During the time course of an athlete’s career there is a high risk of suffering a significant knee or lower limb injury during sports participation. Over the past four years, the cornerstone of my research has been looking at anterior cruciate ligament (ACL) and hamstring strain injuries (HSI) in sport. Many of you may be thinking, “what’s the big deal?”, but what if I told you, that in Australia, ACL injuries cost professional sporting organisations an estimated AUD$17 000 – 25 000 per athlete for surgery and rehabilitation (2, 3), while HSIs cost sport organisations millions of dollars (4, 5).
Understanding lower limb injury
Both ACL and hamstring injuries are major problems in sport and despite some progress in the development of injury prevention and rehabilitation practices, the incidence and recurrence rates of each are still high. Together, hamstring strain and ACL injuries comprise the injuries with the highest incidence (most common) and prevalence (most severe) in sport, respectively (6) (Figure 1). Although, recent trends in elite male professional football suggest that ACL (7) and hamstring injury rates (8) are not declining. However, results may vary in different sports and at different levels of competition.
The hamstring muscle complex
The hamstring muscle complex describes a group of three muscles located on the posterior thigh – semitendinosus, semimembranosus, and biceps femoris; which is further divided into a long and a short head (9) (Figure 2). With the exclusion of biceps femoris short head, the hamstrings are biarticular muscles because they cross both the posterior aspects of the hip and knee joints (10), which allow the long hamstrings to perform flexion at the knee and extension of the hip during concentric contraction. Collectively, the hamstrings’ common actions are hip extensors and knee flexors; however, each muscle exhibits significant differences in morphology (e.g., physiological cross sectional area), architecture (e.g., fascicle length and pennation angles) and function (e.g., activation patterns) (9, 11). Hamstring muscles play important roles in running and side-cutting movements which are sometimes associated with hamstring strain and ACL injuries (12, 13). Eccentric strength training for the hamstrings plays an important role in reducing strain injuries to those muscles (14-16) and the knee flexors are also the primary form of muscular support for the ACL, acting to stabilise the knee and prevent excessive anterior tibial translation (17-19). Consequently, strengthening the knee flexors is considered an important component of ACL (19, 20) and hamstring injury prevention practices (15, 16, 21).
MECHANISMS(S) OF HAMSTRING STRAIN AND ACL INJURIES
HSIs usually involve damage to the muscle tendon unit and are thought to be caused by forceful eccentric contraction or from an excessive stretch of the muscle tendon unit (22). The proposed mechanisms for HSIs include kicking, tackling, cutting, stooping while running and excessive stretching (23-26). Though, the majority of HSIs in running based sports occur during high-speed activities (25-29). During eccentric contractions at medium to long muscle lengths, there are moderate to high levels of muscle strain (muscle length) (30). Strong eccentric actions also expose muscle-tendon units to high degrees of muscle stress, measured as force per unit of cross-sectional area (30).
Most ACL tears are non-contact injuries that occur during landing, changes of direction or deceleration while hip adduction, internal femoral rotation, external tibial rotation and knee valgus is often identifiable at the time of injury (31, 32). ACL reconstructions are almost always considered essential for restoring adequate knee joint stability (33, 34), and they usually involve one of two autogenous grafts harvested from either the ipsilateral semitendinosus and gracilis or the patella tendon (35-37). However, the semitendinosus muscle provides support to the ACL by resisting external tibial rotation and anterior tibial translation (38, 39), and the impact of semitendinosus tendon grafts on the long term function of this muscle is unclear (40).
A PREVIOUS LOWER LIMB INJURY IS THE BIGGEST RISK FACTOR FOR A SUBSEQUENT INJURY IN THE FUTURE
A history of HSI is a very strong predictor of future HSI (41-44). One prospective study in elite soccer players has shown that players who suffered a HSI in the previous season were 11.6 times more likely to sustain a recurrent HSI in the subsequent season (45). The mechanism(s) increasing the risk of future HSI in individuals with a history of this injury remain unclear; however, it is likely to occur from the contribution of a number of maladaptation’s following HSI (46, 47) or the persistence of risk factors that contributed to the initial injury (48-50). Meanwhile, a history of ACL injury is also associated with an increased risk of a new knee injury (51, 52). A history of ACL injury is a strong risk factor for subsequent re-injury and this is due to many factors, including; sub-optimal surgery, muscular weakness and imbalances, weakening of ligaments, altered kinematics and diminished proprioception after initial injury (53-55). A history of ACL injury results in deficits in proprioception and range of motion which may alter the coordination of previously learnt movements (56, 57). Therefore, it is possible that the common hamstring graft reconstruction technique leaves athletes more prone to hamstring strain injury. The biomechanical changes that occur after ACL injury (41, 58) and hamstring muscle weakness (59) are thought to contribute to the increased risk of future hamstring injury after ACL reconstruction and rehabilitation.
KEY TAKE HOME MESSAGES:
- A prior ACL injury is the strongest predictor of future hamstring strain injury, followed by a recent (12 month) history of hamstring strain injury.
- Athletes who undergo an ACL reconstruction (irrespective of graft technique) have an augmented risk of future hamstring strain injury if they have also suffered a recent (12 month) history of hamstring strain injury.
- ACL rehabilitation programs need to be improved given athletes are at significantly elevated risk of hamstring strain injury despite successfully returning to sport, often with ~12 months of rehabilitation.
By Daniel Messer.
Anyone who would like to know more information regarding exercise selection in ACL and hamstring injury prevention and rehabilitation programs should contact Daniel.
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