title: Predicting the Functional Roles of Knee Joint Muscles from Internal Joint Moments.
abstract: Knee muscles are commonly labeled as flexors or extensors and aptly stabilize the knee against sagittal plane loads. However, how these muscles stabilize the knee against adduction-abduction and rotational loads remains unclear. Our study sought 1) to classify muscle roles as they relate to joint stability by quantifying the relationship between individual muscle activation patterns and internal net joint moments in all three loading planes and 2) to determine whether these roles change with increasing force levels.
  
  
  
  A standing isometric force matching protocol required subjects to modulate ground reaction forces to elicit various combinations and magnitudes of sagittal, frontal, and transverse internal joint moments. Surface EMG measured activities of 10 lower limb muscles. Partial least squares regressions determined which internal moment(s) were significantly related to the activation of individual muscles.
  
  
  
  Rectus femoris and tensor fasciae latae were classified as moment actuators for knee extension and hip flexion. Hamstrings were classified as moment actuators for hip extension and knee flexion. Gastrocnemius and hamstring muscles were classified as specific joint stabilizers for knee rotation. Vastii were classified as general joint stabilizers because activation was independent of moment generation. Muscle roles did not change with increasing effort levels.
  
  
  
  Our findings indicate muscle activation is not dependent on anatomical orientation but perhaps on its role in maintaining knee joint stability in the frontal and transverse loading planes. This is useful for delineating the roles of biarticular knee joint muscles and could have implications in robotics, musculoskeletal modeling, sports sciences, and rehabilitation.
authors.affiliation: 1School of Rehabilitation Sciences, University of Ottawa, Ottawa, ON, CANADA; 2Department of Neuroscience and Pharmacology, University of Copenhagen, Copenhagen, DENMARK; 3Section of Sportstraumatology M51, Bispebjerg Hospital, Copenhagen, DENMARK; and 4School of Human Kinetics, University of Ottawa, Ottawa, ON, CANADA.
authors.forename: Teresa E
authors.initials: TE
authors.lastname: Flaxman
authors.forename: Tine
authors.initials: T
authors.lastname: Alkjær
authors.forename: Erik B
authors.initials: EB
authors.lastname: Simonsen
authors.forename: Michael R
authors.initials: MR
authors.lastname: Krogsgaard
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Benoit
mesh_terms: D000328:Adult; D004576:Electromyography; D005260:Female; D000070633:Hamstring Muscles / Q000502:physiology; D006621:Hip Joint / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D007719:Knee Joint / Q000502:physiology*; D035002:Lower Extremity / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D052097:Quadriceps Muscle / Q000502:physiology; D012044:Regression Analysis