Original Research Internal Load From Hard Training Sessions Is Related to Changes in Performance After a 10- Week Training Period in Adolescent Swimmers Rasmus Pind, 1 Evelin M ¨ aestu, 1 Priit Purge, 1 Janno J ¨ urgenson, 2 Mati Arend, 1 and Jarek M ¨ aestu 1 1 Institute of Sport Sciences and Physiotherapy, Faculty of Medicine, University of Tartu, Tartu, Estonia; and 2 Tartu Swimming Club, Tartu, Estonia Abstract Pind, R, M ¨ aestu, E, Purge, P, J ¨ urgenson, J, Arend, M, and M ¨ aestu, J. Internal load from hard training sessions is related to changes in performance after a 10-week training period in adolescent swimmers. J Strength Cond Res XX(X): 000–000, 2019—The purpose of this study was to investigate the association between session rating of perceived exertion (sRPE) and its categorization with the changes in swimming performance in adolescent swimmers. National level swimmers (age 16.4 6 2.1 years, V ̇ O 2 max 50.0 6 4.0 ml·min 21 ·kg 21 ) participated in a 10-week period preceding the National Winter Championships. Pre-training and post-training physiological parameters and swimming performance were measured. All training bouts with sRPE data were recorded. In addition, trainings were categorized as light, moderate, or hard. For calculating the corresponding internal training load (ITL), 2 fixed categorizations were applied based on earlier published sources and an individualized categorization based on ventilatory thresholds. During the 10-week training period, large to very large increases were found in aerobic (effect size [ES] 5 1.58) and anaerobic threshold power output W·kg 21 (ES 5 2.46), respectively, and smaller, but significant, improvement in 100-m swimming performance (ES 5 0.36). Total ITL during the 10-week period was significantly related to changes in anaerobic threshold (r 5 0.81). Fixed and individualized sRPE methods from hard sessions were associated with changes in V ̇ O 2 max (r 5 0.77) and performance of only 100-m leg kicks (r 5 0.76), respectively. Internal training load was related to changes in performance variables during the 10- week study period. Using individualized sRPE values, the accumulation of the ITL from hard training sessions was associated with changes in V ̇ O 2 max and swimming performance. The aforementioned categorization could help coaches in appropriate application of ITL for assessing the total training load in adolescent swimmers. Key Words: training load, exercise intensity, rating of perceived exertion, swimming Introduction Training programs have traditionally been described by the ex- ternal training load (ETL) i.e., training time, covered distance, or lifted weight. However, changes in performance depend on the individual adaptation, and therefore, it is important to monitor the athletes’ physiological responses from external stressors (i.e., internal training load [ITL]). Furthermore, obtaining the optimal training load needs careful planning and monitoring, especially in children and adolescents to prevent both under- training and overtraining (20), and an optimal balance between training and recovery has to be maintained to maximize physical performance (22). Monitoring ITL requires specific equipment for measuring blood lactate, oxygen uptake, power output, etc., which can be inconvenient for application in a practical setting, especially when young athletes are involved. Heart rate (HR) is currently the most widely practiced method for calculating ITL (11). However, it is not suitable for evaluating training intensity and ITL in interval, intermittent, weight or plyometric training, or for specific dis- ciplines such as swimming (1). Different types of training methods (low and high intensity, interval, or maximal speed) are used in competitive swimming to improve performance. There is a need for training programs that optimally combine aerobic and anaerobic training and manipu- late the training load to correspond to the respective race distance and time (28). Training session rating of perceived exertion (sRPE) reflects an individual’s response to training. With the sRPE method, ITL is calculated by multiplying the athlete’s RPE (0–10-pt scale) by the duration of the session. The method has been shown to be valid and reliable, with correlations between sRPE and HR by up to r 5 0.90 (7). The concept was proposed by Foster et al. (7,9), and it is currently one of the most frequently used methods for monitoring and controlling training in various sports (2,6,10,17,18). The perception of effort can be defined as the subjective effort, strain, or fatigue perceived during the training session (24). The training intensity can be categorized into different intensity zones to plan and provide feedback for specific type of trainings (long-distance low-intensity trainings, interval trainings, etc.). Recently, considerable research (5,23,26,27,30) has been pub- lished for different sports on the concept of 3 intensity zones—low lactate zone (below aerobic threshold or ventilatory threshold (VT), lactate steady-state zone (between aerobic and anaerobic threshold or between VT1 and VT2), and lactate accumulation zone (intensities higher than anaerobic threshold or VT2). Using the latter method, Seiler and Kjerland (26) indicated that sRPE and session-goal HR methods were in agreement with 92% in elite-level cross-country skiers if the RPE scale was categorized into 3 zones with ratings of 0–4 indicating trainings below aerobic threshold, 5–6 indicating trainings between aerobic and Address correspondence to Rasmus Pind, Rasmus.Pind@ut.ee. Journal of Strength and Conditioning Research 00(00)/1–7 ª 2019 National Strength and Conditioning Association 1 Copyright © 2019 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.