Lower Extremity Fatigue,Sex, and Landing Performance in a Population With Recurrent Low Back Pain

Context:Low back pain and lower extremity injuries affect athletes of all ages. Previous authors have linked a history of low back pain with lower extremity injuries. Fatigue is a risk factor for lower extremity injuries, some of which are known to affect female athletes more often than their male counterparts.Objective:To determine the effects of lower extremity fatigue and sex on knee mechanics, neuromuscular control, and ground reaction force during landing in people with recurrent low back pain (LBP).Design:Cross-sectional study.Setting:A clinical biomechanics laboratory.Intervention(s):Fatigue was induced using a submaximal free-weight squat protocol with 15% body weight until task failure was achieved.Results:Fatigue altered landing mechanics, with differences in landing performance between sexes. Women tended to have greater knee-flexion angle at initial contact, greater maximum knee internal-rotation angle, greater maximum knee-flexion moment, smaller knee-adduction moment, smaller ankle-inversion moment, smaller ground reaction force impact, and earlier multifidus activation. In men and women, fatigue produced a smaller knee-abduction angle at initial contact, greater maximum knee-flexion moment, and delays in semitendinosus, multifidus, gluteus maximus, and rectus femoris activation.Conclusions:Our results provide evidence that during a fatigued 0.30-m landing sequence, women who suffered from recurrent LBP landed differently than did men with recurrent LBP, which may increase women''s exposure to biomechanical factors that can contribute to lower extremity injury.Key Words: clinical biomechanics, rehabilitation, female athletes, anterior cruciate ligament injuries

Key Points

• Sex differences in landing mechanics (fatigued and unfatigued) and neuromuscular control in men and women with recurrent low back pain are similar to the sex differences seen in individuals without a history of low back pain.
• Women experienced a greater knee-flexion angle at initial contact and maximum knee internal rotation, greater maximum knee-flexion moment, smaller maximum knee-adduction and ankle-inversion moments, smaller ground reaction forces at impact, and earlier multifidus activation.
• Reduced knee abduction at initial contact, increased maximum knee-flexion moment, and delayed activation of the semitendinosus, multifidus, gluteus maximus, and rectus femoris muscles were found in both men and women when landing after lower extremity fatigue.
• These changes are consistent with an increased risk of lower extremity injury for women, particularly when landing while fatigued.

Low back pain is a common occurrence in athletes. Estimates of the incidence vary, depending on the sport but range from 10% to 80%.¹ Despite apparent advances in the diagnosis and management of low back pain (LBP), this disorder continues to place a large burden on individuals and society.² Similarly, injuries to the lower extremity frequently affect athletes of all ages, accounting for approximately 53% of all injuries in collegiate athletes.³ Recognizing those at increased risk for back and lower extremity injury and discovering interventions that may reduce that risk are important research goals.Recently, authors^4–8 have proposed a neuromuscular model linking the function of the low back and the lower limbs. Alterations in the operation of this kinetic chain linkage proximally may increase injury risk at more distal regions. Because pelvic stability is influenced by activity of the trunk muscles through their attachments to the pelvis, an inability to properly activate those muscles may create an unstable pelvic base and contribute to altered lower extremity neuromuscular control. Previous studies have shown that activation of the trunk musculature affects lower extremity mechanics. For example, activation of the transversus abdominis significantly decreases activity of the lumbar erector spinae muscles, increases activity of the gluteus maximus and medial hamstrings, and decreases anterior pelvic tilt during prone active hip extension.⁹Trunk-muscle function is altered in LBP sufferers.¹⁰ Therefore, those individuals may not be able to produce sufficient pelvic stability to provide a stable base for lower extremity motion and control. The relationship between LBP and altered lower extremity movement control has been observed in several studies. Individuals with LBP have diminished lower extremity strength, flexibility, and range of motion,^11–13 as well as altered lower extremity biomechanics and neuromuscular control.^14,15 Those changes may increase the risk of lower extremity injury.Authors of prospective clinical studies have linked LBP history with lower extremity injuries. Zazulak et al¹⁶ found that a history of LBP was a significant predictor of knee injury in females and knee-ligament injury in males. Nadler et al¹³ observed that athletes with a history of lower extremity overuse or ligamentous injury were more likely to be treated for LBP during the following year. Additionally, football players with 2 or more of 3 risk factors (trunk-flexion–hold times of less than the median for the team, Oswestry Disability Index scores of 6 or more, or wall–sit-hold times of less than the median for the team) related to low back dysfunction and trunk-muscle endurance were at twice the risk for back and lower extremity injuries than were those with fewer than 2 factors.¹⁷Female athletes are up to 8 times more likely than male athletes to experience an anterior cruciate ligament (ACL) injury and are more prone to injuries from noncontact mechanisms.^18,19 The higher risk for ACL rupture among female athletes has been explained by hormonal, mechanical, neuromuscular, skeletal, and genetic factors.²⁰ The increased incidence of knee-ligament injuries in female athletes is multifactorial; which factors are dominant is currently unknown.²¹ Although both intrinsic and extrinsic factors may contribute, the injury occurs during a loading event, which can be moderated by mechanical and neuromuscular factors.^19,22 Landing technique and neuromuscular function can be improved with training and may potentially reduce the risk of ACL injury.²² Previous investigators have suggested that an increase in quadriceps activation²⁰ and a discrepancy between quadriceps and hamstrings strength may contribute to ACL injuries.²³Female athletes are more likely to sustain certain lower extremity injuries, such as ACL tears. Additionally, females more often develop those injuries as a result of noncontact mechanisms,¹⁸ which may reflect a failure of neuromuscular control because the injury occurs during loading.^19,22 It is unknown whether the occurrence of LBP affects lower extremity biomechanical and neuromuscular responses differently in males versus females.The effects of fatigue on lower extremity control responses in people with recurrent LBP are unknown. Fatigue serves as a major risk factor for lower extremity injury by altering muscle shock-absorbing capacity and coordination of the locomotor system.²⁴ Fatigue can affect neuromuscular input and output pathways.²⁵ Neuromuscular alterations that occur during fatigue potentially increase the risk of injury,^22,23 and muscle fatigue has been linked to a variety of lower extremity injuries.^24–26 Previous researchers²³ have suggested that the order of muscle activation may not change during fatigue, but muscle premotor and reaction phases may be noticeably greater, suggesting a possible compromise in their protective role. Muscle fatigue moderates lower extremity muscle-activation patterns during landing by altering muscle-burst activation, duration, and intensity, as well as the ability of the lower extremity muscles to absorb repetitive shock or stress.^27–29The effects of sex and fatigue on performance and injury risk are well documented.^19,22–24 Recurrent LBP has been established in the literature as a significant predictor of lower extremity injury.^16,17 However, limited information is available on the effects of lower extremity fatigue and sex on lower extremity control during landing in people with recurrent LBP. The purpose of our study was to determine the effects of lower extremity fatigue and sex on knee mechanics, neuromuscular control, and ground reaction force (GRF) during landing in people with recurrent LBP.