Fatigue is one factor that affects lower limb dynamic joint stability during athletic tasks. Research has reported greater lower-extremity injuries because playing time and fatigue are increased in sports such as rugby, soccer, and field hockey. This can affect athletic performance. It has been thought that reduced muscle force, less coordination, delayed neuromuscular activation, increased knee shear forces and moments, and impaired joint stability are key factors that are responsible for lower-extremity injury rates during fatigued conditions. The specific fatigue-induced mechanisms thought to be responsible for increased knee injury, especially anterior cruciate ligament (ACL) tears, remain unknown for both sexes. Fatigue involves both peripheral and central nervous system factors, which are difficult to recreate in a laboratory (Ortiz 2012). Several studies have looked at the effects of fatigue with varied fatigue protocols. Less knee flexion , decreased vertical jump height, increased electromyographic (EMG) activity of quadriceps and hamstrings, and impaired balance have been found by some investigators after the onset of fatigue (Ortiz 2012). Other studies have reported no differences in biomechanical and performance measures between non fatigued and fatigued conditions. The effects of fatigue not only could increase the risk for injury but also might impair jumping performance and athletic execution by decreasing knee power, jump height, and muscle work.

Ortiz (2012) determined the effects of metabolic fatigue on knee muscle activation, peak knee joint angles, and peak knee internal moments in young women during 2 jumping tasks. Fifteen women (mean age: 24.6 ± 2.6 years) participated in one non fatigued session and one fatigued session. During both sessions, peak knee landing flexion and valgus joint angles, peak knee extension and varus/valgus internal moments, electromyographic (EMG) muscle activity of the quadriceps and hamstrings, and quadriceps/hamstring EMG. The tasks consisted of a single-legged drop jump from a 40-cm box and a 20-cm, up-down, repeated hop task. The fatigued session included a Wingate anaerobic protocol followed by performance of the 2 tasks. Although participants exhibited greater knee injury–predisposing factors during the fatigued session, such as lesser knee flexion joint angles, greater knee valgus joint angles, and greater varus/valgus internal joint moments for both tasks, only knee flexion during the up-down task was statistically significant.
Metabolic fatigue may perhaps predispose young women to knee injuries by impairing dynamic knee joint stability. Training strength-endurance components and the ability to maintain control of body movements in either rested or fatigued situations might help reduce injuries in young women athletes. Metabolic fatigue has been identified as one of the possible contributing factors to impair dynamic control and lower-extremity injuries (Ortiz 2012). Strength and conditioning coach must be aware of how athletic performance can be affected by fatigue and associated mechanisms leading to increased risk for injuries. It is important that the human body might be able to protect the lower extremities against unstable conditions through neuromuscular compensations, such as an increase in muscular activation during the onset of fatigue. This may be the case because certain evidence exists in which fatigue may decrease predisposing factors for lower-extremity injuries. It is for this reason that strength and conditioning specialists should not underestimate training endurance and strength-endurance components in all athletes as means to reduce injuries and maximize performance (Ortiz 2012). It might be possible that the use of repetitive functional exercises help the athletes develop neuromuscular strategies that could be used in situations where fatigue might pose a risk for injury.
Reference:
1. Ortiz A, Olson SL, Etnyre B, Trudelle-Jackson EE, Bartlett W, Venegas-Rios HL. Fatigue effects on knee joint stability during two jump tasks in women. J Strength Cond Res. 2010 Apr;24(4):1019-27.
Written by Chris Barber, CPT