USE OF INDIVIDUAL RELATIVE THRESHOLDS TO ASSESS ACCELERATION IN YOUNG SOCCER PLAYERS ACCORDING TO INITIAL SPEED FRANCISCO I. MARTI ´ NEZ-CABRERA, 1 FRANCISCO J. NU ´ N ˜ EZ-SA ´ NCHEZ, 1 JOSE LOSADA, 2 CARLOS OTERO-ESQUINA, 3 HUGO SA ´ NCHEZ, 3 AND MOISE ´ S DE HOYO 4 1 Department of Sports and Computing, Sport Faculty, Pablo de Olavide University, Seville, Spain; 2 CEDIFA, Andalusian Football Federation, Seville, Spain; 3 Fitness Section, Sevilla Football Club, Seville, Spain; and 4 Department of Physical Education and Sport, University of Seville, Seville, Spain ABSTRACT Martı ´nez-Cabrera, FI, Nu ´n ˜ ez-Sa ´ nchez, FJ, Losada, J, Otero- Esquina, C, Sa ´ nchez, H, and De Hoyo, M. Use of individual relative thresholds to assess acceleration in young soccer play- ers according to initial speed. J Strength Cond Res XX(X): 000–000, 2018—The aims of the current study were (a) to analyze the characteristics of acceleration efforts using individ- ual relative thresholds according to the initial speed during official matches in elite young soccer players according to player position and (b) to compare the differences between absolute and relative thresholds in assessing high-intensity acceleration. Player acceleration profiles were assessed using an individual relative threshold based on their acceleration capacity at different initial speeds (standing, 6, 10.8, and 15 km$h 21 ), and the number of accelerations (.75% of the max- imal acceleration) performed during soccer matches was divided into 3 categories attending to the initial speed. (S1 = 0–7 km$h 21 ; S2 = 7.1–14 km$h 21 ; and S3 = $14.1 km$h 21 ). Within-group analyses showed that the number of accelera- tions performed in each category was higher when the effort started from a static or walking position than at moderate- or high-intensity running (S1 . S2 . S3; very likely to almost certain). Between-group analyses revealed substantial differ- ences between some playing positions according to initial speed. In S1 and S3, central defenders had the lowest number of accelerations (likely to almost certain), whereas midfielders had the greatest number of high-intensity accelerations in S1 and S2. There were also substantial differences between the other playing positions (possibly to almost certain). Regarding relative and absolute thresholds (.3 m$s 22 ), the results showed that absolute threshold overestimated the number of high-intensity accelerations compared with the individual relative threshold in S1 and underestimated the results in S2 and S3 (almost certain). The use of an individual relative threshold to measure acceleration demands allows to distinguish between the numbers of accelerations in function of the initial speed and playing positions. In addition, the absolute acceleration threshold could overestimate or underestimate the acceleration demands in young soccer players as a function of the initial speed. Then, the absolute acceleration thresholds should be taken with caution in the assessment of acceleration activities. KEY WORDS acceleration profile, high intensity, positional role INTRODUCTION I n the past several decades, the early identification of talent in young soccer players has become an interest- ing issue for professional soccer clubs due to the obvi- ous associated financial benefits (6). Thus, optimal resources to recruit these players and adequate strategies to improve their technical, tactical, and physical skills are essential for the development of the players’ talent. Under- standing the physiological demands on young soccer players according to age and playing position is necessary to develop appropriate long-term training to achieve their max- imal potential (6). Although interest in the analysis of move- ment patterns during soccer matches has been growing in recent years (3,10,11,27), little information regarding elite young soccer players during competitive matches is available (8,27). For instance, previous studies have shown that the number of accelerations and decelerations may also be a dis- criminating factor between youth and senior players (31). It is known that in prepubescent children, physical capacities, which have an important influence on factors, such as acceleration, maximum running speed, or repeated-sprint performance, are influenced by growth and maturity (18,19,23). Papaiakovou et al. (23) observed that maximal speed improves with growth, whereas Mendez-Villanueva et al. (19) Address correspondence to Francisco I. Martı ´nez-Cabrera, fimarcab@ outlook.com. 00(00)/1–9 Journal of Strength and Conditioning Research Ó 2018 National Strength and Conditioning Association VOLUME 00 | NUMBER 00 | MONTH 2018 | 1 Copyright ª 2018 National Strength and Conditioning Association