Quadriceps contractile properties and fatigue in senior and youth rofessional soccer players

Soccer is a high-intensity intermittent exercise that requires aerobic and anaerobic capacities, as well as eliciting a high level of force production in several situations. Soccer players experience fatigue following high-intensity phases of matches; however the mechanisms responsible for this are not completely understood. Advances in electro-stimulation techniques have made it possible to determine the contractile properties of specific peripheral muscles. The aim of the present study was to determine the quadriceps contractile properties in three different populations of soccer players and to examine between group differences in peripheral neuromuscular fatigue induced by a standardized 5 minute high-intensity intermittent running exercise (HIE). The mechanical response of the quadriceps muscle was measured via paired electrical stimuli of the femoral nerve, delivered at a firing frequency of 10Hz and 100Hz. Peak torque (PT), maximal rate of torque development and maximal rate of torque relaxation were established for both firing frequencies before and after the HIE. Blood lactate (La-), hydrogen ion (H+) and rate of perceived exertion (RPE) were measured immediately after the HIE. Data was collected during the competitive season from the senior (AP, n=24), Under 18 (U18, n=29) and Under 16 (U16, n=37) soccer squads. RPE and La- was significantly lower (P<0.003) in AP than both U18 and U16 (RPE: 3.2±0.6, 4.6±1.0 and 4.7±1.2 au and La- 6.1±2.4, 7.6±2.4 and 8.4±2.6 mmol•L-1 respectively). Blood H+ was significantly higher (P=0.029) in U16 with respect to AP and U18. No significant differences in quadriceps mechanical responses were found between the 3 groups at rest (P>0.05). Percentage decreases in PT at 100Hz was significantly higher (P<0.001) for U16 than the AP and U18 groups (-15.6±5.0%, -8.6±7.1% and -10.7±5.5% respectively). Whereas the percentage decrease found in PT at 10Hz significantly differed (P<0.001) between all 3 groups (-24.3±10.4%, -30.3±12.7% and -36.1±8.5% for AP, U18 and U16 respectively). The decrease in ratio 10/100 was higher in the U18 and U16 groups (P=0.021). The results suggested that HIE exerted a higher level of peripheral fatigue on the youth players. This fatigue is partially related to the differing metabolic intensities sustained during the exercise. However as suggested by long-lasting fatigue indicators other factors, like failure in the excitation-contraction coupling system, play a role in the development of fatigue.