1. Introduction
Approximately 4.1% of the people in the world play football, which has led to the sport becoming very popular in various communities. Football is a popular global sport and many studies have tried to discover the reasons for the success of players in this popular sport [1, 2].
In recent years, football has transformed from a fascinating sport to one of the most widely accepted games in the world, which has led coaches and players to constantly seek new scientific approaches to better prepare for training and potential performance during competitions [3].
In football, the players are placed in certain positions to perform their specific tasks. Both the tactics and the position of the players on the field are essential for organizing a football match. In most studies, football players are divided into four groups: Strikers or forwards, midfielders, defenders, and goalkeepers [4-7], who perform various technical and tactical skills in the game [8, 9]. It has been suggested that football sports activities may be directly related to the position of players on the field [10-12].
 Accordingly, it is not easy to identify who and in what position can play a role in football. Today, there are so many talents in football that due to the lack of knowledge of coaches about their biomechanical conditions and employing them in non-specialist positions, they are either injured or left out playing football. Football players have different roles such as Strikers or forwards, defenders and playmakers, which is directly related to the position of the players on the field. Each of these players can have different percentages of offensive, defensive and playful roles due to different biomechanical conditions of the body. Accordingly, sufficient knowledge of the biomechanical conditions of the players can be very helpful in better understanding the conditions or the dominant role of the player on the field and make the player the most efficient on the field. Accordingly, it is hoped that by reviewing these studies we will become more familiar with the importance of biomechanical studies in football considering the positions of the players.

2. Methods
The study is a review of the field of biomechanical studies of football players according to the game position. For this purpose, Google Scholar, PubMed and Science Direct search engines were used to search for articles related to the keywords of football, football player, biomechanics, assessment, and analysis. All articles from 1996 to 2020 were reviewed (Table 1).


Initially, 428 articles were found and after reviewing the titles and abstracts of articles, considering that the approach of this research was related to the position of players on the playing field, 52 articles were selected. Inclusion criteria were articles that performed biomechanical and anthropometric assessments in players according to the game position, and exclusion criteria were articles that did not have this trend. Then, after reviewing the full text, 8 articles were selected considering the objectives of the research. A total of 3 articles were selected by reviewing the sources in the 8 articles which selected and added; finally, 11 articles were approved.

3. Results
According to the purpose of the research and considering that the results of the reviewed articles have different aspects, the section related to the findings of this research was presented in two parts: Anthropometric evaluations and biomechanical evaluations.

Anthropometric assessments
Abdullah et al. (2016) in a study entitled “Relationship Between Physical Characteristics in Performance According to Play Position, Parameters Such As Weight, Height, Sitting Height, Fat Percentage and Body Mass Index” in 209 elite Malaysian footballers including 20 goalkeepers, 78 defenders, 71 midfielder, and 40 strikers were examined [3]. The results of this study showed that goalkeepers and defenders are taller and heavier than midfielders and attackers and the fat content of goalkeepers and defenders is higher than that of midfielders [3].

Biomechanical assessments
Jadczak et al. (2019) conducted a study on static and dynamic analysis with respect to the game position in 101 elite footballers [19]. The tests were performed with eyes open and eyes closed with superior and non-superior legs. There was a significant difference in static and dynamic balance with respect to game position. In the static balance, this difference was seen between goalkeepers and full-backs and midfielders. In the dynamic balance, this difference was observed between midfielders and center-backs, goalkeepers, strikers and midfielders. Midfielders had a better dynamic balance, and this balance was better at the non-superior foot of the players [19].

4. Discussion and Conclusion
It has recently been suggested that the anthropometric variables of elite footballers in different positions of the game may be predictors that players of a certain size and shape are appropriate for specific positions in the game [24]. Athletes may develop specific physical characteristics based on the demands of individual positions [11]. The role of players on the field, for example speed or power players, causes them to adjust their characteristics based on the demands in the desired role. Accordingly, the isokinetic strength of the muscles of football players in different positions can be different. It was observed that the static and dynamic balance of the players also showed a significant difference and the midfielders had a better balance [19]. 
As mentioned before, one of the reasons for better balance can be attributed to the distance from the center of gravity to the ground, which can be considered logical considering the results of anthropometric variables in relation to the short stature of midfielders compared to players in other positions.
It seems that biomechanical evaluations are very important in selecting players for different football positions on the field, as well as identification of the football talents sport, which leads to sufficient knowledge of players’ performance in providing specific training programs according to specific characteristics in individuals to improve the performance of players. However, the lack of a comprehensive biomechanical evaluation, including anthropometric, kinetic, kinematic, and electromyographic studies was observed in the studies.

Ethical Considerations
Compliance with ethical guidelines
This article is a meta-analysis with no human or animal sample.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflict of interest.

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