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Abstract

An anterior cruciate ligament (ACL) injury is an important cause of rest in athletes. In most cases, ACL injuries do not require external contact and they are associated with biomechanical risk factors that increase ACL tension. The increase of the hip flexion angle (HF) is included within these. The ACL requires cooperation of the periarticular musculature of the knee, muscle groups, hip stabilizers and CORE muscles; consequently, fatigue caused by exercise would alter the balance and put this ligament at risk.

The objective of the study is to determine the angular behavior for HF before and after a physical load (a standardized training) in children between 11 and 12 years old.

A non-randomized clinical study was carried out. The sample consisted of 50 soccer school students born between 11 and 12 years old. The angular behavior of HF was compared before and after performing a training session. The angular behavior was measured through the Drop Jump test (DJ), with data obtained by inertial sensors.

After the exercise, there was a significant increase in HF. It was concluded that the angular behavior of HF increases significantly in both extremities after training and that preventive measures must be applied for neuromuscular control of the hip.

Introduction

The anterior cruciate ligament injury (ACL) is an important cause of lost days for athletes and it is the most common reason for injuries that require more than eight weeks of recovery1. This injury usually occur while practicing sports and it is more common during competition than during training2. The predominant mechanism is a pivot shift type injury, which occurs without contact in the support leg. This mechanism is a combination of hip and knee flexion associated with internal rotation and initial valgus of the knee and, finally, an external rotation of the tibia 134.

Biomechanical risk factors have been proposed for ACL tear such as an increase in knee valgus moment, increased hip flexion and lesser knee flexion, also greater ground reaction force during the fall would increase ACL tension, bringing it closer to its moment of failure567891011.

Bowerman et al demonstrated that fatigue is related to a decrease in motor balance, which in turn causes a decrease in neuromuscular response, thus increasing the risk of injury to the stabilizing structures12. A study examined non-athletic and healthy adult subjects before and after a workout, and they observed that both in men and women, fatigue produced an increase in the anterior translation of the tibia13. However, these findings have not been systematically reproduced. Increased tibial translation after conducting strenuous training only on the left knee are also a finding in Skinner studies14, but this not happen when isokinetic exercises were execcuted15.

On the other hand, epidemiological studies have shown that the majority of ACL injuries occur during the last 15 minutes of the first half and the last 30 minutes of the second half of a game, so, once again, fatigue could be an important risk factor.

These contradictions in the findings can be explained by the type of training applied to the subjects under study and by the manner in which the deficit produced by fatigue was measured16Nevertheless, there are no studies in the literature of the effects of everyday training on the biomechanics of hip flexion. This must be the initial approach for measuring the risk of suffering lower extremity injuries for each athlete. The objective of this work is to analyze the effect of soccer training on the biomechanics of hip flexion and to relate these findings to the risk of ACL tear.

Results

Fifty male individuals were included. The demographic data of the participants is summarized in Table 1. Hip flexion increased after bilateral exercise. The angular peak mean in right HF was 24.80° ± 10.28° prior to training and 29.82° ± 12.81°after training. For left HF, the mean was 27.24° ± 10.19° prior to training and 32.26° ± 12.76° after training. The post-exercise increase of the hip flexion, both for the right and left lower limbs, was significant, obtaining a probability of 0.01 and 0.02, respectively Figure 1, Figure 2, Figure 3.

Table 1. Demographic data

Variable	Variable Mean ± Standard Deviation
Age	11,34 ± 0,51 years
Weight	46,51 ± 8,71 Kg
Size	152,04 ± 8,68 cm
Width	150,2 ± 8,65 cm
Foot size	25,8 ± 1,38 cm

Figure 1. Comparison of HF pre physical load with HF post physical load. Figure 2. Comparison of HF right pre physical load with right HF post physical load Figure 3. Comparison HF left pre physical load with left HF post physical load

Discussion

In the present study, 11 to 12 year old male subjects belonging to four children s soccer schools in the city of Santiago were evaluated. Within the strengths of the research, we identified the use of inertial sensors, which are highly reliable for measuring the biomechanical variables under study1920. Standardized training is similar to the training that the subjects perform week by week; therefore, its use is valid for establishing the degree of fatigue that they reach and identifying the risk that they are exposed to. In addition, they wore the usual clothing (shoes) and trained on the same type of surface they normally train on. Finally, the use of DJ allows measurement of the biomechanical variables, a common gesture in the sports practice of soccer. The results show that the load subjected to during training causes significant changes in the angular peak of hip flexion. Boden and his colleagues studied the mechanism of ACL tear by video photogrammetry and found that the angular peak increased in those subjects who had an injury, compared to those who were not injured4. Given that the subjects of this work were subjected to an everyday and not maximum load, it may be considered that the increase of hip flexion peak is the first event in the sequence of alterations that fatigue produces in the biomechanical factors that predispose injuries. The cause of this increase in flexion can be attributed to a decrease in CORE muscle activation, which is consistent with what was posed by Hoshikawa et al. They reported significant improvements in unilateral hip muscle strength and jump performance in adolescent soccer players after working out for 6 months in order to strengthen CORE muscles and overall strength, in addition to regular soccer training, in comparison to those who only performed soccer training25. Furthermore, on the neuromuscular level, significant but small-sized relationships were demonstrated between trunk activation levels and lower limb muscles during jumping tasks26.

This work evidences the effect of regular hip flexion training in relation to knee injuries, specifically ACL in 11 to 12 year old children. We consider this extremely important; given that the anthropometric and laxity changes that occur during puberty put athletes at risk of having knee injuries; to objectify that motor control reached at an early age with regular training helps prevent injuries. A recent meta-analysis of ACL injury prevention studies concluded that greater success in reducing knee injury in athletes was achieved through preventive neuromuscular training begun before the onset of neuromuscular deficits and the peak incidence of knee injuries in adolescences. This is ideally in the pre-pubertal stage2728.

The training tasked in a standard way to the students of the participating soccer schools is similar to the one that is commonly executed on practice days, both in terms of duration and type of work to be carried out. In addition, the place where the evaluation took place is where they usually carry out their soccer practice, which will positively influence several types of variables, such as the moderators (footwear and training surface), since the individuals used the same footwear they regularly train with. In addition, the surface used for the test was the same as the type used to carry out all their practices. The unknown variables (psychological state, stress and motivation) are also positively influenced by the fact that they were in a known environment, during their normal practice; this, added to the motivation received by their teachers and parents, for which the psychological state and stress were favored with the control of these variables.

Conclusion

Sports training in our study group showed that hip flexion is modified by the fatigue achieved after normal training. Hip flexion could be the biomechanical factor that is first altered by fatigue, which should be handled by the technical team in order to prevent ligament injuries.

Journal of Sports and Exercise Medicine