The purpose of this investigation was to evaluate the effects of fatigue on knee joint angles, internal knee moments, and neuromuscular activation during the landing phase of 2 jumping tasks performed by young women. As hypothesized, greater knee injury-predisposing factors during the fatigued condition were observed but solely during the up-down hop task. Decreased knee flexion joint angles might increase the likelihood for greater knee ligamentous stress by decreasing the ability of the hamstrings to resist anterior shear forces on the tibia (5). Smaller peak knee flexion joint angles after the onset of fatigue might be a compensation strategy to prevent collapse of the center of mass because of fatigue in the quadriceps muscle group (22,28). This compensation can be observed by minimal smaller knee joint extension moments during fatigue as a strategy to sustain performance by preventing deep knee flexion and subsequent collapse of the center of mass (23,28,29). This compensatory strategy reduces knee moments by increasing joint stiffness, further allowing the subjects to perform faster concentric contractions toward a jump (13,28,29,34). In addition, given the inherent difficulties of landing maneuvers, greater knee stiffness during landing single legged from a jump may be not only a strategy to prevent collapse of the knee joint but also an indication of strategies to stabilize anterior-posterior trunk movements keeping the trunk centered within the base of support (7). Among the possible reasons influencing the magnitude of findings in knee joint flexion between tasks could be the repetitive nature of the up-down hop test. It can be hypothesized that the up-down hop task presents a greater challenge to the motor system given the high speed of movement required to maintain control and balance. Although not statistically significant, the greater varus/valgus moment exhibited by the subjects in this investigation during the fatigue session agreed with previous findings (5,20). These greater varus/valgus moments in combination with smaller knee joint flexion angles are important factors for generating high knee loads that could reach injurious magnitudes (5,20). Therefore, it seems that fatigue places the knee in an unstable situation by decreasing its dynamic stability in the sagittal and frontal planes.
No previously published studies reported the effects of fatigue on task performance by using the cocontraction ratio. Investigators have suggested that the quadriceps/hamstrings cocontraction is used as the main strategy to stabilize the knee joint and resist external loads toward varus/valgus and flexion/extension during high-level athletic tasks (2,19). This strategy has been hypothesized to help protect knee ligaments against stresses that can induce ligament strain and failure by enhancing neuromuscular activation of stabilizing muscles (2,19). Therefore, although the tasks in this investigation have been shown to provide high lower-extremity external loads (10,13,16) in flexion/extension and varus/valgus, it would be reasonable to observe a similar or increased cocontraction ratio between quadriceps and hamstrings during the fatigued condition to prevent ligamentous injury. A different explanation for the observed results of the cocontraction ratios pertains to the operational definition used for the landing phase. Knee ligament injuries in women athletes tend to occur immediately before push off into a cutting or jumping maneuver (10), whereas others have reported the beginning stages of the landing cycle as specifically responsible for ACL injuries (3,4,6). In this investigation, the landing phase was defined as the entire time the subjects were in contact with the force plate, from the moment of initial contact to push off, without differentiating among preactivation (9), weight acceptance (eccentric) (2-4), and push off (concentric) subphases during contact time (2-4,6). The preactivation phase is defined as a motor control phase in which muscle activation is enhanced (9) with the purpose of reducing external impact forces upon landing (13). The eccentric (negative) phase is reflexive in nature and the one in which a rapid response from all stabilizing muscles is created to provide stability during potentially injurious situations (9) and resist the downward collapse of the center of mass (13). The takeoff (push off) phase is the component in which the subjects exert a concentric contraction to recover from the jump and prepare to perform an upward vertical jump (29). Therefore, further work assessing neuromuscular cocontraction between quadriceps and hamstrings should consider different phases (concentric/eccentric) of the landing cycle. Additionally, a successful trial was defined as one where the subject was able to perform the vertical jump after landing. All subjects performed more than 5 trials during the fatigued session as the first or second or both trials were repeated because the subjects fell or were not able to recover from the landing. The exclusion of unsuccessful trials from the analyses might have concealed the real impact of fatigue in landing mechanics.
Although the purpose of EMG and kinematics normalization procedures is to standardize all data, performance of 2 sessions in different days may have introduced error because of the nature of placing skin retroreflective markers and EMG electrodes differently on both occasions. Voluntary adjustments in landing mechanics during preplanned activities might have affected neuromuscular recruitment strategies because the subjects knew in advance the maneuver to be performed (10). Both tasks used in this study were planned maneuvers in which each subject was aware of the movement patterns needed to perform each trial. Besier et al. (2,3) reported differences in movement patterns between planned and unplanned maneuvers. The performance of unplanned tasks placed more stress on the knee joint, increasing the possibility of injury, if improper neuromuscular strategies are used (2,3). These findings might indicate that even though the tasks used in this study are capable of creating high loads to the knee joint (16), the difficulty level may not have been high enough to be altered by metabolic fatigue. Albeit the 60 Hz frequency sampling rate used may have introduced variability into the measurement for both tasks, frequency contents were screened with a residual analysis and filtered through a low-pass Butterworth filter (6 Hz).
Metabolic fatigue has been identified as one of the possible contributing factors to impair dynamic control and lower-extremity injuries. Strength and conditioning specialists need to be aware of how athletic performance can be affected by fatigue and associated mechanisms leading to increased risk for injuries. It is apparent 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. However, this is not always the case because certain evidence exists in which fatigue may exacerbate 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. In addition, 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.
This study was supported in part by an institutional grant from Texas Woman's University (Research Enhancement Program), and the Research Center for Minority Institutions-Clinical Research Infrastructure Initiative (RCRCII) award, 1P20RR11126 and, G12RR03051 and R25RR017589, from the National Center for Research Resources, National Institutes of Health, and The National Strength and Conditioning Association Foundation.
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