The use of real-time monitoring during flywheel resistance training programmes: how can we measure eccentric overload? A systematic review and meta-analysis

1 THE USE OF REAL-TIME MONITORING DURING FLYWHEEL RESISTANCE TRAINING PROGRAMS: HOW CAN WE MEASURE THE ECCENTRIC OVERLOAD? A SYSTEMATIC REVIEW AND META-ANALYSIS HEAD TITLE: ECCENTRIC OVERLOAD IN FLYWHEEL RESISTANCE TRAINING DEVICES ABSTRACT Objectives: This systematic review and meta-analysis aimed to analyze the technologies and main training variables used in the literature to monitor flywheel training devices in real-time. In addition, as the main research question, we investigated how eccentric overload can be effectively monitored in relation to the training variable, flywheel shaft type device and the moment of inertia selected. Methods: The initial search resulted in 11,621 articles that were filtered to twenty-eight and seventeen articles that met the inclusion criteria for the systematic review and meta-analysis, respectively. Results: The main used technologies included force sensors and rotary/linear encoders, mainly to monitor Peakor Mean-Force, -Power or -Speed. Not always an eccentric overload was achieved using flywheel devices. The eccentric overload measurement was related to the main outcome selected. While Mean-Force (p = 0.011, ES= - 0.84) and Mean-Power (p<0.001, ES= -0.30) favored the concentric phase, Peak-Power (p<0.001, ES= 0.78) and Peak-Speed (p<0.001, ES= 0.37) favored the eccentric phase. In addition, the lower moments of inertia (i.e., from 0.01 to 0.2 kg·m2) and a cylindrical shaft type (i.e., vs conical pulley) showed higher possibilities to achieve the eccentric overload. Conclusions: a wide variety of technologies can be used to monitor flywheel devices, but to achieve eccentric overload, a flywheel cylindrical shaft type with low moments of inertia are advised to be used. 2 Keywords: monitoring, physiologic, resistance training, muscle strength, exercise INTRODUCTION Resistance training (RT) methods traditionally use free weights [1], weight stacks [2], isokinetic dynamometers [3], and/or the athletes’ own body mass [4] to induce overload and associated positive adaptations, in relation to muscle strength development. In addition, strength training can be specifically focused on improving the concentric (CON) or eccentric (ECC) phases of the movement [5]. Concentric training is important to enhance acceleration characteristics in some particular sports contexts, such as linear sprinting [6]. However, in other sports (i.e., team sports), where changes of direction frequently occur and the force is applied in multiplanar movements, the use of eccentric training can be beneficial [7]. Furthermore, increasing the athletes’ capacity to break the kinetic energy produced during the concentric phase during the eccentric phase, can reduce the injury risk [8,9]. Intensity and volume are two typical training variables that can be modified over time and can also be monitored in real-time [10]. Of these variables, the training intensity can mostly determine the chronic adaptations [11]. A minimum training stimulus (i.e., intensity threshold) is necessary to evoke positive adaptations [12]. Hence, objective knowledge of the training intensity is of paramount importance. A typical method to indicate the training intensity in RT is the use of the one-repetition maximum (1-RM) and its relative percentages [13]. Moreover, recent works suggest the use of bar speed to objectively control the training intensity, known as velocity-based training (VBT) [13,14]. Traditionally, strength exercises have been monitored using force sensors, such as force platforms [15–17] or strain gauges [2], linear encoders [18– 20] and, more recently, accelerometers [21]. Although they can be used to describe the exercise performance via mechanical outputs, nowadays, there is increasing interest in their use as 3 biofeedback [13,14]. Despite VBT has gained an increasing audience in recent years, its applications are mainly focused on the CON phase of the exercises. Considering eccentric training, an interesting paradigm that has been developed over the last 20 years [22] is the flywheel training. Compared to free weights, the main difference is related to the influence of gravity on the loading; while free weights are gravity-dependent (isoinertial translational movements on the vertical or other planes), flywheel resistance training devices (FRTD) are gravity independent [23], due to the rotary inertial setting of the exercise (isoinertial rotary movement). Hence, while the 1-RM occurs in traditional training models, in the flywheel paradigm, it is not measurable or achievable. Another interesting characteristic of FRTD is that the shaft-type (i.e., vertical cone (VC) or horizontal cylinder (HC)) and its radius [19] determine the final mechanical output, together with the moment of inertia [15,24]. Surprisingly, only recently, there has been increasing interest in characterizing the mechanical overload of different exercises using these devices [15,19,24,25]. As with free weights, the literature shows a mechanical overload when the moment of inertia is increased [15,26]. However, the different types of FRTD, based on their shaft-type, and the absence of 1-RM make it challenging to compare across different RT programs and exercises. Thus, the provision of accurate mechanical descriptive outputs is of importance to quantify the neuromuscular performance of a given training session using FRTD. Different mechanical outputs have been reported in the literature when FRTD were used as part of a RT program. While initial works measured FRTD with force sensors [15,22,27], goniometers [23,28], and electromyography (EMG) [1,15,29] to describe the mechanical or neuromuscular loading, nowadays the use of encoders is more common [2,20,30]. Consequently, several mechanical variables derived from encoders (i.e., force, power, or 4 velocity) have also been reported in association with FRTD. Of interest, the stretch-shortening cycle (SSC) has recently been quantified, because one of the main characteristics of these devices is the coupled concentric-eccentric actions [23]. Furthermore, FRTD have been widely used with the objective of achieving eccentric overload (EO), also known as enhanced negative work-based [31]. Typically, EO is achieved if the ECC output is higher compared to the CON output [23,32]. An early work suggested that a higher EO can be achieved with FRTD compared to free weights [23]. However, the EO is achieved only during small windows or brief episodes of the eccentric movement phase [23,33]. This is explained by the deliberate delay in the voluntary brake at the end of ECC to achieve a higher mechanical peak, proposed as a technique to achieve the EO [32]. The maneuver to elicit EO must be learned by the individual. Nevertheless, the EO does not always occur using FRTD, even when ensuring this learned breaking technique [19]. Therefore, it is crucial to determine the extent to which researchers are successful in inducing a real EO, because this can have practical implications for the training routines that incorporate FRTD. Of note, the term “eccentric overload” appears in the title of several papers related to FRTD [8,19,27,34]. However, EO has not been evidenced in all works using FRTD. Therefore, the purpose of this systematic review with meta-analysis was to characterize the monitoring technologies and related real-time mechanical output during the concentric and/or eccentric movement phases using FRTD. In addition, we propose the following research question: is it possible to achieve an EO (i.e., force; velocity; power) while using FRTD? 5 MATERIALS AND METHODS Registry of Systematic Review Protocol The systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) recommendations [35]. The study protocol was registered at the International Prospective Register of Systematic Reviews (PROSPERO) database (number CRD42020187386). Search Strategy Original articles published in PubMed, Web of Science, Scopus, and Cochrane electronic databases were searched. The search strategy used a combination of appropriate terms with Boolean operators: flywheel training OR flywheel exercise OR flywheel inertia OR flywheel resistance training OR flywheel resistance exercise OR variable inertial OR rotary inertial OR inertial training OR inertial exercise OR isoinertial training OR isoinertial exercise OR eccentric overload OR eccentric overload training OR enhanced eccentric OR gravity independent. There were no language or year restrictions. The final search was carried out in June 2020. In addition, a complementary search was carried out in the reference lists of preselected primary articles and other sources to identify relevant studies which were manually included when applicable [36]. Eligibility criteria and selection process The studies were considered eligible if they met the following criteria: a) included full-text original papers (reviews, letters, opinions, case report, book chapters will be excluded); b) fulltext manuscripts published in English; c) used a FRTD during exercise; d) used technology which provides mechanical (i.e., force, power or velocity) or muscle activation outputs (i.e. EMG) for the CON and ECC movement phases; e) published in a peer-reviewed journal; f) 6 included only healthy adults as participants (>18 years); g) used resistance training; and h) described the training intervention with the following specifications: FRTD equipment description, type of exercise, moment of inertia (i.e., external load), volume and training intensity. For meta-analysis purposes, we selected only the papers which provided information regarding mean and peak mechanical outputs (i.e. force and/or power and/or velocity) during both CON and ECC movement phases. The results obtained from the different databases were grouped and duplicates were removed (Figure 1). Two researchers (AM and PG) screened studies by analyzing titles and abstracts. The screening was performed using the Endnote software (v.X9, Thompson Reuters) and Rayyan online platform [37] (v. 1.0). The full-text analysis of previously selected articles was conducted by both researchers independently based on the inclusion criteria. The discrepancies were analyzed by a third researcher (FF). Any remaining disagreements were resolved by a fourth researcher (FN). Data Extraction Data extraction was performed by three researchers (FF, FN, and AM). Included studies were read to extract the following variables: authors, year of publication, descriptive information of sample, device shaft type, exercise evaluated, assessment device, moment of inertia, volume, real-time mechanical variables monitored, and outcomes evaluated. The mean and standard deviation (SD) values of measures of interest were extracted by two independent researchers (FF and FN). When the data of interest could not be obtained directly from the articles, we contacted the authors to request the information. If no response was obtained, validated (r = 0.99, p <0.001) online software (WebPlotDigitizer, v. 4.3, Pacifica, California USA) was used to extract the data from the figures [38]. Two independent researchers (FF and FN) obtained 7 the data using the aforementioned software. Extracted data were analyzed by a third researcher (AM). Methodological Quality Assessment The quality of the cross-sectional studies was individually assessed by means of the modified version of the Downs and Black checklist [39]. Fourteen items from the original scale were included in the quality assessment. In addition, item 10 of the original scale referring to “Have actual probability values been reported (e.g. 0.035 rather than <0.05)?” was modified to “Reports sufficient descriptive statistical data rather than just p-values”. Items 1-10 refer to “reporting”, item 12 refers to “external validity”, items 15-25 refer to “internal validity”. The quality analysis was conducted by two independent researchers (PG and FN; agreement = 89.5%), and disagreements were resolved through analysis by a third reviewer (AM). We characterized the quality of evidence for each study according to criteria previously used in the literature [40]. The “Quality Index” was obtained by dividing the individual score of each study by 14 and multiplying by 100. Studies with a Quality Index >66.7% were classified as low risk of bias, between 50% and 66.6% as moderate risk of bias, and with < 50% as high risk of bias. The risk of bias assessment and quality of evidence of longitudinal studies were evaluated using the Physiotherapy Evidence Database - PEDro scale [41]. The assessment was independently conducted by two researchers (PG and FN; agreement = 87.8%). A third reviewer (AM) was consulted in case of disagreements. Scoring criteria adopted in previous studies were used to analyze the quality of evidence [42]. The studies were considered as being of “excellent” (9 - 10 points), “good” (6 - 8 points), “fair” (4 - 5 points), or “poor” methodological quality (≤ 3 points)[43]. 8 Statistical Analysis Descriptive results are shown as pooled mean ± pooled SD. Although two studies can be used in meta-analyses, considering that reduced sample sizes are common in the sport science literature [44], a meta-analysis for a given flywheel-derived outcome (i.e., power, force, velocity) was conducted if at least three studies provided sufficient data for the calculation of effect sizes (ES) [43]. Means and standard deviations (SD) for a measure of flywheel-derived outcomes from the CON and ECC were converted to Hedges’ g ES (corrects the typical Cohens’ effect size multiplying it by a correction factor to avoid problems with small samples [45] and corrects biases due to sample size differences across study groups). In all analyses, we used the random-effects model to account for differences between studies that might impact the treatment effect[46]. The ES values are presented alongside their respective 95% confidence intervals (CIs). Calculated ES were interpreted using the following scale: <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 [47]. Heterogeneity was assessed using the I2 statistic, with values of <25%, 25-75%, and >75% considered to represent low, moderate, and high levels of heterogeneity, respectively. The risk of bias was explored using the extended Egger’s test [48]. In the case of a significant Egger test, correction procedures (i.e., trim and fill method) were performed. In addition to the main analyses, we used the flywheel shaft type and flywheel inertia as moderators to explore their influence on the results. Specifically, shaft type was divided into VC and HC, while inertia (kg·m2) was grouped into four categories: 0.01 to 0.1, 0.1 to 0.2, 0.2 to 0.3, and 0.3 to 0.4. We have created those categories according to simple and easy thresholds which include all the moments of inertia used in the literature, to our knowledge. All analyses were carried out using the Comprehensive Meta-Analysis program (v. 2; Biostat, Englewood, NJ, USA). The statistical significance threshold was set at p< 0.05. 9 RESULTS Search results The database search yielded a total of 11,621 potential studies (Figure 1). Twenty eight studies were included in the systematic review (qualitative synthesis) [1,2,24–30,33,49,50,3,51–58,16– 20,22,23], and seventeen were included in the meta-analysis (quantitative synthesis) [2,17,51,53–58,19,20,24–27,30,50]. Most of the papers were not included in the meta-analysis because they did not provide specific information about the moment of inertia used [3,16,18,22,23,33,49]. In addition, two studies [25,28] did not provide data from both CON and ECC movement phases. Finally, two studies [1,29] were excluded because they only monitored EMG. Study Characteristics The study characteristics are shown in Table 1. The majority of the studies (n= 19) included male or female physically active subjects [1,2,29,30,49–53,56–58,3,16,19,22,23,25–27]. The rest of the studies (n= 8) used team sports athletes [18,20,24,28,50,54], resistance training naive participants [55], and national level sprinters [17]. The most frequently used flywheel shafttype device was HC (n= 21) [1,2,28–30,49–51,54,56–58,19,20,22–27], while the VC type was less frequently used (n= 6) [16–19,52,53]. Most of the studies were designed as cross-sectional interventions (n= 21), using one or two sessions with variations in training volume from 1 to 10 sets with 6 to 30 repetitions each. The longitudinal studies (n= 7) included training interventions of between 4 and 7 weeks, with two or three training sessions per week. <<INSERT TABLE 1 NEAR HERE>> The studies showed different purposes when resistance flywheel devices were used. The first published flywheel research were focused on mechanical and electrical muscle activity when 16 fact that is impossible to be achieved in FRTD. In the second scenario, the duration of the ECC is higher, thus increasing the tension that muscle fibers must sustain [69]. This has been highlighted as an interesting option for injury prevention purposes [9]. However, in FRTD, as previously shown, instead of increasing the duration of the ECC phase, to achieve EO, the opposite should be performed. In our opinion, this is a possible explanation for the fact that EO can only be achieved by using Peak Power or Peak Velocity. Concerning the fact that Peak Force did not show EO, Alkner et al. [29] suggested that muscles are at rest during ECC, but are working maximally in CON, at least in greater knee angles where higher forces can be developed in a leg press device, for example. In agreement, if to achieve EO, the kinetic energy must be braked at the last third of the movement, sometimes, a biomechanical disadvantage will exist related to muscle levers, explaining why our results did not show an EO in Peak Force. Other possible reasons why EO is not achieved in Peak Force are related to the explanations above, and also to the exercise direction of vector [65]. Consequently, although many authors have used FRTD with EO purposes, we showed that many of them did not prove that EO was achieved and, in some cases, it was not. However, many papers that measure muscle EMG showed higher ECC activation when using FRTD [1,22,29,49–51,70]. Hence, FRTD offer higher ECC activation, even at high velocities [15], compared to other training equipment [1]. Interestingly, Carroll et al. [56] showed that although the CON muscle electrical activation increased with the progressive overloading in FRTD, the opposite pattern was found in ECC muscle activation. In addition, kinetic and kinematic overloading is observed when progressive testing is performed in FRTD [15,24,25]. These results show a possible influence of the moment of inertia on EO. Accordingly, our results demonstrated that the moment of inertia determined the EO, even in Peak Force, where an EO was not observed when all shaft-types and moments of inertia where considered. Furthermore, Mean Force also showed a moderate EO in the 0.01 17 to 0.1 kg·m2 sub-group, but large overload in CON for higher moments of inertia. In addition, the second lowest moment of inertia sub-group (0.1 to 0.2 kg·m2) showed a moderate EO in Peak Power. As shown by previous works [15,24,25], lower moments of inertia led to higher velocity and power. Thus, it is easier to achieve EO with lower moments of inertia (from 0.01 to 0.2). However, it must be acknowledged that the FRTD shaft type also has an influence on the EO. Nuñez et al. [19] showed that although no EO was achieved, the shaft type influences the force and velocity exerted with the same moments of inertia. In agreement, our results showed that when an HC device is used, EO is achieved only in Peak Velocity, but these results were extracted from a single study [50]. In contrast, a higher moderate CON character was observed when the VC was used regarding Mean Force. This can be explained by the variation in the VC shaft type, from a wider to a narrower radius in CON, and the opposite in ECC, allowing the individual pull with less effort at the beginning of CON. In summary, EO was not proven to be achieved in most of the papers studied. Indeed, of the works which measured it, not all demonstrated that it had been achieved. Our results showed that EO measurement is dependent on the monitored mechanical variable. What is more, it is influenced by the FRTD shaft-type and moment of inertia used. Limitations and future directions Some potential limitations of this meta-analysis should be acknowledged. Firstly, additional analyses regarding shaft type or inertia were not always possible as less than three studies were available for at least one moderator. Secondly, even though the included studies did not specify any negative responses associated with the intervention, it is unclear if there was an attempt by the researchers to record all possible adverse events comprehensively. Therefore, future studies 18 are encouraged to be fully transparent regarding any injuries, pain, or other adverse effects occurring as a result of flywheel use. Thirdly, although most of the included studies in our metaanalysis were classified as low-moderate risk of bias and moderate-good methodological quality, none of the studies were classified with excellent methodological quality. Future studies on this topic should strive for greater methodological quality in their designs. CONCLUSIONS For many years, FRTD have been used with the objective of producing EO. However, we showed that EO is only shown by Peak variables, more specifically in power or velocity. Indeed, it is more suitable to use lower moments of inertia (i.e., from 0.01 to 0.2 kg·m2) and an HC device to achieve EO. In contrast, a VC can help in achieving more significant CON outputs. These results are relevant for real practice to decide the best option regarding the FRTD type and load to be used, especially when the main purpose is to achieve EO during RT programs. Furthermore, the calculation of the E:C-r can provide interesting insights to quantify the EO, even in real-time. In addition, although no EO is achieved during the movement, this ratio can express the eccentric character of the execution, which is also important for physical conditioning. REFERENCES 1. Norrbrand L, Pozzo M, Tesch PA. Flywheel resistance training calls for greater eccentric muscle activation than weight training. Eur J Appl Physiol. 2010;110(5):997– 1005. 2. Fernandez-Gonzalo R, Lundberg TR, Alvarez-Alvarez L, De Paz JA. Muscle damage responses and adaptations to eccentric-overload resistance exercise in men and women. 19 Eur J Appl Physiol. 2014 May;114(5):1075–84. 3. Coratella G, Chemello A, Schena F. Muscle damage and repeated bout effect induced by enhanced eccentric squats. J Sports Med Phys Fitness. 2015;56(12):1540–6. 4. Loturco I, Ugrinowitsch C, Roschel H, Tricoli V, González-Badillo JJ. Training at the optimum power zone produces similar performance improvements to traditional strength training. J Sport Sci Med. 2013; 5. Roig M, O’Brien K, Kirk G, Murray R, McKinnon P, Shadgan B, Reid WD. The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: A systematic review with meta-analysis. British Journal of Sports Medicine. 2009. 6. Maćkała K, Fostiak M, Kowalski K. Selected determinants of acceleration in the 100m Sprint. J Hum Kinet. 2015;45(1):135–48. 7. Gonzalo-Skok O, Tous-Fajardo J, Valero-Campo C, Berzosa C, Bataller AV, ArjolSerrano JL, Moras G, Mendez-Villanueva A. Eccentric-Overload Training in TeamSport Functional Performance: Constant Bilateral Vertical Versus Variable Unilateral Multidirectional Movements. Int J Sports Physiol Perform. 2016 Aug;12(7):951–8. 8. de Hoyo M, Pozzo M, Sañudo B, Carrasco L, Gonzalo-Skok O, Domínguez-Cobo S, Morán-Camacho E. Effects of a 10-week in-season eccentric-overload training program on muscle-injury prevention and performance in junior elite soccer players. Int J Sports Physiol Perform. 2015;10(1):46–52. 9. Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sport. 2003;13(4):244–50. 10. Slimani M, Paravlic A, Granacher U. A meta-analysis to determine strength training related dose-response relationships for lower-limb muscle power development in 20 young athletes. Front Physiol. 2018;9. 11. Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW. Strength and Hypertrophy Adaptations Between Lowvs. High-Load Resistance Training. J Strength Cond Res. 2017;31(12):3508–23. 12. American College of Sports Medicine. American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med Sci Sports Exerc [Internet]. 2009;41(3):687–708. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19204579 13. González-Badillo JJ, Sánchez-Medina L. Movement velocity as a measure of loading intensity in resistance training. Int J Sports Med. 2010 May 23;31(5):347–52. 14. Conceição F, Fernandes J, Lewis M, Gonzaléz-Badillo JJ, Jimenéz-reyes P, GonzalézJJ, Jimenéz-reyes P, Gonzaléz-Badillo JJ, Jimenéz-reyes P. Movement velocity as a measure of exercise intensity in three lower limb exercises. J Sports Sci [Internet]. 2015 Jun 17 [cited 2019 May 6];0414(September):1099–106. Available from: http://www.ncbi.nlm.nih.gov/pubmed/26395837 15. Carroll KM, Wagle JP, Sato K, Taber CB, Yoshida N, Bingham GE, Stone MH. Characterising overload in inertial flywheel devices for use in exercise training. Sport Biomech. 2018;3141:1–12. 16. Chiu LZFF, Salem GJ. Comparison of Joint Kinetics during Free Weight and Flywheel Resistance Exercise. J Strength Cond Res [Internet]. 2006 Aug [cited 2019 May 23];20(3):555. Available from: http://www.ncbi.nlm.nih.gov/pubmed/16937968 17. Vázquez-Guerrero J, Moras G, Baeza J, Rodríguez-Jiménez S. Force outputs during squats performed using a rotational inertia device under stable versus unstable conditions with different loads. PLoS One. 2016;11(4):1–13. 18. Javier Núñez F, Suarez-Arrones LJ, Cater P, Mendez-Villanueva A, Núñez FJ, Suarez- 21 Arrones LJ, Cater P, Mendez-Villanueva A. The high-pull exercise: A comparison between a versapulley flywheel device and the free weight. Int J Sports Physiol Perform [Internet]. 2017 Apr [cited 2019 Sep 7];12(4):527–32. Available from: http://www.ncbi.nlm.nih.gov/pubmed/27705034 19. Núñez FJ, Galiano C, Muñoz-López A, Floria P. Is possible an eccentric overload in a rotary inertia device? Comparison of force profile in a cylinder-shaped and a coneshaped axis devices. J Sports Sci. 2020; 20. Raya-González J, Castillo D, Domínguez-Díez M, Hernández-Davó JL. Eccentricoverload production during the flywheel squat exercise in young soccer players: Implications for injury prevention. Int J Environ Res Public Health. 2020;17(10). 21. Ferro A, Floría P, Villacieros J, Muñoz-López A. Maximum velocity during loaded countermovement jumps obtained with an accelerometer, linear encoder and force platform: A comparison of technologies. J Biomech [Internet]. 2019;95:109281. Available from: https://doi.org/10.1016/j.jbiomech.2019.07.025 22. Berg HE, Tesch PA. A gravity-independent ergometer to be used for resistance training in space. Aviat Sp Environ Med. 1994;65(8):752–6. 23. Tesch PA, Ekberg A, Lindquist DM, Trieschmann JT. Muscle hypertrophy following 5-week resistance training using a non-gravity-dependent exercise system Acta Physiologica Scandinavica Volume 180, Issue 1. Acta Physiol Scand [Internet]. 2004;180(1):89–98. Available from: http://onlinelibrary.wiley.com/doi/10.1046/j.00016772.2003.01225.x/abstract 24. Sabido R, Hernández-Davó JL, Pereyra-Gerber GT. Influence of different inertial loads on basic training variables during the flywheel squat exercise. Int J Sports Physiol Perform. 2018;13(4):482–9. 25. Worcester KS, Baker PA, Bollinger LM. Effects of Inertial Load on Sagittal Plane 22 Kinematics of the Lower Extremity During Flywheel-Based Squats. J Strength Cond Res. 2020;1. 26. Martinez-Aranda LM, Fernandez-Gonzalo R. Effects of inertial setting on power, force, work, and eccentric overload during flywheel resistance exercise in women and men. J Strength Cond Res. 2017 Jun;31(6):1653–61. 27. Norrbrand L, Fluckey JD, Pozzo M, Tesch PA. Resistance training using eccentric overload induces early adaptations in skeletal muscle size. Eur J Appl Physiol. 2008;102(3):271–81. 28. Aagaard P, Simonsen EB, Trolle M, Bangsbo J, Klausen K. Moment and power generation during maximal knee extensions performed at low and high speeds. Eur J Appl Physiol. 1994;69(5):376–81. 29. Alkner BA, Bring DKIK-IKIK-I. Muscle activation during gravity-independent resistance exercise compared to common exercises. Aerosp Med Hum Perform. 2019;90(6):506–12. 30. Piqueras-Sanchiz F, Martín-Rodríguez S, Martínez-Aranda LM, Lopes TR, RayaGonzález J, García-García Ó, Nakamura FY. Effects of moderate vs. high iso-inertial loads on power, velocity, work and hamstring contractile function after flywheel resistance exercise. PLoS One. 2019 Feb 7;14(2):e0211700. 31. Coratella G, Beato M, Cè E, Scurati R, Milanese C, Schena F, Esposito F, Ce E, Scurati R, Milanese C, Schena F, Esposito F, Cè E, Scurati R, Milanese C, Schena F, Esposito F. Effects of in-season enhanced negative work-based vs traditional weight training on change of direction and hamstrings-to-quadriceps ratio in soccer players. Biol Sport. 2019;36(3):241–8. 32. Berg HE, Tesch PA. Force and power characteristics of a resistive exercise device for use in space. Acta Astronaut. 1998;42(1):219–30. 23 33. Pozzo M, Alkner B, Norrbrand L, Farina D, Tesch PA. Muscle-fiber conduction velocity during concentric and eccentric actions on a flywheel exercise device. Muscle and Nerve. 2006;34(2):169–77. 34. Gual G, Fort-Vanmeerhaeghe A, Romero-Rodríguez D, Tesch PA, Gabriel G, Azahara F-V, Daniel R-R, A. TP. Effects of in-season inertial resistance training with eccentric overload in a sports population at risk for patellar tendinopathy. J Strength Cond Res. 2016 Jul;30(7):1834–42. 35. Liberati A, Altman DG, Tetzlaff J, Mulrow C, Gøtzsche PC, Ioannidis JPA, Clarke M, Devereaux PJ, Kleijnen J, Moher D. The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: Explanation and elaboration. PLoS Medicine. 2009. 36. Horsley T, Dingwall O, Sampson M. Checking reference lists to find additional studies for systematic reviews. Cochrane Database Syst Rev. 2011;(8). 37. Ouzzani M, Hammady H, Fedorowicz Z, Elmagarmid A. Rayyan---a web and mobile app for systematic reviews. Syst Rev [Internet]. 2016;5(1):210. Available from: http://dx.doi.org/10.1186/s13643-016-0384-4 38. Drevon D, Fursa SR, Malcolm AL. Intercoder Reliability and Validity of WebPlotDigitizer in Extracting Graphed Data. Behav Modif. 2017; 39. Downs SH, Black N. The feasibility of creating a checklist for the assessment of the methodological quality both of randomised and non-randomised studies of healt. J epi. 1998;377–84. 40. Batacan RB, Duncan MJ, Dalbo VJ, Tucker PS, Fenning AS. Effects of high-intensity interval training on cardiometabolic health: A systematic review and meta-analysis of intervention studies. Br J Sports Med. 2017;51(6):494–503. 41. Maher CG, Sherrington C, Herbert RD, Moseley AM, Elkins M. Reliability of the 24 PEDro scale for rating quality of randomized controlled trials. Phys Ther. 2003;83(8):713–21. 42. Grgic J, Rodriguez RF, Garofolini A, Saunders B, Bishop DJ, Schoenfeld BJ, Pedisic Z. Effects of Sodium Bicarbonate Supplementation on Muscular Strength and Endurance: A Systematic Review and Meta-analysis. Sports Medicine. 2020. 43. García-Hermoso A, Ramírez-Campillo R, Izquierdo M. Is Muscular Fitness Associated with Future Health Benefits in Children and Adolescents? A Systematic Review and Meta-Analysis of Longitudinal Studies. Sports Medicine. 2019. 44. Valentine JC, Pigott TD, Rothstein HR. How many studies do you need? A primer on statistical power for meta-analysis. J Educ Behav Stat. 2010;35(2):215–47. 45. Dupuy O, Bherer L, Audiffren M, Bosquet L. Night and postexercise cardiac autonomic control in functional overreaching. Appl Physiol Nutr Metab. 2013;38(2):200–8. 46. Kontopantelis E, Springate DA, Reeves D. A Re-Analysis of the Cochrane Library Data: The Dangers of Unobserved Heterogeneity in Meta-Analyses. PLoS One. 2013; 47. Hopkins W, Marshall S, Batterham A, Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc. 2009 Jan;41(1):3–13. 48. Egger M, Smith GD, Schneider M, Minder C. Bias in meta-analysis detected by a simple, graphical test. Br Med J. 1997; 49. Caruso JF, Hernandez DA, Porter A, Schweikert T, Saito K, Cho M, De Garmo N, Nelson NM. Integrated electromyography and performance outcomes to inertial resistance exercise. J Strength Cond Res. 2006;20(1):151–6. 50. Tous-Fajardo J, Maldonado RA, Quintana JM, Pozzo M, Tesch PA. The flywheel legcurl machine: offering eccentric overload for hamstring development. Int J Sports Physiol Perform. 2006 Sep;1(3):293–8. 25 51. Norrbrand L, Tous-Fajardo J, Vargas R, Tesch PA. Quadriceps muscle use in the flywheel and barbell squat. Aviat Space Environ Med. 2011 Jan;82(1):13–9. 52. Carmona G, Guerrero M, Cussó R, Padullés JM, Moras G, Lloret M, Bedini JL, Cadefau JA. Muscle enzyme and fiber type-specific sarcomere protein increases in serum after inertial concentric-eccentric exercise. Scand J Med Sci Sport. 2014;25(6):e547–57. 53. Moras G, Vázquez-Guerrero J. Force production during squats performed with a rotational resistance device under stable versus unstable conditions. J Phys Ther Sci [Internet]. 2015 [cited 2019 Jun 6];27(11):3401–6. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4681914/pdf/jpts-27-3401.pdf 54. Sabido R, Hernández-Davó JL, Botella J, Navarro A, Tous-Fajardo J, Hernandez-Davo JL, Botella J, Navarro A, Tous-Fajardo J. Effects of adding a weekly eccentricoverload training session on strength and athletic performance in team-handball players. Eur J Sport Sci. 2017 Jun;17(5):530–8. 55. Illera-Domínguez V, Nuell S, Carmona G, Padullés JM, Padullés X, Lloret M, Cussó R, Alomar X, Cadefau JA, Illera-Dominguez V, Nuell S, Carmona G, Padulles JM, Padulles X, Lloret M, Cusso R, Alomar X, Cadefau JA. Early Functional and Morphological Muscle Adaptations During Short-Term Inertial-Squat Training. Front Physiol. 2018 Sep 10;9. 56. Carroll KM, Wagle JP, Sato K, Taber CB, Yoshida N, Bingham GE, Stone MH. Characterising overload in inertial flywheel devices for use in exercise training. Sport Biomech. 2019;18(4):390–401. 57. Castillo D, Domínguez R, Rodríguez-Fernández A, Raya-González J, Dominguez R, Rodriguez-Fernandez A, Raya-Gonzalez J. Effects of Caffeine Supplementation on Power Performance in a Flywheel Device: A Randomised, Double-Blind Cross-Over