Influence of fatigue on sprint acceleration mechanics: is there a connection with hamstring injury?

Introduction The relationship between hamstring injury and their capacity to produce a force are no longer to demonstrate: decreased ability to produce strength after muscle injury [1], increased risk of injury in case of weakness and/or isokinetic imbalance [2-4], eccentric strength weakness increasing the risk of muscle damage [5-7],… Several studies have also highlighted the fact that fatigue induced by sports activities would increase the risk of hamstring injuries [8, 9]. During a sprint, the ability to orient the forces horizontally, telltale of the effectiveness of the foot strikes [10], is related to the force production capacity of the hamstrings [11]. Moreover, two studies, a case report [12] and a preliminary study [13], seem to indicate that an alteration in horizontal force production during sprint occurs before and after hamstring injury. Is a progressive induction of fatigue lead to a decrease in the athlete's ability to produce horizontally oriented forces during a sprint and in this case could increase the risk of injury? Methods Seven amateur soccer player (22.7 ± 1.3 years, 179.3 ± 5.5 cm, 75.4 ± 4.6 kg) realized the Soccer-specific Aerobic Field Test (SAFT90) [9, 14] with three maximal 50m sprints before, one every each 15 minutes during and three after the protocol. The force- and power-velocity relationships and mechanical effectiveness of force application during sprint running are calculated from anthropometric and spatio-temporal data acquired with a Stalker ATS II radar [15]. Results The Repeated Measures ANOVA reveals a significant (p < 0.001) time dependent decrease in theoretical maximal velocity (v0) (-11.0%), in maximal velocity reached at the end of the acceleration (vHmax) (-10.2%) and in ratio of the net horizontal force (RF0) (-10.5%). Conversely, there's no time dependent modification in theoretical maximal force (F0) (-9.8%), in acceleration time constant (τ) (-18.2%) and in resultant ground reaction forces (GRF) (-3.3%). Discussion Our results, time dependent decrease in RF0, revealed that the fatigue, induced by SAFT90, impacts particularly the hip extensors since at the same time the GRF, resultant ground reaction forces, isn't significantly reduced by the induction of fatigue. Based on previous studies [12, 13], these findings about decreased strength production capacity of hamstring refine our knowledge of the relationships between exhaustion, decreased performance and increased predisposition to hamstring strain injury as the soccer game progresses. Indeed, this is the first time, at our knowledge, that the strength production capabilities of hip extensors are measured accurately during the sprint, the pattern responsible for the greatest number of hamstring injuries in football [16]. Acknowledgements The authors wish to thank the Wallonia-Brussels Federation for their assistance in this study. References 1. Maniar, N., et al., Hamstring strength and flexibility after hamstring strain injury: A systematic review and meta-analysis. British Journal of Sports Medicine, 2016. 2. Croisier, J.L., et al., Strength imbalances and prevention of hamstring injury in professional soccer players: A prospective study. Am J Sports Med, 2008. 36(8): p. 1469-75. 3. van Dyk, N., et al., Hamstring and quadriceps isokinetic strength deficits are weak risk factors for hamstring strain injuries: A 4-year cohort study. Am J Sports Med, 2016. 44(7): p. 1789-95. 4. Yeung, S.S., A.M. Suen, and E.W. Yeung, A prospective cohort study of hamstring injuries in competitive sprinters: Preseason muscle imbalance as a possible risk factor. Br J Sports Med, 2009. 43(8): p. 589-94. 5. 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