Exploring the role of football specific fatigue on dynamic knee stability in elite female youth football players

Young female athletes have a greater relative risk of sustaining an anterior cruciate ligament injury than their male counterparts (Renstrom et al., 2008) and this injury tends to occur towards the end of a game when fatigue is present. The functional hamstring/quadriceps (FH/Q) ratio describes normal knee function by expressing the hamstrings eccentrically and quadriceps concentrically during knee extension and provides information regarding the competence of the hamstrings to provide adequate dynamic joint stability (Aagaard et el., 1998). It is recognised that both muscular and neuromuscular mechanisms are important aspects in dynamic knee stability, however little research has focused on young female athletes. The aim of this study was to explore the effect of football specific fatigue on muscular and neuromuscular components of dynamic knee stability in elite female youth football players. Participants were 36 females from an FA licensed youth academy (U13= 14; U15=9; U17=13) and all tests were performed before and after a football specific fatigue protocol. Torque was determined in a prone position at three angular velocities (60, 120 and 180°/s) during concentric/concentric and eccentric/eccentric cycles. During the eccentric cycle, electromyography was used to determine the electromechanical delay (EMD) of the biceps femoris, semitendinosus and gastrocnemious muscles. Twenty consecutive sub maximal hops were performed in order to calculate leg stiffness. Mixed Model ANOVA indicated a significant time x angle interaction (F2,34=3.914, p=0.030) for FH/Q ratio with post hoc tests indicating that only at 0-10° of knee flexion did the ratio decrease from pre to post fatigue (-14.1%). A significant time x group interaction effect (F2,34=4.295, p=0.022) was reported for leg stiffness and indicated that the U13 age group decreased from pre to post fatigue (-4.9%), the U15 remained similar (+1.5%) and the U17 increased (+12.3%). A significant time x group interaction effect (F2,34=3.404, p=0.046) was reported for EMD indicating that EMD was significantly longer in the U13 age group and this difference was greater post fatigue. Irrespective of angle and velocity, a significant main effect for time (F1,35=10.031, p=0.000) and group (F2,34=6.356, p=0.005) were also observed revealing EMD to be longer post fatigue (+58.4%). The findings suggest that muscular functioning is not dramatically impaired following fatigue however impairments to neuromuscular functioning were present in all age groups and may be responsible for the increased relative risk of knee injury in female athletes