title: Voluntary enhanced cocontraction of hamstring muscles during open kinetic chain leg extension exercise: its potential unloading effect on the anterior cruciate ligament.
abstract: BACKGROUND
  A number of research studies provide evidence that hamstring cocontraction during open kinetic chain knee extension exercises enhances tibiofemoral (TF) stability and reduces the strain on the anterior cruciate ligament.
  
  
  OBJECTIVE
  To determine the possible increase in hamstring muscle coactivation caused by a voluntary cocontraction effort during open kinetic chain leg-extension exercises, and to assess whether an intentional hamstring cocontraction can completely suppress the anterior TF shear force during these exercises.
  
  
  METHODS
  Descriptive laboratory study.
  
  
  METHODS
  Knee kinematics as well as electromyographic activity in the semitendinosus (ST), semimembranosus (SM), biceps femoris (BF), and quadriceps femoris muscles were measured in 20 healthy men during isotonic leg extension exercises with resistance (R) ranging from 10% to 80% of the 1-repetition maximum (1RM). The same exercises were also performed while the participants attempted to enhance hamstring coactivation through a voluntary cocontraction effort. The data served as input parameters for a model to calculate the shear and compressive TF forces in leg extension exercises for any set of coactivation patterns of the different hamstring muscles.
  
  
  RESULTS
  For R≤ 40% 1RM, the peak coactivation levels obtained with intentional cocontraction (l) were significantly higher (P < 10(-3)) than those obtained without intentional cocontraction (l 0). For each hamstring muscle, maximum level l was reached at R = 30% 1RM, corresponding to 9.2%, 10.5%, and 24.5% maximum voluntary isometric contraction (MVIC) for the BF, ST, and SM, respectively, whereas the ratio l/l 0 reached its maximum at R = 20% 1RM and was approximately 2, 3, and 4 for the BF, SM, and ST, respectively. The voluntary enhanced coactivation level l obtained for R≤ 30% 1RM completely suppressed the anterior TF shear force developed by the quadriceps during the exercise.
  
  
  CONCLUSIONS
  In leg extension exercises with resistance R≤ 40% 1RM, coactivation of the BF, SM, and ST can be significantly enhanced (up to 2, 3, and 4 times, respectively) by a voluntary hamstring cocontraction effort. The enhanced coactivation levels obtained for R≤ 30% 1RM can completely suppress the anterior TF shear force developed by the quadriceps during the exercise.
  
  
  CONCLUSIONS
  This laboratory study suggests that leg extension exercise with intentional hamstring cocontraction may have the potential to be a safe and effective quadriceps-strengthening intervention in the early stages of rehabilitation programs for anterior cruciate ligament injury or reconstruction recovery. Further studies, including clinical trials, are needed to investigate the relevance of this therapeutic exercise in clinical practice.
authors.affiliation: Department of Experimental Medicine, University of Perugia, Perugia, Italy andrea.biscarini@unipg.it.
authors.forename: Andrea
authors.initials: A
authors.lastname: Biscarini
authors.affiliation: Department of Neurological and Movement Sciences, University of Verona, Verona, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Benvenuti
authors.affiliation: Department of Experimental Medicine, University of Perugia, Perugia, Italy.
authors.forename: Fabio M
authors.initials: FM
authors.lastname: Botti
authors.affiliation: Department of Clinical Orthopedics and Traumatology, University of Perugia, Perugia, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Brunetti
authors.affiliation: Department of Experimental Medicine, University of Perugia, Perugia, Italy.
authors.forename: Orazio
authors.initials: O
authors.lastname: Brunetti
authors.affiliation: Department of Experimental Medicine, University of Perugia, Perugia, Italy.
authors.forename: Vito E
authors.initials: VE
authors.lastname: Pettorossi
mesh_terms: D000328:Adult; D016118:Anterior Cruciate Ligament / Q000502:physiology*; D001696:Biomechanical Phenomena; D004576:Electromyography; D015444:Exercise / Q000502:physiology*; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007866:Leg / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology*
keywords: electromyography; hamstring coactivation; knee extension; therapeutic exercise; tibiofemoral shear force