Brief Summary
McNally et al 2023
Hamstring strain injuries (HSI) pose significant challenges in terms of performance, economic impact, and player availability across various sports. In Australian football, they account for the highest percentage of time lost due to injuries, constituting up to 15% of all injuries in rugby and 19% in professional football. Despite recent efforts to implement strategies for reducing HSI risk, the associated costs have continued to rise. This increase is primarily attributed to injury cases doubling from 2001 to 2022 and the compounding effect of escalating athlete salaries.
Despite an improved understanding of risk factors, HSI recurrence rates remain elevated and subsequent injuries often result in more significant impairments and prolonged recovery periods for athletes. Together these HSI trends suggest current screening and rehabilitation practices are insufficient.
An improved understanding of the demands placed on the hamstring muscle group and its synergists during common injury mechanisms could facilitate the development of more targeted prevention and rehabilitation strategies. As a result, recent research has investigated hamstring muscle activation and related joint kinetics across a range of locomotor speeds and tasks in an attempt to ensure athletes are exposed to the necessary gym and field-based loads during general conditioning and specific rehabilitation programs.
This review consolidates existing literature on hamstring activation and associated kinetic demands during high-speed running. Professionals in performance and rehabilitation can utilise these data to formulate evidence-based conditioning and rehabilitation programs.
EMG Studies
Analysis of five studies revealed diverse patterns of muscle activation during running. Some studies describe a relatively linear relationship, with 66% of maximal run speed equating to ~ 66% of maximal lateral hamstring activation. Others report a nonlinear relationship, with 70% of maximal run speed corresponding to approximately 30% of maximal medial hamstring activation. In several instances, sub-maximal running speeds resulted in activation levels surpassing 100% of maximum voluntary isometric contraction (MVIC). In this work, the highest activation level reported during maximal sprinting was 145% of MVIC for the lateral hamstrings.
These data sets are difficult to compare due to differing normalisation. While some authors have normalised to maximum EMG levels expressed during sprinting, others have utilised MVIC. In the case of the latter, authors have also utilised significantly different protocols with joint angles and both the point and degree of fixation varying greatly. Clearly, more research with better control of EMG normalisation and MVIC test protocols is required. However, while it is difficult to draw definitive conclusions about the correlation between sub-maximal running speeds and regional hamstring activation from these EMG data alone, consideration of kinetic studies can assist the clinician in understanding locomotor demands and therefore conditioning and rehabilitation needs.
Kinetic Studies
The examination of athletes running at sprinting speeds in this review describes knee flexion jointmoments of up to 2.9 Nm/kg at a running speed of 9.8 m/s and hip extension moments of 8.2 Nm/kg at 9.7 m/s. The authors highlight the more conservative data gleaned from subject specific modelling where joint moments of 2.2 Nm/Kg and 4.8 Nm/Kg have been provided for knee flexion and hip extension respectively with subjects running at above 8.5m/s.
These sagittal plane kinetic demands across the knee and hip provide additional context for clinicians looking to determine if athletes have sufficient capacity to generate sufficient torque across the knee and hip to prepare them for sprinting and / or return to play. For instance, they may choose to recommend threshold levels of MVIC strength are reached without symptomology, across both the knee and hip, prior to progressing beyond target straight line running speeds and / or moving into more uncontrolled ballistic training drills where even high torques are produced.
Table 1 from McNally et al 2023 highlights the increases in torque across the knee and hip associated with increasing locomotor speed. Note that torque across the knee and therefore hamstring demands above 7m/s increase exponentially.
Importantly, these points of reference and appropriate supporting technology can assist clinicians seeking to expand their use of monitoring / testing strength data from a rudimentary ‘is the athlete back to their individual pre-injury strength levels’ to:
- ‘can the athlete generate torques required during late swing phase when sprinting’,
- ‘does the athlete have the capacity to produce these torques for an entire game’,
- ‘how does the athlete compare to appropriately normalised data for ‘like athletes’ with no HSI history / high levels of resilience’.
Medical and Performance Practitioners can utilise KT360 to screen and monitor knee flexion and hip extension strength and the fatigability of these muscle groups accurately. The system also normalises these data and allows an athlete’s strength, fatigability and training efficacy to be tracked over time and compared to relevant populations and subgroups as required.
While the focus of this article review has remained limited to the risk associated with sprinting, HSI’s also occur during lunging, kicking and change direction events when lumbo-pelvic rotation and hip abduction / rotation mechanics differ significantly. These mechanisms and relative states of fatigue are significantly different and likely to vary, placing altered demands on the hamstring muscle group and its synergists. As such, utilising KT360 to assess the neuromuscular capacity of other lower limb muscle groups is often useful in understanding hamstring and general lower limb injury risk.
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