The Effect Of Sequential Fatigue On Lower Extremity Biomechanics During A Crossover Task

Abstract

Fatigue has been shown to augment the risk for anterior cruciate ligament (ACL) tears in female athletes. However, few studies have analyzed the biomechanics of common soccer maneuvers at various stages of the fatiguing process. PURPOSE: The aim of this study was to examine the effect of a sequential fatigue protocol on the lower extremity biomechanics of a crossover task (CO). METHODS: Eighteen injury-free female collegiate soccer players with no history of ACL injury (age=19.2±0.9 years, mass=61.6±5.1 kg, height =1.66±0.5 m) volunteered for the study. Subjects alternated between a fatiguing protocol and 2 consecutive unanticipated CO trials until fatigue was reached. The fatiguing protocol consisted of 3 squats, agility (5-10-5), 3 vertical jumps, and step-ups. When the subjects could not perform at least 1 acceptable vertical jump for 2 successive cycles or they sustained 90% of their calculated maximum heart rate for 3 sequential cycles, fatigue was acknowledged. The unanticipated CO task was triggered by an automated system that projected a scenario onto a screen simulating an actual soccer event. Lower extremity biomechanics were evaluated during the CO task using a 3D motion capture system and 2 force plates. Separate repeated measures ANOVAs were conducted to analyze the difference between pre-fatigue, 50% and 100% of fatigue for each dependent variable. Alpha level was set at 0.05. RESULTS: Knee flexion at initial contact (KF-IC) at pre (-31.6±8.9°) and 50% (-28.5±10.9°) of fatigue were significantly higher than KF-IC at 100% (-21.9±9.0 °) of fatigue (p<0.001 & p=0.015 respectively). Knee abduction at IC (KAB-IC) at pre- (8.8±4.7°) and 50% (7.9±4.3°) of fatigue were significantly higher (p=0.006 & p=0.049 respectively) than KAB-IC at 100% (6.3±4.0°). No other statistically significant difference was attained (p≥0.05). CONCLUSIONS: The sequential fatigue protocol had a negative effect on knee biomechanics during the CO task specifically from 50% to 100% of fatigue. The fatiguing process caused a detrimental effect primarily to the sagittal and frontal knee angles. A decreased knee flexion angle has been shown to put an athlete at an increased risk for ACL injury. Since most of the biomechanical decline occurs after 50% of fatigue, intervention programs should also be implemented while athletes are fatigued.