Strain within the anterior cruciate ligament during hamstring and quadriceps activity*

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Abstract
The objectives of this study were to measure strain in the ACL during simulated: (1) hamstring activity alone, (2) quadriceps activity alone, and (3) simultaneous quadriceps and hamstring activity. Seven knee speci mens removed from cadavers were studied. Heavy sutures applied to load cells were attached to the hamstring and quadriceps tendons. Loads were then applied manually (hamstrings) and/or with an Instron testing machine (quadriceps) to simulate isometric con tractions of the various muscle groups. Strain was measured using a Hall effect transducer. Acting alone, the isometric hamstring activity decreased ACL strain relative to the passive normal strain at all positions tested. Thus, hamstring exercises are not detrimental to ACL repairs or reconstruction and can be included early in the rehabilitation program after ACL surgery. Acting alone, at flexion angles of 0° to 45°, the quad riceps significantly increased the strain within the ACL relative to the passive normal strain. Strain in the ACL during simultaneous hamstring and quadriceps activity was significantly higher than that during passive normal motion from full extension to 30° of flexion. The ham strings are not capable of masking the potentially harm ful effects of simultaneous quadriceps contraction on freshly repaired or reconstructed ACLs unless the knee flexion angle exceeds 30°.

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