Lower limb muscle activation during sprinting and hamstring strength training exercises : Implications for mitigating hamstring injury risk in Rugby Union players

This research investigates lower limb muscle activation during sprinting and training exercises in a sample of high level rurgby players. Furthermore, the research aims to establish the effect of load on muscle activation during commonly employed hamstring exercises.

Hamstring strain injuries(HSIs) are highly prevalent in sprinting-based sports, including rugby union, and have a high rate of recurrence, indicating the importance of injury prevention programmes. Exercise-based interventions are commonly used as a means of decreasing HSI incidence; however, the persistent injury rates suggests that current injury prevention practices could be improved. Comparing lower limb muscle activity during sprinting and hamstring exercises could improve exercise specificity and better inform exercise selection for training programmes aimed at minimising HSI risk. Therefore, the aim of this thesis was to examine the pattern and magnitude of lower limb muscle activation during sprinting and unloaded and loaded hamstring strength training exercises in rugby union players. 

The first three studies were conducted with male players from an international rugby seven’s team. Study 1 (Chapter 4) analysed the activity of biceps femoris long head (BFlh), semitendinosus (ST), gluteus maximus (GM), rectus femoris (RF) and medial gastrocnemius (MG) during the early stance and late swing phases of sprinting in international rugby seven’s players (n=  5). No significant interactions or main effects were observed for peak muscle activity for sprint phase and muscle (all p≥0.05). A large (d≥ 0.80) and small mean difference (d≤0.2-0.49) was observed between BFlh and ST peak activity during early stance and late swing respectively, with higher BFlh activity relative to ST being observed. A significant main effect (p≤ 0.001) for integrated electromyography (iEMG) was observed for sprint phase only with higher iEMG occurring during the late swing phase compared to early stance, this was associated with a large effect size (η2≥ 0.14). Overall, there was a trend for higher BFlh activity during sprinting compared to ST which may contribute to the BFlh muscle’s susceptibility to injury and the higher incidence of injury observed in the lateral hamstring.

Study 2 (Chapter 5) examined lower limb muscle activity during a series of hamstring strength training exercises including the Nordic hamstring exercise (NHE), single leg prone hamstring curl, single leg bridge, slider and single leg Roman chair hold (peak activity n = 7; iEMG n = 5). A significant interaction (p≤ 0.05), and a large effect size (η2≥ 0.14) for exercise and muscle was observed for all findingsand significant main effects (p≤ 0.05) and large effect sizes (η2≥ 0.14) for normalised peak activity were observed for exercise and muscle (p≤ 0.05) and co- activation iEMG. Significantly  greater BFlh and GM activity was observed completing the single leg Roman chair hold exercise with a weight-lifting bar when compared to using body weight only. The single leg Roman chair hold bar exercise generatedthe highest normalised iEMG and peak activity for BFlh and GM, with the peak activation exceeding the 100% reference value of sprinting. Collectively, BFlh activation was higher than ST during the majority of exercises and the highest ST peak and iEMG was generated during asingle leg prone hamstring curl and single leg bridge exercises respectively. These findings suggest that these exercises could be considered for hamstring training programmes.

Study 3 (Chapter 6) investigated the relationship between muscle activation during different strength training exercises and sprinting (peak activity n = 7; iEMG n = 5). Limited significant relationships were observed for peak muscle activity and no significant findings were observed for iEMG; this was influenced by the small sample size. Biceps femoris long head activation during late swing and training exercises demonstrated a trend of positive relationships (peak activity r = 0.08 to0.58; iEMG r = 0.02 to0.65) while negative relationships were largely seen between BFlh activity during the early stance phase and exercises (peak activity r = -0.18 to-0.26; iEMG r = -0.08 to-0.57). The majority of exercises demonstrated positive relationships with sprinting for peak ST activity. The iEMG of ST during early stance showed a negative relationship with all exercises, while positive relationships were observed between ST iEMG during the late swing phase and all exercises. Overall, the findings demonstrated that hamstring muscle activity during the early stance and late swing phases of  sprinting demonstrated stronger relationships with exercises that were not eccentrically biased. 

Study 4 (Chapter 7) investigated the effect of load on lower limb muscle activation during hamstring strength training exercises in British University and College Sport (BUCS) rugby union players (n =  30). The exercises analysed included a double leg prone hamstring curl, single leg bridge and single leg Roman chair hold and three different loads were used for each exercise. The results for normalised peak activity and iEMG showed a significant interaction (p≤ 0.001) and a large effect size (η2≥ 0.14) for load and exercise, and exercise and muscle. Significant main effects (p≤ 0.05) were observed for muscle, exercise and load and all significant main effects were associated with large effect  sizes (η2≥ 0.14). The  majority of exercises generated a minimum of 70% of peak activity for the BFlh and ST muscles and ST activation was greater relative to BFlh during all exercises analysed. A continued increase in muscle activation in response to increased loading was observed during the double leg prone hamstring curl and single leg Roman chair. Loading did not however have a significant influence on muscle activation during the single leg bridgeand it generated the highest peak BFlh and ST activity, with the medial hamstring reaching values in excess of 100%. 

Collectively, the findings illustrate the single leg bridge as the exercise of choice to generate high levels of BFlh and ST activity. The findings of this thesis extend current knowledge regarding lower limb muscle activation during hamstring strength training exercises and the effect of load on activation relative to sprinting in rugby union players. To confirm the potential of the exercises identified in this thesis, additional research using different populations is necessary to further increase our understanding of hamstring activity during exercises and to better inform HSI prevention strategies.