title: Changes in quadriceps and hamstring cocontraction following landing instruction in patients with anterior cruciate ligament reconstruction.
abstract: METHODS
  Pretest/posttest controlled laboratory study.
  
  
  OBJECTIVE
  To determine changes in the neuromuscular activation of the quadriceps and hamstrings following instructions aimed at improving knee flexion during a single-limb landing task in persons who have undergone anterior cruciate ligament reconstruction (ACLR).
  
  
  BACKGROUND
  Clinicians advise patients who have undergone ACLR to increase knee flexion during landing tasks to improve impact attenuation. Another long-standing construct underlying such instruction involves increasing cocontraction of the hamstrings with the quadriceps to limit anterior shear of the tibia on the femur. The current study examined whether cocontraction of the knee musculature changes following instruction to increase knee flexion during landing.
  
  
  METHODS
  Thirty-four physically active subjects with unilateral ACLR participated in a 1-time testing session. The kinetics and kinematics of single-leg landing on the surgical limb were analyzed before and after instruction to increase knee flexion and reduce the impact of landing. Vastus lateralis and biceps femoris activities were analyzed using surface electromyography and normalized to a maximal voluntary isometric contraction (MVIC). Cocontraction indices were integrated over the weight-acceptance phase of landing.
  
  
  RESULTS
  Following instruction, peak knee flexion increased (preinstruction mean ± SD, 56° ± 11°; postinstruction, 77° ± 12°; P<.001) and peak vertical ground reaction forces decreased (preinstruction, 3.50 ± 0.42 body mass; postinstruction, 3.06 ± 0.44 body mass; P<.001). Cocontraction also decreased following instruction (preinstruction, 30.88% ± 17.68% MVIC; postinstruction, 23.74% ± 15.39% MVIC; P<.001). The change in cocontraction was correlated with a decrease in hamstring activity (preinstruction, 23.79% ± 12.88% MVIC; postinstruction, 19.72% ± 13.92% MVIC; r = 0.80; P<.001).
  
  
  CONCLUSIONS
  Landing instruction produced both a statistically and clinically significant change in landing mechanics in persons post-ACLR. Conscious improvement of the absorptive power of the surgical limb was marked by decreased hamstring activity and cocontraction during single-limb landing.
authors: School of Physical Therapy and Rehabilitation Sciences, University of Montana, Missoula, MT.
authors: Audrey R C
authors: AR
authors: Elias
authors: Curt D
authors: CD
authors: Hammill
authors: Ryan L
authors: RL
authors: Mizner
mesh_terms: D000293:Adolescent; D000328:Adult; D000842:Ankle / Q000502:physiology; D059549:Anterior Cruciate Ligament Reconstruction / Q000534:rehabilitation*; D001696:Biomechanical Phenomena; D002648:Child; D004576:Electromyography; D005260:Female; D006615:Hip / Q000502:physiology; D006801:Humans; D007717:Knee / Q000502:physiology*; D008297:Male; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000502:physiology*; D013647:Task Performance and Analysis; D055815:Young Adult
keywords: EMG; biomechanics; knee; lower extremity; motor control/learning