Comparing the Knee Joint Kinematic Parameters During Landing at Different Minutes of Soccer Game

* Corresponding Author: Abdolrasoul Daneshjoo, PhD. Address: Department of Biomechanics and Corrective exercise, East Tehran Branch, Islamic Azad University, Tehran, Iran. Tel: +98 (912) 2061034 E-mail: phdanesh@yahoo.com 1. Assistant Professor, Department of Biomechanics and Corrective exercise, East Tehran Branch, Islamic Azad University, Tehran, Iran. 2. Master Degree in Sport Injuries and Corrective Exercise, East Tehran Branch, Islamic Azad University, Tehran, Iran. *Abdolrasoul Daneshjoo1 , Maryam Mohseni2


Introduction
ne of the mechanisms of lower limb injury is landing. This movement can create a force as large as 2 to 12 times the bodyweight; the skeletal system must moderate such mechanical shock [1]. Repeated increasing impact forces during landing provide the basis for structural damage to the soft tissue around the joint [2]. O Numerous factors influence the landing mechanics of football players. Some of the most essential causes of serious knee injuries are gender, age, and fitness level. Besides, some physiological factors, such as fatigue, increased knee valgus angle, and decreased knee flexion during landing, might affect them. The superior leg is stronger than the non-superior one; however, the Anterior Cruciate Ligament (ACL) injury is greater in this leg, compared to the nonsuperior leg [2]. Researchers reported that landing with decreased lower limb and knee flexion angles causes more significant anterior tibial shear force; thus, they suggested that athletes flex their trunk and knee joints more frequently during landing [8].
No study was found to examine the kinematic parameters of the lower limb during landing at different minutes of a football play. Therefore, this study aimed to compare the kinematic parameters of lower extremities during landing at different minutes of a soccer game.

Participants and Methods
In total, 15 youth players of FC Mes of Kerman Industry with the Mean±SD age of 17.55±1.12 years, the weight of 62.21±7.22 kg, and height of 175.43±7.88 cm volunteered to participate in the current study. The players performed the Benzo Buddies Fatigue Protocol, i.e. similar to soccer. Then, they made two reverse jumps every 15 minutes. After recording the video with three cameras (Sony A7S), we extracted the landing data through the Kinovea software. Using SPSS, we compared the kinematic performance of the study subjects in different minutes of a football game. We also used the repeated measures Analysis of Variance (ANOVA) to compare time intervals (Table 1).

Results
Paired comparisons of 0, 15, 30, and 75 minutes with other measurements were statistically significant (P<0.05); however, other comparisons were not significant (P>0.05). Table 2 compares the mean scores of repeated measures ANOVA of the maximum valgus-varus knee angle during landing between the superior legs.

Discussion
This study investigated the risk factors of ACL injuries during landing at different minutes of a soccer game. The obtained results suggested that knee flexion before initiating the game was significantly less than that of the other time intervals. After this time, knee flexion has been relatively high during landing. After a half-time break, the knee flexion decreased again in the 60 th minute (after the second half started). There was less flexion in the knee joint be-  fore starting the first and second halves, compared to other times. Therefore, this was due to lower body temperature and muscle minutes before the start of each half of the game. This result is in line with Mohr's research [3].

Conclusion
The achieved results revealed that the knee flexion process during the soccer game had an upward course. In other words, the first few minutes of each half had the lowest knee flexion rate during landing; accordingly, the odds of ACL injury incidence was higher during these intervals.
According to the previous hypothesis, the highest valgus angle of the superior leg occurred at 0 and 60 minutes of a football play. This finding indicates a relationship between lower knee flexion and increased valgus angle. Furthermore, the early minutes of each half-time are potentially dangerous for ACL injury. Moreover, fatigue affected the kinematics of the superior leg during landing; i.e. in the 105th minute of play, the superior leg knee has increased knee valgus and enhanced the risk of ACL injury. The nonsuperior leg of the players appeared to be less likely to have an ACL injury. This might be because of the lack of knee valgus at any time interval and that a knee varus angle has always occurred. This might have resulted from non-superior leg neuromuscular adaptations.

Compliance with ethical guidelines
All subjects voluntarily participated in the study and signed the written consent form. IR. IAUETB.96141 Article Code of Ethics

Funding
This article is taken from Ms. Maryam Mohseni's Master of MS. in Physical Education and Sport Sciences, Islamic Azad University, East Tehran Branch.

Authors' contributions
The whole process of editing the article was the responsibility of the first author, and Ms. Maryam Mohseni was responsible for conducting tests and preparing the raw data.

Conflicts of interest
The authors declared no conflict of interest.