Soccer is a situational team sport in which players are interconnected in a complex system characterized by technical-tactical components that are supported by physical and physiological factors 1. Soccer performance is described by unpredictable intermittent cyclic and acyclic activities characterized by different physical demands 2. Understanding the physical demands required during soccer matches is important for coaches in order to appropriately schedule each training program with the aim of maximizing the effect of training while minimizing the risk of injury.
High-speed running, accelerations and decelerations are considered determinant factors related to physical performance that should be taken into consideration during the training schedule process 3. As a matter of fact, during a match, players usually perform around 150–250 different actions and 1100 changes of direction 4. Moreover, players’ physical activity characteristics change every 4–6 seconds based on the different players position on the field 4,5 resulting in a high rate of changes in velocity. Therefore, attention should be given to both the accelerations and decelerations that induce a high metabolic cost and high mechanical load, respectively, due to eccentric contractions 3,6. Indeed, Osgnach et al. found that taking into consideration the accelerations the high-intensity activity is three times higher compared to the one that considered only the running speed 7. Moreover, it has been found that players increase their metabolic load when a large amount of soccer-related events take place in small places. This result suggests that accelerations, decelerations and changes of direction occur even when speed is low 7. To improve soccer players’ performance while reducing injury risk, coaches should assess training and match loads in relation to players’ position on the field to optimize training planning 8–17. Specifically, external load (EL) is usually described by the total distance, range of speed covered, accelerations, metabolic power 7, and other derived measures. Global Positioning System (GPS) technology has been largely used by coaches to assess EL allowing them to make time-motion analysis in technical-tactical tasks 11,18,19.
The main specific tasks proposed by coaches during soccer training include Small Sided Games (SSG), match-based exercises (MBE) and friendly matches (FM). Coaches could schedule different tasks in order to provide their players with different EL stimuli inducing different individuals’ adaptations. In fact, different ELs induce specific internal load (IL) responses because of different psychological, physiological, biochemical, metabolic and biomechanical stress stimuli 20,21. In particular, different characteristics are shown in each soccer training task. SSGs represent tasks with specific technical-tactical rules performed in small spaces of the pitch with a reduced number of players compared to regular matches and elicit physical, tactical, psychological and techniques components 22–28. Whereas MBE represents tasks more similar to regular matches which are organized in larger spaces and with a larger number of players than SSG. Moreover, ~ 300 m2 is the theoretical match reference density of a regular match 29,30 while soccer-specific tasks are performed using ~ 100 m2 or ~ 200 m2 per player 30–32. Due to these differences, different intensities are detected in each of them 18,33. Usually, the intensity is assessed through some EL indicators referred to as time (e.g., distance per minute, number and distance of accelerations and decelerations per minute, high-speed running distance per minute) 18,33, while other authors assess the intensity as IL parameters (e.g., rating of perceived exertion, RPE and percentage of heart rate, % HR) 26. Different authors observed that both EL and IL can be affected by some variables such as game rules, objectives, number of players per team, pitch size and coaches verbal stimulation 28,34,35. Some studies focused on accelerations and decelerations involving both metabolic and neuromuscular systems and described the physiological load requests in SSG 28,36,37. These studies showed that in small spaces there are higher number of accelerations, decelerations and changes of direction, while in larger spaces high metabolic components were detected because of high-speed running achieved by players 29. Moreover, higher peak speeds and very high speeds were detected during official and friendly matches compared to SSG 37–39. Hence, since the SSG tasks show high-intensity activity that involves neuromuscular factors, some differences were detected compared to matches 37,40. Therefore, metabolic power, defined as the product of the energetic cost of acceleration running (EC, J · kg− 1 · m− 1) and speed (v, m · s− 1), should be considered for EL monitoring as it considers both speed and acceleration factors 7.
It is worth noting that physical demand is highly related to players’ positions on the pitch due to the fact that roles have specific technical-tactical requests strictly related to different physical, physiological, energetic and biomechanical components 2,17,41. For example, the longest distance covered at high intensity is achieved by wide midfielders and fullbacks 9,42.
To the best of our knowledge, there are no studies that have investigated differences in EL indicators between official matches (OM) and soccer-specific tasks (i.e., SSG, MBE, and FM) considering soccer players different position on the field. Hence, the aim of this study was to investigate differences in EL indicators, specifically strength and metabolic indicators, between OM, FM, SSG, and MBE in different field positions of semi-professional soccer players.