Effects of hip extensor fatigue on lower extremity kinematics during a jump-landing task in women: A controlled laboratory study
Introduction
The incidence of anterior cruciate ligament (ACL) injuries and patellofemoral pain syndrome (PFPS) is several times higher in women than men (Boling et al., 2010, Renstrom et al., 2008). Although reasons for the increased incidence of knee injuries in women are not certain, altered lower extremity (LE) kinematics have been implicated as risk factors (Ireland et al., 2003, Krivickas, 1997, Powers, 2003, Tiberio, 1987). Medial femoral rotation accompanied by increased knee valgus is theorized to be a risk factor for ACL injury and PFPS (Boden et al., 2000, Krosshaug et al., 2007, Powers, 2003). Impaired hip muscle strength may contribute to these altered kinematics (Finnoff et al., 2011, Fredericson et al., 2000, Fulkerson, 2002, Fulkerson and Arendt, 2000, Ireland et al., 2003, Noyes et al., 2005) because the hip may adduct and/or rotate medially, resulting in increased knee valgus and medial rotation (Powers, 2003). These altered motor behaviors may increase stresses sufficiently to cause injury (Huberti and Hayes, 1984, Lee et al., 2003).
In addition to diminished hip muscle strength, impaired neuromuscular control may also contribute to increased medial hip rotation and knee valgus. Neuromuscular control may be defined as the unconscious activation of dynamic restraints (i.e., muscles) in preparation for or in response to joint motion and joint loading for the purpose of maintaining joint stability (Riemann and Lephart, 2002). Electromyography, the recording technique that allows the study of muscle recruitment through the electromyogram (EMG), reflects central nervous system input that drives muscle activation and provides insight into neuromuscular control. The amplitude of the gluteus maximus (Gmax) EMG signal during unilateral step-downs, for example, correlates negatively with knee valgus (Hollman et al., 2009). Moreover, women perform unilateral landing activities with less Gmax activation than men (Zazulak et al., 2005), which may partially explain why women demonstrate more medial hip rotation, hip adduction and knee valgus during jump-landings (Ford et al., 2003, Hewett et al., 2004, Malinzak et al., 2001). Findings such as these suggest that altered hip muscle neuromuscular control may influence hip and knee kinematics.
Understanding the extent to which hip muscle strength and neuromuscular control uniquely contribute to LE kinematics may be accomplished in part through studies that employ fatigue protocols. Neuromuscular fatigue, defined as the failure of a muscle to maintain a required or expected force (Bigland-Ritchie and Woods, 1984), temporarily impairs muscle strength and is accompanied by characteristic changes in the EMG signal. During sustained submaximal contractions, the amplitude of the EMG signal typically increases and the median frequency of the EMG signal decreases (Moritani et al., 1986). Examining a motor task before and after localized muscle fatigue can provide insight into the muscle's role in the performance of the task. For example, if motor performance diminishes after localized muscle fatigue is induced, an interpretation may be that reduced strength caused the reduction in motor performance. On the other hand, if motor performance does not change after localized muscle fatigue is induced but one performs the task with greater neuromuscular activation, an interpretation may be that reduced muscle strength has little bearing on the motor task being performed and that improved neuromuscular control can compensate for the reduction in strength. Several studies have provided evidence that lower extremity muscle fatigue induces biomechanical changes—increased valgus moments and anterior shear forces—that may predispose the knee to injury (Chappell et al., 2005, Kernozek et al., 2008, Madigan and Pidcoe, 2003, Mclean et al., 2007). An interpretation of these findings is that impaired muscle strength subsequent to the fatigue-inducing interventions led to the biomechanical changes that were observed. One limitation of these studies is that they used generalized exercise protocols to induce fatigue in multiple lower extremity muscles. It is believed that these protocols did not permit investigators to examine the effect of impaired function at specific hip muscle groups that may influence LE kinematics.
Subsequent fatigue protocols have induced hip strength deficits (Carcia et al., 2005, Geiser et al., 2010, Gribble and Hertel, 2004, Thomas et al., 2011). Thomas et al (2011), for example, induced medial and lateral rotation fatigue and reported that small increases (< 3°) in medial hip rotation occur during post-fatigue single-leg landings but that no sagittal or frontal plane hip or knee motion changes occur. Geiser et al (2010) induced hip abductor fatigue and concluded that only a small increase (< 2°) in knee valgus occurs during post-fatigue side-cutting and jumping tasks. While the effect of fatiguing the hip rotator and abductor muscles on LE kinematics appears to be small, interpreting the findings of those studies is limited by several factors. None examined hip extensor fatigue, none examined EMG to gain insight into neuromuscular control and none assessed fatigue via controlled study designs. The hip extensors, particularly the Gmax, may influence LE kinematics. Since the Gmax extends, laterally rotates and assists in abducting the hip, engaging the Gmax may limit excessive medial hip rotation or hip adduction during weightbearing tasks, thereby limiting knee valgus motions.
Gaining a better understanding of how hip muscles contribute to knee kinematics may facilitate interventions that prevent knee injury or enhance rehabilitation following injury. The purpose of this study was to examine the effects of experimentally-induced hip extensor fatigue on LE kinematics during a jump-landing task. We incorporated EMG to gain insight into Gmax recruitment during the landing task and conducted the study with a randomized controlled design. Assuming that impaired hip extensor strength would influence LE kinematics, we hypothesized that hip medial rotation, hip adduction and knee valgus would increase following an acute bout of hip extensor-fatiguing exercise. Second, we also hypothesized that Gmax recruitment may increase to compensate for participants' loss of strength.
Section snippets
Study Design
This was a randomized, sham-controlled laboratory study with pretest-posttest measures. Participants were randomly allocated to groups and were not made aware as to whether the intervention they performed was considered the experimental or sham condition.
Participants
Active, healthy women between the ages of 18 and 36 were eligible to participate. They reported absence of knee pain while descending stairs, squatting, or during prolonged sitting, could ambulate without assistive devices and could jump and
Participant Characteristics
Of 44 volunteers assessed for eligibility, 40 met criteria for enrollment. Four were excluded because they reported knee pain within 6 months prior to testing. After randomization, the groups were statistically equivalent on demographic characteristics and all participants were moderately to highly physically active (Table 1). The proportion of participants classified as moderately or highly physically active did not differ between groups (χ2 = 2.558, P = 0.110), therefore activity level was not
Discussion
We examined whether hip and knee kinematics would change during a bilateral jump-landing task following an intervention that fatigued the hip extensors and hypothesized that medial hip rotation, hip adduction and knee valgus would increase. Results did not support our hypothesis. While hip extension strength decreased by 25%, no change in hip or knee kinematics occurred that can be attributed to the intervention. Rather, in support of our second hypothesis that Gmax recruitment would increase,
Conclusion
Reduced hip extensor strength following a fatiguing intervention did not lead to changes in hip or knee kinematics during a bilateral jump-landing task. Rather, participants accommodated for the strength deficit by recruiting more Gmax activation to complete the task. Gmax recruitment may compensate when hip extensor strength is impaired, suggesting that improved neuromuscular control can influence motor performance when strength is diminished.
Conflict of interest statement
The authors assert that they have no commercial relationships that may lead to a conflict of interest.
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