Football player’s strength and speed has been widely investigated and correlations found between the variables, with both modifiable factors for ACL and hamstring strains. Fatigue exacerbates muscular weaknesses and asymmetries, influencing injury susceptibility. The study aimed to investigate the effect of a football-specific fatigue protocol (SAFT45) on concentric and eccentric quadriceps and hamstring strength, H:Q muscular imbalances, concentric hip extensor strength, hip flexor to extensor muscular imbalances, limb asymmetries and sprint speed, in relation to ACL and hamstring injury risk. Thirteen male footballers, currently competing in the British University Midlands leagues (age: 20.46 ± 1.51 years; height: 178.47 ± 5.31 cm; mass: 74.38 ± 8.83 kg; football playing age 11.08 ± 3.82 years) volunteered to participate, and participants completed two training sessions (60-90 minutes) and one match (90 minutes) per week. Any participants currently injured or with significant hamstring or ACL injury causing over 4 months out was excluded from the study. Having gained institutional ethics approval, participants were tested once, completing a crossover counterbalanced study of maximal isokinetic contractions for concentric and eccentric knee extensors; knee flexors and concentric hip extensors followed by 30 m maximal sprints, pre and post-SAFT45. A significant difference was reported between pre and post-SAFT45 10 m, 20 m and 30 m sprint performance (all p<.05). Significant differences existed between pre and post-SAFT45 NDL conHiFl:conHiEx ratio (p<.05) showing increased conHiEx:conHiFl post-SAFT45. Linear regression identified DL and NDL H:Q ratios, Hcon, Hecc, Qcon and Qecc limb asymmetries are significant predictors of post-SAFT45 30 m sprints (R2=.993, p=.032). Pearson’s correlation identified no significant relationships between DL or NDL H:Q ratio and conHiFl:conHiEx ratio (all p>.05). Pearson’s correlation identified strong, negative and significant relationships between post-SAFT45 NDL to DL Qcon asymmetries at 60o/s and 10 m (R=-.747, p=.003), 20 m (R=-.827, p<.001) and 30m sprint time (R=-.826, p<.001), however no relationships identified between pre-SAFT45 sprint times and any variables (all p>.05). Pre-SAFT45 and post-SAFT45 no relationship exists between H:Q and conHiFl:conHiEx ratios or asymmetries. The results suggest fatigue to not influence muscular force production, however influencing sprint performance indicating hamstring and ACL injuries to be multifactorial. For future studies, SAFT45 could include football-specific actions such as kicking, tackling, and jumping to closer replicate match-play and elicit muscular fatigue to better assess injury susceptibility.
