Soccer is an intermittent sport in which sudden changes in intensity and type of actions or motor tasks continuously occur (1). Short sprints, rapid acceleration or deceleration, turns, kicks, jumps, and tackles are all essential actions that increase demands on the players’ physical and technical ability (2). These actions are essential for scoring, winning or losing duels, or determining the game’s outcome (3). In modern soccer, the capacity of players to produce varied forceful and explosive actions is essential for optimal performance (4). In addition, the constant development of the game requirements at the elite level increases the demand for effective training systems to enhance the physical and functional abilities of the athletes.

Functional training has become very popular (4) (5). It is a systematic approach to exercises that imitate the fundamental movements of the respective sport for which training plans are provided. Functional training aims to achieve and maintain an optimal balance between specific mobility, stability, strength and agility, helping the athlete perform fundamental movement patterns accurately and efficiently (6).

Steinkjer Sports Arena Football Performance (SSAFP) is a strength and conditioning concept designed to improve soccer performance using functional strategies and exercises based on the principles and methods of “applied functional Science” the Gray Institute developed. To the author’s knowledge, no existing literature investigated the use of “applied functional science” in improving physical performance in soccer. Therefore, this study aims to quantify the effect of a 4-week training intervention on Norwegian sub-elite soccer players using the concept of SSAFP.   

It was hypothesized that SSAFP would improve sprint performance, direction change, and jump tests

2. METHODS
2.0 Experimental approach to the problem
The current study investigated the effect of a 4-week functional strength and power training program on improved performance. Vertical jump height (Countermovement jump), change of direction test (S180°-test) and linear sprint test (10 m and 20 m sprint) were used to quantify the effects of a 4-week strength training period in the players. A quantitative controlled trial design was chosen, and 14 players were recruited. Additional sessions were added to their regular soccer training (3 sessions per week and one match). The additional training (2 sessions a week, each approximately [70 minutes]) consisted of a combination of functional power training (multidirectional plyometric and sprint exercises) and functional training (multidirectional strength exercises). The investigator, who is a certified strength and conditioning coach, trained and supervised the players during the intervention period. All players were familiarized with the test battery one month before the pretest because these tests were part of the already ongoing preseason training in evaluating the players’ physical status. Players were tested by the same investigator and ordered on each of the two testing days.    

2.1 Subjects
Sub-elite male soccer players were recruited from a local club in the North of Trondelag, Norway. The players to be recruited played in the Norwegian 3. division. The players were moderately trained, participating in 3 soccer training sessions per week and one 11-a-side game on a pitch of regular size (approximately 50*100 m), which lasted for 90 minutes (2*45 min with 15 min rest between halves). Each soccer session for the intervention group was generally composed of a 10-minute warmup, 30 minutes of technical training, 45 minutes of tactical and simulated competition training and 5 minutes of cooldown. All players had a good background in regular traditional strength and power training. A total of 14 players were included in the statistical analysis (age: 21,28 ± 3,91 years old; height: 180,30 ± 9,12 cm; and body mass: 77,98 ± 6,73 kg; mean and SD). The exclusion criteria were injury or illness and rehabilitation not allowing the players to participate in regular training or lack of participation during the intervention period. Therefore, at the end of the study, the remaining players were four included in the statistical analysis (age: 23,00 ± 4,08 years old; height: 177,07 ± 6,49 cm; and body mass: 78,22 ± 2,41 kg; mean and SD).

2.2 Procedures
The players did engage in fatiguing training sessions the days before testing because of the preseason. None of them reported any injuries at the time of pretesting, and all could play soccer with high intensity, measured using Tegner’s activity scale (7). All tests were performed on the same weekday for the pre-and post-test. Due to the preseason work, different warmup strategies were used on the two testing days. On the pretest, the participants could perform the tests before the strength training workout, but on the post-test, the players completed the tests as a part of their workout because of logistical challenges. All participants were informed to bring footwear for indoor use in the following sequence: 10 m and 20 m sprint, S-180°, and maximal vertical jump.      

10 and 20 m sprint. Players had to start from a standing position placing their feet just behind the starting line 0,5 m from the first photocell at 0 meters (Brower TCi Timing System, Draper, UT, USA). They performed a 20 m sprint with a stationary start, and the timing started as soon as the first cell was passed. Photocells were positioned at 10 m and 20 m from the first photocells at the height of 0,32 m. There were two trials, and a 1-minute recovery was allowed between each trial. The fastest 20 m sprinting time and the associated 10 m sprinting time were selected for analysis.  

S180°-agility. The setup can be found in the study by Sporis et al. (2007) (8). Players had to start from a standing position placing their feet just behind the starting line 0,5 m from the first photocell at 0 meters (Brower TCi Timing System, Draper, UT, USA). They performed a stationary start, and the timing started as soon as the first cell was passed, where the player was sprinting through the following pattern AB(9 m)-BC(3 m)-CD(6 m)-DB(3 m)-BE(9 m) touching a placed cone on point B, C, D and E (Height 0,39 m), and performing a 180-degree rotational turn against the cone. The setup of the test ensures the players were performing two 180 degrees turn to the right and left, respectively. There were two trials, and a 1-minute recovery was allowed between each trial. The fastest time was selected for analysis.   

Maximal Vertical Jump. Players in a shod condition started from an upright standing position and performed vertical countermovement jumps on a portable platform (Just Jump System, M-F Athletic Company, Cranston, RI, USA). Jump height was determined based on the flight time. Each player performed two jumps interspersed with 1-minute rests in between, and the highest jump was used for the analysis.

2.3 Statistical Analyses
SPSS 28.0.1.1 (IBM, Ehningen, Germany) was used for the statistical analysis. Standard statistical parameters (mean and SD ) were calculated for each trial of the mentioned tests. The Shapiro-Wilk test was used to test the normality of the distribution. Dependent samples t-tests were used to examine the differences between the pre-and post-test within the group for the 10 m linear sprint, 180° agility test and the vertical jump test. The Shapiro-Wilk test showed that the 20 m linear sprint test data were not normally distributed, where a Wilcoxon signed-rank test was used to examine the differences between the pre-and post-test.
A significant level was defined as p < 0,05.         

3. RESULTS
At the end of the 4-week training period, the dependent samples t-test and Wilcoxon signed rank t-test showed no significant training effects on the 10 m sprint test (P = .096).  20 m sprint (P = .273), S180°-agility (P = .551), and vertical jump test (P = .078) in the experimental group.

4. DISCUSSION
This study investigated the effects of a 4-week functional strength and power program in Norwegian sub-elite soccer players quantified using specific tests. Data analysis found no significant changes in the 10 m and 20 m sprint, S180°-agility, and vertical jump test.
The results of this study were not in accordance with this study’s expected outcome. Studies have reported that a short-distance sprint and jumping performance depend on the ability to generate muscular power (9). Especially ankle, knee and hip complex strength and neuromuscular skills are essential to creating a high power output in the acceleration phase to receive maximum speed quickly horizontally or vertically (10) (11). A study has shown a strong correlation between maximal strength, sprinting, and jumping performance in Norwegian elite soccer players, which this program was attended to improve among the participants (12). That is in accordance with a study showing that a 6-week strength training period combining plyometric and strength training significantly improves adult soccer players’ sprint time and vertical jump abilities (13).

However, this study has several limitations according to sample size included in the statistical analysis, compliance and reliability, which can significantly influence the results presented. First, because the experimental group consisted of six U-19 soccer players playing qualification games during the intervention, they could only participate in one strength training workout per week during the four weeks intervention period. Further, four adult players were gone from practice the weeks before the post-test because of illness. Therefore they were excluded from the statistical analysis because they could not meet the inclusion criteria of this study. Finally, the pre and post-test warmup protocols were different because of a strict preseason plan due to time limitations in preparing the 

players physically before the upcoming season. On the pretest, the participants were tested before their strength training workout and post-test after finishing a hard strength and power workout because of logistical challenges in the facilities.

5. CONCLUSION
According to the lack of reliability between the pre-and post-test, sample size and low compliance, a new study is recommended to conclude the effect of using functional strength and power training to improve power performance among Norwegian sub-elite soccer players. 

6. ACKNOWLEDGEMENTS
The author would like to thank the participants for their effort throughout this study. The author has no conflict of interest to declare and base the research on the Danish Code of Conduct principles for research integrity.

7. REFERENCES

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