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Description: INTRODUCTION Everything is set for the national junior championships in the 100-meter sprint – the children have adjusted their starting blocks, got into position and are waiting for the starting signal. “On your marks – set” – bang. The race is close, but as expected, it is won by the favorite. His high investment of time in training and the support by the national talent promotion system—with the goal of developing him into successful adult athlete—seem to pay-off. He and his coach celebrate the title and look ahead together to what appears to be a promising future as a senior athlete. But does his title as junior national champion really have a meaning beyond the present? With the present investigation we address the role of junior athletes’ performance in the long-term development of their peak performance as seniors. We are interested in examining athletes’ performance trajectories to better understand how athletic performance develops through childhood, adolescence, and adulthood, and whether junior performance levels predict senior performance levels in sports. Better scientific understanding of athletes’ performance trajectories has immediate benefits for current talent promotion programs in sports. Our research will inform central principles and decisions in these programs such as (1) how to promote junior development for long-term sustainable senior performance, and (2) how to select junior athletes for talent promotion programs aimed at expanding senior performance talent pools. KEY TERMS To provide context for our protocol, we use the following definitions for each key term: ‘Juniors’ refers to youth athletes competing in under-age categories and ‘seniors’ to adult athletes competing in the highest, open-age category, mostly in their 20s or 30s. A widely used key criterion of selection for talent promotion programs is youth athletes’ current performance (e.g. Fuchslocher et al., 2013). However, it is important to note that performance can be interpreted as an absolute or relative value. In some cgs sports—sports where performance is measured in centimeters, grams, or seconds (e.g. athletics, swimming, triathlon)—absolute performance values are sometimes used (e.g., a defined distance jumped or thrown, weight lifted, or time run or swum). However, in most cases, the relative performance is considered—that is, athletes’ performance relative to competitors’ performance, typically expressed as rank position. Athletes’ relative performance can be determined in all types of sports and facilitates comparisons of performance across different age categories, including junior and senior age. Here, we refer to relative performance (unless indicated otherwise). DIFFERENTATION OF EXISTING EMPIRICAL INVESTIGATIONS Several empirical studies have been conducted to investigate the association between junior and senior performance in sports. However, empirical results from these studies are mixed: Some studies reported strong associations between junior and senior performance whereas others reported weak or no association. This heterogeneity may partly be due to differences in samples, methods of data collection, and approaches to analyzing the data. Regarding samples, the athletes across studies varied in their countries of origin, types of sports, sexes, and junior age categories (e.g. under-14, under-16, under-18). In addition, they considered different ranges and levels of performance both at junior and senior age (e.g., participants, finalists, or medalists at national, continental, world championships or Olympic Games). Regarding methods of data collection, studies varied in whether they used prospective or retrospective methods. Prospective studies track junior athletes’ performance development into adulthood, whereas retrospective studies determine adult athletes’ former junior performance. Regarding data analyses, some studies examined correlations between athletes’ earlier and later performance, whereas others identified proportions of athletes achieving a defined performance level both as juniors and as seniors. The types of approaches of data collection and data analysis thus yields four types of studies (table 1). Tab. 1 Types of studies Data collection Prospective Retrospective Data analysis Correlations Type I Type II Proportions Type III Type IV Type I and II studies may consider absolute or relative performance, whereas type II and IV studies use relative performance only. Type I and II studies provide the strength of the correlation between junior success and senior success—that is, the extent to which individual differences in junior performance predict individual differences in senior performance. Type III and IV studies can be described as transition rate or conversion rate studies. The research question of prospective conversion rate studies (type III) seek the examine the proportion of successful junior athletes who become successful senior athletes. Type III studies allow the quantification of rates of successful transition, i.e., the percentage of junior athletes confirming an equivalent or higher performance level as seniors? Type IV studies also calculate conversion rates, but retrospectively. That is, these studies seek to assess what proportion of successful senior athletes were successful junior athletes? Type IV studies thus also determine how many athletes increased from a lower junior to a higher senior performance level. AIMS OF THIS SYSTEMATIC REVIEW AND META-ANALYSIS The mixed empirical results and the studies’ different designs call for a comprehensive literature review and a differentiated analysis, technically, a systematic review and meta-analysis. The aims of this systematic review and meta-analysis are (1) to describe and critically evaluate the quality of the available evidence, (2) to synthesize the results from all studies that investigated correlations between junior and senior performance, allowing us to evaluate the prognostic validity of junior performance for senior performance, (3) to aggregate the results from all studies that investigated transition rates of successful juniors to successful seniors, and (4) to investigate whether some of the heterogeneity among original study results is explained by moderator variables including types of sports, sex, junior age categories, and performance levels considered. We expect that these procedures can establish more robust and generalizable findings. Assessing the predictive validity of junior success for senior success and understanding potential differences between subgroups may lead to (1) improved coaching of young athletes, (2) better informed selection decisions in talent identification and talent promotion programs, and (3) more effective talent promotion programs.