When it comes to training for sports performance, injury prevention should never be overlooked and should coincide alongside flexibility, strength, power and speed training. However, this might be disregarded amongst personal trainers and sports coaches who have little experience or knowledge with regard to the best strategies wherein injury prevention can be trained; or by not utilising appropriate screening tests to examine the athlete’s readiness to perform fast dynamic movements. Empirical evidence would suggest that training seems to direct more emphasis on speed and acceleration which involve mainly concentric muscle actions, or long drawn out running sessions to improve sports specific endurance. While these have their place, consideration must also be given to eccentric muscle actions or force absorption. Without an adequate amount of eccentric strength and technique training, non-contact injuries can become significantly increased (1).
Anterior cruciate ligament (ACL) injuries tend to be most common, with females rupturing their ACL more frequently than males (1–4). Fifty percent of ACL injuries occur in younger athletes with around 70% of these due to non-contact mechanisms; largely occurring from abrupt deceleration, cutting or landing activities with largely extended knee angles (4,5). Whilst these injuries cannot always be avoided, the prevention rate of the athlete can be increased with appropriate training, facilitating with tolerance to unexpected forces that may occur during the aforementioned manoeuvres. This article will discuss the biomechanics of the ACL, mechanisms related to injuries in females such as technique and hormones, and finish with a critical discussion around training methods for prevention of ACL ruptures.
The ACL is one of four ligaments that connect the femur and the tibia, assisting with stability of the knee joint and minimising anterior translation. In addition, the ACL will prevent excessive tibia medial and lateral rotation, as well as Varus and Valgus stresses (see figure 1). Several biomechanical factors have been associated with ACL ruptures. Firstly, when ACL ruptures happen in field sports or track athletes, the athlete’s foot tends to be pronated in a closed chain position with the tibia internally rotated (6). Attempting to change direction in this position will cause excessive torsional force that can rupture the ACL (6). A second mechanism for ACL ruptures, is increased forces to the ligament whenever forward translation of the tibia occurs, causing ACL strain due to quadricep contraction, usually at knee angles less than 35º (7,8). However, greater hamstring stiffness may resist shear force and limit tibia forward translation (9). Following landing activities, if the hamstrings are weak or have a delay in firing rates compared to the quadriceps, an increase in sagittal plane energy absorption may occur increasing the force applied to the ACL and placing it in potential threat (4). This can happen if the athlete lands or decelerates with an increased knee extension angle (8,9). Also, an increase in internal hip rotation can further increase strain on the ACL (2). Landing forces can increase compression forces on the knee joint which can produce internal tibial rotation resulting in further ACL stress (10). Females regularly display leg dominance, a strength imbalance during lower limb tasks, as a result, lower hamstring torque in the weaker limb. The dominant limb can therefore have an increased stress on the same knee, whereas, the weaker limb will be at risk of injury due to the knee not been capable of withstanding the absorbed forces from the sporting activities (11). A lack of neuromuscular control can therefore result in a lack of stability in the hips and trunk, which can lead to poor control of the athlete’s centre of mass and increase knee abduction and/or adduction.
The previously mentioned biomechanical factors can be an injury risk for females, but changes in hormone concentrations can leave females at higher risk of ACL injuries (6,12,13). Three hormones have been found within the ACL, i.e., estrogen, progesterone and relaxin (6). The female hormone estrogen can change the composition of the ACL during the menstrual cycle, leaving it at higher risk of injury due to changes in joint laxity (14) which occurs from fluctuations in hormone concentrations (13). Estrogen provides bone and muscle function, however high levels can be hazardous due to decreases in ligament and tendon stiffness (15). Upsurges in estrogen can put the ACL at risk due to downregulation of collagen synthesis which has been observed in rodent studies (16). In contrast, upregulation has been observed in vitro studies using fibroblasts from porcine ACL (17). Lee et al (17) reported upsurges in collagen synthesis for type 1 and type 3 collagen after administration of estrogen, but this became inhibited when tensile force was applied, suggesting that mechanical force plays a particular role in inhibiting fibroblast metabolism.
In a normal menstrual cycle, estrogen will increase during the ovulatory phase (Figure 2), peaking around day 13, then decrease in the luteal phase (18). Conversely, upsurges in progesterone will occur during the luteal phase with relaxin increasing mid luteal phase (6,18). These alterations in hormones may be the reason that ligaments become laxly. A systematic review by Zazulak et al (19) reported that knee laxity increased when estrogen levels dropped, which will happen in the follicular phase and again in the luteal phase (Figure 2). Correlations have been found between ACL injuries and the most frequent occurrence at days 1 and 2 of the follicular phase, highlighting these injuries don’t occur by chance (18). Increases in joint laxity due to hormone regulation and the previously mentioned biomechanical factors may explain why ACL ruptures occur more in females.
“The female hormone estrogen can change the composition of the ACL during the menstrual cycle, leaving it at higher risk of injury due to changes in joint laxity”Tweet
Before starting a training program, it’s important to screen athletes to grasp an understanding of their current movement patterns. Females have been shown to display less flexion through the knee with more tibia rotational forces during landing assessments (2). When forces are applied to the knee from a landing with largely extended knees, the accommodating joints are unable to dissipate this force through movement. Consequently, the body is unable to absorb these forces, as a result the ACL develops additional stress (2,4,9). An extended knee angle will hinder the athlete’s ability to absorb any force eccentrically through the quadriceps, thus, leading to an increase in force to the knee joint (8).
It is important as coaches to teach proper technique so athletes can minimise injury risk. Proprioceptive training such as balance training alongside neuromuscular training, such as plyometrics, are successful methods for reducing the risk of ACL injuries in females (20,21) along with a biomechanically informed injury prevention program (22). A long term study by Weir et al (22), have highlighted the importance of a biomechanically informed injury prevention program and how this is more specific to individual training modalities such as plyometrics, strength, technique and proprioceptive training. The design by Weir et al (22) implemented technique and muscular activation exercises on elite female hockey players for 9 weeks of intensive training, followed by 16 weeks of maintenance training. Factors relating to ACL injuries were included in their design, i.e., increasing knee flexion angles upon touch down, improving dynamic control of the trunk and upper body along with strengthening the hip external rotators and gastrocnemius muscle groups. Significant improvements were observed in muscle activation, lasting right into the maintenance phase of the program, weeks 9-25. Weir et al (22) also witnessed significant improvements in peak knee valgus moments following the training intervention.
Additionally, plyometric training has been shown to be an effective method to decrease the risk of ACL injuries in females, with further suggestions referred to balance training alongside plyometrics having greater benefits to injury prevention in this population (11,23). Following 12 weeks of plyometric training on female volleyball players, Enginsu et al (24) observed improvements in knee and hip joint kinematics as well as significant increases in performance tests. Short term programs have also seen significant differences but should not be a long-term solution. Pfile et al (25) reported significant differences in hip and knee joint kinetics and kinematics on teenage girls who performed either plyometric or core stability training after 4 weeks, suggesting both types of training may merit ACL injury prevention. In contrast, both groups displayed a decreased knee flexion angle following the training intervention, this would suggest that the subjects may have increased their chances of injury due to the smaller knee angle upon landing as this has been reported to be a high risk factor (23), therefore 4 weeks training may not be long enough to encourage new movement patterns.
Resistance training alone may not reduce the risk of injury as opposed to resistance training and alongside a balance and plyometric program (11,26). Balance and dynamic stabilisation exercises have been shown to improve the athlete’s ability to absorb force on single leg landings due to developed biomechanics (27). Since most athletes tend to jump of one leg, single leg exercises should take preference. Myer et al (11) compared two groups over 7 weeks of training; a balance training group and a plyometric training group, which were included alongside a neuromuscular training intervention. During single leg landings, the balance training group reduced their impact forces by 7% post intervention, while the plyometric group increased theirs by 7.6%. What is important from the aforementioned findings, both groups had significant improvements in their hamstring strength, as assessed via isokinetic dynamometer. The hamstring muscles have an important role in knee stabilisation due to coactivation between the agonist and antagonists, thus it is necessary for the hamstrings to assist the ligaments with joint stability, and limit anterior tibia translation (9,28). The coactivation patterns between athletes and non-athletes are significantly different (28), with the athletes having greater coactivation due to regular training. Furthermore, the hamstrings will act eccentrically to decelerate the athlete during landing exercises (9).
Jump and balance training has been shown to increase hamstring pre-activation amongst female basketball players (27). In addition, Bai et al (29) reported significant findings in the pre-activity of the hamstrings on healthy females during rotational jumps of 180º and 360º. Bai and colleagues (29) reported that hamstring pre-activity occurred before the quadriceps pre-activity and initial ground contact during rotational jumping. Rotational jump training may be an option for increasing hamstring pre-activation in an ACL injury prevention program and can be used as biomechanical focused training. Regarding biomechanics, Nagano et al (27) reported significant changes in athletes kinematics following 5 weeks of jump and balance training. Following the intervention, knee flexion angles increased from 19.5º to 24.4º upon foot contact during single leg landings. Knee flexion angles less than 30º during bi-lateral landings will increase the likelihood of ACL injury due to greater quadricep and less hamstring contractions (7,8). In contrast, knee flexion angles greater than 60º will increase hamstring activity, therefore reducing anterior tibia translation (9), which may assist with ACL injury prevention.
“resistance training alongside plyometric and balance training will further reduce the changes of ACL injuries in female athletes”Tweet
Rapidly slowing the body is a key to a faster re-acceleration as well as been capable of absorbing ground reaction forces that can led to injury. So it could be suggested that an ACL injury prevention program should be started as young as early teenage years, since most ACL ruptures tend to happen to females in their late adolescent years (32). Plyometric and balance training will lead to kinetic changes in the hips and kinematic changes in the knee joint which will be ideal for younger females, improving their biomechanics. Additionally, resistance training alongside plyometric and balance training will further reduce the changes of ACL injuries in this population. A great training program will have injury prevention incorporated into it, thus, making a stronger more powerful athlete while also reducing their chances of injury.
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