-treat statistical analysis; between-groups statistical comparison; point measures, and variability data (50). A trial was considered of high-quality when the PEDro score was ≥5 of 10 points. All the studies included in the meta-analysis scored between 6 and 7 points, indicating a methodological “high quality” according to the criteria proposed by Moseley et al. (59).

Characteristics of Included Studies

A total of 246 studies were identified in the literature search. After eliminating duplicates and reviewing titles and abstracts, we read 132 articles. Afterward, 120 articles were excluded because they did not meet the selection criteria. The current systematic review consists of 12 articles (Figure 2).

F2
Figure 2.:
Flow diagram of the studies that underwent the review process. Criteria I: article written in English and published in a peer-reviewed scientific journal; Criteria II: athletic population consisted of soccer players younger than 20 years (mean of the group); Criteria III: sprint time (> 5 m) was measured before and after training by an automated electronic machine (e.g., time gate or radar gun); Criteria IV: the experimental group performed sled or vest RST at maximum-velocity: Criteria V: studies reported the load of the sled or vest and Criteria VI: the RST intervention lasted at least 4 weeks.

This systematic review included intervention studies (pre-post) that analyzed the effects of RST performed with sled or vest on performance during the phases of a sprint: acceleration (0–10 m), full sprint (0–30 m) and maximum velocity (30–40 m). A complementary analysis consisted of exploring the influence of the training equipment (sled or vest) and resistive load (<20 or ≥20% of BM) on the changes in sprint performance. A comparison between pre and post was made independently for each study, and also compared against the changes observed in the control group when possible. Only 5 studies included a control group; 3 of them continued with their usual soccer training (62,69,81) and 2 of them performed URS training (15,80).

Procedures

Data Extraction

We extracted from each eligible study, data relating to linear sprints over various distances (acceleration 0–10 m, full sprint 0–30 m, and maximum-velocity phase 30–40 m). Means, standard deviations (SD), and sample sizes (n) were extracted by one author (L.F.) from the included papers and were corroborated by a second author (A.C.). Any discrepancy between the authors was resolved through discussion with a third author (A.G.). Only studies that performed RST and reported the data needed to perform the meta-analysis were included. Seven meta-analyses were performed according to the sprint phases (acceleration 0–10 m, full sprint 0–30 m and maximum-velocity phase 30–40 m). In addition, in each meta-analysis, 3 subgroups were considered: sled versus vest, sled and vest with a load ≥20% of BM versus sled and vest with a load <20% of BM, and sled and vest postintervention versus the control group.

Statistical Analyses

Meta-analysis was conducted using a free software program (RevMan ver 5.3; The Nordic Cochrane Centre, Copenhagen, Denmark). Means, standard deviations, and sample sizes for a measure of postintervention performance within experimental group (pre-vs. posttest) and between groups (experimental vs. control group) were used to calculate an effect size (ES). Effect sizes were calculated by subtracting the preintervention mean value from the postintervention mean value (post–pre) for the experimental (Δ1) and control groups (Δ2). The net treatment effect size was obtained as Δ1 minus Δ2 divided by the pooled SD of baseline values (42). In studies that reported intermediate and postintervention values, only final values were compared against baseline.

Subgroup analyses were performed to evaluate the potential moderating factors of the training equipment (sled or vest) and resistive load (above or below 20% of BM). For continuous variables comparison, the cut-off values based on medians from the full sprint analysis were used as cut-off values for grouping studies. However, in specific cases, the cut-off was established in an arbitrary way (i.e., load). The SD was calculated as the square root of the summation of the squared SDs of the mean time in the known conditions. Standardized mean difference (SMD) can be interpreted as trivial (<0.20), small (0.20–0.49), moderate (0.50–0.79), and large (≥0.80) (78). Significant differences between subgroups were reported when the SMD of one subgroup was outside the 95% confidence interval of the other subgroup. An inverse-variance random-effects model was used because of the heterogeneous study methods and subject populations. Statistical heterogeneity was examined using chi-squared and I2-Index tests (78). A test for heterogeneity examines the null hypothesis that all studies are evaluating the same effect. The quantity, which we call I2, describes the percentage of total variation across studies that is because of heterogeneity rather than chance. An I2 value lower than 25% is considered to exhibit low heterogeneity, 50% as moderate heterogeneity, and 75% as high heterogeneity (37). Statistical significance of the overall result is also expressed with the probability value (p value) in the “test for overall effect.” A p value of ≤0.05 was considered statistically significant.

Results

Description of Included Studies

All included studies (6,10,15,19,27,62,68,69,72,76,80,81) performed RST with a vest or sled and only 5 studies included a control group. A total of 53 effect sizes were computed between postintervention and preintervention sprint times from 12 original studies, and 13 effects from 5 original studies examined the differences in the changes in sprint performance between the experimental and control groups. Of the 12 studies, only Upton (80) analyzed women. The age of the subjects ranged from 10.4 ± 0.8 to 19.8 ± 1.6 years. The number of subjects who performed the sled towing and vest RST intervention was 156 and 63, respectively. Three distances (0–10, 0–30, and 30–40 m) were considered for analyses. The studies included were training interventions that were performed for a duration between 4 and 8 weeks with a total of 12–24 training sessions. The total intervention sprint volume ranged from 560 to 6,240 m. Finally, the resistive load was individualized according to the percentage of subjects' BM and ranged from the 2.5% to the 55% of BM. The main characteristics of the selected articles are shown in Table 1.

Table 1 - Main characteristics of the subjects and resistive training programs of the studies included in the meta-analysis.*
Study N Gender Body mass (kg) Height (cm) Age (y) Variable reported Training
RST vest RST sled CG Post Pre Sessions Weeks S. volume (m) T. Volume (m) % BM Method
Vivas et al. (15) 11 Male 75.90 ± 12.40 180.00 ± 0.05 18.00 ± 1.60 T0–10 m’ S = 2.07 ± 0.06 2.10 ± 0.07 16 8 114.37 m 1830 m 10–20 Vest
T0–30 m’ S = 4.49 ± 0.16 4.55 ± 0.17
T20–30 m’ S = 1.17 ± 0.05 1.19 ± 0.05
13 Male 71.30 ± 7.50 178.00 ± 0.04 18.20 ± 2.20 T0–10 m’ S = 2.08 ± 0.02 2.11 ± 0.03 16 8 114.37 m 1830 m 10–20 Sled
T0–30 m’ S = 4.55 ± 0.07 4.61 ± 0.07
T20–30 m’ S = 1.2 ± 0.03 0.21 ± 0.03
12 Male 71.70 ± 10.90 178.00 ± 0.04 18.40 ± 2.40 16 8 Control
Uthoff et al. (81) 34 Male 58.70 ± 10.80 170.20 ± 7.90 14.00 ± 0.30 T0–10 m’ S = 1.89 ± 0.1 1.9 ± 0.1 16 8 112.50 m 1800 m 20–55 Sled
35 Male 56.30 ± 9.90 168.60 ± 10.10 14.40 ± 0.52 16 8 Control
Osorio et al. (69) 19 Male 63.90 ± 11.50 174.20 ± 8.10 18.80 ± 5.30 T–10 m’ S = 1.76 ± 0.1 1.83 ± 0.07 12 6 100 m 1,200 m 12.50 Vest
T 0–30 m’ S = 4.33 ± 0.21 4.36 ± 0.2 12 6 100 m 1,200 m 50
19 Male 63.20 ± 8.10 173.50 ± 6.10 17.70 ± 3.40 T0–10 m’ S = 1.73 ± 0.09 1.82 ± 0.13 Vest
T0–30 m’ S = 4.32 ± 0.29 4.37 ± 0.32
16 Male 64.70 ± 9.20 164.80 ± 4.00 17.80 ± 4.20 12 6 Control
Esquina et al. (62) 12 Male 69.40 ± 4.20 176.70 ± 2.20 17.00 ± 1.00 T0–10 m’ S = 1.7 ± 0.05 1.7 ± 0.06 14 7 40 m 560 m 20 Sled
12 Male 69.40 ± 4.20 176.70 ± 2.20 17.00 ± 1.00 T0–10 m’ S = 1.69 ± 0.05 1.71 ± 0.05 14 7 40 m 560 m 20 Sled
12 Male 69.40 ± 4.20 176.70 ± 2.20 17.00 ± 1.00 14 7 Control
Raya et al. (68) 8 Male 66.40 ± 4.80 176.90 ± 7.30 16.50 ± 0.30 T0–10 m’ S = 1.84 ± 0.06 1.89 ± 0.08 12 6 242.91 m 2,915 m 0–15 Vest
T0–30 m’ S = 4.33 ± 0.12 4.38 ± 0.18
Borges et al. (10) 9 Male 68.70 ± 9.20 175.00 ± 7.10 16.60 ± 0.60 T0–30 m’ S = 4.26 ± 0.17 4.31 ± 0.11 12 7 170.80 m 2050 m 10–13 Sled
Hoyo et al. (27) 12 Male 73.12 ± 2.50 178.24 ± 1.20 17.00 ± 1.00 T0–10 m’ S = 1.71 ± 0.06 1.72 ± 0.05 16 8 83.75 m 1,340 m 12.60 Sled
T0–30 m’ S = 4.19 ± 0.13 4.22 ± 0.12
T30–50 m’ S = 2.33 ± 0.08 2.37 ± 0.10
Sekine (76) 10 Male 60.30 ± 6.30 167.50 ± 4.90 16.50 ± 0.50 T0–10 m’ S = 2.18 ± 0.15 2.27 ± 0.17 24 8 260 m 6,240 m 20 Sled
Bachero et al. (6) 6 Male 70.20 ± 11.90 175.40 ± 6.70 19.80 ± 1.60 T0–10 m’ S = 1.77 ± 0.15 1.78 ± 0.05 14 7 151 m 2,115 m 20 Sled
T0–30 m’ S = 4.25 ± 0.07 4.28 ± 0.08
T20–40 m’ S = 2.07 ± 0.04 2.07 ± 0.06
Rumpf et al. (72) 14 Male 38.20 ± 15.60 141.00 ± 7.93 10.40 ± 0.80 T0–30 m’ S = 10 10.1 ± 0.96 16 6 220 m 3,520 m 2.50–10 Sled
18 Male 62.70 ± 11.00 173.00 ± 5.32 15.20 ± 1.60 T0–30 m’ S = 6.55 ± 0.44 6.95 ± 0.54 16 6 220 m 3,520 m 2.50–10 Sled
Upton (80) 9 Female 63.40 ± 6.90 166.90 ± 5.90 19.60 ± 0.90 T0–13.7 m’ S = 2.66 ± 0.16 2.65 ± 0.21 12 4 137 m 1,644 m 12.60 Sled
T0–36.6 m’ S = 5.84 ± 0.21 5.9 ± 0.23
T22.9–32 m’ S = 1.87 ± 0.31 1.89 ± 0.32
10 Female 63.40 ± 6.90 166.90 ± 5.90 19.60 ± 0.90 12 4 Control
Clark et al. (19) 6 Male 79.10 ± 5.26 182.25 ± 8.30 19.79 ± 0.90 T18.3–54.9 m’ S = 4.35 ± 0.2 4.41 ± 0.21 13 7 236 m 3,071 m 18.52 Vest
7 Male 87.90 ± 17.30 181.15 ± 6.80 19.73 ± 1.00 T18.3–54.9 m’ S = 4.44 ± 0.19 4.45 ± 0.21 13 7 236 m 3,071 m 10.24 Sled
*CG = control group; N = sample size; RST = resisted sprint training; S. volume = Session volume; T. volume = Total; post = postintervention; pre = preintervention.

The quality of the 12 studies included in the meta-analysis is summarized in Table 2. The median PEDro score was 6 of 7. All studies clearly stated eligibility criteria and provided point estimates for effect size calculation. Nine (75%) studies were randomized, and all studies matched intervention groups at baseline. None of the studies used concealed allocation nor the evaluators were blinded to treatment allocation. Ten (83%) studies reported that >85% of subjects had complied with the intervention. All studies awarded a point in the intention-to-treat analysis and reported that all subjects received treatment or control conditions as allocated. All studies completed between-group analyses.

Table 2 - Quality metrics of included studies.
Study name Eligibility criteria specified Random allocation of subjects Allocation concealed Groups similar at baseline Assessors blinded Outcome measures assessed in 85% of subjects Intention to treat analysis Reporting of between group statistical comparison Point measures and measures of variability reported Overall
PeDro score
Vivas et al. (15) Yes Yes No Yes No No Yes Yes Yes 6
Uthoff et al. (81) Yes Yes No Yes No Yes Yes Yes Yes 7
Osorio et al. (69) Yes Yes No Yes No Yes Yes Yes Yes 7
Esquina et al. (62) Yes No No Yes No Yes Yes Yes Yes 6
Raya et al. (68) Yes Yes No Yes No Yes Yes Yes Yes 7
Borges et al. (10) Yes Yes No Yes No Yes Yes Yes Yes 7
Hoyo et al. (27) Yes No No Yes No Yes Yes Yes Yes 6
Sekine (76) Yes Yes No Yes No Yes Yes Yes Yes 7
Bachero et al. (6) Yes Yes No Yes No Yes Yes Yes Yes 7
Rumpf et al. (72) Yes No No Yes No Yes Yes Yes Yes 6
Upton (80) Yes Yes No Yes No Yes Yes Yes Yes 7
Clark et al. (19) Yes Yes No Yes No No Yes Yes Yes 6

Acceleration Phase (0–10 m)

Twelve effects were analyzed from 9 original studies that compared the acceleration phase performance before and after performing RST. The overall effect of RST considering a total of 166 subjects was a small reduction in acceleration phase time (SMD = −0.41; [95% CI: −0.63 to −0.19], p = 0.0002). Based on the subgroup analyses, improvement in acceleration phase time was greater when the RST was performed using vest (SMD = −0.70; [95% CI: −1.08 to −0.32], p = 0.0003) compared with using sled (SMD = −0.27; [95% CI: −0.54 to 0.00], p = 0.05) (Figure 3). In addition, RST with a load lower than 20% of BM induced a moderate reduction in acceleration phase time (SMD = −0.55; [95% CI: −0.89 to −0.21], p = 0.001), whereas only a small reduction in the acceleration phase time was observed using loads equal of greater than the 20% of BM (SMD = −0.31; [95% CI: −0.60 to −0.02], p = 0.04) (Figure 4).

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Figure 3.:
Standardized mean differences comparing the effects of sled and vest resistive sprint training on acceleration phase performance (sprint time). Note: Forest plot shows pooled standardized mean differences with 95% confidence intervals (CI) separately for the 12 interventions. Subgroup analyses show the results for each type of training equipment: 8 sled (n = 109) and 4 vest (n = 57). The diamond at the bottom of the graph and the subgroups represents the pooled standardized mean difference with the 95% CI for all trials following random effect meta-analyses.
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Figure 4.:
Standardized mean differences comparing the effects of performing resistive sprint training using loads ≥ or < 20% of body mass (BM) on acceleration phase performance (sprint time). Forest plot shows pooled standardized mean differences with 95% confidence intervals (CI) separately for the 12 interventions. Subgroup analyses show the results for the magnitude of the resistive load: ≥ 20% of BM (n = 93) and < 20% of BM (n = 73). The diamond at the bottom of the graph and the subgroups represents the pooled standardized mean difference with the 95% CI for all trials following random effect meta-analyses.

Eight effects were computed from the 5 original studies that included a control group (Figure 5). A trivial and nonsignificant effect on acceleration sprint performance was observed between the experimental (sled or vest RST) and control groups (normal soccer training or URS training) (SMD = 0.05; [95% CI: −0.20 to 0.30], p = 0.68).

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Figure 5.:
Effects of resistive sprint training conducted with sled or vest (experimental groups) compared with soccer training alone or unresisted sprint training (control groups) on acceleration phase performance. Forest plot shows pooled standardized mean differences with 95% confidence intervals separately for 8 controlled trials. The diamond at the bottom of the graph represents the pooled standardized mean difference with the 95% CI for all 5 studies following random effect meta-analyses.

Full Sprint Phase (0–30 m)

Eleven effects were analyzed from 8 original studies that compared the full sprint phase performance before and after performing RST. The overall effect of RST considering a total of 139 subjects was a small reduction in full sprint phase time (SMD = −0.36; [95% CI: −0.60 to −0.13], p = 0.003). A small reduction in full sprint phase time when the RST was performed with sled (SMD = −0.44; [95% CI: −0.75 to −0.13], p = 0.006) and vest (SMD = −0.26; [95% CI: −0.63 to 0.11], p = 0.17) was observed in the subgroup analyses (Figure 6). In addition, RST with a load lower than 20% of BM induced a small reduction in full sprint phase time (SMD = −0.40; [95% CI: −0.66 to −0.14], p = 0.003), whereas a small and nonsignificant reduction in the full sprint phase time was observed using loads equal of greater than the 20% of BM (SMD = −0.21; [95% CI: −0.77 to 0.35], p = 0.46) (Figure 7).

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Figure 6.:
Standardized mean differences comparing the effects of sled and vest resistive sprint training on full sprint phase performance (sprint time). Forest plot shows pooled standardized mean differences with 95% confidence intervals (CI) separately for the 11 interventions. Subgroup analyses show the results for each type of training equipment: 7 sled (n = 82) and 4 vest (n = 57). The diamond at the bottom of the graph and the subgroups represents the pooled standardized mean difference with the 95% CI for all trials following random effect meta-analyses.
F7
Figure 7.:
Standardized mean differences comparing the effects of performing resistive sprint training using loads ≥ or < 20% of body mass (BM) on full sprint phase performance (sprint time). Forest plot shows pooled standardized mean differences with 95% confidence intervals (CI) separately for the 11 interventions. Subgroup analyses show the results for the magnitude of the resistive load: ≥ 20% of BM (n = 25) and < 20% of BM (n = 114). The diamond at the bottom of the graph and the subgroups represents the pooled standardized mean difference with the 95% CI for all trials following random effect meta-analyses.

Five effects were computed from the 3 original studies that included a control group (Figure 8). A trivial and nonsignificant effect on full sprint performance was observed between the experimental (sled or vest RST) and control groups (normal soccer training or URS training) (SMD = 0.08; [95% CI: −0.25 to 0.42], p = 0.64).

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Figure 8.:
Effects of resistive sprint training conducted with sled or vest (experimental groups) as compared to soccer training alone or unresisted sprint training (control groups) on full sprint phase performance. Forest plot shows pooled standardized mean differences with 95% confidence intervals separately for 5 controlled trials. The diamond at the bottom of the graph represents the pooled standardized mean difference with the 95% CI for all 3 studies following random effect meta-analyses.

Maximum-Velocity Phase (30–40 m)

Seven effect sizes were analyzed from 5 original studies that compared the maximum-velocity phase performance before and after performing RST. The overall effect of RST considering a total of 65 subjects was associated with a small and nonsignificant reduction in maximum-velocity phase time (SMD = −0.25; [95% CI: −0.60 to 0.09], p = 0.15). The changes in maximum-velocity phase time were comparable when the RST was performed with vest (SMD = −0.34; [95% CI: −1.02 to 0.33], p = 0.32) and sled (SMD = −0.22; [95% CI: −0.62 to 0.18], p = 0.28) (Figure 9).

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Figure 9.:
Standardized mean differences comparing the maximum-velocity phase performance (sprint time) before and after performing sled and vest resistive sprint training. Forest plot shows pooled standardized mean differences with 95% confidence intervals (CI) separately for the 7 interventions. Subgroup analyses show the results for each type of training equipment: 5 sled (n = 48) and 2 vest (n = 17). The diamond at the bottom of the graph and the subgroups represents the pooled standardized mean difference with the 95% CI for all trials following random effect meta-analyses.

Discussion

The purpose of this meta-analysis was to determine whether RST improves sprint performance in young soccer players and to elucidate whether the long-term adaptations in the different phases of the sprint are affected by the training equipment (vest vs sled) and resistive load (<20% vs ≥ 20% of BM) used in training. The main findings of this study were that: (a) RST is an effective method for improving sprint performance in young soccer players, resulting in a decrease in sprint time of −1.94%; (b) no additional performance benefit was observed when RST influence was compared with that of URS training; (c) the greatest effectiveness of RST with vest and sled was found in the 0–10 m (decrease in mean sprint time of −3.57%) and 0–30 m (decrease in mean sprint time of −2.04%) sprint phases, respectively (d) the use of RST was equally effective when using light (<20% of BM; decrease in mean sprint time of −1.85%) and heavy loads (≥20% of BM; decrease in mean sprint time of −1.65%); and (e) no significant improvements were observed in the maximum-velocity phase between either method (sled or vest). To our knowledge, this is the first systematic review and meta-analysis to analyze the effects of RST conducted with sled or vest on sprint performance exclusively in athletes below 20 years.

Significant improvements were obtained in the acceleration phase after RST performance concurrently with their usual soccer training or URS training (ES = −0.41; p = 0.0002; decrease in acceleration phase time of −1.81%) (Figure 3). These results were expected because resistance training is known to improve lower-body maximal strength (75), which in turn positively affects sprint performance (8,75). These results are similar to those found in other reviews showing the effects of RST in adult subjects (4,65). It is known that high levels of strength to overcome BM inertia are required in the initial phase of the sprint (0–5 m), which is reflected by the significant relationship between relative strength and performance during this phase of the sprint (20). For example, a large correlation (r = −0.613) was reported (20) between 5-m sprint time and relative maximal squat strength. Although relative strength is an important contributor to sprint performance, it is important to note that only one of the studies included in the present review reported relative strength values (15). Therefore, we were unable to explore whether changes in relative strength values were related to the different training adaptations found in the studies included in this review. Furthermore, the results of different reviews that have examined the impact of different training methodologies (e.g., plyometric training, strength training, or sprint training) on sprint performance in both young (55,57,71) and adult athletes (8,24,31,63,73,75) have confirmed the efficacy of these methods for improving sprint performance. Interestingly, the magnitude of the changes reported in these meta-analyses seems to be comparable if not higher than the changes reported in this review. In addition, when training young athletes, we must take into account the different stages of maturity (prepeak, midpeak, and postpeak high velocity), which have a decisive influence on the training methodologies used (53). When the RST group and control group are compared, there was no difference between the impact of these training activities on acceleration phase performance (ES = 0.05; p = 0.68) (Figure 5) as found in other studies (4,77). Therefore, in our opinion, RST should not be used as a substitute for conventional strength training or plyometric training methods, which have consistently been reported to be effective for enhancing soccer-specific explosive actions when compared with only performing soccer training (31). In addition, whereas the effect size of performance improvement generated from training with a vest was moderate (ES = 0.70, p = 0.0003; decrease in mean sprint time of −3.57%), that derived from training with a sled was small (ES = 0.27, p = 0.05; decrease in mean sprint time of −0.90%). These findings are surprising considering the importance of horizontal propulsive forces, especially in the acceleration phase of sprint (66). One possible explanation is that the vest intervention may increase intramuscular coordination and eccentric forces of the leg extensor muscles during the braking phase, which result in an increase in muscle and leg stiffness, thus decreasing the contact time with the ground and therefore increasing the stride speed (23). Furthermore, RST with vest provides a different overload stimulus than that of the same load magnitude in the horizontal direction because of the added effect of gravity, leading to a further increase in maximal power output which limbs can develop (Pmax) being located in the midpoint of the force-velocity (F-V) curve (15). An additional explanation for this finding is that GRF orientation becomes more vertical as long as speed increases during the sprint (48). Therefore, a more horizontal GRF orientation displays a high correlation with soccer sprints, where most sprints start with little or no initial speed (35). Finally, these results match with those from Fitzpatrick et al. (30), who have reported that the force-vector theory, which classifies exercises based on the direction of force expression with respect to the global coordinate frame, is flawed and that the direction of force relative to the athlete is more important (30). This seems evident in sprinting as the athlete maintains a triple flexion position (ankle, knee, and hip flexion) in proper synchrony.

In relation to the load applied to the sled or with a vest, it is likely that the amount of load used can result in changes to the kinematics of the sprint resulting in changes in the stride length or frequency (2,46) and an increase in ground contact time, trunk lean, and hip flexion (46). Mann et al. (54) have reported that the hip flexors are the main muscles that increase gait speed, which lends support for using activities that target the development of greater strength and power (46). In this regard, Lockie et al. (46) conducted a pilot study that analyzed the performance changes produced by 2 loads (12.6 and 32.2% of BM) on sprint kinematics and found that significant changes in hip flexion occurred at loads of 12.6% of BM. However, subsequent increases in load did not seem to alter the hip flexion angle. Thus, researchers commonly recommend an external loading that yields 10% or lower decrement in maximum sprint velocity, or a load of 12.6% of BM or lower (2,46). Based on the available literature, studies using heavy loads showed both positive (1,6,9,39,60) and negative (46) effects on performance during the acceleration phase. These discrepancies found may be because of the different types of methodology used (i.e., load range, number of sessions, initial performance level and familiarization degree of subjects, and the time between the end of training and the post-test and associated tapering). Therefore, it is not clear which training method produces best results and we understand that more research is needed. Based on our analysis studies where the load was ≥20% of BM, the effects of training on performance were small (ES = 0.31; p = 0.04; decrease in mean sprint time = −1.82%). However, when the load was <20% of BM, the performance effects were moderate (ES = 0.55; p = 0.001; decrease in mean sprint time = −1.81%). In this sense, we determined that loads between 10 and 20% of BM produced the best improvements during the acceleration phase in young athletes (15,68,69). This may be explained by the fact that an excessive load can limit the stretch-shortening cycle (SSC) and decrease motor neuron excitability (H reflex), thus impairing the performance of the acceleration phase.

The sled intervention resulted in significant improvements and a positive small effect on full sprint phase performance (ES = 0.44; p = 0.006; decrease in mean sprint time = −2.04%), whereas no significant improvements were noted when using vest resistance (ES = 0.26; p = 0.17) (Figure 6). This improvement is given by a better result in kinetic variables such as a greater stride length (39,47), stride frequency (39,87), increase in trunk angle (77,87), or decrease in ground contact time (77). It was reported that the performance in the acceleration phase depends largely on the propulsive force provided by the hip, knee, and foot extensors (41) including specific adaptations within the neuromuscular system, which allow increased production of impulses and horizontal/vertical GRF, thus improving sprint performance without resistance (46,77). A further explanation may be related to the key role of relative strength during the full sprint phase, which was supported by the large correlations reported by Comfort et al. (21) between relative strength and 20-m sprint times (r = −0.672). In relation to the load, we observe that as in the acceleration phase, when loads were <20% of BM, the improvements were significant and effect sizes were small (ES = 0.40; p = 0.003; decrease in mean sprint time = −1.87%), whereas when they were ≥20% of the subject's BM, nonsignificant improvements were found (ES = 0.21 p = 0.46) (Figure 7). These results are in agreement with those found in adult elite athletes (3,4,27,33,49,77,83). It is likely that the higher loads result in longer contact time with the ground and slower rates of force development (RFDs). Finally, a trivial and nonsignificant effect on full sprint performance was observed between the experimental (sled or vest RST) and control groups (normal soccer training or URS training) (SMD = 0.08; p = 0.64).

In relation to the maximum-velocity phase, we observed positive effects after training with sled (ES = 0.22; p = 0.28; decrease in mean sprint time of −0.91%) and vest (ES = 0.34; p = 0.32; decrease in mean sprint time of −1.57%), but the changes were not statistically significant. It is known that the ability to produce force is important over short distances, but maximum-velocity depends on other factors such as pure speed (13). These results were expected, as other studies have shown that RST does not induce better performance in the maximum-velocity phase than unloaded training (6,19,80). It should be noted that the correlation between acceleration and maximum-velocity found in previous studies (r = 0.56–0.87) (32,43,56,82) suggests that these are specific qualities that require a different approach to intervention (34). In this way, the acceleration phase is influenced by concentric force development, impulse and knee and hip extensor activity (28) where the maximal capacities of the muscles to produce force (F0) are fundamental (38). On the other hand, the objective of maximum-velocity phase is to produce great vertical ground reaction forces related with stretch-shortening cycle, lower-limb stiffness and hip extensor activity (84), being maximum-velocity capacity (V0) fundamental for developing long accelerations and reaching a high sprint velocity (70).

Besides the inherent limitations associated with the meta-analytic technique itself, a number of specific limitations of the current meta-analysis have to be considered. Regarding the primary literature, we must recognize that no important scientific criteria were met in any article, such as that of evaluators blinded to treatment assignment or hidden assignment. This meta-analysis concludes that RST improves sprint performance between 0-10 and 0–30 m, but this should be interpreted cautiously because several variables that were not considered in the studies reviewed in the present study could have on our results. Specifically, aspects that could have affected our findings include (a) the load used in relation to the weight of the subject and more importantly how it relates to individual maximal strength level may affect the variation of force and speed during the race (1,9,60). For example, it is expected that subjects were able to tow a sled with a greater weight than their BM; therefore, the specific weight used in relation to their maximal strength ability was much lower to that which was indicated, ie, a percentage of their BM. (b). Time of the season when the intervention is performed, in the preseason, where the level of the subject is lower, the intervention will be more advantageous than at the end of the season (22). (c). Sample population study and in particular the age of the athletes, since the rate of physical adaptation to training is linked to the subject's fitness profile (51,52) and familiarity with strength training (44). For all these reasons, we believe it is necessary to continue studying this topic in depth and to conduct interventions in subjects below 20 years. For example, it would be interesting to verify in future research, kinetic (GRF) and kinematic (joint angle and stride length) improvements in sprint performance, on different variables such as stride length, flight time, and joint angles. In addition, no significant differences were found in postintervention versus control group. Although it is true that only 5 studies had a control group, we encourage future researchers to go deeper into this aspect.

Practical Applications

According to the findings of the articles reviewed in the present study, coaches should consider RST as a complement to regular soccer training to improve the sprint ability of young soccer players. More specifically, they should note that the vest and loads lower than 20% of BM were more effective than the sled and loads equal to or higher than 20% of BM to improve performance during the initial acceleration phase (0–10 m). They should also consider the inclusion of the sled and loads lower than 20% of the BM rather than vest and loads equal to or higher than 20% of BM to improve performance in the acceleration phase (0–30 m). However, strength and conditioning professionals working with young athletes should also note that no evidence supports that RST induces superior adaptations in the different phases of the sprint than URS training. The results of this meta-analysis could help coaches and strength and conditioning staff involved in the physical preparation of young soccer players to optimize their sprint ability.

Acknowledgments

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. The authors received no financial support for the research, authorship, and/or publication of this article. The results of the present study do not constitute endorsement of the product by the authors or the NSCA. L.M. Fernández-Galván, A. García-Ramos and A. Casado devised the study, L.M. Fernández-Galván, A. Casado and G.G. Haff conducted the data collection, L.M. Fernández-Galván and A. Casado analyzed the data, L.M. Fernández-Galván, and A. Casado drafted the study and literature review. G.G. Haff and A. García-Ramos revised the study critically for intellectual content and approved the final version to be published.

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Keywords:

strength training; acceleration; maximal velocity; ground reaction forces

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