ABSTRACT Background Concussion and hamstring injuries are two of the most frequently diagnosed injuries in Rugby Union. Neck and hamstring strength have been identified as a modifiable risk factor to decrease the severity and incidence of these injuries. As such, there is an increasing interest in strengthening the neck and hamstrings in Rugby Union for injury prevention purposes. Aims The overarching aims of this thesis were to: 1. better understand the role of neck and hamstring strength and imbalances in the context of concussion, neck, and hamstring injury incidence and prevention; 2. scientifically document the status of neck and hamstring strength and imbalances in Rugby Union players using reliable methods; and 3. evaluate the impact of strengthening exercises and feedback strategies on strength outcomes in Rugby Union players. Overall, this thesis sought to shed some light on neck and hamstring strength assessment and strengthening practices to inform injury prevention strategies in Rugby Union players. Methods This thesis is divided into four sections. The first section comprises a systematic literature review addressing neck strength in Rugby Union, a cohort cross-sectional study that compares the two most common methods to test isometric neck strength (“make” versus “break”), and a reliability study of isometric neck strength measures in flexion, extension, and bilateral side flexion. The second section includes a systematic literature review on hamstring injury incidence, risk factors, and prevention in Rugby Union. Thereafter, the reliability of measures from a load cell device to assess Nordic hamstring strength and bilateral balance in Rugby Union players is examined. The third section characterises both neck and hamstring strength in Rugby Union players. Specifically, isometric neck and Nordic eccentric hamstring strength values are examined in a large cohort of male and female semi-professional and professional Rugby Union players. Finally, the fourth section involves a randomised-controlled trial with a control group that examined the effect of a six-week progressive Nordic eccentric and isometric hamstring strength training programme performed with or without visual feedback on hamstring strength and bilateral strength balance in Rugby Union players. Results The neck strength literature review highlighted no consensus regarding standardised test methods implemented in Rugby Union, and limited evidence regarding direct associations between neck strength and concussion. When experimentally comparing the break to the make test, there were no systematic or proportional differences between methods detected based on the Passing-Bablock procedure. Despite the results suggesting these two methods could be used interchangeably to assess isometric neck strength, the make test showed some procedural advantages (e.g., lower potential for test-related injuries, enhanced participant comfort and confidence, and reduced influence of the examiner). Hence, the make test was chosen as the primary neck strength assessment method in the subsequent studies of this thesis. The reliability study on isometric neck strength measures from the make test revealed good-to-excellent intrasession reliability (ICC=0.85–0.95) and fair intersession reliability (ICC=0.51–0.69) for mean and maximal values. The hamstring literature review concluded that it is likely worthwhile to combine strategies to prevent hamstring injuries and their recurrences in Rugby Union, and should include Nordic strength assessments and Nordic strengthening exercises. On this basis, Nordic assessment methods were chosen to assess hamstring strength in this thesis. The reliability study on Nordic eccentric strength assessment measures showed good intrasession reliability (ICC=0.79–0.90) and fair intersession reliability (ICC=0.52–0.64) for mean and maximal values. A total of 342 Rugby Union players (87% male) participated in the characterisation study. Regarding isometric neck strength, male professional players were significantly stronger in all directions than semi-professional players except in extension; and overall, forwards (range:22–39 kg) were significantly stronger than backs (range: 22–31 kg) in all directions. Flexionto-extension ratio was 0.76 and side flexion ratio was 1.0 in males. There were no significant differences between forwards (range: 14–23 kg) and backs (range: 14–22 kg) in semiprofessional females, except in extension (backs > forwards). Flexion-to-extension ratio was 0.83 and side flexion ratio of 1.0 in females. Considering Nordic eccentric strength, there male professionals were stronger than semi-professional players in terms of absolute strength (422.1 vs 398.7 N), relative to body mass was greater in professionals than semi-professionals (4.57 vs 3.87 N·kg⁻¹). Absolute strength was significantly greater in forwards than backs (426.8vs 387.2 N); but relative to body mass, strength was lower (3.78 vs 4.67 N·kg⁻¹). In females, the strength values ranged from 226 ± 41N and 2.8 ± 0.5N·kg⁻¹ to 320 ± 1.0N and 3.8 ± 0.6N·kg⁻¹, forwards ranged from 255 ± 107N to 320 ± 1.0N whilst backs ranged from 226 ± 41N to 295 ± 65N. Regarding the four section a randomised-controlled trial that analysed the effectiveness of a six-week isometric hamstring and Nordic eccentric training performed with or without visual feedback on hamstring strength and bilateral strength balance in Rugby Union players. The time x group interaction from the repeated measures ANOVA was not significant for both peak strength and bilateral strength balance measures, indicating similar changes between groups from Pre to Post timepoints. Additionally, there was no main effect of time on outcomes, indicating no significant difference in strength or balance measures from Pre to Post timepoints. There was a significant main effect of group on peak strength, with controls identified as being significantly weaker than the other two groups during post-hoc pairwise comparisons (94.0 N weaker vs non-feedback and 89.3 N weaker vs feedback). There was no significant main effect of group on balance. Conclusion This thesis presents a compilation of studies focusing on neck and hamstring strength as modifiable risk factors for concussion, neck, and hamstring injuries in Rugby Union. This thesis provides data on isometric neck and Nordic hamstring eccentric strength values using two feasible and reliable test methods that use load cell devices. The data collected from semiprofessional male, professional male, and semi-professional female Rugby Union players can be used to inform practice and establish baseline values for practitioners interested in rehabilitation, return to play, and strengthening of neck and hamstring in Rugby Union. The six-week progressive Nordic eccentric and isometric hamstring strength training programme was not superior to usual training (control group) for improving eccentric bilateral strength or decreasing bilateral strength imbalances, warranting exploration of alternative strength training programmes and strategies.