The authors have declared that no competing interests exist.
The Anterior Cruciate Ligament (ACL) injury is uncommon, but constitutes an important percentage of the lesions requiring more than 8 weeks of recovery in sports. Most ACL injuries do not require external contact and are associated with biomechanical risk factors that stress this ligament ACL: like an increase in knee valgus angle (KV) and hip flexion (HF) and a decreased in knee flexion (KF). The ACL requires muscle cooperation to resist the load; however, when there is fatigue, the protective capacity of the musculature would decrease. The objective of the study is to analyze the angular behavior before and after a physical load for HF in children of 14-15 years old.
The physical load applied in a single training session is enough to change some biomechanical risk factors of ACL injury.
Experimental study
2b
The sample consisted of 50 students from different soccer schools between 14-15 years old. We analyzed the hip angular behavior of hip flexion before and after a training session. The angles where obtained from inertioal sensor while the students performed a Drop Jump test from a 30 centimeters drawer.
Statistically significant differences were found in the differences between pre physical load HF / post-load HF (p = 0.00053), right pre physical load HF / right post- load HF (p = 0.0086) and left post- load (p = 0.0248206).
It was concluded that the angular behavior of HF increases significantly in both post-training limbs, after performing a physical load.
This study analyzed the importance of using prevention injury programs since early stages, and give importance to the eccentric role of hamstring, gluteus and core exercises.
The Anterior Cruciate Ligament (ACL) lesion is classified as an uncommon lesion; however, it is characterized by the fact that a large percentage (47.8%) of the lesions require more than 8 weeks of recovery in sports, are due this kind of injury
In addition, 77.7% of cruciate ligament injuries were associated with sports, with 42.9% occurring during competitions and 34.8% in training
It is known that not all ACL injuries in sports require contact, in fact, a significant percentage occurs without external contact
It is important to mention the existence of biomechanical risk factors which includes increased knee valgus (KV)
Bowerman
The main objective has been to determine the angular behavior of the hip joint in the flexion, before and after applying a physical load, through a kinematic analysis by means of the inertial sensors of a Drop Jump (DJ) in children between 14-15 years old
The hypothesis of this study is whether the physical load in soccer increases the angle of hip flexion, after applying a physical load, in children who practice amateur soccer, between 14-15 years old.
The populations for this study were 4 children's soccer schools located in Santiago, Chile. The inclusion criteria were to be born in the years 2004-2005 and practice 3-5 hours of soccer per week, while the exclusion criteria were to have a prior history of ACL rupture and joint or periarticular pathology in the 3 months prior to the test. The sample consisted of 50 individuals born in the years 2004-2005, who were chosen by convenience sampling. The sample data are summarized in
| Variable | Mean ± Standard deviation |
| Age | 14.34 ± 0.51 years |
| Weight | 46.51 ± 8.71 Kg |
| Size | 152.04 ± 8.68 cm |
| Span (arms opens, from hand to hand) | 150.2 ± 8.65 cm |
| Foot size | 25.8 ± 1.38 cm |
The tool for obtaining data was the MVN motion capture system consisting of 17 MTx sensors plus 2 Xbus Masters sensors. MTx is an inertial sensor and magnetic measurement unit comprising gyros, accelerometers and 3D magnetometers
The initial position between the sensors and the body segments is unknown at the beginning, therefore, a calibration procedure has to be performed in which the alignment and body dimensions are determined
An Experimental, Transversal and Non-random study was carried out. Eight days of measurements were performed maintaining a specific order. First, the players were summoned to their regular training location with their usual soccer shoes. Then, a previous measurement was made in order of arrival in which the first child measured in the previous phase would be measured in the later phase so as to achieve the same amount of training in minutes. The placement of Xsens MVN sensors was done next by means of a protocol to be observed at a later point in the text 2.3.3). Finally the subjects were instructed to perform a drop jump (DJ) test that consisted in the drop of a drawer of 0.3 m height with both feet at the same time on a hoop located 0.5 m distance. Each subject had to perform at least 3 DJ jumps. Once the subject has completed the three jumps, the Xsens MVN sensors are removed in order to be used by the next subject.
It is a test that allows evaluating the reactive force of the lower limbs. The subject must perform a vertical jump action after a fall of a drawer, in this study, the drawer has 30 cms. The fall must be advancing one foot while leaving the rest to the gravitating effect so as not to alter the height of the fall
The sensors were placed as indicated by the manufacturer. Two Master Xbus sensors were placed the posterior superior iliac spine and six MTx sensors located at the functional rotation point of each joint under study, using as reference points the greater trochanter of the femur, tuberosity of the tibia and the ankle-talus joint center, so that by connecting all the joint centers a skeleton is formed in the software used by Wu31, 32,33.
A soccer training session is standardized in order to match the physical loads to which the subjects will be submitted to. This consists of 3 stages which are pre competition, technical physical work and football performance. The stages are detailed as follows
Development of low intensity exercises which combine a slow jog with joint mobility for upper and lower extremities. Afterwards, the intensity of the jog is increased and when the auditory signal (whistle) is given the subjects must perform actions such as jumps, dribbling and changes of direction. Then, there are 3 sets with 4 repetitions of 35 meter sprints. The final exercise consists of the subjects forming lines and when a tactile, auditory or visual signal is given, they must perform speed sprints; 3 series of 5 repetitions at a distance of 15 meters. This stage lasts approximately 25 minutes.
An important physical component is combined with a technical one, which consists of 3 different tasks with duration of 12 minutes each. The first exercise is an aerobic component of controlling the soccer ball in addition to controlled passes in a circuit of 20 meters one way and 20 meters return. The second exercise consists of a coordination work along with technical sequences and vertical jumps that end with 30 meter speed sprint races (3 series). The third exercise consists of reduced soccer (4 vs 4) which has duration of 15 minutes.
Soccer is played in dimensions that match the age (50x30 meters). This stage lasts 20 minutes per time with a break of 5 minutes. After the last measurement, the final exercise consists of the children performing a 15-minute regeneration jog followed by passive-active type stretching for all muscle groups most used in soccer practice such as the hip and knee flexors and extensors, abductors and sural triceps in 3 sets of 20 seconds per muscle group.
The angular peak corresponds to the maximum angle reached by a joint at any given moment during any movement. The angular peaks in the hip joint were obtained in the hip flexion (HF) for the left and right legs. Each test subject performed 3 jumps of DJ pre physical load and post physical load, although for purposes of this study, only 1 angular peak was taken which was the highest value reached in any of the 3 executions, leaving 1 angular peak for the following variables. HF pre-physical load 26.27 ± 10.39°, HF post physical load 31.04 ± 12.91°.
For this study, the variables were divided into right and left extremities, thus working with 4 more variables whose values are right HF pre physical load 24.80 ± 10.28 degrees, left HF left pre physical load 27.24 ± 10.19º; right HF post physical load 29.82 ± 12.81°, left HF post physical load 32.26 ± 12.76°, Summary in
| Variable. | Mean/Standard deviation. |
| HF pre physical load. | 26.27± 10.39º |
| HF post physical load. | 31.04± 12.91º |
| HF right pre physical load | 24.80± 10.28 |
| HF left pre physical load | 27.24± 10.19º |
| HF right post physical load. FR right pre physical load | 29.82±12.81º63.50± 11.58º |
| HF left post physical load. FR left pre physical load. | 32.26± 12.76º65.67±10.27º |
The value to be analyzed will be the difference between the angular peak post physical load and the angular peak pre physical load. In order to determine the normality of the sample, the Shapiro Wilk test will be applied. The variables that present normal distribution will be analyzed with the t student test for paired samples, whereas those that do not show a normal distribution will be analyzed with the Wilcoxon test. A statistically significant difference was observed for a p <0.05 value. The statistical analysis was performed using the Statgraphics Centurion XVI.I., data analysis program.
The use of inertial sensors is supported by the Cuesta-Vargas
The study participants had to sign an informed consent as well as their tutors which were reviewed and approved by the ethics committee of the Metropolitan University of Education Sciences on September 10, 2018.
In the comparison of HF pre physical load with HF post physical load, it was observed that the angular average for the pre-load phase was 26.27º, while the angular average for the post-load phase was 31.04º, with the 4.77 ° increase being statistically significant (
In the present study, we evaluated male subjects from 14 to 15 years of age, belonging to 4 children's soccer schools. The participants are in a formative age of the trained sport, for which it has a recreational objective as well as the future objective of reaching their maximum performance in adulthood, for which they are improving the physical performance, acquiring technique and perfecting motor gestures.
The tool used to evaluate the angular behavior was the Xsens MVN inertial sensor, which as demonstrated in previous research has a high reliability in the measurement of the studied biomechanical variable.
The participants were subjected to standardized training that is similar to the one performed each week, which seeks to control the load exerted on them. Furthermore, the gesture with which it was evaluated, the Drop Jump (DJ), requires the landing, that is also an activity that is frequently requested in soccer, reason why they were not submitted to tasks other than the commonly done. Finally, all the evaluations were made in a familiar atmosphere, the training place they use to practice and with their usual equipment.
As for the results obtained in hip flexion, it was observed that in the total of the sample there was an increase of 4.77°, with 5° in the left leg and 4.6° in the right leg. These were statistically significant increases, which is why the increase of the biomechanical variable is associated with the physical load exerted
Regarding the reference values that exist for the angular behavior, these are derived from measurements made in adult subjects, and therefore they are not applicable for the study population. In spite of this it is of utmost importance to establish the existing relationship for the correction of motor gestures that are being acquired.
Regarding hip analysis, although it is a variable that could be related to a potential risk of ACL injury, is the least studied in the literature. However, increased hip flexion may be due to decreased eccentric strength of gluteus maximums and hamstring muscles, since according to the study
Another important factor at hip level is the position since according to the results obtained in the study
There a lack of studies in children aged 10 to 14 years in the literature concerning prevention of this type of injury. Soccer is among the sports most commonly played by patients from 5 to 17 years of age who tear their ACLs
The training done in a standard way to the students of the participating soccer schools is similar to the one commonly executed on training days, either in terms of duration and type of work to be performed. Furthermore, the place where the evaluation was carried out is where they routinely carry out their training sessions, which will positively influence various types of variables such as moderators (footwear and training surface) since the individuals used the same footwear they occupied in all their training sessions. In addition to the latter, the surface used for the test is the same as the one used to perform all their training sessions. The unknown variables (psychological status, stress and motivation) are also positively influenced by the fact that they were in a well known environment during their normal practice, in addition to the motivation received by their teachers and parent. Psychological and stress factors were favored in the control of these variables.
Regarding the use of the Xsens MVN sensor equipment, the children were consulted about any discomfort they might have presented with their use, so that this would not interfere with the jump process. Furthermore, the weight of the sensors (less than 2 kg.) that was distributed throughout the lower extremity was negligible for the individuals. No discomfort was experienced when using the Xsens MVN sensors at the time of jumping performance.
In reference to the bias of our study, we found some variables that were not controlled, such as the time it takes to train each child in his respective school, so that the level of experience or expertise in the movements he/she can perform are not fully controlled thus, having individuals with better technical gestures than others and therefore better control of the motor gesture. The type of footwear that the children used were not controlled either, so it was not the same for all thus creating variations when the DJ was performed. Other variables of interest are the state of fatigue, which could be measured with electromyography or fatigue scale (Borg), which is suggested to be controlled in future investigations, as well as a postural evaluation seeking pelvic antevertion.
The resulting physical load after training improved the hip flexor angle in boys aged 14 to 15 years playing amateur soccer.
Our results reflect the effects of the fatigue of a training session thus alter the biomechanical risk variables of a ACL injury, increasing the hip flexor angles, this could reflect a lack of eccentric strength in the gluteus maximums and hamstrings, as well as a lack of trunk control in men aged 14 to 15 years practicing amateur soccer.