Chronic Effects of Rotational Inertial Devices on Adolescents’ Physical Capacities in Team Sports: A Systematic Review

Abstract

Inertial training is one of the most popular training methodologies in recent years and one of the objects of study in recent literature. The aim of this systematic review is to evaluate the current literature surrounding the chronic effect of rotational inertial devices on the physical capacities of team sports athletes through jumping performance, sprinting time, and change of direction performance. This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocols. Three databases were screened up to January 2023. Eight studies were included in the final analysis. The results revealed the effectiveness of rotational inertial devices as flywheels or conic pulleys, showing significant improvements (from trivial to large effect size) in jump performances, significant improvements in some tests for change of direction ability and non-consistent results for sprint tests with significant improvements mainly in short distances. In conclusion, inertial training has been shown to be a useful way to improve performance in young athletes in team sports.

1. Introduction

Strength training (ST) is one of the most common strategies to improve different actions that are key in team sports performance, such as jumps, sprints, accelerations, or changes of direction (CODs) [1]. Studies largely support these findings in young populations where it seems clear that ST induces great improvements in strength, power output, speed, jumps, or kicking [2,3]; indeed, young athletes have shown improvements in athletic performance and body composition with self-loading [4].

In recent years, eccentric overload training (EOT) has become a popular method for the athlete population due to its benefits for athletic performance in youth athletes [5]. In fact, the effect of EOT showed improvements in young soccer players, and it had an impact on muscle injury incidence and severity [6]. Inertial training (IT) is probably the most commonly used method to achieve eccentric overload, and it is also known for its capability to stimulate the stretching-shortening cycle (SSC) [7,8]. The eccentric (ECC) phase of the muscle action has emerged as an alternative method that may produce greater muscle adaptations [9]. IT provides us with the opportunity to perform the optimal strength output; therefore, power and strength can develop further than with traditional ST methods [10]. Rotational inertial devices like flywheels or conic pulleys have been increasing in popularity recently [8,11]. Although these devices were created in 1994 to reduce atrophy in astronauts in space [12], it was only in the last decade, when they were used for athletes, that we learned that exercises using non-gravity-dependent devices produced similar, if not greater, benefits than using free weight exercise [7].

Due to the aforementioned studies, EOT has been extensively reviewed in the scientific literature [9,10,13]. Besides chronic performance enhancement, IT is a valid way to achieve acute improvements in specific sports actions [13]. Some researchers have suggested that this eccentric overload provides a great mechanical stimulus for both the muscular and tendinous tissues, which benefits early neuromuscular (e.g., strength and power increases) and performance (e.g., jumping and COD ability) adaptations [14]. Resistance programs that incorporate flywheel exercises are one of the most effective methods for improving sport-specific performance in sporting populations [15]. These benefits can be explained because this strategy induces changes in maximum neural activation that promote a greater recovery [10]. In addition, optimization of resistance training using a strictly EOT regime is rather complex and technically difficult to apply [9]. Several authors have reported the need to apply certain strategies in order to provide instructions that encourage the participants to delay the braking action to the last third of the ECC phase [8]. Furthermore, these devices give the coach and athletes the ease to constantly modify the load in any exercise repetition, which means better adaptations of the neuromuscular system [13]. The purpose of this research was to investigate the effects of an IT intervention in actions that play a key role in team sports performance in youth athletes. Therefore, our hypothesis is that the use of rotational inertial devices could improve the power and strength of young athletes, thus improving jump, sprint, and COD performance.

2. Materials and Methods

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocols [16].

2.1. Search Strategy

A systematic and computerized search of the databases Web of Science and Scopus was conducted by two separate reviewers (AR and MO), using a date filter from 1 January 2019 to 30 January 2023, although, additionally, earlier studies published on the topic were screened for further potentially relevant information to help the researchers introduce the topic and make the discussion of this article. Only full-text articles from peer-reviewed studies written in English or Spanish were included.

The search included the following keywords collected through expert opinion: “isoinertial”, “flywheel”, “conic pulley”, “eccentric overload”, “training”, “team sports”, “soccer”, “futsal”, “handball”, “basket”, “hockey”, “rugby”, and “volleyball”. The specific Boolean search algorithm was [“isoinertial” OR “flywheel” OR “conic pulley” OR “eccentric overload”] AND [“training”] AND [“team sports” OR “soccer” OR “futsal” OR “handball” OR “basket” OR “hockey” OR “rugby” OR “volleyball”].

In order to extend the review, a second systematic and computerized search of the databases PubMed was conducted by two separate reviewers (AR and SL) using the same filters. Following the same search strategy, 49 research were found (32 duplicates and 17 different articles, but no one met the eligibility criteria).

2.2. Eligibility Criteria

Studies meeting the inclusion criteria (

Table 1. Eligibility criteria.

Age	Participants included were between 12 and 20 years old.
Injury status	Participants were free from injury or illness.
Subjects	Participants included were male or female team sports athletes of various training levels (academy amateur or academy elite).
Team sports	Basketball, soccer, futsal, handball, hockey, volleyball, and rugby union.
Training	The study utilized an inertial device (e.g., flywheel or conic pulley).
Training period	The intervention period was ≥4 weeks.
Test/metrics	The measures come from specific functional tests (CMJ, COD, sprint…)
Article type	Peer-reviewed publication
Article language	English or Spanish

) were included, focusing the review on healthy team sport adolescents who have been trained with EOT for a period of 4 weeks or longer with inertial devices (flywheel or conic pulley) to elicit chronic adaptations. The training intervention and load (volume and intensity) needed to be quantified. Moreover, data was collected through at least one specific functional test such as a sprint test (e.g., 10 m, 20 m, 40 m, RSA), power test (e.g., jump height), or change of direction test (e.g., t-test, Illinois test).

Studies that did not meet any of the previous criteria were excluded from the review. For example, we excluded studies with no outcome pertaining to EOT in relation to functional test performance, such as isokinetic, TMG, EMG, or body composition, and questionnaires or studies where inertial devices were only used for testing instead of training. An intervention duration of less than four weeks was a further reason to exclude a study.

2.3. Study Selection

Initially, in order to avoid duplicates, a first filter was carried out because there is usually approximately a 46.9% coincidence [17]. Thus, all studies during the initial search were uploaded to a reference manager software (Zotero, version 6.0.23, Corporation for Digital Scholarship, Vienna, VA, USA), reviewed, and screened for duplicates. Based on the study title, author, year of publication, and DOI, duplicates were identified and merged using the “Duplicate Items” function.

Secondly, an assessment of eligibility was performed in an unblinded manner by two reviewers (AR and MO) separately. Titles and abstracts of the articles identified through the initial search were screened against the eligibility criteria (Table 1). Potentially relevant articles were retrieved for an evaluation of the full text. Interrater agreement was assessed using Cohen’s kappa (κ = 0.82). If there was uncertainty about whether a study met the standard for inclusion, that was clarified with a third reviewer (SL). The three reviewers determined the final pool of articles included in the review. The study selection process is presented in Figure 1.

2.4. Quality Assessment

Preventing the risk of bias and providing a quality assessment of the research are critical factors for a systematic review [18]. There are several scales to assess the methodological quality of studies, like the PEDro scale, the Delphi scale, or the Cochrane scale. However, previous studies have demonstrated that strength and conditioning studies specifically, or non-healthcare studies in general, usually score low using these methodological scales [19,20].

Consequently, following Allen et al. [19], who used methods similar to Brughelli et al. [21], the eight selected studies were evaluated separately by the same two reviewers (AR and MO) using an evaluation derived from the aforementioned scales. This scale utilizes 10-item criteria, and the reviewers select between 3 options (0 = clearly no; 1 = maybe; and 2 = clearly yes, scoring each study from 0 to 20. To determine the study quality, previous researchers [19] have proposed three different levels: high quality (score > 15), moderate quality (score 10–15), and low quality (score < 10). In the same way as during study selection, any differences between reviewers were clarified and settled with a third reviewer (SL).

The individual scores for the quality assessment could be reviewed (

Table 2. Methodological quality of studies.

Study	Inclusion Criteria	Random Allocation	Intervention Defined	Groups Tested for Similarity at Baseline	Control Group	Outcome Variable Defined	Assessments Practically Useful	Duration Intervention Practically Useful	Between-Group Stats Analysis Appropriate	Point Measures of Variability	Total Score Quality Assessment
Gonzalo-Skok et al., 2019 [22]	2	2	2	2	0	2	2	2	2	2	18 (high)
Murton et al., 2021 [23]	2	2	2	2	0	2	2	2	2	2	18 (high)
Nunez et al., 2019 [24]	2	2	2	2	2	2	2	1	2	2	19 (high)
Gonzalo-Skok et al., 2022 [25]	2	2	2	2	0	2	2	2	2	1	17 (high)
Stojanovic et al., 2021 [26]	2	2	2	2	2	2	1	1	2	2	18 (high)
Fiorilli et al., 2020 [27]	2	2	2	2	0	2	2	2	2	2	18 (high)
Arede et al., 2020 [28]	2	2	2	2	0	2	2	2	1	2	17 (high)
Raya-Gonzalez et al., 2021 [15]	2	2	2	2	2	2	2	2	2	2	20 (high)

). The average score was 18 points (high quality), with values ranging from 17 to 20 points; all of them were categorized as high quality, although the lack of control groups in some studies could be interpreted as a source of bias.

3. Results

The reviewers extracted data from the included studies in a standardized template created with Microsoft Excel in order to code and organize the information and compare the results.

3.1. Participants

A total of eight studies met the inclusion criteria and were included in the review, with a summary of the participant characteristics provided in

Table 3. Study characteristics.

Authors	Sample Size	Gender	Age (Years)	Height (cm)	Body Mass (kg)	Sport	Level	Groups
Gonzalo-Skok et al., 2019 [22]	35	Male	15.4 ± 0.7	174.9 ± 5.8	64.2 ± 7.0	Soccer	Academy players	SVW (Same Volume Weaker) = 10 DVW (Double Volume Weaker) = 11 SVS (Same Volume Stronger) = 14
Murton et al., 2021 [23]	16	Male	18.0 ± 1.0	--	93.0 ± 13.1	Rugby Union	Elite Academy players	FIT (Flywheel Inertial Training) = 8 TRT (Traditional Resistance Training) = 8
Nunez et al., 2019 [24]	20	Male	17.0 ± 1.0	178.1 ± 2.3	62.8 ± 6.6	Soccer	Elite Academy players	CPG (Conic-Pulley Group) = 10 CG (Control Group) = 10
Gonzalo-Skok et al., 2022 [25]	24	Male	16.0 ± 1.0 (VUH) 16.0 ± 1.0 (VUL)	190.1 ± 10.1 (VUH) 191.2 ± 10.8 (VUL)	83.2 ± 9.9 (VUH) 84.2 ± 10.1 (VUL)	Basket	Elite Academy players	EOT VUH (Unilateral Horizontal) = 12 EOT VUL (Unilateral Lateral) = 12
Stojanovic et al., 2021 [26]	36	Male	17.58 ± 0.52 (FST) 17.52 ± 0.58 (TST) 17.56 ± 0.54 (CON)	190.54 ± 4.98 (FST) 190.58 ± 6.56 (TST) 192.81 ± 3.99 (CON)	75.53 ± 5.43 (FST) 78.78 ± 8.01 (TST) 80.00 ± 8.76 (CON)	Basket	Academy players	FST (First Experimental Group) = 12 TST (Second Experimental Group) = 12 CON (Control Group) = 12
Fiorilli et al., 2020 [27]	34	Male	13.21 ± 1.21 (FEO) 13.36 ± 0.80 (PT)	165.21 ± 10.00 (FEO) 168.36 ± 7.00 (PT)	51.25 ± 6.71 (FEO) 52.10 ± 5.23 (PT)	Soccer	Academy players	FEO (Flywheel Eccentric Overload) = 18 PT (Plyometric Training) = 16
Arede et al., 2020 [28]	19	Female	15.0 ± 0.5	165.7 ± 5.4	61.7 ± 7.3	Team Sports	Academy players	EOT Variable = 8 EOT Standard = 11
Raya-Gonzalez et al., 2021 [15]	22	Male	--	--	--	Soccer	Elite Academy players	EG (Experimental Group) = 11 CG (Control Group) = 11

. A total of 206 adolescents were recruited and included in the analysis, but only one study recruited female athletes (19 participants). The control group included 33 of the total participants. Participants took part in a range of team sports, including soccer, rugby, and basketball. Academy athletes were recruited in four studies [22,26,27,28], whereas athletes from elite academies were recruited in another four studies [15,22,23,24].

3.2. Intervention

Intervention characteristics are provided in

Table 4. Intervention characteristics.

Authors	Weeks	EOT/Week	Exercises	Sets and Reps	Inertial Device	Inertia Load	Test/Measures
Gonzalo-Skok et al., 2019 [22]	10 weeks	1 session	Unilateral lateral squat	Weeks 1–2 (2 sets × 6 reps) Weeks 3–6 (2 sets × 8 reps) Weeks 7–10 (2 sets × 10 reps) 30″ rest between legs 3′ rest between sets	Conic Pulley (VersaPulley, Costa Mesa)	0.27 kg·m2	SLH (single-leg horizontal jump test) TSLH (triple-leg horizontal jump) CMJ L/R (unilateral) CMJ (bilateral) *
Murton et al., 2021 [23]	4 weeks	2 sessions	Squat Romanian deadlift Bulgarian split squat	Week 1 (4 sets × 8 reps) Week 2 (5 sets × 6 reps) Week 3 (4 sets × 8 reps) Week 4 (5 sets × 8 reps)	K-box (Bromma, Sweden)	0.05 kg·m2	CMJ (countermovement jump) SJ (squat jump) DJ (drop jump) **
Nunez et al., 2019 [24]	9 weeks	1 session	Front-step acceleration	2–3 sets × 6 reps (each leg)	Conic Pulley	0.22 kg·m2	20 m linear sprint test ***
Gonzalo-Skok et al., 2022 [25]	6 weeks	2 sessions	VUH exercises (side step, backward lunges, crossover cutting, landing, and backward lunges) VUL exercises (lateral squat, defensive-like shuffling steps, lateral crossover cutting, 90° lunge)	Week 1–2 (1 set × 6 reps) Week 3–4 (1 set × 8 reps) Week 5–6 (1 set × 10 reps)	Conic Pulley (VersaPulley, Costa Mesa)	0.27 kg·m2	CMJ (countermovement jump) LJ (lateral jump) HJ (horizontal jump) 25 m linear sprint test COD (180° test) COD (V-cut test) ****
Stojanovic et al., 2021 [26]	8 weeks	2 sessions	Romanian deadlift Half squat	Week 1–2 (2 sets × 8 reps) Week 3–6 (3 sets × 8 reps) Week 7–8 (4 sets × 8 reps)	Flywheel (D11, Desmotec)	0.075 kg·m2	CMJ (countermovement jump) 5 m linear sprint test (SPR5m) 20 m linear sprint test (SPR20m) COD (t-test)
Fiorilli et al., 2020 [27]	6 weeks	2 sessions	Multidirectional–unilateral COD Shooting movement	4 sets × 7 reps 120″ rest between sets	Flyconpower conical machine (Cuneo; Italy)	Not reported	SJ (squat jump) DJ (drop jump) 7R-HOP (7 repeated hop test) COD (Y-agility test) COD (Illinois test) 60 m linear sprint test Shot test (Loughborough soccer shooting)
Arede et al., 2020 [28]	6 weeks	2 sessions	Backward lunges Defensive-like shuffling steps Side step (The participants included in the variable group were instructed to perform the movement randomly in one of the three directions (0°, 45° right, and 45° left))	1 set (5–6 reps each leg)	Eccommi (Byomedic System)	0.0315 kg·m2	CMJ (countermovement jump) SLCMJ (single-leg countermovement) SLRJ (single-leg rebound jump) COD (t-test) 10 m linear sprint test (0–10 m) *****
Raya-Gonzalez et al., 2021 [15]	10 weeks	1 session	Lateral squat	Week 1 (2 sets × 8 reps) Week 2–3 (2 sets × 10 reps) Week 4 (3 sets × 8 reps) Week 5–6 (3 sets × 10 reps) Week 7–8 (4 sets × 8 reps) Week 9 (3 sets × 8 reps) Week 10 (2 sets×8 reps) 180″ rest between sets	K-Box 4 (ExxentricTM, Sweden)	0.025 kg·m2	CMJ (countermovement jump) 10 m linear sprint test (SPR10) 20 m linear sprint test (SPR20) 30 m linear sprint test (SPR30) COD10 (5 + 5 m) COD20 (10 + 10 m) COD90 (90°)

* Unilateral test can add L (left), R (right), W (weaker) or S (stronger); ** Test adds H (height), PF (peak force), or PP (peak power); *** T10m (time at 10 m), T20m (time at 20 m), and T10–20m (time between 10 m and 20 m) are registered; **** All jumps and 180COD add L (left) or R (right); ***** All jumps add L (left) or R (right).

. Training programs lasted from 4 to 10 weeks (7.3 mean ± 2.2 SD), including 8 to 16 training sessions (11.1 mean ± 2.5 SD). The studies with fewer weeks of intervention (4–8 weeks) had two sessions per week, whereas the longest studies (9–10 weeks) had only one. Studies utilized several inertial devices: conical pulley—VersaPulley used in three studies [22,24,25], K-Box in two studies [15,23], flywheel D1 Desmotec in one study [26], Flyconpower conical machine in one study [27], and Ecconomy Byomedic [28] in one study. The inertial load was highly different for every device. The prescribed training volume ranged from 1 to 5 sets of 6–10 repetitions, being gradually increased every 1–2 weeks in five studies [15,22,23,25,26] and maintaining the same load in three studies [24,27,28]. During intervention protocols, a huge variety of exercises were used, which included unilateral lateral squats, backward lunges, defensive-like shuffling steps, Romanian deadlifts, Bulgarian split squats, front-step acceleration, side steps, crossover cutting, landing, half squats, or multidirectional–unilateral CODs. Backward lunges and lateral squats were the most used.

3.3. Outcome Measures

The intervention characteristics and the outcome measures of the functional tests are presented in Table 4 and

Table 5. Results and conclusions.

Authors	Results	Results Summary	Conclusions
Gonzalo-Skok et al., 2019 [22]	Within groups: Possibly to likely improvements in CMJ and CMJW (all groups) Possibly CMJ asymmetry reduction (all groups) Possibly to very likely improvements in SLHW, TSLHR, TSLHL, TSLHS, and TSLHW (SVW and DVW groups) Possibly CMJL improvement (DVW and SVS groups) Substantially improvement in CMJR (SVW group) Substantially TSLH asymmetry reduction (DVW group) Substantially SLH asymmetry increment (DVW and SVS group) Between groups: The improvement in TSLH asymmetry was substantially greater in DVW than in SVW A substantially greater SLHR, TSLHR, TSLHS, and TSLHW in SVW and DVW in comparison to SVS Substantial greater improvements in SLH asymmetry and CMJR in SVW compared to SVS Substantially greater performance in TSLHL in DVW than SVS Correlational analysis At pre-test, negative relationships were found between SLHR and SLHL with single-leg horizontal asymmetry, between TSLHL with triple single-leg horizontal asymmetry, and between CMJR with CMJ asymmetry At post-test, no significant relationships were found between asymmetries and jumping performance	There are improvements in jump performances and reductions in asymmetries for all groups, but mainly in the DVW group	Unilateral strength training programs were shown to substantially improve bilateral jumping performance
Murton et al., 2021 [23]	Within groups: Significant improvements for CMJ-H (moderate) and SJ-H (moderate) in TRT group Significant improvements for CMJ-PP (small) with a trend for improvement in CMJ-H (small) in FIT group Between groups: No statistical significance for all measures Greater improvements for CMJ-PF, CMJ-H, SJ-H in TRT group Greater improvements for SJ- PP in FIT group	There are improvements in jump performances for both groups but higher for TRT (traditional) training	TRT may be favorable to FIT. In well-trained youth male adolescent athletes, increases in lower-limb strength and power measures can occur within as little as four weeks following either TRT or FIT
Nunez et al., 2019 [24]	Within groups: Substantially enhanced T10m and T20m in the CPG Substantially enhanced T10–20m and T20m in the CG Between groups: At pre-test, no substantial differences in any of the variables with the lower limb power test At pre-test, substantially better T10m, T10–20m, and T20m for CG than the CPG	Improvements in sprint performance and lower limb power for the CPG group	Adding a weekly one-step acceleration exercise with a conical pulley device provides insufficient data for an improvement in the ability to sprint in 10 m and 20 m, compared to strength training with the use of sled training, full squats, and plyometric exercises
Gonzalo-Skok et al., 2022 [25]	Within groups: Substantial improvements in CMJL, HJR, HJL, LJL, 180CODR, 180CODL in both groups Substantial enhancement in CMJR and 5 m split time in the VUH group Substantially better LJR in VUL group Between groups: Substantially greater for LJR and LJL in VUL group than VUH group Possibly greater performance in CMJR and 5 m split time in VUH group than VUL group	No substantially improved bilateral vertical jumping performance in any group Unilateral vertical jumping performance was substantially improved in both groups Lateral and horizontal unilateral jumps related to linear sprinting and COD performance VUH group achieved a substantial improvement in 5 m Both training programs induced substantial improvements in COD 180° performance V-cut test was not substantially improved in any group	A specific force vector training program induced substantial improvements in both specific and non-specific inter-limb asymmetries and functional performance tests, although greater improvements in lateral and horizontal variables may depend on the specific force vector targeted
Stojanovic et al., 2021 [26]	Within groups: Improvements for CMJ in FST, TST, and CON groups (very large, large, and trivial effect sizes, respectively) Improvements for SPR5m in FST, TST, and CON groups (very large, moderate, and moderate effect sizes, respectively) Improvements for t-test in FST, TST, and CON (very large, large, and moderate effect sizes, respectively) Between groups: No significant differences in pre-test for any variable analyzed Significant differences in CMJ between FST and TST group, FST and CON group, and CST and CON group Significant differences in SPR5m between FST and TST and FST and CON groups No significant differences in SPR5m between the TST and CON groups No significant differences for SPR20m Significant differences for t-test between the FST and CON, TST and CON, and FST and TST groups	Flywheel group displayed significantly higher improvements in strength, vertical jump, 5 m sprint time, and COD ability compared to the control group Neither training modality was proved effective for enhancing 20 m sprint performance	Eight weeks of flywheel training (1–2 sessions per week) induces superior improvements in CMJ, 5 m sprint time, and COD ability than an equivalent traditional weight training in well-trained junior basketball players
Fiorilli et al., 2020 [27]	Within groups: Significant differences for DJh, DJct, 7R-HOPh, SJh, ILL, YT, SPRINT, and SHOT No differences for DJRSI, 7R-HOPtc, and 7R-HOPRSI Between groups: Differences between groups in DJh, 7R-HOPh, SJh, ILL, and SHOT Significant interactions in DJh, ILL, YT, SPRINT, and SHOT No differences in DJct, DJRSI, 7R-HOPh, 7R-HOPtc, 7R-HOPRSI, and SJh *	FEO (flywheel eccentric overload) group significantly improved jumps, CODs, and sprints	Positive effect of flywheel training shows greater improvements in these tests compared with the plyometric training
Arede et al., 2020 [28]	Within groups: Significant improvements for CMJL, CMJR, LJR, LJL, HJL, SLRJL, 0–10 m in EOT Standard Significant improvements for CMJL, CMJR, SLRJL, SLRJR, t-test in EOT Variable Between groups: Differences for LJL favoring EOT Variable	EOT significantly improved jumps, CODs, and sprints	The rotational flywheel training includes improvements
Raya-Gonzalez et al., 2021 [15]	Within groups: Significant improvements for CMJd, CMJnd, COD (all metrics), and CODdef in EG group Improvements for COD10d and CODdef10d in CG group Between groups: Differences between groups in CMJd and CMJnd Differences for COD10d, COD10nd, CODdef10d, COD20nd, CODdef20d, and CODdef20nd in favor of EG group No differences between groups in SPR10 and SPR30 **	EG significantly improved jumps and CODs but no improvements in sprints	One flywheel training session per week, over 10 weeks, can effectively enhance jump and COD performance without affecting the reported well-being state in U16 elite soccer players in-season

* It can be added h (height), ct (contact time), tc (contact time), RSI (reactive strength index); ** It can be added d (dominant leg), nd (non-dominant leg), def (deficit—additional time that a COD needs in comparison to a linear sprint test over the same distance).

, respectively.

3.3.1. Power

Lower limb power was measured using a variety of tests, including counter-movement jump (CMJ) unilateral and bilateral, single-leg horizontal jump (SLH), triple single-leg horizontal jump (TSLH), squat jump (SJ), drop jump (DJ), rebound jump (RJ), seven repeated hop test (7R-HOP). CMJ was the most used functional test, and only one of the eight selected studies did not include a lower-limb power test, showing significant and substantial improvements in jump performances in the other seven studies (from trivial to large effect size). However, Murton et al., 2021 [23] showed greater results for the traditional training group than for the inertial training group.

3.3.2. Change of Direction

Five of the total studies included tests to measure the change of direction ability. t-test, Y-agility test, Illinois test, V-cut test, 180° test, and 90° test were mainly used, including a description of the protocol and setup in the articles. The heterogeneity of the test makes comparison difficult as to which is the best training to increase the performance. On the other hand, some studies showed significant improvements in the change of direction ability for the inertial training group. In this regard, Arede et al., 2020 [28] and Stojanovic et al., 2021 [26] showed significant improvements in the t-test (from moderate to very large effect size), Fiorilli et al., 2020 [27] showed significant improvements in Illinois and the Y-agility test, Raya-González et al., 2021 [15] showed significant improvements in all COD tests and Gonzalo-Skok et al., 2022 [25] showed substantial improvements in the 180° test, whereas there was no effect in the V-cut test.

3.3.3. Sprint

Sprint actions were evaluated in six studies, throughout several linear sprint tests with different distances (5 m, 10 m, 20 m, 25 m, and 60 m), thus a different stimulus for the athletes. Short distances are related to power and acceleration processes, whereas long distances are more focused on maximum speed. Nunez et al., 2019 [24], Gonzalo-Skok et al., 2022 [25], Arede et al., 2020 [28], and Stojanovic et al., 2021 [26] found significant improvements in sprint performance in short distances. Whereas Stojanovic et al., 2021 [26] did not show inertial training as an effective way to enhance 20 m sprints, Fiorilli et al., 2020 [27] showed greater improvements in long-distance sprints (60 m linear sprint), and Raya-González et al., 2021 [15] did not find significant improvements in sprint performance.

4. Discussion

Following a comprehensive literature search, the most recent studies that made an intervention with at least four weeks of training using inertial devices were analyzed to know how this methodology can help coaches and athletes achieve enhancements. The primary findings suggest IT is a useful way to improve performance variables such as jumps, sprints, or CODs, although there are a few controversies in some studies.

In recent years, previous research has analyzed the effect of IT in adult populations [15,19,20,29,30,31,32], whereas it is not so common in youth athletes. The structural benefits from IT seem to be clear, such as improvements in strength also appear to occur alongside rapid structural and strength changes, thus improving the morphological characteristics of athletes. ST programs that adequately load the lengthening phase of movements, called eccentric training, might induce superior neuromuscular adaptations (faster cortical activity, inversed motoneuron activity pattern, improved muscle–tendon unit morphology and structure) compared with traditional strength training [26].

The usefulness of IT for enhancing jumping ability, sprint, and COD performance seems to be clear [32]. Our revision is in line with these findings, and the same results have been observed in young athletes [26,27,28]. Previous studies in which IT was performed also showed enhancement in strength and body composition in youth soccer players [27]. Inertial devices could be a great tool to perform ST in young athletes because it is an easy way to work in different vectors [14], and it is not necessary to use a weighted load, given that this method is characterized by the use of their own force produced [7]. The force produced in the eccentric-concentric transition generates a great stimulus for SSC, which may explain better adaptations in sports actions [27].

Jumping performance is commonly used as a key indicator for lower-limb power and strength [30], and it is a usual action in team sports; in fact, it plays a key role in performance in team sports and an enhancement in high-intensity actions, such as jumps can be a determinant to achieve success in competition. In this sense, IT has been shown to be an effective tool for improving muscular power. Our findings are in line with previous research in the adult population; all the studies that were analyzed showed improvements in jump ability. Nevertheless, vector force and the specificity of the exercise are quite important [11]; actually, bilateral CMJ did not show significant improvement, whereas unilateral and different vector jump abilities were improved in studies that did not perform bilateral exercises [25]. This is also supported by previous authors who indicate the importance of specificity. Exercises with IT to achieve improvements in specific actions such as jump ability are also of great importance [30]. On the other hand, Raya-González et al., 2021 [15] suggest that improvement in jumping performance is explained by the nature of flywheel devices, with the squat being one of the most analyzed exercises in the literature due to its similarity with the jump pattern. The similarity of the movement and the production of the stretch-shortening cycle may be related to the transition from eccentric to concentric phases during flywheel training, which could have a positive transfer to jumping performance [30]. In fact, Murton et al., 2021 [23] demonstrated that only four weeks are necessary to achieve enhancement in jump performance, a better way than traditional ST. According to our findings, improvements in jump ability show important improvements after IT protocols.

Sprinting plays a key role in performance in team sports as well; a lot of successful actions are preceded by sprinting. In this case, the literature shows controversial results, which are also supported by some of our findings, displaying greater improvements in sprint [24,26,27,28], and other findings either not demonstrating improvements or showing little improvement in 5 m linear sprint instead of 10 m, 20 m, and 25 m linear sprints [25]. These results could be explained by the volume of training. Gonzalo-Skok et al., 2022 [25] performed one set; meanwhile, Fiorilli et al., 2020 and Stojanović et al., 2021 [26,27] even performed four sets. On the other hand, Arede et al., 2020 [28] performed one set as well, but their subjects had the additional stimulus of regular soccer training, where sprints are more habitual than in basketball training. Moreover, Raya-González et al., 2021 [15] performed one weekly training session, whereas the rest of the studies had a minimum of two training sessions per week. The training volume can be relevant to improve sprint performance in youth athletes. For achieving enhancements in sprint athletes, there needs to be an adequate volume per week, but more studies are necessary to know the volume for improving sprint performance.

CODs are commonly performed as well in many situations during competition in team sports [33]. During COD, an athlete needs a great SSC because of the ability to generate an eccentric force to rapidly decelerate and concentric strength to accelerate in a new direction [27]. Indeed, Flywheel devices have been utilized to replicate similar movement patterns and transition from eccentric to concentric phases, which are believed to be particularly beneficial for improving change of direction results [30]. Incorporating flywheel exercises in ST seems to be one of the most effective methods to improve sports actions in athletes [15]. The results of the reviewed studies with younger athletes suggest the same conclusions as previous literature on adult athletes. In fact, a weekly training session may be enough to improve COD in elite young soccer players [15]. Flywheel training appears to improve performance by reducing braking time and enhancing braking impulse during COD movements [30]. This better exploitation of the SSC may have allowed a greater training stimulus to occur over time, resulting in improved cutting performance [28]. IT has shown improvements in COD performance in the youth athlete population. On the other hand, another important enhancement that plays a key role in sports was found, such as shoot [27] or asymmetry of lower limbs [22], as an indicator of injury risk.

Between-study differences might be due to the training volume performed, the season moment, or the participants’ training experience/age [25]. To optimize training outcomes, it is recommended that practitioners individualize (i.e., create inertia–power or inertia–velocity profiles) and periodize flywheel training using the latest guidelines [30].

Finally, our systematic research supports the use of IT as a great tool for improving jump, sprint, and COD performance in youth athletes. Coaches can take advantage of the benefits of this training methodology, combining strength training and specific training on the pitch. Two sessions per week could be enough to achieve enhancements. At least two to three exercises and two to three sets per exercise should be necessary to improve young athletes. However, some limitations were found in this study, including the tests used to analyze were not exactly the same and the level of athletes was different. Another limitation is the number of databases used for this study (Web of Science, Scopus, and PubMed). Future research could include more databases such as Latindex, Cochrane Library, or Sage. It would be interesting with new research to clarify an adequate volume to achieve improvements in sprint performance.

5. Conclusions

This systematic review analyzed eight studies with an IT intervention and the effects on jump, sprint, and COD performance pre- and post-test on youth athletes. In our review, we support the usefulness of rotational inertial devices for enhancing the ability of players in high-intensity actions such as jumps, sprints, and CODs; however, future research is needed to determine the adequate volume to develop sprint performance.

The results of our study showed that IT can be a useful tool to improve important abilities in team sports performance in young athletes, like jumps, changes of direction, or sprints. From our point of view and based on our findings, the direction, volume, and laterality (bilateral or single strength) of the exercise have an impact on test performance. For this reason, the methodology of training is quite important for achieving enhancements; the selection of exercises, volume, and load play a key role in the variables that we want to improve.