Biomechanical response to hamstring muscle strain injury
Introduction
Hamstring strains are common injuries [1], [2], most of which occur whilst sprinting [2], [3]. In order to optimise the rehabilitation and prevention of hamstring strains, exercise interventions must be specific to the mechanism of injury [4]. An understanding of the biomechanical conditions that cause the hamstrings to fail during sprinting is therefore of clinical significance.
The hamstrings are active throughout terminal swing and initial stance of the sprinting cycle [5], [6], [7]. Conjecture exists regarding the precise point when hamstring strains occur [4], [8]. Some researchers have argued that the hamstrings are most biomechanically susceptible to injury during terminal swing [7], [9], [10], [11]. Others have proposed initial stance to be the critical point [12], [13]. As all of these studies are based on either theoretical rationale [10] or analyses of asymptomatic subjects [7], [9], [11], [12], [13], they are unable to definitively establish when in the sprinting cycle the hamstrings fail.
In order to identify the biomechanical circumstances that lead to injury during sprinting, in vivo experimental data of an acute hamstring strain are ideally required. Unfortunately, such data are virtually impossible to generate practically. It is therefore not surprising that there exists only one published case study reporting biomechanical data of a running athlete captured at the time of an acute hamstring strain [14]. A 130 ms interval during terminal swing was identified in this study as the most likely time of injury. Whilst a unique insight into the potential timing of hamstring strains was obtained, the study was associated with several limitations. First, the injury occurred whilst the subject was running at a sub-maximal speed (5.36 m/s) on an inclined treadmill. Whether these results can be generalised to overground sprinting is difficult to determine. Second, as the subject was running on a treadmill, no ground reaction force (GRF) data were collected and thus relevant stance phase dynamics were not considered. Finally, the results are limited to the single subject evaluated. Before the conclusions can be scientifically accepted, verification by further independent experimental investigations is required.
In the current study, bilateral kinematic and GRF data were captured from a sprinting athlete prior to and immediately following a right hamstring strain. These data were obtained unexpectedly during a routine quantitative gait analysis assessment conducted prior to the athlete returning to competition following previous right hamstring strains. The specific aims were to: (a) investigate whether pre-injury biomechanical asymmetries existed; (b) evaluate the biomechanical response to the injury and; (c) identify the timing and segmental location of the initial response. It was anticipated that this information would prove useful for generating hypotheses regarding the likely time of occurrence of the injury.
Section snippets
Subject
The subject was an elite Australian Rules male football player (height: 186.0 cm; body mass: 91.5 kg; age: 20.3 years). Written informed consent was obtained to analyse data for research purposes and approval was obtained from the institutional Human Research Ethics Committee. The subject was participating in a quantitative gait analysis assessment. He was suffering from recurrent right hamstring strains. The first injury occurred 67 days prior to the assessment and was re-aggravated 42 days
Pre-injury trials
The subject's average (±1S.D.) sprinting speed for the nine pre-injury trials was 7.44 ± 0.10 m/s. The critical period during sprinting for understanding hamstring muscle function is from mid-swing until mid-stance. From mid-swing onwards, the hip and knee joints were both extending, with knee extension occurring at a faster rate than hip extension. During terminal swing, the hip continued to extend, whilst the knee reached peak extension and began flexing just prior to foot-strike (Fig. 1, top
Discussion
Given that the subject had recently suffered two right hamstring strains but had never injured his left hamstring, the quantitative gait analysis assessment was conducted to determine if biomechanical asymmetries were present in his sprinting gait. It was thought that such knowledge might prove useful clinically to identify potential contributing factors and develop subject-specific therapeutic interventions. Interestingly, asymmetries were found in the data for the pre-injury trials (Table 1
Conflict of interest statement
There are no conflicts of interest associated with this research.
Acknowledgements
There were no sources of funding associated with this research. We wish to thank Professor David L. Morgan for reviewing an earlier version of this manuscript.
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