Since the proximal tendon of the gastrocnemius muscle wraps around the posterior aspect of the tibia, its contraction could
potentially strain the anterior cruciate ligament (ACL) by pushing the tibia anteriorly. However, the relationship between contraction
of the gastrocnemius muscle and ACL strain has not been studied in vivo. The objectives of this study were to evaluate the
ACL strain response due to isolated contractions of the gastrocnemius muscle and to determine how these strains are affected by cocontraction
with the hamstrings and quadriceps muscles. Six subjects with normal ACLs participated in the study; they underwent
spinal anesthesia to ensure that their leg musculature was relaxed. Transcutaneous electrical muscle stimulation (TEMS) was used to
induce contractions of the gastrocnemius, quadriceps and hamstrings muscles while the strains in the anteromedidl bundle of the
ACL were measured using a differential variable reluctance transducer. The ACL strain values produced by contraction of the
gastrocnemius muscle were dependent on the magnitude of the ankle torque and knee flexion angle. Strains of 2.8'!% and 3.5% were
produced at 5" and 15" of knee flexion, respectively. The ACL was not strained at 30" and 45". Changes in ankle angle did not
significantly affect these strain values. Co-contraction of the gastrocnemius and quadriceps muscles produced ACL strain values that
were greater than those produced by isolated activation of either muscle group when the knee was at 15" and 30". Co-contraction of
the gastrocnemius and hamstrings muscles produced strains that were higher than those produced by the isolated contraction of the
hamstrings muscles. At 15" and 30" of knee flexion, the co-contraction strain values were less than those produced by stimulation of
the gastrocnemius muscle alone. This study verified that the gastrocnemius muscle is an antagonist of the ACL. Since the gastrocnemius
is a flexor of the knee, this finding may have important clinical ramifications in ACL rehabilitation since flexor torques
are generally thought to be protective of a healing ACL graft. 0 2001 Orthopaedic Research Society. Published by Elsevier Science
Ltd. All rights reserved.