IndexIntroductionReviewConclusionPractical applicationsIntroductionIn recent years, youth tennis has become increasingly competitive at all levels and at all ages. Recognition of the importance of physical preparation and development in youth tennis has also grown. Therefore, effective modalities that enhance performance-relevant physical attributes are crucial. The serve is strategically considered the most important shot. Consequently, improving serve performance is a major goal of youth tennis programs. A key aspect of service performance is service speed. The serve involves multiple body segments that produce force simultaneously through complex coordinated muscle activations; called the “kinetic chain”. Fett, Ulbricht, and Ferrauti (2018) found that in elite youth tennis players, predictors of strength and power explained 41–66% of the variance in serve speed in boys and 19–45% in girls. Therefore, to increase serve speed and improve tennis performance, youth programs must aim to attenuate strength and power throughout the kinetic chain. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an Original Essay Plyometric training is an established, appropriate, and safe modality for improving power, movement speed, and performance of explosive actions in young athletes. Plyometric exercise causes muscles to rapidly lengthen and shorten, a phenomenon called the "stretch-shorten cycle" (SCC). Plyometric training induces neural adaptations resulting in a better ability to utilize the SCC and generate greater tension and subsequent strength. Plyometric training can provide specific stimuli through the manipulation of movement patterns, speed, loads and metabolic demands. Chu (2003), for example, proposed guidelines for tennis-specific plyometric training. Despite the theoretical potential of transference to service, this has not been fully clarified in the literature. Therefore, the purpose of this critical evaluation is to review studies on plyometric training interventions on serve speed in young tennis players. Papers were retrieved using SPORT Discus, PubMed, Google Scholar and relevant references from acquired studies. Search terms included “plyometric training,” “service speed,” “youth,” and “tennis.” The Boolean modifiers “AND” and “OR” were used to narrow the search and include alternative phrases such as “plyometrics,” “serve speed,” and “youth.” Studies had to include a plyometric training intervention lasting at least six weeks, a control group, a sample of young tennis players, and outcome measures including serve speed. Review All but one study measured peak service speed. Fernandez-Fernandez and Ellenbecker (2013), Fernandez-Fernandez, De Villarreal, Sanz-Rivas and Moya (2016) and Pardos-Mainer, Ustero-Perez and Gonzalo-Skok (2017) all had players perform eight maximum serves and had used the highest recorded serve velocity for subsequent analyses: to be recorded, the serves had to land in the service box. In contrast, Behringer et al. (2013) measured average serve speed, recording over twenty serves with no accuracy requirements. Behringer and colleagues argue that this increases the practical relevance and ecological validity of the study. On the one hand, fatigue during matches reduces the speed of the serve. Therefore, it could be suggested that improvethe average speed on numerous services is of greater importance than increasing the maximum speed on fewer services. However, this method essentially measures the effects of plyometric training on fatigue resistance during repeated submaximal serve performances, rather than on maximal serve velocity. Furthermore, the lack of precision requirements negates the potential ecological benefits of this method. If a player can serve with a higher average speed over 20 serves but fails to land in the service area, the performance simply does not improve. Furthermore, Behringer and colleagues' findings are weakened by other limitations. This study reported a significant increase in service speed compared to the post-intervention control. However, the average speed decreased during the study period in the control group (-5.3%), while it simultaneously increased in the plyometric group (2.9%). This suggests that the significant difference reported by the authors was caused by uncontrolled external variables and not exclusively by the intervention. Furthermore, the authors did not report significant within-group improvement from pre- to post-intervention in the plyometric group. Therefore, the results of Behringer et al. (2013) should be interpreted with great caution. Behringer et al. (2013) and all other studies in this review did not blind the interventions. Blinding is a crucial methodological tool for reducing bias. Failure to use may cause the Avis and/or John Henry effect. This is where participants assigned to the control condition are disappointed and underperform (John Henry effect), or are motivated and outperform the intervention group (Avis effect). Although it is difficult to effectively “blind” a plyometric intervention, it is possible to reduce these effects by manipulating the study design. For example, using certain repeated measures designs, a wait list control, or a staggered baseline. However, the studies in this review did not use any of these tools. This reduces the ability to interpret the results of these studies as strong evidence of the effects of plyometric training on serve speed. Furthermore, the John Henry effect may partly explain the decline in service speed observed in the control group in the study by Behringer et al. (2013). Intervention programs in most studies were similar and deemed suitable and appropriate for adaptation. Interventions lasted six to eight weeks, training two to three times per week with sessions that included upper and lower body exercises with familiarity and coaching throughout. Fernandez-Fernandez et al. (2016) and Pardos-Mainer et al. (2017) used similar programs, adhering to traditional plyometric programming principles. These two studies reported contrasting but relatively positive results. Behringer et al. (2013) exercises based on NSCA recommendations, a needs analysis by Reid et al. (2003) and tennis-specific plyometric training recommendations from Chu (2003). However, these exercises were then grouped into one "circle" for the upper body and two for the lower body, consisting of three or four exercises. These were then performed in an interval training style, as opposed to traditional plyometric training. Fernandez-Fernandez and Ellenbecker (2013) used a variety of exercises including plyometric, core and elastic exercises, and medicine ball exercises. This intervention could be considered a program that is not strictly plyometric, making it unsuitable. It might be suggested that the adaptations resulting from these two interventions somewhatinappropriate would not be consistent with plyometric training. Both of these studies reported significant improvements in the plyometric group compared to control and post-intervention. However, it is difficult to interpret these studies as strong evidence of the effects of specifically plyometric training on serve speed in young tennis players due to these methodological shortcomings. Neither Fernandez-Fernandez and Ellenbecker (2013), Pardos-Mainer et al. (2017) or Behringer et al. (2013) controlled for training volume between the intervention and control groups. Participants in these plyometric groups performed the program in addition to the standard seasonal regimen. Therefore, these participants were exposed to a greater training volume than the control. The improvements in service speed reported by these studies may be due to the increased volume of training, rather than the intervention alone, making the results of these studies difficult to interpret. On the other hand, Fernandez-Fernandez et al. (2016) controlled for training volume across groups. This was achieved by having participants complete plyometric sessions in lieu of tennis training periods. This provides a simple and effective way to equalize training volume between groups. However, many coaches would prefer significant training time focused on the technical aspects of the game, especially with young players, for whom skill development is of paramount importance (Reid et al., 2007). Therefore, implementing plyometric training in this way may not be suitable for youth tennis programs; reducing the applicability of the results of this study. Predicted age at peak height velocity (PHV) is crucial when attempting to recognize whether power developments are caused by training interventions or natural improvements that occur during maturation and growth. The onset of this natural increase in potency usually occurs about a year to a year and a half before PHV. Biological age of maturity can be calculated to identify how many years earlier or later PHV participants are at the time of measurement. Differences in biological age of maturity between groups may be a confounding variable, explaining some of the differences in serve speed between the control and plyometric groups. Therefore, it is critical that studies evaluate the biological age of maturity of the sample in order to control for these confounding pediatric factors. Behringer et al. (2013) used a pubertal self-report test to measure maturation status. This produced nominal data, whereby participants were assigned to one of five pubertal stages. This method provides insight into differences in pubertal stage between groups, but is limited in scope for statistical analyses. Pardos-Mainer et al. (2017) only measured age, height, weight, and BMI. In contrast, Fernandez-Fernandez and Ellenbecker (2013) and Fernandez-Fernandez et al. (2016) measured age of maturity and reported no significant differences between the control and plyometric groups. This suggests that the improvements in service speed found in this study are not caused by differences in maturity. However, the underlying mechanisms and adaptations are still hypothetical due to the absence of direct physiological measures such as EMG. As a result, it is difficult to confidently interpret changes in serve speed as a result of plyometric training alone (even if maturity is controlled for). Conclusion The purpose of this critical evaluation was to review the effectiveness of using plyometric training as an intervention.