Football is a sport that needs intermittent, non-continuous exercises which includes many sprints of different intensities especially during a match. Unfortunately, high-speed running has also been associated with injury and classified as an intrinsic risk factor. Higher speed of play during competitions may exert more loads or forces on the knee joint, which in turn may raise frequency of injury.
The knee is one of the most commonly injured sites. Most knee injuries in soccer players are the non-contact injury involving planting, pivoting, or landing. A smaller amount of knee-flexion joint angle (within 10–30°), greater knee valgus joint angle, and greater vertical and posterior ground reaction forces during landing increase knee joint loading and injury risks. Other determining factor of knee injury is kinetic and kinematic asymmetry between two legs.
Surprisingly, not many studies have investigated the effects of sprinting on knee’s kinetics and kinematics of soccer players. It is unknown whether sprint activity followed by plyometric and explosive activity, i.e. jumping and landing, would change the kinetic and kinematics variables of soccer players.
The main purpose of this present study was to analyze jumping-landing maneuvers after different run-up speeds using related kinetics (GRF and knee valgus moment) and kinematics (knee flexion and valgus joint angles) risk factor measures in male soccer players.
Twenty-three male soccer players (age 22.1 ± 1.7 years, range 20–24 years, body mass 64.1 ± 7.1kg; height 168.7 ± 7.6 cm) were recruited from University of Malaya soccer team. The players performed 4 tasks: vertical jump (VJ), vertical jump after slow running (VJSR), vertical jump after fast running (VJFR) and double horizontal jump after fast running (HJFR).
Movement was recorded using Vicon motion analysis system (100 Hz) with five cameras, and two Kistler force platforms (1000 Hz) which were embedded in the floor in the middle of the capture volume. Motion analysis system and force platforms were synchronized and calibrated prior to testing. VICON plug-in gait was used to process motion capture and ground reaction force data. In order to capture the movement, 16 reflective markers were affixed to bony landmarks of the lower limb.
To compare the variables between the four conditions (VJ, VJSR, VJFR, HJFR) and legs (dominant and non-dominant), the 4 ×2 (condition vs leg) repeated measures ANOVA was used. The effect sizes of each variable were tested using partial eta (η) squared (0.01 = small effect, 0.06 = medium effect, and 0.14 = large effect).
For knee flexion joint angle no differences were found between conditions and leg dominance in the jumping phase. For knee valgus joint angle, results showed significant main effect between leg dominance (p = 0.006, η = 0.31). This result suggested bilateral deficit exists between right and left lower limb in this phase.
For knee flexion joint angle, the results showed significant differences between the four conditions (p = 0.014, η = 0.15) with large effect size in the landing phase. For knee valgus moment in the landing phase, the results showed significant differences between conditions (p = 0.001, η = 0.71) with large effect size.
The greater valgus moment was found in HJFR than other conditions, while the smaller knee valgus moment indicated in VJ than other conditions. Relatively greater knee valgus moment seen after landing following fast running (HJFR) has higher predisposition to knee injury due to greater force transfer to the knee.
The main findings in this study were greater knee valgus moment and less knee flexion joint angle proceeding sprint (HJFR & VJFR) rather than no sprint condition (VJ). The results support the theory that fast sprint increases knee valgus moment during landing in soccer players. This presents an increased risk for knee injuries.
The findings of this research may be helpful for coaches and trainers who can strategize training programs to reduce knee injury risk factors of male soccer players as well as reducing bilateral different knee valgus joint angle.