Hamstring strain injury : rate of torque development and previous hamstring strain injury

Hamstring strain injuries (HSI) are extremely common in football and are the primary cause of re-injury in football players. Although the isokinetic strength ratios have often been used to predict hamstring injuries in football players, the rate of torque development hamstring to quadriceps ratio (RTD H/Q) has rarely been considered in previous reports. Furthermore, little consideration has been given to selective hamstring lengths and their influence on torque production and knee muscle balance, especially under muscle fatigue condition in previously injured hamstrings. Thus, the present dissertation contemplated two approaches. The first, presented in Chapter 3, which aimed to determine the RTD H/Q at long hamstring lengths and the conventional (concentric/concentric) and functional (eccentric/concentric) hamstrings/quadriceps (H/Q) ratios. The second approach, presented in Chapter 4, aimed to determine the effect of muscle fatigue, induced by a specific protocol seeking to mimic the loads and characteristics of team sports, on the knee muscle balance, given by the RTD, conventional and functional hamstrings/quadriceps (H/Q) ratio. Twenty-four professional men's football players, divided into two experimental groups, with (PI) and without (UG) previous HSI, competing in the 1st and 2nd national leagues, participated voluntarily in the studies (approved by the local ethical council – CEFMH: 13/2018). Players performed both quadriceps and hamstring maximal voluntary isometric contractions at 30º of knee flexion (long hamstring lengths) and maximal concentric isokinetic contractions at 180°.s-1 and 60°.s-1. Isokinetic eccentric contractions at 180°.s- 1 and 60°.s-1 of hamstring muscles were also performed. Conventional and functional H/Q ratios based on peak torque throughout the entire isokinetic range of motion and at 30° of knee flexion were calculated. Long hamstring lengths RTD H/Q ratios were extracted at incrementing time periods of 50 milliseconds (ms) from the onset of contraction (50-250 ms). The main findings showed that players with a previous HSI (PI) have a small effect towards lower eccentric torque and RTD H/Q at the initial period of contraction (50 ms, p> 0.05, d=0.4). The PI also showed small to moderate (0.4>d100 ms) as well. Following fatigue, significant differences were found between groups (PI and UG) in the RTD H50/Q50 ratio (p 0.05; 0.3>d<0.6) lower RTD H/Q ratios in early time intervals (<150 ms) as well (<150 ms). In conclusion, to the best of our knowledge, these were the first studies to investigate the rapid H/Q ratio at long hamstring lengths in professional football players with previous HSI. The findings presented in this dissertation may have important implications for future injury prevention and rehabilitation programs and for the explosive strength improvement of the hamstrings. Future studies should also investigate whether lower RTD H/Q ratios are an adaptation to injury, or if it’s a risk factor to future HSI. Also, future studies should seek to investigate whether training in a fatigue state reduces the injury incidence, and, if successful redirect new injury prevention guidelines, to help reduce the large (re-)injury rates of HSI in professional football.As roturas dos isquiotibiais (RISQ) são extremamente comuns no futebol e representam a primeira causa de recidiva de lesão em jogadores de futebol. Apesar dos rácios isocinéticos entre isquiotibiais e quadricípites (rácios ISQ:QUAD) serem amplamente divulgados na literatura como preditores de RISQ, o rácio para a taxa de produção de força (TPF ISQ:QUAD) tem sido muito raramente referenciado em investigações anteriores. Além disso, pouca relevância tem sido dada à influência do estado de alongamento dos ISQ na capacidade de produção de força e no equilíbrio funcional do joelho, sobretudo num contexto de fadiga muscular em isquiotibiais previamente lesionados. Deste modo, a presente dissertação contemplou duas abordagens. Uma primeira, (Capítulo 3), procurou determinar o TPF ISQ:QUAD em condição de alongamento dos ISQ, assim como os tradicionais rácios ISQ:QUAD convencional (ISQconc/QUADconc) e funcional (ISQexc/QUADconc). A segunda abordagem (Capítulo 4), procurou determinar o efeito da fadiga muscular, induzida por um protocolo específico envolvendo ações caraterísticas da modalidade, no equilíbrio muscular e funcional da articulação (rácios ISQ:QUAD convencional, funcional e da TPF). Vinte e quatro jogadores profissionais de futebol, divididos em dois grupos experimentais, com (PI) e sem (UG) historial prévio de RISQ, com prática competitiva atual nos campeonatos das 1ª e 2ª ligas nacionais, participaram voluntariamente nos estudos. Foram realizadas contrações isocinéticas concêntricas (QUAD e ISQ) e excêntricas (ISQ) a 180°.s-1 e 60°.s-1 e contrações isométricas máximas (QUAD e ISQ) a 30º de flexão do joelho. Os rácios convencional e funcional foram calculados com base no momento de força máximo obtido em toda a janela isocinética e a 30º de flexão do joelho. O TPF ISQ:QUAD foi calculado em intervalos incrementais de 50 milissegundos (ms) a partir do instante inicial de contração (50-250ms). Os principais resultados revelaram que jogadores com historial de RISQ (PI) apresentam ligeiro efeito para um reduzido momento de força excêntrico e do TPF ISQ:QUAD no período inicial de contração (50ms; p>0,05; d=0,4). O PI revelou ainda um pequeno a moderado efeito (p>0,05; 0,4>d100ms). Em condição de fadiga, foram observadas diferenças significativas entre grupos no rácio TPF ISQ:QUAD aos 50ms (p<0,05; d=1,0). Foram ainda encontradas pequenos a moderados efeitos (p>0,05; 0,3>d<0,6) para rácios TPF ISQ:QUAD inferiores nos períodos prematuros da contração (