Football is a team sport where players move the ball beyond the goal line into the opposing goal. Therefore, they often compete fiercely for the ball to prevent the opposing players from scoring, especially in the area in front of the goal. In a professional match, collisions and fouls are committed so many times, sometimes more than one hundred times from both teams, which shows how fierce, competitive and physical-strength-consuming a football match can be. A match of male five-a-side football lasts 40 minutes, and sometimes it extends to the penalty shootout to determine the winner [1-3]. In such matches, every player has to move around a lot while performing many techniques with and without the ball, which consumes a great deal of energy. Therefore, a fierce match consumes a lot of players’ energy [4, 5]. Consequently, it is vital to study methods to develop professional physical fitness for footballers, especially from their tweens.

Helgerud et al. [6] studied how to improve the strength and endurance of players in leading football teams in the pre-season period by implementing a highly intense training program to maximize the endurance. 21 players were selected to participate in the study in 8 weeks. The results showed that combining strength and endurance training with professional training regularly would remarkably improve players’ physical capacity. Bujnovsky et al. [7] identified if players’ speed, agility and aerobic ability depend on their positions. Through the research on 123 footballers, including goalkeepers, left/right defenders, central defenders, left/right midfielders, center midfielders, and attacking midfielders. The results demonstrated the notable impact of positions on linear sprint performance, strategically useful for coaches and doctors to make specific adjustments to each position. Karthick et al. [8] studied the impact of SAQ training on the physical parameters and capacity of middle school footballers. 30 players were selected for this study and its results showed that thanks to SAQ training, such physical parameters as their speed, agility and capacity had improved dramatically. Chen [9] analyzed advanced footballers’ physical characteristics. Through relevant data gathering, it was concluded that the level of physical fitness training is closely related to the players’ morphological and functional characteristics Atakan et al. [10] investigated the link between anthropometric measurements and the performance of young football players. A total of 17 young male footballers volunteered to participate in the study. The measurements of weights, mineral content, fat mass, etc., before and after the training were carried out to find the correlations among these indicators. Finally, the authors concluded that to football players, changes in body composition play an important role in sports activities, especially sprint and agility. Asian et al. [11] examined motion characteristics over time of football teams in La Liga. And the outcomes demonstrated the difference in physical performance between various competitive levels. Both successful and unsuccessful teams imposed the same running requirements at higher speeds. These findings provide valuable information about physical requirements, which depend on the teams’ standing at the league table. Published studies show us the importance of professional physical fitness development for footballers as one of the decisive factors to a match’s result [12-17].

The playing field of a five-a-side football is rectangular, with a maximum length of 42m and a minimum of 25m; a maximum width of 25m and a minimum of 15m. The penalty area is marked from the horizontal line of each part of the field, taking the posts of the two goalposts as the center and draw a quarter of a circle with a radius of 6m. The connecting of the endpoints of the two arcs is a 3.16m long straight line which parallels the goal line (goal line) 6m. The area within the bounds by those lines is the penalty area. In the middle of each goal line, there is a goal frame consisting of two vertical columns perpendicular to the ground and 3 meters apart (from the inside edge). They are connected by a parallel crossbar and 2 meters from the floor (from the bottom edge). The width and thickness of the vertical posts and crossbars must be 8cm in size.

According to the actual situation of Vietnamese football, this paper introduces several professional physical training exercises for U10 five-a-side football teams in Vietnam. The aim is to evaluate the players’ professional physical fitness development before and after 6 months of training, which serves as a reference and scientific basis of general physical fitness development in football for kids. Additionally, it contributes to building up training programs and improving the effectiveness of youth football training in Vietnam.

Pre-teens start to witness drastic growth and change in terms of both mentality and physicality. Therefore, at this age, the typical changes are a significant increase in height, fast growth in spine and limb bones, and slow growth in ribs, which results in a narrow chest compared to the height [18,19]. During this period, muscular strength increases yet is not accompanied by endurance development. This is the reason for excessive muscle tension, where pre-teens start to perform activities stretching their muscles much more intensely and longer than their capacities, resulting in bone structure disorders or scoliosis. To avoid that, it is essential to pay special attention in sports activities and physical training. Players should do physical exercises suitable for their age. With the above characteristics, pre-teens should not work on exercises requiring excessive strength, sudden movements, compression and high-speed running exercises. Exercises with moderate muscle intensity and endurance are much more appropriate [20, 21].

Based on the mental and physical characteristics of preadolescence, as well as suggestions of experts, coaches and teachers, we introduce 5 tests, including long jump on the spot, 15-meter sprint, 5x30m sprints, 5-minute running and 1-minute rope skipping.

To carry out the experiment, we selected 14 players from U10 five-a-side male football team of Binh Tan District Children Center, Ho Chi Minh City, Vietnam. 6 months of training was divided into 3 phases with four 90-minute training sessions/week. Phase 1 was aimed for the players to adapt to normal training, phase 2 was to develop the maximum focus speed, and phase 3 was to develop professional physical strength. Through training, we observed a growth in their general physical strength. The final results were outstanding and statistically significant with t_calculated > t_standard =2.145 at P < 0.05. The results are presented in Table 1 and Fig. (1).

Fig. (1). The growth rate of professional physical fitness development of the U10 five-a-side male football team after 6-month training. Note: Test 1 is a long jump on the spot, Test 2 is a 15-meter sprint, Test 3 is 5x30m sprints, Test 4 is a 5-minute running, and Test 5 is a 1-minute rope skipping.

Table 1. The test results of U10 five-a-side male football team before and after 6-month training (n1=n2=14).

Test	Testing Time		S	Cv (%)	ε	W (%)	t	P
Long jump on spot (cm)	Before training	170.02	7.52	4.42	0.025	1.99	13.44	<0.05
	After training	173.44	6.93	4.00	0.023
15-meter sprint (s)	Before training	2.72	0.12	4.41	0.025	2.72	6.83	<0.05
	After training	2.65	0.09	3.33	0.019
5x30m sprints (s)	Before training	31.72	1.79	5.66	0.032	3.05	6.82	<0.05
	After training	30.77	1.57	5.09	0.029
5-minute running (m)	Before training	920.84	68.10	7.40	0.042	2.21	10.63	<0.05
	After training	941.21	67.24	7.14	0.041
1-minute rope skipping (time)	Before training	65.36	5.49	8.39	0.048	9.47	12.22	<0.05
	After training	71.86	6.19	8.61	0.049

Note: is the average value, S is the standard deviation, C_v is coefficient of variation, ε is relative error of mean value W is the growth rate, t is the test value of 2 related samples, P is the correlation coefficient.

The test of long jump on spot: The average result before training was = 170.02cm, with S₁ = 7.52cm and the average result after training was = 173.44cm, with S₂ = 6.93cm. After 6-month training, the players’ results of long jump on spot were 3.42 cm longer, with the growth rate W = 1.99%. The growth was statistically significant because t_calculated = 13.44 > t_standard = 2.145, at the possibility P <0.05.

The test of 15-meter sprint: The average result before training was = 2.72s, with S₁=0,12s, and the average result after training was = 2.65s, with S₂ = 0.09s. After 6-month training, the players’ results of 15-meter sprint were 0.07s faster, with the growth rate W= 2.72%. The growth was statistically significant because t_calculated = 6.83 > t_standard = 2.145, at the possibility P <0.05.

The test of 5x30m sprints: The average result before training was = 31.72s, with S₁ = 1.79s and the average result after training was = 30.77s, with S₂ = 1.57s. After 6-month training, the players’ results of 5x30m sprints were 0.95s faster, with the growth rate W= 3.05%. The growth was statistically significant because t_calculated = 6.82 > t_standard = 2.145, at the possibility P <0.05.

The test of 5-minute running: The average result before training was = 920.64m, with S₁ = 68.1m and the average result after training was = 941.21m, with S₂ = 67.24m. After 6-month training, the players’ results of 5-minute running were 20.57m longer, with the growth rate W= 2.21%. The growth was statistically significant because t_calculated = 10.63 > t_standard = 2.145, at the possibility P <0.05.

The test of 1-minute rope skipping: The average result before training was = 65.36 times, with S₁ = 5.49 times and the average result after training was = 71.86 times, with S₂ = 6.19 times. After 6-month training, the players’ results of 1-minute rope skipping were 6,5 times more, with the growth rate W = 9.47%. The growth was statistically significant because t_calculated =12.22 > t_standard=2.145, at the possibility P <0.05.

The above analysis shows that after 6 months of training, the participants’ results in all tests have witnessed growth. The growth was statistically significant because t_calculated > t_standard at the possibility P < 0.05.

The results show that the physical fitness of the male athletes in all the tests has increased after 6 months of training; the growth was statistically significant because the t_calculated > t_standard and the possibility P < 0.05.

In a football match which has five players in each team, the number of participants is large. The coordination between attack and defense is relatively difficult and much more complicated than in other sports such as volleyball, table tennis, badmintonSoccer players must use the least flexible parts of the body - the legs to dominate and control the ball to make it move all the time. Therefore, it is quite difficult to practice and master football techniques.

An official match lasts 40 minutes; sometimes, the two teams have to take a penalty shootout to determine the winner. In a fierce match, each player has to move a lot. In addition, there are many technical moves with and without the ball. The body's energy expenditure is huge.

Laws and principles in football are relatively simple; so are the field and training equipment. In general, training is rarely restricted by time, number of players, stadiums, etc. Only in official competition applies strict rules are set, for example, goalkeeper’s holding time, time to make a corner kick, direct free kicks, indirect free kicks, second penalty. All must take no more than 4 seconds.

Nguyen [23] studied the physical fitness development of the U10 futsal team at primary school. Hoang [24] assessed the basic technical and physical fitness for U12 male football athletes after one year of training. These studies are based on some standards to evaluate the ability of athletes after a period of training. Although specific scores have been given to athletes, they have not shown the reliability of the tests. Our research has shown exercises to improve athletes' fitness, and also given the accuracy and reliability of the tests. The research results are the basis for coaches to offer appropriate exercises for young Vietnamese athletes. However, in reality, it is necessary to expand research on the development of physical strength and psychology of athletes.

From the research results, we have selected a mix of appropriate tests to evaluate the professional physical fitness development of U10 five-a-side male football teams. This study has built up the grading scale and criteria of classification to examine the general physical strength of young male football players before and after 6-month training. In addition, it also helps to assess exercises’ effectiveness in reality. Its results are the basis for coaches and PE teachers to refer and build training plans for U10 five-a-side male football teams.