INJURY PREVENTION FOOTBALL


Biomechanical and neuromuscular risk factors Biomechanical risk factors are particularly important in knee injuries. Landing from a jump with the hip and knee in minimal flexion increases the load transmitted to the knee and increases the shear force from the quadriceps (Figure 1), thus imparting stress upon the ACL. More et al. (30) used a cadaveric model with loaded


quadriceps and hamstrings muscles to simulate squat exercises. When the hamstring was loaded, anterior tibial translation during flexion was significantly reduced in addition to a reduction in internal tibial rotation. During a squat, hamstring muscles function synergistically with the ACL to provide anterior knee stability. McLean et al. (31) compared knee kinematics and gender in 30 high-performance athletes during lateral side-cutting manoeuvres. Women displayed greater inter-trial variability for axial internal rotation patterns during cutting than men, but gender was not the main determining factor. The differences were directly related to the athlete’s level of experience. It is important to note that these were high-performance athletes, so the results may have been affected by selection bias (31). (Selection bias exists when only a certain group of individuals are selected for a study who do not reflect the population as a whole.) In a follow-up study, McLean et al. (32) studied 10 male and 10 female athletes during cutting manoeuvres with random perturbations (that is, unannounced and unanticipated forces) at initial contact (n = 5000). Injury to the ACL in the sagittal plane is incurred with an anterior drawer force (on the tibia in a forward motion) of more than 2000 N. The neuromuscular perturbations produced significant increases in extrinsic forces on the anterior knee, on valgus moments and on internal rotation moments. During the study, the anterior drawer force never exceeded 2000 N in any study model. Valgus loads were great enough to rupture the ligament, and occurred more frequently in females than males. McLean concluded that forces on the sagittal plane of the knee joint cannot rupture the ACL during side-step cutting, primarily because the muscle and joint mechanics and external ground-reaction forces in that plane protect the upward limit of ligament loads. They can be protected up to a certain amount of force, but once this force is exceeded the ligament will tear. They suggested that valgus loading is the more likely injury mechanism, especially in females (32). In a separate study, Malinzak et al. (33) compared knee motion patterns in male and female recreational athletes. Three-dimensional coordinates and electromyographic data were collected for knee flexion–extension, valgus–varus, and internal–external rotation angles. Female athletes showed less knee flexion and greater knee valgus when landing from a jump and with cutting manoeuvres. These female athletes also had greater quadriceps activity, together with decreased hamstring activity. The frequency and intensity of their hamstring activity was less than in male athletes. The females typically contracted their hamstring fibres 50 ms slower than their male counterparts (200 ms vs 150 ms, respectively) and with less intensity (55.2% vs 71.8%, respectively, at initial contact).


Anterior cruciate ligament (ACL) injuries The ACL is one of the major stabilising intracapsular ligaments in the knee joint. This ligament prevents excessive anterior translation of the tibia on the femur and plays a


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Femoral adduction


Dynamic valgus


Ankle eversion Knee abduction


midline Figure 1: Landing from a jump with minimal hip/knee flexion can result in ACL injury secondary role of limiting internal rotation of the tibia.


ACL rupture Injuries to the knee joint are common. In the USA, there are approximately 250,000 ACL injuries every year. This is a heavy economic burden given the costs of MRIs, reconstructive surgery, post-operative bracing and rehabilitation, resulting in an annual cost in excess of 2 billion dollars (34). The psychological impact of such an injury can be devastating – an athlete may miss 6–12 months of competitive play with an ACL injury and may have to undergo reconstructive surgery and rehabilitation. However, this does not reflect the large discrepancy that exists between males and females, whereby female athletes have from


Posterior cruciate ligament (PCL)


Medial collateral ligament (MCL)


Figure 2: Main ligaments of the knee 17


©2010 Primal Pictures


Anterior cruciate ligament (ACL)


Lateral collateral ligament (LCL)


Lateral meniscus


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