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The mechanism of hamstring injuries–a systematic review. 

Danielsson, A., Horvath, A., Senorski, C., Alentorn-Geli, E., Garrett, W.E., Cugat, R., Samuelsson, K. and Hamrin Senorski, E., 2020. The mechanism of hamstring injuries–a systematic review. BMC musculoskeletal disorders, 21, pp.1-21.

In today’s letter

• Overview of the hamstring muscle injury mechanism

• Rapid Results = Findings from studies investigating stretch-type injury mechanisms consistently indicate that injuries result from pronounced hip flexion (forward leaning) coupled with knee hyperextension

• Professional takeaway = Pelvic control, hip flexor strength nordic hamstring exercise - Are all good starting points for strengthening & prevention

Whats new is SportsMed U

Aim of study

• To Conduct a systematic review to explore the mechanism of hamstring injuries

• Investigating the systematic review of hamstring injury mechanisms

Background info

• Hamstring injuries exhibit a common occurrence across various sports, with an incidence ranging from 1.2 to 4 injuries per 1000 hours of athlete exposure.

• In disciplines such as athletics and Gaelic football, hamstring injuries contribute to 17-21% of total reported injuries, and it is estimated that around 22% of football players sustain such injuries each season.

• On average, hamstring injuries lead to a time loss of 24 days, impacting athletes' participation and performance.

• Despite the prevalence of hamstring injuries, there is no consensus on the specific mechanism behind them. Askling et al. proposed two scenarios, identifying high-speed running and stretching movements as potential causes.

• High-speed running injuries predominantly affect the long head of the biceps femoris and generally have a shorter recovery time compared to stretching-type injuries, which often affect the semimembranosus muscle.

• Notably, in Australian football, 81% of hamstring injuries occur during sprinting (running type), while kicking (stretching type) contributes to 19% of reported injuries

 Methods

• Eligibility criteria for this systematic review included an evaluation of all original studies exploring the mechanism of hamstring injury or the biomechanical properties of the hamstrings.

• The systematic review specifically excluded hamstring injuries with avulsion fractures, defining hamstring injury as a strain injury to the hamstring muscle group

Results

• A comprehensive database search retrieved a total of 4264 studies, with 318 from the Cochrane Library, 2053 from EMBASE, and 1893 from PubMed.

• Following the elimination of 1423 duplicate studies, the remaining 2841 underwent screening based on abstract and title to determine eligibility.

• Eligible studies, identified through the initial screening, underwent a thorough full-text assessment, leading to the inclusion of 21 studies in the final systematic review.

• In the course of the full-text assessment, an additional 52 previously unidentified studies were discovered through reference lists, with five of them meeting the criteria for inclusion in the systematic review

Discussion

• In studies examining runners, a frequently proposed injury mechanism is the occurrence of eccentric strain, particularly during the late swing phase of the running gait cycle.

Stretch-related hamstring injuries

• Findings from studies on stretch-type injuries consistently indicate that these injuries are a result of extensive hip flexion combined with simultaneous knee extension.

• In Australian football, 19% of hamstring injuries occur during kicking, a typical stretch-type injury where the end of a kick involves both flexed hips and extended knees

• Additionally, Worth et al proposes that attempting to pick up a ball from the ground while running at full speed is a prevalent situation leading to hamstring injuries in Australian football.

• It's important to note that researchers did not directly observe any of the hamstring injuries; instead, patients recalled the injury situations, introducing a potential bias. Therefore, caution is advised in interpreting findings related to stretch-type hamstring injuries.

Hamstring injuries during running

• The majority of studies addressing hamstring injuries during running consistently highlight the late swing phase as the period when hamstrings are most susceptible to injury, primarily due to eccentric loading.

• Contradictorily, some studies propose that hamstring injuries are more likely to occur during the stance phase, challenging the consensus on injury timing.

• Authors of certain studies specifically attribute hamstring injuries to the late swing phase, basing their conclusions on early signs of injury, including neuromuscular latencies, and assessing factors such as hamstring length, force, velocity, and negative work.

• Supporting this perspective, a recent literature review aligns with the idea that hamstring injuries during the late swing phase result from elevated muscle excitation and strain.

• Findings from a separate study indicate that a forward trunk lean during the stance phase, particularly with a fully extended knee, increases strain on hamstring muscles, resembling the stretch-type injury mechanism.

• Notably, the impact of a forward trunk lean, elongating the hamstrings and intensifying strain, is attributed to poor activation and control of core and hip muscles, heightening the risk of hamstring injuries.

• Static stretching emerges as a potential preventive measure, reducing ground reaction forces in the early stance phase and decreasing strain on the biceps femoris long head during the late swing phase.

• The reduction in ground reaction forces leads to diminished peak joint torque at the hip and knee, accompanied by increased force productions of the biceps femoris at longer muscle lengths, suggesting that stretching may mitigate hamstring injury risks.

  **But have performance detriments? - Link 

• These findings gain significance as preventive studies on the Nordic hamstring exercise, focusing on eccentric training, demonstrate efficacy in reducing the risk of hamstring injuries.

• The preventive effect of the Nordic hamstring exercise is linked to its ability to increase muscle fascicle length, addressing the association between short hamstring fascicles and an elevated risk of hamstring injury.

• Despite the prevailing understanding that hamstring injuries during running occur predominantly in the late swing phase, further research is essential to confirm and deepen these insights.

• Notably, the estimated occurrence of hamstring injuries during the late swing phase is attributed to heightened strain on the hamstring muscles while running or sprinting.

• The need for additional research persists to substantiate and expand upon the existing knowledge regarding the timing and mechanisms of hamstring injuries during running. 

Strength-related hamstring injuries

• Retrospective studies examining hamstring strength in relation to injury mechanism yield inconclusive results.

• Fatigue, as reported in one study, was linked to a reduction in eccentric hamstring strength, potentially elevating the risk of hamstring injuries, while lower hamstring strength endurance was associated with a heightened likelihood of hamstring re-injury.

• A study comparing muscle activity in athletes with previously injured and uninjured hamstrings revealed that those with prior injuries exhibited lesser hamstring activation, contributing to diminished overall hamstring strength.

• The observed findings are likely more connected to risk factors for subsequent injuries, potentially offering insights to enhance rehabilitation strategies, rather than directly informing the mechanism of hamstring injuries.

Limitations

• The prevailing approach in the majority of studies involves drawing conclusions based on estimations of the hamstring injury mechanism.

• The publications addressing the hamstring injury mechanism are limited in number, and diverse methods have been employed to assess this mechanism.

• Each study group included in the analysis consists of a limited number of studies, each with distinct methodological limitations, contributing to uncertainties in the systematic review's results.

• Several biomechanical studies were excluded from consideration, as they did not provide conclusive insights into the mechanism of hamstring injury.

Conclusion

• “A stretch-type injury to the hamstrings is caused by extensive hip flexion with an extended knee. Hamstring injuries during sprinting are most likely to occur due to excessive muscle strain caused by eccentric contraction during the late swing phase of the running gait cycle”

Clinical Implication

• Comprehensive understanding of this injury type is crucial and can be applied in hamstring injury prevention and rehabilitation programs, emphasising the incorporation of hip and core strengthening exercises alongside conventional hamstring exercises.

• Key components for consideration in such programs include Nordic hamstring curls and the integration of proprioception, neuromuscular control, and velocity-based training techniques