title: Association of quadriceps and hamstrings cocontraction patterns with knee joint loading.
abstract: BACKGROUND
  Sex differences in neuromuscular control of the lower extremity have been identified as a potential cause for the greater incidence of anterior cruciate ligament (ACL) injuries in female athletes compared with male athletes. Women tend to land in greater knee valgus with higher abduction loads than men. Because knee abduction loads increase ACL strain, the inability to minimize these loads may lead to ACL failure.
  
  
  OBJECTIVE
  To investigate the activation patterns of the quadriceps and hamstrings muscles with respect to the peak knee abduction moment.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Neuromuscular research laboratory.
  
  
  METHODS
  Twenty-one recreationally active adults (11 women, 10 men).
  
  
  METHODS
  Volunteers performed 3 trials of a 100-cm forward hop. During the hop task, we recorded surface electromyographic data from the medial and lateral hamstrings and quadriceps and recorded lower extremity kinematics and kinetics. Lateral and medial quadriceps-to-hamstrings (QratioH) cocontraction indices, the ratio of medial-to-lateral QratioH cocontraction, normalized root mean square electromyographic data for medial and lateral quadriceps and hamstrings, and peak knee abduction moment were calculated and used in data analyses.
  
  
  RESULTS
  Overall cocontraction was lower in women than in men, whereas activation was lower in the medial than in the lateral musculature in both sexes (P < .05). The medial QratioH cocontraction index (R(2) = 0.792) accounted for a significant portion of the variance in the peak knee abduction moment in women (P = .001). Women demonstrated less activation in the vastus medialis than in the vastus lateralis (P = .49) and less activation in the medial hamstrings than in the lateral hamstrings (P = .01).
  
  
  CONCLUSIONS
  Medial-to-lateral QratioH cocontraction appears to be unbalanced in women, which may limit their ability to resist abduction loads. Because higher abduction loads increase strain on the ACL, restoring medial-to-lateral QratioH cocontraction balance in women may help reduce ACL injury risk.
authors.affiliation: University of Michigan, Ann Arbor, MI 48109, USA. riannp@umich.edu
authors.forename: Riann M
authors.initials: RM
authors.lastname: Palmieri-Smith
authors.forename: Scott G
authors.initials: SG
authors.lastname: McLean
authors.forename: James A
authors.initials: JA
authors.lastname: Ashton-Miller
authors.forename: Edward M
authors.initials: EM
authors.lastname: Wojtys