title: A sex comparison of reactive knee stiffness regulation strategies under cognitive loads.
abstract: Sex differences may exist in cognitive faculties and neuromuscular strategies for maintaining joint stability. The purpose of this study was to assess whether preparatory and reactive knee stiffening strategies are affected differently in males and females exposed to sex-biased cognitive loads.
  
  
  
  20 male and 20 female volunteers were tested for knee joint stiffness and quadriceps and hamstring muscle activation patterns throughout a rapid eccentric knee extension perturbation. Participants were tested under 3 cognitive loads (Benton's Judgment of Line Orientation; Symbol Digit modalities Test; and Serial 7's) and a control condition. Apparent knee joint stiffness and muscle activation amplitude and timing were quantified throughout the perturbation across the 4 conditions.
  
  
  
  Reactive knee stiffness values were significantly less during the cognitive tasks compared to the control condition (Judgment of Line Orientation=0.034Nm/deg/kg, Symbol Digit Modalities Test=0.037Nm/deg/kg, Serial 7's=0.037Nm/deg/kg, control=0.048Nm/deg/kg). Females had greater normalized total apparent stiffness than males. The quadriceps muscles had faster and greater activation than the hamstring muscles; however, no group differences were observed. No overall differences in muscle activation (magnitude and timing) were found between the cognitive loading tasks.
  
  
  
  Cognitive loading may decrease the ability of healthy individuals to reactively stiffen their knee joint and appears to interfere with the normal stiffness regulation strategies. This may elucidate an extrinsic risk factor for non-contact knee ligament injury.
authors.affiliation: Department of Orthopaedics, University of Pennsylvania Health System, 3737 Market St., Philadelphia, PA 19104, USA.
authors.forename: Allison S
authors.initials: AS
authors.lastname: Kim
authors.affiliation: Department of Health & Exercise Science, Appalachian State University, ASU Box 32071, Boone, NC 28608, USA. Electronic address: needlear@appstate.edu.
authors.forename: Alan R
authors.initials: AR
authors.lastname: Needle
authors.affiliation: Department of Kinesiology, Temple University, 1800 N Broad St, Philadelphia, PA 19121, USA.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Thomas
authors.affiliation: Department of Psychology, Loyola University, 4501 N. Charles St, Baltimore, MD 21210, USA.
authors.forename: Christopher I
authors.initials: CI
authors.lastname: Higginson
authors.affiliation: Department of Kinesiology & Applied Physiology, University of Delaware, 541 S. College Ave, Newark, DE 19716, USA.
authors.forename: Thomas W
authors.initials: TW
authors.lastname: Kaminski
authors.affiliation: Department of Kinesiology & Applied Physiology, University of Delaware, 541 S. College Ave, Newark, DE 19716, USA.
authors.forename: C Buz
authors.initials: CB
authors.lastname: Swanik
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D000070598:Anterior Cruciate Ligament Injuries / Q000503:physiopathology; D003071:Cognition / Q000502:physiology*; D004576:Electromyography; D005260:Female; D006801:Humans; D007718:Knee Injuries / Q000503:physiopathology; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D052097:Quadriceps Muscle / Q000502:physiology; D012018:Reflex / Q000502:physiology; D012727:Sex Characteristics; D012737:Sex Factors; D055815:Young Adult
keywords: ACL prevention; Dual-task; Knee stability; Neuromuscular control; Reaction times