The number of professional and amateur female soccer players has been steadily increasing over the years 97.

Soccer is a high intensity and complex contact sport and therefore it is associated with the risk of sustaining injuries during practice. These injuries are associated with a number of consequences for the players and health systems 12. According to De Loes 8, out of a total of 3,611 athletes from different teams, knee injuries were 3,864, at a total cost of U.S. $ 4,268,014. The cost per treatment of knee injuries in female soccer players averaged the U.S. $1,861 per injury. This information indicates that injuries are a serious cause of concern for the football clubs and therefore, it is necessary to introduce prevention programs 811.

Studies in professional and amateur female soccer players in Sweden have shown an incidence of up to 24 injuries per 1,000 playing hours and up to 7 injuries per 1,000 hours of training 15.

Analyses of risk factors for soccer players are scarce and a study carried out by Faude 11 sought to identify these factors in elite female soccer players. One of the findings that can be highlighted in the research is the suggestion that anthropometric characteristics are an important indicator when analyzing the risk factors for sustaining an injury. As per the research carried out by the author, ligamentous injuries are the most common; among which the anterior cruciate ligament (ACL) is the most recurrent 8.

The incidence of ACL injuries is higher in women than in men 2. In soccer, these lesions are commonly seen with a ratio of 9:1, of which approximately 70% of ruptured ACL injuries are not due to contact 2.

In Chile, no record of neither studies nor statistical data on the number of injuries that occur in male and female soccer players have been found. European studies indicate that the application or implementation of structured warm-up programs can contribute to the formation and development of the soccer players to such a point that they would be directly associated to a decrease of injuries 222325.

The Federation Internationale de Football Association (FIFA) created a training program known as the "11+", designed to amend morpho-functional parameters such as lower extremity alignment, strength, intra, and intermuscular coordination and balance, among others 20222325.

Players initially evaluated were 25, 5 of them were eliminated from the study due to the aforementioned exclusion criteria. The mean characteristics of the athletes evaluated during the study were: age 15.8 years, weight 56 kilos, height 1.6 meters and BMI 22. 5. All the results obtained are shown in the following Table 1.

"The 11+" warm-up program has been applied internationally and this study is an implementation on a national scale in order to analyze the results.

In relation to the characteristics of the population under study (weight, height, BMI), there were no significant changes during the 3 months duration of the program.

The soccer players subjected to “The 11+" program for 3 months showed favorable changes in all the morphological and functional variables. Statistically significant improvements in the speed and thigh girth test were observed. Improvements in the jump and balance tests were observed.

Based on the data obtained, it can be seen that there was a statistically significant change in the variable studied after the application of “The 11+", meaning, the thigh circumference increased in comparison to the initial assessment. This change is attributed to the fact that the initial assessment was carried out after the athletes had a rest period (after vacations), which may have had an influence in the loss of muscle mass with the consequent functional atrophy, which was quickly reversed during the time when they were subjected to the warm-up and training program itself.

It is noteworthy to mention that, "The 11+" directly works the strength of the quadriceps and hamstring, and specific functional strengthening exercises 14 are developed. Therefore, both situations that have been described may have had a direct influence on the significant variation of the thigh circumference.

This increase in thigh circumference is associated with an increase in muscle mass (hypertrophy) and thus, greater strength 24. This condition facilitates greater knee stability during the different external demands it will be exposed to 3 as a result of the different movements that are required during a game (Figure 1). Therefore, greater knee stability would be helpful in reducing the risk factors for non- contact injuries 18.

Based on the data obtained, statistically significant favorable changes were observed in relation to the variable under study after implementing "The 11+" program, meaning, race times decreased with regards to the initial evaluation. These changes can be attributed to the same reasons discussed in the thigh circumference test since, during a break from training, a decrease in immediate acceleration speed (10 m) and sustained acceleration speed (30 m) is expected (Figure 2, Figure 3).

Nevertheless, it appears that "The 11+" recommends eccentric muscle strengthening exercises for hamstrings and as the strength is increased, the sprint or race becomes more efficient 24, with less energy being used and an increase in the time period before muscle fatigue occurs. It has been shown that eccentric muscle strengthening decreases the risk of tearing616.

It can be seen that there was a favorable change, however, it is not statistically significant in relation to the variable studied after implementing “The 11+".

The structured warm-up program includes among its objectives the development of strength and muscle power in an indirect manner and this can be evaluated through the jump test (functional assessment) 5.

However, the average between the initial and final evaluation improves. The overall trend is towards improving the jump and thus, the strength and power 5.

According to the literature, there is a correlation between jumping, strength, and power 5, and therefore, if there is an increase in the jump variable, this will beat tribute to the increase in strength and power. As mentioned previously, the improvement of these characteristics affects the stability of the knee, being that the more stable the joint, the lesser the risk of injury.

Moreover, studies conducted by Urabe et al 171926 reported that an increased activation of the quadriceps on hamstrings is a factor that would have an impact on the increase on the risk of a knee injury given that during the post-jump buffer stage, the quadriceps tend to anteriorize the tibia generating an increase in ACL strain, stressing them beyond their capabilities and enhancing their risk of injury. From this, and considering the controversy that exists in the literature stating that the quadriceps is a protective factor against a knee injury 2 or that it can increase the risk of injury of the same 20, it is argued that it is not imperative to only increase the strength of quadriceps and hamstring, but to maintain proper synergy and intra and inter-muscular coordination at the thigh (quadriceps – hamstrings ratio) 2202127.

The modified SEBT corresponded to one of the functional assessments and included in these assessments was the objective to evaluate balance ability and how the players controlled their knees and ankles.

According to Filipa 13 who analyzes the improvements in neuromuscular training performance in this test, there is a critical value when predicting the risk of injury and its relationship with the score obtained during a modified SEBT. It argues that a distance less than 94 % in relation to the length of the athlete’s leg indicates that it is 6.5 times more likely that an injury makes occur in the lower extremity.

From this premise, and considering the previously analyzed results, it can be seen that most of the football players had scores higher than 94% with both extremities during the initial evaluation. Thus, if the players started off with a good performance of the modified SEBT (over 94% in relation to the length of the limb), it can be expected that once the program was applied, there would be no significant changes. The changes are not attributed solely to the application of the warm-up program but to the combination of the program and the training carried out on a daily basis by the National Football Team.

Junior football players were used as the model in the study and it is unclear whether the results can be generalized to both sexes, other age groups, and other sports. However, similar prevention programs were effective in older athletes 41 for both sexes or in other sports 2610.

The application of " The 11+" warm-up program for 3 months, along with proper training for junior female football players generated morpho-functional changes related with the increase in thigh circumference and improvement in speed.

This study was developed at Sports Center of the National Football Association (ANFP)

Level of Evidence: IV, case series.