title: Sex comparison of hamstring structural and material properties.
abstract: BACKGROUND
  Musculotendinous stiffness provides an estimate of resistance to joint perturbation, thus contributing to joint stability. Females demonstrate lesser hamstring stiffness than males, potentially contributing to the sex discrepancy in anterior cruciate ligament injury risk. However, it is unclear if the sex difference in hamstring stiffness is due to differences in muscle size or to inherent/material properties of the musculotendinous unit. It was hypothesized that hamstring stiffness, stress, strain, and elastic modulus would be greater in males than in females, and that hamstring stiffness would be positively correlated with muscle size.
  
  
  METHODS
  Stiffness was assessed in 20 males and 20 females from the damping effect imposed by the hamstrings on oscillatory knee flexion/extension following joint perturbation. Hamstring length and change in length were estimated via motion capture, and hamstring cross-sectional area was estimated using ultrasound imaging. These characteristics were used to calculate hamstring material properties (i.e., stress, strain, and elastic modulus).
  
  
  RESULTS
  Stiffness was significantly greater in males than in females (P<0.001). However, stress, strain, and elastic modulus did not differ across sex (P>0.05). Stiffness was significantly correlated with cross-sectional area (r=0.395, P=0.039) and the linear combination of cross-sectional area and resting length (R(2)=0.156, P=0.043).
  
  
  CONCLUSIONS
  Male's hamstrings possess a greater capacity for resisting changes in length imposed via joint perturbation from a structural perspective, but this property is similar across sex from a material perspective. Females demonstrate lesser hamstring stiffness compared to males in response to standardized loading conditions, indicating a compromised ability to resist changes in length associated with joint perturbation, and potentially contributing to the higher female ACL injury risk. However, the difference in hamstring stiffness is attributable in large part to differences in muscle size.
authors.affiliation: Neuromuscular Research Laboratory, University of North Carolina at Chapel Hill, 124 Fetzer, CB # 8700, Chapel Hill, NC 27599-8700, USA. troyb@email.unc.edu
authors.forename: J Troy
authors.initials: JT
authors.lastname: Blackburn
authors.forename: David R
authors.initials: DR
authors.lastname: Bell
authors.forename: Marc F
authors.initials: MF
authors.lastname: Norcross
authors.forename: Jeff D
authors.initials: JD
authors.lastname: Hudson
authors.forename: Megan H
authors.initials: MH
authors.lastname: Kimsey
mesh_terms: D001696:Biomechanical Phenomena / Q000502:physiology; D055119:Elastic Modulus; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000209:etiology; D007719:Knee Joint / Q000502:physiology; D008297:Male; D009129:Muscle Tonus; D018482:Muscle, Skeletal / Q000033:anatomy & histology* / Q000502:physiology*; D009929:Organ Size / Q000502:physiology; D016059:Range of Motion, Articular; D012307:Risk Factors; D012727:Sex Characteristics; D013314:Stress, Mechanical; D013710:Tendons / Q000033:anatomy & histology / Q000502:physiology; D013848:Thigh / Q000033:anatomy & histology* / Q000502:physiology*; D055815:Young Adult