Effects of jump training on jumping performance of handball players: A systematic review with meta-analysis of randomised controlled trials

Abstract

This study aimed to assess the effects of jump training on the jumping performance of handball players compared with a control condition. The data sources utilised were PubMed, MEDLINE, Web of Science Core Collection and SCOPUS. Only peer-review randomised controlled trials were included. The inclusion criteria comprised: a jump training programme of at least 2 weeks; a control group; the assessment of the countermovement jump. The Physiotherapy Evidence Database scale was used to assess the risk of bias and methodological quality of eligible studies included in the meta-analysis. Risk of publication bias across studies was assessed using the extended Egger’s test. Cohen’s d effect sizes (ESs) were calculated from the countermovement jump and presented together with 95% confidence intervals (CIs). From 6108 records initially identified through database searching, 5 were eligible for meta-analysis. A significant improvement in countermovement jump height was observed, corresponding to 6.4 cm (95% CI = 4.9–7.9; Z = 8.4, p < 0.001), showing moderate heterogeneity (I² = 51.4%). The magnitude of the main effect was very large (ES = 2.2 (95% CI = 0.95–3.4), Z = 3.5, p < 0.001). Jump training is effective in increasing vertical jump performance in handball players. However, the insufficient number of studies conducted precluded analyses of moderator variables. In future, researchers are advised to conduct jump training studies of high methodological quality (e.g. randomised controlled trials) and assess different jump exercise prescriptions across handball players of different sexes, ages and competitive levels to analyse if exercise prescription and player characteristics may influence training responses.

Keywords

Ballistic movement plyometrics stretch-shortening cycle vertical jump

Introduction

Handball has become more popular over the past few decades and, therefore, research on this sport has increased considerably in recent years.¹ The impact of this ‘demographic’ change has brought a secular increase in the potential talent pool of athletes who compete in elite handball.² As a result, there has been significant improvement in player technical and physical skills, placing great emphasis on better development of speed and power abilities.³ In this context, speedy and fast-pace activities are common characteristics of both defensive and offensive game situations.⁴ Accordingly, modern handball requires players to effectively and repeatedly perform numerous explosive motor tasks such as acceleration, deceleration, and vertical jumps.^5–7 Studies involving the physical development of handball athletes have primarily focused on improvement in muscle power, which is popularly evaluated and developed through the systematic use of vertical jump exercises,^8–10 a training method known as jump training (JT).

JT has been proven to be an efficient and practical strategy to increase vertical jump performance in subjects with different training backgrounds.^11–13 In addition, jump exercises seem to also be effective to reduce the risk of injury and improve a wide range of distinct physical capacities.^2,14,15 JT usually involves the musculotendinous units undergoing intense stretch-shortening cycles,^16,17 induced by different jump types,^18–20 being used as a regular and continuous muscle power training method.^2,21,22 Since the times of ancient Greece²³ to the end of the 20th century, JT has evolved, culminating in numerous articles published in peer-reviewed journals in recent years,^10,24–26 in line with the increased scientific productivity in handball.

As a consequence of its positive effects, several reviews and meta-analysis related to JT have been published^{11,12,25–29} evidencing the effectiveness of this training strategy to improve distinct power related abilities in athletes from numerous sports disciplines. However, to our knowledge, no reviews have been conducted regarding the effects of JT on vertical jumping performance of handball players. Given the increased scientific awareness of the relevance of JT, and the lack of review studies targeted at handball players, it is appropriate to analyse studies focused on describing these effects. The aim of this review was to assess the effects of JT on the vertical jumping ability of handball players.

Methods

Procedures

A systematic review and meta-analysis were conducted following the guidelines of the Cochrane Collaboration.³⁰ Findings were reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).³¹

Inclusion and exclusion criteria

To qualify for inclusion in the meta-analysis, studies were required to include (i) a JT programme of at least two weeks, defined as lower body unilateral or bilateral bounds, jumps and hops that commonly utilise a pre-stretch or countermovement which incites usage of the stretch–shortening cycle,^13,32,33 (ii) cohorts of healthy handball players, with no restriction for sex or age, (iii) a control group of handball players and (iv) a measure of muscular power,^33,34 usually some form of countermovement jump (CMJ).

Literature search

Database searches were conducted using PubMed, MEDLINE, Web of Science Core Collection and SCOPUS electronic databases from inception until 1 July 2019. Only articles published in the English language were considered. The search was conducted using the Boolean expressions AND, OR, and the following keywords: ‘ballistic’, ‘training’, ‘complex’, ‘explosive’, ‘force’, ‘velocity’, ‘plyometric’, ‘stretch’, ‘jump’, ‘shortening’, ‘handball’, and ‘cycle’. Following is an example of a PubMed search: (((((((((‘randomised controlled trial’ [Publication Type]) OR ‘controlled clinical trial’ [Publication Type]) OR ‘randomised’ [Title/Abstract]) OR ‘trial’ [Title]) OR ‘clinical trials as topic’ [MeSH Major Topic]) AND ‘handball’ [Title/Abstract]) OR ‘handball players’ [Title/Abstract]) OR ‘handball/physiology’ [Title/Abstract]) AND ‘training’ [Title/Abstract]) OR ‘plyometric’ [Title/Abstract]. Duplicates were removed and the search results were analysed according to the eligibility criteria. In selecting studies for inclusion, a review of all relevant titles was conducted before examination of the abstracts and then the full texts. Only peer-reviewed articles were included in the meta-analysis. Following the formal systematic searches, additional hand-searches were conducted.

Study selection

Two authors (RRC and AGH) independently screened the titles and abstracts of potentially eligible studies identified by the search strategy. Potential discrepancies between the two reviewers were resolved by consensus with the third author. Full-text articles excluded, with reasons, were recorded.

Data extraction

Data were extracted from included articles by two authors (RRC and AGH) independently, using a form created in Microsoft Excel (Microsoft Corporation, Redmond, WA, USA). In cases where the data required were not clearly or completely reported, the authors of the article were contacted for clarification.^35,36

Data items

The CMJ is an appropriate measure of muscular power as it is performed with a fast transition between the downward and upward stages of the action, requiring utilisation of the stretch–shortening cycle.³⁷ The measure also presents very high test–retest reliability³⁸ and was therefore chosen on the basis of establishing a degree of consistency between analysed studies. The CMJ outcome is commonly reported as jump height (cm), although it may also be reported as power (W), velocity (m.s⁻¹), or a similar unit of measure.

Assessment of risk of bias in individual studies

The Physiotherapy Evidence Database (PEDro) scale was used to assess the risk of bias and methodological quality of eligible studies included in the meta-analysis. This scale evaluates internal study validity on a scale from 0 (high risk of bias) to 10 (low risk of bias). As in a similar previous meta-analysis,²⁸ the quality assessment was interpreted using the following 10-point scale: ≤3 points was considered poor quality, 4–5 points moderate quality and 6–10 points high quality. Two independent reviewers (RRC and AGH) performed this process. If trials had already been assessed and listed on the PEDro database, these scores were adopted. However, methodological quality was not an inclusion criterion. It should be noted that the Cochrane Collaboration has previously discouraged the use of these scales, stating that the practice is not underpinned by empirical evidence and assessment criteria may apply inaccurate study weights,³⁹ such that the subjectivity of personal opinion may undermine the accuracy of these scales. Previous JT systematic reviews^28,40,41 suggest that studies are typically of low to medium quality. In relation to this point, previous reviews^13,27,42 highlighted the difficulty in using blinding in training studies. Indeed, a recent review⁴³ that applied the PEDro scale found that only 4 of 43 resistance training studies were able to meet the associated criteria, potentially preventing a meta-analysis occurring in the first instance.

Summary measures and synthesis of results

For analysis and interpretation of results, meta-analyses were conducted if at least three studies provided effect sizes for the same parameter.^33,44,45 Meta-analytical comparisons were carried out in the comprehensive meta-analysis program (version 2; Biostat, Englewood, NJ, USA). Means and standard deviations for a measure of post-intervention performance were used to calculate the weighted mean difference. The inverse variance random-effects model for meta-analyses was used as it allocates a proportionate weight to trials based on the size of their individual standard errors⁴⁶ and facilitates analysis while accounting for heterogeneity across studies.⁴⁷ In addition, Cohen’s d effect size (ES) calculations were represented by the standardised mean difference and are presented with 95% confidence intervals (CIs). The calculated ESs were interpreted using the conventions outlined for standardised mean differences by Hopkins et al.⁴⁸ (<0.2, trivial; 0.2–0.6, small; >0.6–1.2, moderate; >1.2–2.0, large; >2.0–4.0, very large; >4.0, extremely large). To gauge the degree of heterogeneity amongst the included studies, the I² statistic³¹ was referred to as low, moderate, and high levels of heterogeneity corresponding to I² values of 25%, 50% and 75%, respectively.⁴⁹

Publication bias

Risk of bias across studies was assessed using the extended Egger’s test.⁵⁰ Sensitivity analyses were conducted to assess the robustness of the summary estimates to determine whether a particular study accounted for the heterogeneity. Thus, to examine the effects of each result from each study on the overall findings, results were analysed with each study deleted from the model once. It is acknowledged that other factors, such as differences in trial quality or true study heterogeneity, could produce asymmetry.

Results

Study selection

From 6108 records initially identified through database searching, 14 JT studies^{10,24,35,36,51–60} conducted with handball players were found (Figure 1). However, from these, only five studies^{10,24,35,59,60} incorporated a randomisation-controlled approach, and provided mean and standard deviation post-intervention data for the main outcomes. From the nine studies excluded, five studies did not have control groups, two were not randomised, one study was not exclusively performed with handball players and one study did not provide sufficient data (mean and standard deviation of the main outcome) to be included in the analysis. Therefore, five studies were included in the meta-analysis.^{10,24,35,59,60}

Figure 1.

PRISMA flow diagram of literature screening process.

Study characteristics

The characteristics of the participants from the studies incorporated in the meta-analysis are displayed in Table 1, while the programming parameters of the JT interventions from the included studies are indicated in Table 2.

Table 1.

Characteristics of the participants of the studies included in the meta-analysis.

Study	Group	n	Sex	Age (years)	Body mass (kg)	Height (cm)	Baseline (cm)^a	Follow-up (cm)^a
Chelly et al.¹⁰	Experimental	12	Male	17.1 ± 0.3	80.1 ± 11.9	181 ± 6.4	42.0 ± 4.0	46.0 ± 4.0
Chelly et al.¹⁰	Control	11	Male	17.2 ± 0.4	78.0 ± 11.4	177 ± 5.3	41.0 ± 3.0	42.0 ± 3.0
Hammami et al.⁶¹	Experimental	14	Male	14.5 ± 0.3	69.3 ± 3.1	178 ± 5.0	28.7 ± 4.4	39.3 ± 2.7
Hammami et al.⁶¹	Control	14	Male	14.6 ± 0.2	66.9 ± 4.7	172 ± 7.0	28.2 ± 3.5	30.3 ± 3.7
Hammami et al.⁶²	Experimental	14	Female	16.6 ± 0.3	60.8 ± 4.7	163 ± 4.0	28.1 ± 1.6	33.8 ± 1.6
Hammami et al.⁶²	Control	14	Female	16.6 ± 0.3	60.4 ± 4.3	164 ± 4.0	27.0 ± 3.5	28.1 ± 3.4
Hermassi et al.³⁵	Experimental	14	Male	20.1 ± 0.3	85.8 ± 3.1	189 ± 3.0	44.1 ± 1.3	49.0 ± 1.7
Hermassi et al.³⁵	Control	10	Male	20.1 ± 0.2	88.6 ± 1.1	190 ± 2.0	41.9 ± 0.7	42.3 ± 0.8
Karadenizli²⁴	Experimental	14	Female	15.6 ± 0.8	56.4 ± 3.7	161± 3.4	37.5 ± 5.6	41.6 ± 7.3
Karadenizli²⁴	Control	12	Female	15.4 ± 0.9	55.9 ± 4.6	160± 4.9	36.2 ± 5.3	37.3 ± 8.9

Data are presented as mean ± standard deviation.

^aBaseline and follow-up data indicating vertical jump height performance.

Table 2.

Characteristics of the jump training (JT) programmes.

Study	Training frequency (per week)	Duration (weeks)	Intensity	BH (cm)	Jumps per programme	Combined^a	RBS (s)	RBR (s)	PO	TP
Chelly et al.¹⁰	2	8	Maximal RSI	40	860	No	NR	5	V, I, T	IS
Hammami et al.⁶¹	2	8	NR	30–40	1536	Sprint and change of direction drills	90	NR	V	IS
Hammami et al.⁶²	2	10	NR	30–40	1920	Resistance training and sprint	60–120	repeated jumps	No	IS
Hermassi et al.³⁵	2	8	Maximal RSI	40–60	940	No	180	5	V, I, T	IS
Karadenizli²⁴	2	10	NR	40	2336	Running and change of direction drills	60–180	NR	V, T	IS

BH: box height (for those drills that required the use of a box, not necessarily applied to drop jumps); IS: in-season; NR: not reported; PO: progressive overload, in the form of either volume (i.e., V), intensity (i.e., I), type of drill (i.e., T), or a combination of these; RBR: rest time between repetitions (only when the JT programme incorporated non-repeated jumps); RBS: rest time between sets; RSI: reactive strength index; TP: training period

^aOf note, it is common to incorporate JT interventions with other forms of explosive and resistance training drills as part of a more comprehensive training approach. However, to be incorporated in this study, the JT intervention represented ≥50% of the experimental drills.

Risk of bias within studies

The studies included in the meta-analysis achieved a quality assessment of 5–6 points, interpreted as moderate-high quality (Table 3). The average total score was 5.8, with a range from 5 to 6. Four studies scored 6 points and one study scored 5 points according to the PEDro criteria.

Table 3.

Physiotherapy Evidence Database (PEDro) scale ratings.

	Scale item no. 1	Scale item no. 2	Scale item no. 3	Scale item no. 4	Scale item no. 5	Scale item no. 6	Scale item no. 7	Scale item no. 8	Scale item no. 9	Scale item no. 10	Scale item no. 11	Total^a
Chelly et al.¹⁰	Yes	Yes	No	Yes	No	No	No	Yes	Yes	Yes	Yes	6
Hammami et al.⁶¹	Yes	Yes	No	Yes	No	No	No	Yes	Yes	Yes	Yes	6
Hammami et al.⁶²	Yes	Yes	No	Yes	No	No	No	Yes	Yes	Yes	Yes	6
Hermassi et al.³⁵	Yes	Yes	No	No	No	No	No	Yes	Yes	Yes	Yes	5
Karadenizli²⁴	Yes	Yes	No	Yes	No	No	No	Yes	Yes	Yes	Yes	6

Note: A detailed explanation for each PEDro scale item can be accessed at https://www.pedro.org.au. In the next lines a brief explanation is provided: Item 1, eligibility criteria were specified; item 2, subjects were randomly allocated to groups; item 3, allocation was concealed; item 4, the groups were similar at baseline; item 5, there was blinding of all subjects; item 6, there was blinding of all therapists; item 7, there was blinding of all assessors; item 8, measures of at least one key outcome were obtained from more than 85% of the subjects initially allocated to groups; item 9, all subjects for whom outcome measures were available received the treatment or control condition as allocated or, data for at least one key outcome was analysed by ‘intention to treat’; item 10, the results of between-group statistical comparisons are reported for at least one key outcome; item 11, the study provides both point measures and measures of variability for at least one key outcome.

^aTotal from a maximal of 10.

Synthesis of results

A significant improvement in CMJ height was observed, corresponding to 6.4 cm (95% CI = 4.9–7.9; Z = 8.4, p < 0.001), showing moderate heterogeneity (I² = 51.4%). These results are displayed in Figure 2.

Figure 2.

Forest plot of increases in countermovement jump height (cm) in handball players participating in jump training compared to controls. Values shown are weighted difference in means with 95% confidence intervals (CI).

Across all included studies, there was a very large, significant improvement in CMJ height (ES = 2.15 (95% CI = 0.95–3.36), Z = 3.5, p < 0.001). The relative weight of each study in the analysis varied between 16.8% and 21.5%, demonstrating an equilibrated weight distribution.

Risk of bias across studies

Evidence suggesting publication bias was not found using Egger’s test (p = 0.6). In the sensitivity analysis, with each study deleted from the model once, the results remained consistent across all deletions.

Adverse effects

Among the studies included in this review, none reported injury or other adverse effects. In general, the studies did not report clear methods for identifying adverse effects, most often providing statements such as ‘no adverse events or injuries were observed in the training sessions’.

Discussion

The aim of this review was to assess the effects of JT on the vertical jumping ability of handball players. The main finding of our analysis was that JT is an effective strategy to increase the CMJ height in handball players, with a very large (i.e. 6.4 cm) main effect being found. This reinforces previous reviews supporting the use of JT for the improvement of lower limb muscle power in both youth female⁶¹ and male²⁷ athletes.

The improvements in vertical jump height after JT probably encompass potential mechanisms, being attributed to enhanced neural drive to agonist muscles, alterations in musculotendinous stiffness, increases in muscle size and architecture, improved intermuscular coordination, greater excitability of the stretch reflex, and changes in muscle fibre mechanics^14,62; some mechanisms that have been partially identified in handball players.³⁶ In fact, a six-week study not included in the current meta-analysis (i.e. due to the impossibility of assessing its data) conducted with young male handball players³⁶ (24 JT sessions, 4 sessions per week) compared to a control group, found a significant increase of 13.2% in CMJ height. This improvement is in line with the two studies included in the meta-analysis^10,24 but lower than the ∼20%–37% improvements found by Hammami et al.^59,60 Differences between JT programmes (e.g. intensity and volume) may partially explain the different magnitudes of CMJ improvements between studies. In addition, the characteristics of the participants may also explain differences in the magnitude of changes between distinct studies. For instance, in the studies of Hammami et al.,^59,60 the initial CMJ height performance of the participants was <29 cm. In contrast, the athletes in the study of Chelly et al.¹⁰ had an initial CMJ height of 42 cm, and in the study by Karadenizli²⁴ the athletes had an initial CMJ height of 37.5 cm. Overall, the evidence suggests that significant improvements in CMJ height can be achieved in youth male and female handball players with different initial levels of performance, although the relative gain in CMJ height appears to be greater for those with lower initial jumping performance.

Although the initial search for studies identified 14 studies including handball players, only 5 were eligible for this meta-analysis. The relatively low number of studies conducted with handball players is somewhat striking, especially considering the importance of high intensity activities such as vertical jumping in handball and the fact that handball is an Olympic sport played worldwide,⁶³ with an estimated ∼18 million players in ∼150 international federations.⁶⁴ The lack of studies in handball has indirectly been reported previously.^11,25,26 In these meta-analyses, the authors described and compared the effects of JT interventions on jumping, strength, and sprinting performance in different sports, but none of the comparisons included studies in handball players. Although between the years of 2000 and 2017 scientific publications on JT have increased 25-fold compared with any previous period,¹³ the low number of controlled and randomised studies conducted in handball is remarkable. However, this problem is not exclusive for this sport, since JT studies missing an active or passive control group and randomisation accounted for ≥40% of overall studies.¹³ Therefore, more efforts should be undertaken to overcome these limitations, increasing awareness about the potential impact and benefits of JT on handball practitioners. Although some limitations are difficult to address (e.g. subject blinding), investigators should strive to conduct more randomised and controlled studies, whilst also being mindful that the nature of the control condition will depend heavily on the applied setting in which the study is conducted. For example, researchers must consider the use of physically active control groups in athletic settings, whilst passive controls are more appropriate in clinical settings (e.g. handball player rehabilitation after injury). When using a control group is not possible, a washout period or cross-over design might serve as potential alternatives in a field setting, although this may consider the physical maturation state in youth athletes.⁶⁵

It was interesting to observe that the participants of four of the five included studies in the meta-analysis had mean ages between 15 and 17 years, with the other study reporting a mean age of 20 years. In the past, it was thought that resistance training was detrimental to youth athletes and it was specifically avoided out of concern for damage caused by the high forces exerted on the adolescent skeleton, which could result in increased physical injuries and the potential for stunted growth.⁶⁶ From the 68 individuals involved in the 5 studies analysed in the current review, no acute injuries due to JT were reported, underlining the relative safety of this type of exercise for youth athletes. This finding is similar to those reported in previous reviews and meta-analyses.^11,13,25,26 Furthermore, JT has been proven to be safe even in older adults.³³ Finally, JT may even help to reduce risk factors and prevent injuries in team-sport athletes,^67–69 including handball players.^70,71 It is likely that sufficiently qualified coaches can greatly reduce the possible risks associated with this type of exercise by following some general recommendations.⁷² For example, youth athletes who perform JT should receive a programme that is appropriate for their physical condition and training history. To this end, technical competency should be assessed, with adequate coaching provided to demonstrate an appropriate technique before the major components of the exercise prescription, including exercise complexity, intensity and volume are progressed.^73,74 Nevertheless, it must be highlighted that the studies in this meta-analysis were carried out in healthy individuals. Moreover, as only five studies were included in the current meta-analysis, the results should not be taken as evidence to support the safety of JT in all handball players.

For future research purposes, researchers may carry out specific JT interventions independent of any other forms of exercise training. A previous review¹³ stated the difficulty of distinguishing the effects of multiple concurrent training types on outcome variables in experimental studies. This makes it difficult to determine the true effects of JT in this population, as this study design was a feature of all but one investigation¹⁰ in the current analysis. Additionally, only five controlled-randomised studies were included in the current meta-analysis. The relatively low number of studies carried out with handball players was surprising, more so when considering the extensive number of studies that have been carried out in recent years regarding JT,¹³ and this represents a viable avenue of investigation. As a result of the low number of studies included in the current meta-analysis, statistical analysis of the effects of moderator variables was precluded. This is a line of inquiry that should be further pursued, particularly regarding the relevance of JT prescription characteristics such as intensity, volume, frequency and type of exercise. Moreover, from the studies included in this meta-analysis, only one reported the physical maturity of the athletes. Considering the potential modulator role of maturity on JT-induced physical fitness improvements,^27,61,65 researchers should always provide data for males and females separately for any main effects in JT studies.

Although the effects of moderator variables were precluded due to the low number of available studies, considering that the type of jump included in JT interventions may affect the magnitude of the vertical jump improvement,¹¹ a discussion regarding this issue follows. In the studies of Karadenizli²⁴ and Hammami et al.,⁵⁹ mean vertical jump improvements of 4.1 cm and 10.6 cm were reported, respectively, after similar JT interventions, that included unloaded hopping and jumping drills, involving hurdle heights up to 40 cm, aside from one- and two-legged jumps, using different directions (horizontal, vertical and lateral), as repeated- or non-repeated jumps. In the studies of Hermassi et al.³⁵and Chelly et al.,¹⁰ mean vertical jump improvements of 4.9 and 4.0 cm were reported, respectively, after JT interventions that included unloaded 40–60 cm hurdle jumps and 40 cm drop jumps. In the study of Hammami et al.,⁶⁰ a mean jump improvement of 5.7 cm was reported, after a JT intervention (mixed with resistance training drills) that included 30–40 cm hurdle jumps, horizontal jumps and one-legged hopping. Overall, the evidence suggests that the moderator role of the JT type of drill on vertical jump height gains in handball players should probably be considered, although in line with other key JT moderator variables such as volume and intensity (among others).^11,13

Finally, it must be acknowledged that different assessment techniques were used among the included studies in order to measure CMJ. Essentially, researchers used contact mats,²⁴ infrared photo cell mats^59,60 and force platforms.^10,35 These differences may have potentially affected the results derived from the current meta-analysis, as some of these methods have been found to present questionable validity when compared to more consistent force platform methods.⁷⁵ However, we ran a moderator analysis (not included in the results section, as <3 studies were available for one of the moderators) for those studies which measured CMJ height using force platforms^10,35 and compared to with those that measured CMJ height using flight time-based methods.^24,59,60 We found that a significant improvement in CMJ height was observed using both force platform (ES = 2.9; 5.7 cm; 95% CI = 3.1–8.3; Z = 4.4, p < 0.001; I² = 65.1%) and flight time-based methods (ES = 1.8; 6.8 cm; 95% CI = 4.1–9.6; Z = 4.9, p < 0.001; I² = 60.3%), without between-group differences (p = 0.549). In addition, in the current meta-analysis publication bias was not found using Egger’s test (p = 0.6). Furthermore, only a moderate heterogeneity (I² = 51.4%) was observed. The lack of publication bias and heterogeneity, despite different measurement protocols being used to assess jump height, may be related to the high reliability reported among the included studies (i.e. ICC = 0.93–0.95), and the fact that all the studies reported jump height using the CMJ test. Independently from the above, researchers are advised to use valid protocols for the assessment of vertical jump height among handball players, including ecologically valid outcomes. For researchers conducting systematic reviews with meta-analysis, it is important to select adequate outcomes, based on a logically defensible rationale.^33,34 In this sense, the CMJ has been advocated as an appropriate measure of lower-limb maximal-intensity short-duration effort.³⁷ The measure also presents very high test–retest reliability³⁸ and, therefore, can be chosen on the basis of establishing a degree of consistency between analysed studies.

The current study presents some limitations, and, hence, the current results should be interpreted with caution. For subgroup analyses, it is obvious that the reduced number of included studies limited the possibility of conducting a robust subgroup analysis in order to determine the potential effects of moderator variables. Previous studies recommended conducting meta-analyses only if at least three studies provided effect sizes for the same parameter.^33,44,45 In this sense, in the current meta-analyses only ≤2 studies were available for each moderator. Moreover, the current systematic review with meta-analyses was not registered a priori, however, we followed the guidelines of the Cochrane Collaboration³⁰ and the PRISMA.³¹ Future studies may seek to include a registered protocol.

The lack of adverse responses to JT is encouraging, although it is possible that in some cases the authors did not report this information. Although current evidence increasingly points towards the safety of JT exercises for populations including youth athletes, including handball athletes, practitioners should take a cautious approach to programming. In addition, the reader must consider the lack of uniformity in how training programmes were prescribed. Practitioners are advised to consider general guidelines when formulating JT programmes, according to the current scientific evidence, as well as making them appropriate to the individual(s) with whom they are working.^76–79 Researchers are encouraged to describe how data related to injury, pain, or any other potential adverse effect were collected, clarifying the procedures, definitions and diagnosis by qualified medical practitioners, among other key aspects.

In summary, JT seems to be effective for improving the vertical jump performance of handball players. Nonetheless, the insufficient number of investigations limits definitive conclusions regarding the potential role of moderator variables. In future, researchers are advised to conduct JT studies of high methodological quality (e.g. randomised controlled trials) that better isolate the effects of JT on CMJ performance and compare different forms of JT exercise prescription within the same study. The identification of these dose-response relationships following JT requires longer-term interventions and follow-ups, especially in youth handball athletes.

Conclusion

In conclusion, this systematic review shows that JT is effective for increasing the vertical jump height of handball players. Our findings reinforce previous recommendations and support the use of this practical training strategy to improve lower limb power performance in handball players. On average, we found a very large (ES = 2.2) increase in the vertical jump height of handball players after a period of JT when compared with control (i.e., average increase in jump height = 6.4 cm (95% CI: 4.9–7.9)). Despite the significant improvement observed in the jump performance of handball players, the low number of studies conducted on this topic precludes additional analyses of moderator variables. Importantly, none of the included studies reported the occurrence of any injury or other adverse effects in handball players after a period of JT, which also confirms the safety and efficacy of this training method.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Rodrigo Ramirez-Campillo

Justin WL Keogh

Felipe García-Pinillos

Irineu Loturco

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