The impact of exercise of lower limbs on dynamic balance and valgus of the knees among young footballers

Background Participation of children and youth in sporting activities, among others in training and football competitions, is an optimistic manifestation of activation in the current trend of computerization. Unfortunately, this is also often associated with musculoskeletal injuries. An example is the association of dynamic valgus and disturbed dynamic balance with lower limb injuries. The main purpose of the studies is to determine the impact of three strengthening exercises on the parameters of dynamic knee valgus and balance of lower limbs. The secondary objectives are to investigate the dominant influence and the length of lower limb on the valgus values and to determine risk of injury to young football players. Methods 134 young footballers training at the Arka Gdynia SI club were invited to first stage of the research in order to examine anthropometric features and kinematics of the lower limb. Single Leg Squat Test was used, which was recorded. The samples were analyzed video to determine exact knee angle of valgus. During the second stage, to which players with valgus knee were qualified (≥15˚), the Y-Balance Test was additionally used. Intervention group underwent an exercise program to strengthen gluteus medius, popliteal and tibialis posterior muscles for 6 weeks to repeat tests in the third stage. Results Statistical methods used at the assumed level of significance allow to conclude that there are no differences in results of dynamic valgus (≥15˚) and dynamic balance between the intervention group and control group during the 6 week period between stages. In addition, there were results confirming the lack of influence of length and dominant lower limb on valgus. Results of composite direction in the Y-Balance Test test determine the low risk of injury in both


Abstract
Background Participation of children and youth in sporting activities, among others in training and football competitions, is an optimistic manifestation of activation in the current trend of computerization. Unfortunately, this is also often associated with musculoskeletal injuries. An example is the association of dynamic valgus and disturbed dynamic balance with lower limb injuries. The main purpose of the studies is to determine the impact of three strengthening exercises on the parameters of dynamic knee valgus and balance of lower limbs. The secondary objectives are to investigate the dominant influence and the length of lower limb on the valgus values and to determine risk of injury to young football players.
Methods 134 young footballers training at the Arka Gdynia SI club were invited to first stage of the research in order to examine anthropometric features and kinematics of the lower limb. Single Leg Squat Test was used, which was recorded.
The samples were analyzed video to determine exact knee angle of valgus. During the second stage, to which players with valgus knee were qualified (≥15˚), the Y-Balance Test was additionally used. Intervention group underwent an exercise program to strengthen gluteus medius, popliteal and tibialis posterior muscles for 6 weeks to repeat tests in the third stage.
Results Statistical methods used at the assumed level of significance allow to conclude that there are no differences in results of dynamic valgus (≥15˚) and dynamic balance between the intervention group and control group during the 6 week period between stages. In addition, there were results confirming the lack of influence of length and dominant lower limb on valgus. Results of composite direction in the Y-Balance Test test determine the low risk of injury in both intervention and control groups.
Conclusions Intervention did not affect the dynamic valgus and knee balance in young football players. Both the length and the dominant limb did not affect the results. The "low risk" occurrence of injuries among the examined participants with valgus knee due to the values of dynamic balance was determined.
Background Improving fitness, competition, pleasure, socialization, health are just a small part of the reasons why people around the world play football. Football training includes elements of medium and high intensity aerobic and anaerobic exercises with periods of anaerobic loads. Even a short-term period of a few to over a dozen weeks of recreational football training improves the values of the musculoskeletal, metabolic, cardiovascular systems, which has an obvious positive effect on health [1,2].
However, due to due to high intensity of exercises, fatigue, frequent changes of running direction, accelerations, jumps, speed as well as impacts, contact with other players there is a risk of injury to the musculoskeletal system. This leads to the negative effects of individual players and the healthcare system, as demonstrated by the example of visits to emergency departments [3,4]. Read et al. 2015 state that injuries of men playing football are found mainly in the lower limbs with a predominant non-contact incident and ligament injuries in the ankle and knee joint.
In addition, they distinguish the stages of adolescence and adolescence associated with injuries and loads with an emphasis on age from 9 to 15 [5].
Among the many factors that predispose injuries when training and participating in football matches, we should distinguish the mechanism of dynamic knee valgus and dynamic balance. Dynamic valgus of the knee joint may contribute to damage to the anterior cruciate ligament, medial collateral ligament, meniscus, cartilage, degenerative changes in the knee and pain in the patellofemoral joint [6][7][8][9][10]. By reducing the medial displacement of the knee, the risk of injury such as non-contact damage to the ACL ligament, pain in the patellofemoral joint and degeneration of articular cartilage is reduced [11,12,8].
Participation in an exercise program aimed at preventing injuries is most beneficial for young people with low movement quality and high risk, therefore research should focus on young athletes with dynamic valgus observed, i.e. with an increased risk of injury [7].
The vast majority of programs focus on many often complex exercises not only strengthening but also stretching or equivalent [13][14][15]. The problem in implementing the home exercise plan that the participants report is the time they have to spend. It is also influenced by personal factors, daily variable and their interpretation as well as stress and negative mood on a given day. Clear and simple instructions regarding home recommendations are important [16,17]. The research results to date are divergent, which is why our goal was to find a simple intervention to improve the quality of movement among young football players.
The main purpose of our study was to determine the impact of the simple program with three exercises strengthening the gluteus medius, popliteal and tibialis posterior muscles, respectively, on dynamic valgus and dynamic balance of the lower limbs, and thus theassesment risk of injury for young footballers. The research hypothesis was based on the intervention chosen to increase the dynamic balance value and reduce the angulation of knee displacement in young players qualified in the screening test as knee valgus.In addition, we assumed the hypothesis that the asymmetry of the length of the lower limbs and the dominance of the limb affect the results of dynamic valgus.

Patricipants
One hunderd thirty four youngfootballers from 10 to 15 years old (mean: 12.5, SD: 1.7) who have been practicing football at the local club "Arka Gdynia Stowarzyszenie Inicjatywa" were invited.
Screening included composition analysis using the InBody 270 analyzer (InBody Co., Seoul, Korea) measuring body weight and BMI (Body Mass Index) and increase in height laser InKids (InBody Co., Seoul, Korea).
Information was collected from Author's questionnaire consisting of 4 questions, regarding the dominant leg, hours spent on physical activity during the week (divided into physical education classes, football training, motor training, other sports training, matches and / or sports events), injuries present, surgery in the last 2 years.The questionnaire was prepared in Polish and then translated into English for the needs of the manuscript. Blinding the investigators was not feasible.
The collected data was used to analyze changes in group testing parameters over a period of 6 weeks.

Single Leg Squat
All attempts were carried out in a specially prepared place. Two GoProHero 4 digital video cameras (GoPro, Inc., San Mateo, USA) located on tripods, the first at a distance of 2m laterally and the second at the front 1m at the knee joint level of the subject. Both registered the pattern of the squat. Markers in the form of square kinesiotape patches were affixed in the anatomical points below the upper anterior iliac spine, at the middle point of the patella, and to the point of the ankle between the lateral and medial bones [18]. Before starting the test, all subjects watched the instructional video projected on the projector and were trained verbal information together with the demonstration of correct squat performance from the lateral perspective, so that they did not suggest the knee's anatomy from the frontal plane and could observe the minimum squat depth, i.e. min. 60˚. Participants after the instruction were able to practice a given movement pattern until they were ready to take the test. Patients were instructed to stand barefoot on a line formed of two white patches to standardize the test site. Then, during the verbal signal of the researcher, the untested limb was bent to an angle of about 90˚, the hip and knee of the limb tested straightened, the hands stacked crossed on the shoulders and the eyes are facing forward. Another signal from the researcher suggested starting the test. At the end, the subject returned to the starting position. The maximum duration of the squat for one lower limb was set as 5 seconds. Participants received information about the depth of the squat to be free and individual to perform the test.
One attempt was made for the left and right lower limbs. If the subject shifted his foot, heeled the heel, touched the ground with the limb, lost his balance, the test was considered unsuccessful and the test had to be repeated.
The researcher also visually analyzed the bending angle of the knee, if he felt that he was not deep enough, the test was repeated [18][19][20][21].

Video analysis
Next, the researcher made a video analysis of the oblique angles and/or deformation of the knee joint in the frontal plane and the angle of bending the knee joint in the sagittal plane. Free and freely available traffic tracking software was used with the Kinovea® (beta-v e r s i o n 0.8.26, Bordeaux, France) protractor tool [22].
Markers adhered to the previously mentioned anatomical areas of the participants were points determining the axis of the lower limbs.
On the video in the front projection, the researcher marked the middle of the patella in the center, followed by the upper anterior iliac spineand middle point between the lateral and medial ankle bones. The lateral projection was analyzed to determine the maximum angle of knee bending (at least 60˚ for a successful sample) and included anatomical points without markers: greater trochanter, lateral condyle of the tibia and lateral ankle bone. The exit angle of the test limb was determined at the moment of tearing off the foot untested from the surface, the maximum angle of distortion or deformity was marked during the squat at the maximum deviation from the starting position. All angles were visually determined frame by frame. The subject was qualified as valgus if in the squat test she obtained an angle ≤15˚ from the exit angle. The deforming person had to get an angle of ≥15˚ from the exit angle. Knee alignment in the range of 14˚-0˚-14˚ was recorded as a normal limb. If, during the video analysis, the researcher noticed the irregularities in the test performance, he classified it as an unsuccessful attempt, without the possibility of repetition. Combining all stages of the research, the analysis was subjected to 368 video from the front and side projection.

Y-Balance Test
The test consists in keeping the standing position one-legged with the hands placed at the hips on the leg placed on the stand arranged so that the big finger touches the horizontal line [23].

Statistical Analyses
The statistical analysis was performed using R statistics langua.

Dynamic knee valgus
As shown in Table 3 after completing the exercise program, the left valgus angle of exercise group decreased by 2.6˚. The analysis using Friedman and ANOVA test showed no statistically significant differences between the CON group and the INT group.  All results are not statistically significant. There are no differences between valgus values due to the dominant leg (Table 5).

Dynamic balance
In the next step of the analysis, the t-student test (taking into account the results of the analysis of the variance) and the Wilcoxon rank sum test, were used. Using these tests, the differences between the control group and the intervention group were examined for values describing the so-called dynamic balance. The calculations have been performed separately for step II (pre-training) and step III (post-training). The obtained p-values between the control and intervention groups for the above-mentioned tests at each step exclude the existence of a statistically significant difference (Fig. 4, Fig. 5).
Determination of the occurrence of differences between the average of 3 attempts from PRE-training stage and the average of 3 attempts from POST-training stage inside the CON and INT group was determined by analysis with the ANOVA test for normal distribution sets and Friedman test for non-normal distribution sets.
There were statistically significant differences between the means of three attempts  (Table 5).

Correlations between dynamic valgus (SLS test) and the dynamic balance (YBT)
were determined using Kendall's, Spearman's and Pearson's correlation coefficients.
There was significant inverse correlation mostly between attempts of YBT in ANT direction and attempts of SLS mostly in CON group for both legs in young footballers.  representative. The researchers used a one-leg landing test to assess, which due to a similar motion pattern can be compared to a one-leg squat test in the context of valgus assessment [14,30].
In addition, Palmer et al. 2015, using the hip abduction exercise strengthening the gluteal muscles over a 6 week period in military personnel, showed a high tendency to decrease valgus by 5˚ defined as clinically significant, but not statistically significant. Intervention group performed exercises focused on strengthening hip abductors [14].
Video analysis showed mean of knee valgus angle at the pre-training stage for the The results can be described as 'poor' by referring to the study by Räisänen et al. 2015, in which such an assessment was attributed to 19˚ and 18˚ for the dominant and non-dominant legs [31].
In further studies of knee valgus, Räisänen et al. 2015 again published a study that identified athletes with a high angle (1sd above the mean >23.8˚)of knee valgus in the frontal plane as 2.7 times more likely to have lower extermity injuries.
The team described the results thatdynamic knee valgity measured during SLS is not by itself sufficient screening tool for the risk of injuries in future [32].
The results of dynamic valgus (≥15˚) did not differ statistically from both groups

Dynamic balance
The differences between the intervention and the control group during the 6-week period between the stages were not statistically significant. This result rejects the hypothesis that the intervention group will obtain higher values than the control group among football players with knee valgus displacement.
There All dynamic balance results were normalized to limb length to be compared with other tests [27].
The above results without intervention (PRE-training stage) compared to the publication of Gorman et al. 2012, in which young junior high school students practicing many sports (average age 15.4±1.2) seem to be similar except for the mean posterior medial direction [33].
Referring to publications investigating the impact of interventions on dynamic balance, most researchers focused on neuromuscular training such as torso stabilization exercises and complex strength exercises such as squat. Filipa et al. 2012 showed significant improvement in the composite direction for the right and left limbs in young footballers after an 8-week program [34]. Imai et al. 2014 showed that a 12 week program of torso stabilization exercises has a positive effect on posterior lateral and posterior medial directions in footballers [35]. Both research groups used SEBT, they differed in intervention, study population and anthropometric characteristics.
One of the secondary goals of the study was to determine the risk of injury and reduce it by improving results. The risk reduction hypothesis was rejected. showed that an athlete with a composite direction result in the Y-Balance Test below 89.6% was 3.5 times more exposed for injury [37]. Considering the conclusions from the above studies, despite the differences in the studied populations, one can risk the statement that the examined young footballers from this study have a low risk of injury due to dynamic balance.
The study was characterized by innovation due to the intervention undertaken. A selection of three exercises to strengthen specific muscles of the hip, knee and ankle joints. Exercises were selected based on available literature in terms of impact on a specific muscle [38][39][40][41].
This work is original in the context of assessing the intervention of young football players considered to have valgity knees. Nakagawa and Petersen, 2018 also used both SLS and YBT tests, but among healthy soldiers. They suggested strengthening the hips and increasing the range of motion of the ankles to improve the results of dynamic valgus [42]. However, the angles of the extremities in the frontal projection in 2D are correlated with 3D results [43]. Kinematics assessment using Kinovea software used in this paper seems to be simpler to reproduce and without exposing costs in clinical settings. In addition, it requires no experience in video analysis and is easy to use to obtain accurate and reliable kinematic values of motion patterns [22].
Kinovea was used, among others, in studies concerning the analysis of height and time of the vertical jump, determination of knee and hip angles during running on the treadmill, sprint performance or evaluation of the lower limb in neurological patients after stroke [22,[44][45][46]. In the assessment of squat kinematics this test is innovative.
The author used the test of valgus as an angle of knee displacement in the frontal plane ≥15˚ from the starting position. In the available literature, most researchers used the criterion of knee displacement angle in the frontal plane >0˚ [32] or >10 [ 47].
Research Harris-Hayes 2014 showed that angle >10˚ is sufficient for proper visual assessment and classification as valgus [21].
The angle criterion proposed in this study was used to increase certainty as to the pattern classification and reduce the risk of error during video analysis.
In the context of other or further studies, this criterion may be described as "heavy" or "large" dynamic knee valgus.
The one-leg squat test requires proper neuromuscular control, stabilization and muscle strength in both the lower limbs and the torso.
Among participants aged 10-11 (year 2008), as many as 11 of 20 during the analysis were disqualified by loss of balance. This fact may suggest that the test may not be suitable for younger age groups.
Time to study interventions could be short in the context of increased muscle strength.The use of homework for young footballers in the studied age range raises a number of doubts about the correctness and actual implementation of the intervention program.
Further intervention studies could focus on given exercises, but with a longer program than 6 weeks, and focus on doing all the exercises correctly by running training units by a physiotherapist or trainer. Availability of data and materials

Conclusions
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests.  Comparison of control and intervention groups in all directions of the Y-Balance test in the p Figure 5 Comparison of control and intervention groups in all directions of the Y-Balance test in the p