The Relationship Between Muscular Co-Contraction and Dynamic Knee Stiffness in ACL-Deficient Non-Copers

Gardinier, Emily S.
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University of Delaware
Anterior cruciate ligament-deficient (ACL-D) individuals who fail to dynamically stabilize their knee (termed non-copers) are reported to adopt a neuromuscular strategy characterized by reduced knee flexion excursions, reduced external knee flexion moments and generalized co-contraction during gait. This unsuccessful neuromuscular strategy has been qualitatively described as the “knee-stiffening strategy,” although dynamic knee stiffness has not been measured in the non-coper population. While the generally ascribed function of muscular co-contraction is to increase joint stiffness and stability, its relationship to dynamic knee stiffness during gait has not been evaluated. Establishment of a reliable relationship between these two measures would render stiffness a simpler means by which to infer co-contraction among individuals with ACLD knees. The purpose of this study was to determine (1) whether a strategy of generalized, increased co-contraction was present in the ACL-deficient limb, (2) whether ACL-D noncopers walk with a quantifiably stiffer involved knee and (3) whether dynamic knee stiffness correlates with muscular co-contraction in this dynamically unstable population. Kinematic and kinetic data were collected from 42 ACL-D non-copers (male, N=31; female, N=11; age, 27.8±10.1 yrs) during the weight acceptance phase of gait for calculation of dynamic joint stiffness. Electromyography for 6 lower extremity muscles (M/L vastus, M/L hamstring, M/L gastrocnemius) were also collected for determination of co-contraction indices. No significant differences in co-contraction indices and dynamic knee stiffness were found between the involved and uninvolved limbs in this sample. Furthermore, dynamic knee stiffness did not correlate with co-contraction during gait. The lack of elevated co-contraction in the ACL-D limb is contrary to recent reports in the literature. However, the lack of differences in dynamic stiffness between limbs suggests that during gait, absence of the ACL does not result in a quantifiably discernible “knee-stiffening strategy.” Because stiffness values did not correlate with co-contraction, we conclude that dynamic stiffness cannot be used as a surrogate for co-contraction during gait.