Modifiable risk factors of hamstring strain injury : assessment, performance and training

Ripley, N ORCID: 2021, Modifiable risk factors of hamstring strain injury : assessment, performance and training , PhD thesis, University of Salford.

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Hamstring strain injuries (HSIs), which are typically sustained during high velocity movements (i.e., sprinting or high velocity running), commonly involves the bicep femoris long head (BFLH). To date, the Nordic hamstring exercise (NHE) is the most effective training modality for reducing HSIs within team sports, however, compliance to training prescription is commonly low which can influence the effectiveness of the intervention. Alternative training modalities have been advocated within the literature as being effective at decreasing the risk of sustaining a future HSI by increased eccentric hamstring strength and BFLH fascicle length (FL). There are several alternative training modalities, including hip dominant exercises and sprinting, which are relatively unexplored within the literature, with between just one and two previous investigations of these modalities, respectively. Additionally, previous studies commonly lack ecological validity, whereby the use of a single exercise would be unusual within sport practice. The overarching aim of this thesis was to observe the HSI modifiable risk factors within a cyclical-practice approach, which could be used within practice, this includes measurement and identification, identifying their influence on performance and observing the effect of known exercises and training. With specific objectives which will aim to answer gaps within the literature, the objectives were to firstly examine the methods used to assess BFLH FL (Chapter 3 and 4). Secondly, the thesis investigated potential mechanisms of HSIs during running when accounting for eccentric hamstring strength and BFLH FL (Chapter 5). Thirdly, the thesis identified characteristics such as kinematics, electromyography (EMG) and in-vivo dynamics of the NHE and variations (Chapter 6). Finally, the thesis observed the effect of a short-term (seven-week), lower-limb resistance training intervention including a hip dominant exercise, with the addition of the either the NHE or sprinting, on eccentric hamstring strength, BFLH FL and measures of athletic performance (Chapter 7). There is a potential for large degree of error in the measurement of BFLH FL, therefore, investigations were required to determine the most appropriate estimation equation (if required) and imaging field of view (FOV). Therefore, the focus of Chapter 3 and 4, was to determine the most reliable, accurate and low-cost methods of assessing BFLH FL using the ultrasound (US). Between three commonly used methods to estimate BFLH FL, Chapter 3 demonstrates trivial differences between all methods, in 13 male team sport athletes (left and right BFLH n = 26), although the methods cannot be used interchangeably. Chapter 4 aimed to identify if any differences exist between different imaging FOV (6-cm vs. 10-cm) within 16 male team sport athletes (left and right BFLH n = 32). Trivial to small differences were noted between 6-cm and 10-cm FOV, however, there was minimal agreement and incongruency between the two FOV. Although, one estimation equation presented congruent differences between FOV, with the 6-cm FOV consistently overestimating BFLH. In Chapter 5, physically active males (n = 18) were grouped (high- and low-risk) by relative measures of eccentric hamstring strength and BFLH FL, due to nearly perfect associations between the two measures observed within the present thesis. Peak and waveform kinematic and hamstring EMG characteristics were observed during treadmill running at various running velocities (8-16 kmhr-1). Significant and meaningful differences were observed between the two groups for peak and waveform knee and hip kinematics and BFLH EMG measurements. The differences observed in Chapter 5 could be defined as being unfavourable, especially as the differences occur both within the take off and late swing phases, as being defined as areas of interest for the incidence of HSI, with alterations in pelvic control and velocity and activation of the hamstrings under lengthening conditions. The NHE is one of the most extensively researched hamstring exercises, however, to date it is currently unknown what is occurring within the muscle during the exercise and potential variations. Within Chapter 6, kinematic, EMG and in-vivo ultrasound imaging was performed, while 13 participants (10 males, 3 females) with resistance training experience, performed NHE variations (0, -20 and +20 horizontal plane performance angles). There were significant and meaningful differences in kinematic, neuromuscular and BFLH in vivo muscle mechanics, with changes in NHE performance angle manipulating the lever arm through which the centre of mass from the knee up is acting. Differences in instantaneous knee angle, MTU length and neuromuscular contributions of the BFLH and the ST to the task at break point could be related to the altered starting position of each performance angle. There were meaningful differences identified within the early-mid range of movement (0-40% time normalized to break point between variations), where greater fascicle shortening was observed within the decline and flat NHE variations. This is potentially explained by the contractile components within the BFLH, to take up more slack which would be present within the elastic component (i.e., distal tendon), which is under less strain within the early stages of the movement. Whereas there were no likely or meaningful differences identified between variations at the mid-end range (40-100% time). Therefore, all variations result in a similar magnitude of relative fascicle lengthening, which may indicate that similar positive adaptations in eccentric hamstring strength and BFLH FL would be attained from there utilisation. Finally, seven-weeks of lower limb resistance training alone (control n = 10), incorporating a hip dominant exercise alone, results in significant and meaningful increases in eccentric hamstring strength and BFLH FL. However, both the addition of either the low volumes of NHE (n = 15, 2 x 4 with progressive intensity, twice per week) or sprinting (n = 13, 200-350 m per week) resulted in a greater magnitude of increase in eccentric hamstring strength and BFLH FL. The NHE was the more effective at increasing both eccentric hamstring strength and BFLH FL than sprinting, however, the multi-modal approach (NHE or sprinting plus a hip dominant based exercise) as used within the present thesis and would be more frequent in elite practice, is superiorly effective than using the single modality alone. Additionally, and unsurprisingly, a seven-week lower-limb resistance training program was effective at increasing measures of athletic performance (sprint, jump and isometric mid-thigh pull), while simultaneously only inducing low-moderate delayed onset muscle soreness.

Item Type: Thesis (PhD)
Contributors: McMahon, JJ (Supervisor) and Comfort, P (Supervisor)
Schools: Schools > School of Health and Society
Depositing User: Nicholas Ripley
Date Deposited: 06 Jul 2021 13:48
Last Modified: 06 Jul 2021 13:48

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