Soccer training consists of structurally and systematically performed general and specific exercises to improve physical abilities and acquire skills. The adaptations imposed by a training stimulus take place on an anatomical, physiological, biochemical, molecular, and functional level. The content of a training session is usually prescribed by the coach and can be defined as the external training load. The total external training load comprises all of the players` actions during a training session and is generally quantified using tracking technology. The external training load elicits an internal physiological stress, or the internal training load. However, the internal training load is not only dependent on the imposed external training load, but also on the players` individual characteristics (ICs). It is possible to assess the internal training load through quantification of a training session`s duration and intensity. Duration is quantifiable in time and relatively easy to measure. On the other hand, intensity can be quantified using different methods, such as heart rate (HR) monitoring, blood lactate concentrations, and the (session) rating of perceived exertion (sRPE).2 HR monitoring is widely used in soccer, but it has been suggested that HR monitoring underestimates or overestimates the intensity during intermittent activities. Furthermore, HR monitoring requires both technical and physiological expertise to make an appropriate analysis.6 The blood lactate concentration is not often monitored during soccer training for practical reasons. On the other hand, the sRPE is a simple and practical tool that represents the players` own perceptions of training stress, which include both physiological and psychological stress. The sRPE has shown to be a valid indicator of intensity in soccer and a more valid marker of exercise intensity in soccer over a broad range of activities than HR monitoring. The external training load is the first factor influencing the internal training load and has previously been investigated by Gaudino et al. They confirmed that the sRPE is related to external load indicators (ELIs), such as high-speed distance, impacts, and accelerations. Second, the players` ICs influence the internal training load. Because of the variability in adaptations to a training stimulus, coaches should consider the players` ICs when prescribing the external training load. It has been suggested that fitter athletes within a team may not receive the optimal stimulus for physiological adaptations through extensive use of group training exercises.On the other hand, players with inferior fitness may be overstressed through group training sessions. To provide an appropriate training stimulus, it is important to establish a level of agreement between the athlete`s sRPE and the coach`s sRPE. However, previous research has shown that, in several sports, coaches tend to underestimate or overestimate the athlete`s sRPE. Based on the previous statement, it seems that coaches can experience difficulties controlling the external training load and may put athletes at risk of maladaptive responses to training,16 such as fatigue, injury, and a reduction in performance. To further understand the underlying indicators of external training load, internal training load, and ICs contributing to the sRPE, this study aims to predict the sRPE and identify the main predictors of the sRPE.
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