applied sciences  
The Profile of the Internal Load of Amateur Soccer Players during Official Matches with Formation 1-4-3-3 and Relationships with Indexes of External Load  
[PERSON]  
1 Laboratory of Evaluation of Human Biological Performance, Department of Physical Education and Sports Science, Aristotle University of Thessaloniki, 57001 Thessaloniki, Greece; [EMAIL_ADDRESS] (T.B.); [EMAIL_ADDRESS] (T.L.M.) Department of Physical Education and Sports Sciences, Aristotle University of Thessaloniki, 62100 Serres, Greece; [EMAIL_ADDRESS] (V.K.); [EMAIL_ADDRESS] (I.G.)  
Footnote  
and volume will determine the effectiveness of the training process. Therefore, monitoring the load during a match can help determine the volume and intensity of loads to be used during the training microcycle [2].  
The external load in a match is the set of actions that a soccer player takes during it. Recent studies have reported specific ratios for various external load parameters in relation to the match, providing coaches with thousands of data points from a single match. Commonly used parameters include total distance (TD), distance in different speed zones, accelerations, and decelerations [3,4]. The organism's response to psycho-physiological stimuli during a match constitutes the internal load [5]. Internal load measurement involves indicators related to the cardiovascular system (heart rate), neuromuscular system (rating of perceived exertion--RPE), and metabolic function (lactic acid) [6,7]. Additionally, several training impulse models (TRIMP) have been developed for estimating internal load, such as the [PERSON] model, usually measured in arbitrary units [8]. According to the TRIMP method of [PERSON] (1993) [9] (TRIMP\({}_{\text{Edw}}\)), to calculate the internal load, they use accumulated time in five arbitrary heart rate zones multiplied by a weighting factor (1-5).  
The external and internal load a player experiences during a match is influenced by their playing position and the team formation [10]. More specifically, previous studies have reported that central midfielders cover the longest distance compared to other positions [4,11], while wide midfielders and full backs cover the longest distance at high speed and sprint [12,13]. Also, the formation with which the team plays seem to affect the running performance of soccer players [14]. [PERSON] et al. (2019) [14] observed that players in the 1-4-5-1 formation covered more distance at high intensity than in the 1-3-5-2 formation.  
In previous studies investigating the effect of position on formation, they observed that defenders in the 1-4-4-2 formation cover shorter overall distance and distance at high intensity compared to 1-4-3-3 and 1-4-5-1 formations [15]. Another study reported that central midfielders accelerate more often in the 1-4-2-3-1 formation and cover longer distances at high intensity in the 1-4-4-2 formation compared to other formations [16].  
Regarding the internal load on soccer players in relation to the playing position or formation they play, studies are limited [7,17]. More specifically, [PERSON] et al. (2015) [7] and [PERSON] et al. (2016) [17] observed that wide midfielders showed the lowest average heart rate and the highest internal load index compared to other positions, while center defenders showed a lower internal load index than all other positions in the 1-4-4-1-1 formation. Most studies in this area focus on professional or developmental-level soccer, leaving a gap in understanding the loads experienced by amateur players seeking performance improvement.  
Therefore, the aim of this study was to investigate the internal load profile of amateur soccer players when playing in the 1-4-3-3 formation and explore possible relationships between internal load indicators and selected external load indices.