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Effect of Varying Hamstring Tension on Anterior Cruciate Ligament Strain During in Vitro Impulsive Knee Flexion and Compression Loading
Thomas J. Withrow, PhD1; Laura J. Huston, MS2; Edward M. Wojtys, MD3; James A. Ashton-Miller, PhD4
1 Department of Mechanical Engineering, Vanderbilt University, VU Station B 351592, Nashville, TN 37235. E-mail address: thomas.j.withrow@vanderbilt.edu
2 Vanderbilt Orthopaedic Institute, Medical Center East, South Tower, Suite 4200, Nashville, TN 37232-8774. E-mail address: laura.huston@vanderbilt.edu
3 MedSport, 24 Frank Lloyd Wright Drive, Ann Arbor, MI 48106. E-mail address: edwojtys@umich.edu
4 Department of Mechanical Engineering and Applied Mechanics, Biomechanics Research Laboratory, University of Michigan, G.G. Brown 3208, Ann Arbor, MI 48109-2125. E-mail address: jaam@umich.edu
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Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the National Football League Charities Foundation and the National Institutes of Health. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Presented at the Annual Meeting of the American Orthopaedic Society for Sports Medicine (AOSSM), Hershey, Pennsylvania, July 2006. Winner of the 2006 AOSSM Excellence in Research Award.
Investigation performed at MedSport, Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, Michigan

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2008 Apr 01;90(4):815-823. doi: 10.2106/JBJS.F.01352
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Background: The hamstring muscles are well positioned to limit both anterior tibial translation and anterior cruciate ligament strain during the knee flexion phase of a jump landing. We hypothesized that systematically increasing or decreasing hamstring tension during the knee flexion phase of a simulated jump landing would significantly affect peak relative strain in the anterior cruciate ligament.

Methods: Ten cadaveric knees from four male and six female donors (mean age [and standard deviation] at the time of death, 60.3 ± 23.6 years) were mounted in a custom fixture to initially position the specimen in 25° of knee flexion and simulate axial impulsive loading averaging 1700 N to cause an increase in knee flexion. Quadriceps, hamstring, and gastrocnemius muscle forces were simulated with use of pretensioned linear springs, with the tension in the hamstrings arranged to be increased, held constant, decreased, at "baseline," or absent during knee flexion. Impulsive loading applied along the tibia and femur was monitored with use of triaxial load transducers, while uniaxial load cells monitored quadriceps and medial and lateral hamstring forces. Relative strain in the anterior cruciate ligament was measured with use of a differential variable reluctance transducer, and tibiofemoral kinematics were measured optoelectronically. For each specimen, anterior cruciate ligament strains were recorded over eighty impact trials: ten preconditioning trials, ten "baseline" trials involving decreasing hamstring tension performed before and after three sets of ten trials conducted with increasing hamstring tension, constant hamstring tension, or no hamstring tension. Peak relative strains in the anterior cruciate ligament were normalized for comparison across specimens.

Results: Increasing hamstring force during the knee flexion landing phase decreased the peak relative strain in the anterior cruciate ligament by >70% compared with the baseline condition (p = 0.005). Neither a constant hamstring muscle force nor the absence of a hamstring force significantly changed the peak strain in the anterior cruciate ligament relative to the baseline condition.

Conclusions: Increasing hamstring muscle force during the knee flexion phase of a simulated jump landing significantly reduces the peak relative strain in the anterior cruciate ligament in vitro.

Clinical Relevance: It may be possible to proactively limit peak anterior cruciate ligament strain during the knee flexion phase of jump landings by accentuating hip flexion, thereby increasing the tension in active hamstring muscles by lengthening them.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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