The Effects of Flywheel Training with a Portable Device on Physical Performance in Soccer Players

Abstract

Soccer is a team sport in which players expend high-intensity intermittent efforts that require the production of sprints, accelerations, decelerations, changes of direction, and jumps. The aim of this study was to analyze the effects of training with portable and low-cost flywheel devices, using multi-directional exercises over 8 weeks, on the performance of linear sprint, curve sprint, and change of direction in senior soccer players. Thirty-three players participated in the research, divided into a control group and an experimental group. The latter group performed the training protocol in addition to the technical–tactical soccer training. Before and after the application of the training protocol, the linear and curve sprint and change of direction (V-cut) were evaluated. At the end of the training program, significant improvements (p < 0.05) were found in change of direction and in curve sprint with respect to the control group. The results suggested that resistance training with flywheel devices with low training volumes improves performance in change of direction and in curve sprint, which are key performance variables in soccer. Therefore, this type of training could be added to technical–tactical soccer training to enhance the variables that are essential to reach high-performance metrics in soccer.

1. Introduction

Soccer is a team sport in which players expend many high-intensity efforts through predominantly unilateral actions [1] and in different planes of movement [2]. One of the most repeated actions during a match is change of direction (COD), with more than 800 changes of direction (CODs) per match [3]. There are multiple factors on which the COD ability depends—neuromuscular coordination, strength, and power of the lower limbs [1,4,5,6,7,8]. Among these changes of direction, we can differentiate those involving a deceleration phase followed by acceleration in a different direction [9], typically characterized by directional changes at angles of less than 90° [10], and, by contrast, the curve sprint (CS), which is defined by a gradual and continuous change of direction throughout the entire route [10,11]. This sprint can be performed to the right, with the right leg remaining on the inside of the curve, or to the left, with the left leg remaining on the inside of the curve. Filter et al. [12] point out the existence of a strong side and weak sides in the CS, finding a similar performance between the CS towards the strong side and the linear sprint. The CS is a neuromuscular action determining performance in soccer [11]. Around 85% of the sprints performed in a soccer match are executed in a curved trajectory; all outfield players complete CSs during the match [11]. It is a resource used by players to avoid offside and to evade, follow, or lure an opponent [13]; therefore, it is highly important in both offensive and defensive situations.

Eccentric strength is particularly important to CODs [9]. The scientific literature has shown that the COD correlates with the eccentric strength of the lower limb [14,15]. To target the eccentric phase of the movement, training devices known as rotary inertial devices (RIDs) have been designed [16,17]. RIDs provide a source of linear resistance from a tether wrapped around a horizontal axis in the shape of a cylinder or a vertical axis in the shape of a cone [18]. Concentric action unwinds the tether belt by rotating the flywheel in one direction, and eccentric action occurs during rewinding by breaking the generated motion [19]. The literature shows that for the same working load, RIDs generate more resistance to eccentric phase motion than traditional means [19]. Eccentric overload training using RIDs has been shown to enhance muscle activation in soccer players during various tasks involving CODs [14]. Following the implementation of a training protocol with RIDs, significant improvements in COD performance were observed in soccer players [6,20]. Repeated actions under eccentric overload may result in improvements in braking capacity, a critical factor in COD performance [18]. The use of RIDs to overload multidirectional movements yields greater performance enhancements compared to conventional soccer training [21]. Additionally, isoinertial resistance training demonstrated superior results compared to cable resistance training methodologies [22]. However, one of the drawbacks of the use of RIDs in soccer training is the difficulty of transporting them to different places where players train and their high cost, which makes their use almost unattainable for modestly funded soccer teams [23].

Recently, new, easily portable, and low-cost RID technology has been developed [24]. The device features compact dimensions (12 × 11 × 11 cm) and a lightweight design (<1 kg), making it easily transportable in a backpack [25]. RIDs can operate with either a conical or a cylindrical pulley system. In this case, the portable RID uses a conical pulley, operating similarly to traditional conical RIDs while offering a lower cost and enhanced portability. The conical shape of the RID enables the execution of specific multi-directional and multi-joint movements with optimal power [21].

To the best of our knowledge, no previous scientific publication has investigated the application of this training system to enhance physical performance in soccer players. Therefore, this study aims to analyze the effect of using these devices on the improvement of sprint performance, COD, and CS in senior soccer players. We hypothesize that through training with a low-cost, portable RID, improvements in the COD and CS will be obtained.

2. Materials and Methods

2.1. Participants

In total, 33 senior soccer players from federated, non-professional clubs, all competing in the same division, voluntarily participated in this research. None of the participants had prior experience with strength training using RIDs. The players were randomly divided into an experimental group (EG) and a control group (CG). The characteristics of the participants are presented in Table 1. All players participated in 3 soccer training sessions per week of one and a half hours, plus a weekly league match. Only male players over 18 years of age and without injuries were included in the study. Players who did not participate in 80% of the sessions were excluded from participation in the research. Finally, 23 players were included in the analysis (EG, n = 11; CG, n = 12). All subjects were informed about the potential risks and benefits of participation in the research and gave written informed consent. The study was approved by the Virgen Macarena y Virgen del Rocio University Hospitals ethics committee (0398-N-17) and was conducted in accordance with the Declaration of Helsinki.

2.2. Procedures

In this study, a pre-test–post-test cohort design was conducted to analyze the effects of training with low-cost, portable RIDs twice a week, for 8 weeks, on sprint, CS, and COD performance. The players had a familiarization session with the tests performed prior to the pre-test. Prior to the tests, a standardized warm-up was performed, consisting of 5 min of continuous running, 5 min of joint mobility, 2 sets of 10 repetitions of squats (body weight), 2 sets of 3 repetitions of countermovement jumps, 3 sets of 20 m accelerations (80%, 90%, and 100% of maximum speed), and a maximum acceleration of 10 m. The tests were always performed in the same order with a 5 min recovery time between them—(1) 20 m sprint; (2) 20 m CS; and (3) V-Cut. All measurements were performed at the same time (20:00 h ± 30 min), on the same surface (3rd generation artificial turf), and in similar weather conditions (no rain, no wind, and similar atmospheric temperatures). Participants were verbally motivated during all the tests to encourage maximal effort.

2.2.1. The 20 m Linear Sprint

Two 20 m sprints separated by a passive rest of 3 min were performed, and the best attempt was selected for analysis. Wireless photocells (Polifemo Radio Light, Microgate, Bolzano, Italy) were placed at the start, 10 m and 20 m from the start, to obtain sprint times at 10 m and 20 m. The starting position was upright, with one foot forward and placed 1 m behind the first photocell, without allowing for any oscillations. Once given the go-ahead, the subject could start the sprint when they felt ready; he did not have to react to any sound signal.

2.2.2. The 20 m Curve Sprint

The trajectory used in the CSs was the penalty area curved line [11]. Two 20 m curve sprints to the right (CS20r) and two to the left (CS20l), separated by a passive rest of 3 min, were performed, and the best attempt was selected for analysis. To determine the time used, wireless photocells (Polifemo Radio Light, Microgate, Bolzano, Italy) located at the start, 10 m and 20 m from the start, were used to obtain the sprint times at 10 m (CS10) and 20 m (CS20). The player stood in an upright position, with one foot forward and positioned 1 m behind the first photocell, without allowing any oscillations. The subject could start the sprint when he felt ready; he did not have to react to any sound signal.

2.2.3. V-Cut Change of Direction Test

Players performed 2 sprints of 25 m with a 45° COD every 5 m for a total of 4 CODs, alternating right and left [26]. The best attempt was selected for analysis. For the test to be valid, players had to step with one foot on a line established by 2 cones 0.7 m apart [26]. Wireless photocells (Polifemo Radio Light, Microgate, Bolzano, Italy) located at the start and finish were used to determine the time taken. The player stood in an upright position, with one foot forward and positioned 1 m behind the first photocell, without allowing for any oscillations. The subject could start the sprint when he felt ready; he did not have to react to any sound signal.

2.3. Training Program

Prior to the soccer training, the players performed 2 exercises, firstly with a front start (Figure 1) and secondly with crossover cutting (Figure 2), resisted by an RID (Handy Gym PRO, Vigo, Spain) with 0.08 kg/m² moment of inertia. All players performed 2 sets of 6 repetitions with each leg for both exercises, with 2 min rest between sets, 2 days per week, with 48 h recovery between each session, for 8 weeks. Before each training session, the same warm-up was performed as in the tests.

2.4. Statistical Analyses

Data are presented as the mean ± SD. Before performing statistical analyses, we checked the data distribution using a Shapiro–Wilk normality test. Test–retest reliability was assessed using the coefficient of variation (%CV) and the intraclass correlation coefficient (ICC) with 95% CI. For the statistical analysis, a 2 × 2 Mixed ANOVA was performed, with one between-group factor (group) with two levels (experimental, control) and one within-group factor (time) with 2 levels (pre-test, post-test). In case of differences between groups in the pre-test, we performed an ANCOVA, establishing the pre-test as a covariate. In case there was a Time × Group interaction, we compared different pairs using a post hoc analysis with Bonferroni adjustment. In addition, the effect size (ES) was calculated for each of the selected variables using Hedges’ g. For the intra-group comparison between the left and right side of the curve sprint, a t-test for related samples was performed. For these analyses, we used the statistical package SPSS version 25.0 (SPSS, Inc., Chicago, IL, USA).

3. Results

Test–retest reliability values are shown in Table 2. The intra-group and inter-group data comparisons for the different variables are shown in Table 3. The data corresponding to the intra-group comparison between the left and right sides in the CS are presented in Table 4.

There was a Time × Group interaction (p < 0.001) for the variable V-Cut. The EG significantly reduced the time in the V-Cut (−4.12%), producing significant differences with respect to the CG (p = 0.007; ES = 1.47), which did not improve.

There was a Time × Group interaction (p = 0.037) for the variable CS10r. The EG significantly reduced the time in CS10r (−3.28%), producing significant differences (p = 0.002; ES = 1.39) with respect to the CG, which did not improve. There was a Time × Group interaction (p = 0.049) for the variable CS20r. The EG significantly reduced the time in CS20r (−2.58%), producing significant differences (p = 0.003; ES = 1.36) with respect to the CG, which did not improve. There was a Time × Group interaction for the variable CS20l (p = 0.047). The EG showed signs of significance (p = 0.07) and a significantly greater reduction in time (−1.27%) (p = 0.007; ES = 1.21) than the CG, which did not improve.

There was no Time × Group interaction for the other analyzed variables.

4. Discussion

This study analyzed the effects of training with portable, low-cost RIDs on the performance of senior soccer players when performing sprints, CS, and COD. The main finding of this study is that training resisted by portable and low-cost RIDs improved the COD and CS ability of senior soccer players.

Change of direction (COD) is a crucial skill that determines a soccer player’s performance during a match. In many situations, gaining an advantage requires the ability to change direction faster than the opponent [1,7]. In fact, more than 700 CODs have been recorded in a single professional soccer match [27]. In our study, we obtained a V-Cut improvement of 4.12% over 8 weeks of training. These improvements are similar to those obtained in handball players who resisted the movement with traditional RIDs, using a training period duration and weekly frequency similar to ours and a daily training volume above that used in this study [22]. Other studies have reported findings that are similar to those observed in this investigation regarding the COD variable following the implementation of a training program with RIDs. For instance, Núñez et al. [28] observed significant improvements in 90-degree COD performance in young team-sport athletes following RID training performed both bilaterally and unilaterally. Notably, only unilateral training led to enhancements in both the dominant and non-dominant legs. These findings reinforce the exercise selection in this current study, where both exercises in the training program were performed unilaterally. Specifically in soccer players, Fiorilli et al. [21] reported results similar to those of this study, demonstrating significant improvements in COD performance in the group that trained with RIDs after six weeks of training. Additionally, Bloomfield et al. [27] determined that approximately 80% of CODs performed in Premier League matches occurred at angles between 0 and 90 degrees. Consequently, the V-CUT test appears to be a suitable method for assessing COD performance in soccer players, as it reflects the demands of the sport. Both this current study and the research conducted by Tous-Fajardo et al. [29] reported significant improvements in the V-CUT test, which translate into enhanced performance in CODs involving angles of 90 degrees or less.

This is the first study to apply a low-cost, portable RID training protocol and analyze its effects on CS performance. The CS is a resource used by players to avoid offside situations and to evade, follow, or draw the opponent [12]. As a result, it plays a crucial role during a match, both in offensive and defensive situations. In this study, a significant performance improvement was obtained in the CS performed to the right (−2.58%), and there were indications of significance in the CS performed to the left (−1.27%). According to Filter et al. [12], we observed that in our sample there was a weak side in CS, with this side being the one on which greater time was spent to complete. A possible explanation for these results is that the percentage of right-footed players in both the EG (72.7%) and the CG (83.3%) was higher than that of left-footed players. It is known that soccer players have a better COD ability with the dominant leg [30]. This could explain why, in our sample, the performance in the CS in the pre-test was better towards the left side (CS10l and CS20l). In the post-test, these differences between the two sides were balanced, but only in the EG was it due to an improvement in CS performance towards the weak side (CS10r and CS20r). Further studies are needed to confirm if the improved balance between the strong and weak side of the CS relates to player performance enhancement.

Based on this study, a low-to-moderate, non-variable training volume and a fixed external load may be sufficient to achieve positive adaptations in COD and CS in senior soccer players. We obtained similar improvements to those of other studies, which assessed the COD after applying a training program with traditional RIDs, using half the training volume per session and the same weekly training frequency [6,21,28,31]. It is possible that the similar improvement obtained with half the volume per session was due to the starting strength levels of the different samples. The literature has shown that subjects with more experience achieved better performance with the use of RIDs [32]. However, it has been shown that, both in experienced and inexperienced subjects, when comparing the effects of strength training with traditional systems to training with RIDs, rotary inertial devices yield greater performance improvements in power-related variables [33]. Including a higher volume would increase the total time dedicated to this training and, therefore, decrease the total time available for technical–tactical training. Moreover, RIDs induce a high level of variability per se. Therefore, increasing the level of variability in the exercise will not add benefits to physical performance and training outcomes [34]. Regarding the external load used in inertial systems, the resistance generated by the device is proportional to the force applied by the subject during the concentric phase. This means that if a subject applies less force for any reason, the device will generate proportionally less resistance, and therefore the load is individualized in each set and repetition performed by each subject [35]. As a result, there is no need to manually adjust external loads as would be done in traditional weight training. Further research is needed to corroborate the findings of this study.

Sprinting is one of the high-intensity actions that occur in soccer matches [36,37]. Currently, the literature does not clarify if RID training enhances linear sprint performance. After applying a training program in which the EG included a front start exercise with RIDs in addition to traditional strength exercises, the authors found no significant differences with respect to the CG in the 20 m sprint [38]. De Hoyo et al. [39] also used the front start exercise with RIDs and found improvements in the 10 m sprint but without differences compared to the group that trained without RIDs. On the other hand, Tous-Fajardo et al. [29] did not find conclusive results on the effect of training with RIDs for the improvement of the sprint. In our study, sprint performance even worsened. One possible explanation may be that linear sprints are not the high-intensity action that occurs most often in soccer, since in more than 85% of these high-intensity actions there is a COD [10]. It is possible that the use of a test that is non-specific to the habitual movement of the player may be influenced to a greater extent by the variability in its execution. Another possibility is that the proposed exercises may lead to a selective strengthening of the intrinsic foot musculature [20], with this musculature being more important in the improvement of the COD. Further research is needed to determine the influence of the test specificity, as well as to check whether the type of exercise used can improve the intrinsic foot musculature and consequently the COD performance.

This study has some limitations. Firstly, there was no daily control of the training intensity. While the moment of inertia remained consistent throughout the program (0.08 kg/m²), the player’s acceleration of the device during exercise execution could result in varying levels of effort. Secondly, this study investigated the effect of training with a portable RID on senior amateur soccer players, where it was not possible to quantify the load and volume of soccer training for each player. Thirdly, the strength levels of the participants were not explicitly assessed before and after applying the training protocol.

In conclusion, a multi-directional training protocol, with a low volume and resisting movement with portable and low-cost RID, performed during the season, and combined with technical–tactical soccer training twice a week, improved the performance in COD and CS in senior soccer players.

5. Practical Applications

Considering that a training program like the one implemented in this study, or similar ones, will not induce high levels of fatigue, will not take up a long duration of the training session, and will lead to improvements in key performance variables for soccer without interfering with technical–tactical training, strength, and conditioning, coaches could incorporate this type of training into their players’ technical–tactical routines to enhance essential variables for achieving high performance on the field. Finally, another important aspect for coaches to consider is that this type of training should be added to regular soccer sessions twice per week, on separate days.

Author Contributions

The individuals who contributed to this paper are listed as follows: Conceptualization, J.L.N.-G. and F.J.N.; Methodology, J.L.N.-G., F.J.N., J.V.R., C.V.B. and F.H.A.; Formal Analysis, C.V.B.; Investigation, J.L.N.-G. and J.V.R.; Resources, F.J.N., C.V.B. and F.H.A.; Data Curation, J.L.N.-G. and J.V.R.; Writing—Original Draft Preparation, J.L.N.-G. and J.V.R.; Writing—Review and Editing, J.L.N.-G., F.J.N., J.V.R., C.V.B. and F.H.A.; Supervision, F.J.N.; Project Administration, F.J.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

This study was approved by the Virgen Macarena y Virgen del Rocio University Hospitals ethics committee (0398-N-17 15, November 2017) and was conducted in accordance with the Declaration of Helsinki.

Informed Consent Statement

Informed consent was obtained from all subjects involved in this study.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Front start exercise.

Figure 2. Crossover cutting exercise.

Table 1

Descriptive data of the players. Values presented as mean ± SD.

	EG (n = 17)	CG (n = 16)
Age (years)	21.94 ± 2.66	21.56 ± 1.7
Body mass (kg)	73.54 ± 7.12	72.77 ± 8.29
Height (m)	1.79 ± 0.06	1.78 ± 0.07
Years of training (soccer)	14.88 ± 3.39	14.13 ± 2.06

Table 2

Test–retest reliability of all measured variables related to players’ physical performance.

Variables	ICC (95% CI)	CV
V-Cut	0.902 (0.772–0.958)	6.75%
T10	0.966 (0.921–0.985)	7.64%
T20	0.930 (0.838–0.970)	7.96%
CS10l	0.927 (0.830–0.969)	5.98%
CS10r	0.913 (0.799–0.963)	8.28%
CS20l	0.890 (0.744–0.953)	7.17%
CS20r	0.937 (0.853–0.973)	6.70%

V-Cut = change of direction test time; T10 = 10 m sprint time; T20 = 20 m sprint time; CS10l = 10 m left curve sprint time; CS10r = 10 m right curve sprint time; CS20l = 20 m left curve sprint time; CS20r = 20 m right curve sprint time; ICC = intraclass correlation coefficient; CI = confidence interval; CV = coefficient of variation.

Table 3

Changes in the physical performance variables assessed. Values presented as mean ± SD.

Variable	EG			CG
	Pre-Test	Post-Test	ES	Pre-Test	Post-Test	ES
V-Cut (s)	6.52 ± 0.13	6.25 ± 0.11 *§	2.16	6.49 ± 0.26	6.59 ± 0.29 *§	0.35
T10 (s)	1.58 ± 0.09 #	1.69 ± 0.05	1.51	1.70 ± 0.06 #	1.74 ± 0.09	0.51
T20 (s)	2.85 ± 0.11 #	2.95 ± 0.08	1.01	2.98 ± 0.11 #	3.03 ± 0.11	0.44
CS10l (s)	1.73 ± 0.04	1.71 ± 0.05	0.43	1.75 ± 0.08	1.75 ± 0.05	0.00
CS10r (s)	1.77 ± 0.06	1.69 ± 0.05 *§	1.40	1.79 ± 0.08	1.77 ± 0.06 *	0.28
CS20l (s)	3.08 ± 0.08	3.04 ± 0.08 *	0.48	3.11 ± 0.11	3.14 ± 0.08 *	0.30
CS20r (s)	3.11 ± 0.09	3.03 ± 0.08 *§	0.91	3.17 ± 0.10	3.15 ± 0.09 *	0.20
CS10l–CS10r (%)	2.10 ± 2.42	−1.05 ± 1.83	1.43	2.37 ± 3.26	1.38 ± 3.10	0.30
CS20l–CS20r (%)	0.97 ± 0.89	−0.42 ± 1.40	1.17	1.95 ± 1.85	0.30 ± 1.86	0.86

ES = effect size; s = seconds; V-Cut = change of direction test time; T10 = 10 m sprint time; T20 = 20 m sprint time; CS10l = 10 m left curve sprint time; CS10r = 10 m right curve sprint time; CS20l = 20 m left curve sprint time; CS20r = 20 m right curve sprint time; CS10l–CS10r = difference in percentage between 10 m right and left curve sprint; CS20l–CS20r = difference in percentage between 20 m right and left curve sprint. * Significant differences (p < 0.05) between groups on post-test; # Significant differences (p < 0.05) between groups on pre-test; § Significant intra-group differences pre-test vs. post-test.

Table 4

Comparison between right and left side in the curve sprint test *.

Variables	EG				CG
	CS10r Pre	CS10r Post	CS20r Pre	CS20r Post	CS10r Pre	CS10r Post	CS20r Pre	CS20r Post
CS10l pre	0.039				0.023
CS10l post		0.104				0.088
CS20l pre			0.010				0.004
CS20l post				0.349				0.653

CS10l = 10 m left curve sprint time; CS10r = 10 m right curve sprint time; CS20l = 20 m left curve sprint time; CS20r = 20 m right curve sprint time. * The data represented in the table correspond to the p-value resulting from the statistical analysis between the variables.

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