Abstract
The purpose of this series of studies was to (1) establish a standing and weight-bearing target-matching task to characterize neuromuscular control and (2) characterize the altered neuromuscular control and morphology resulting from ACL injury and surgical reconstruction using a quadruple-bundle hamstring autograft and allograft. First, a standing target-matching neuromuscular control task was developed and the neuromuscular control strategies used by uninjured athletes was established using electromyography and a weight-bearing protocol to examine the effect of limb task. Visual feedback was given of the forces and moments of the "mobilizer" limb, while the "stabilizer" limb produced nearly identical forces and moments was given no visual feedback. The biarticulate muscles were the primary directional activators in this task, and the uniarticulate vasti muscles acted synergistically to brace the knee in both hip flexion and extension force postures, while subjects produced bidirectional controlled static isometric forces by pushing on two force-platforms. The lateral hamstring was recruited with more refined control while acting as a mobilizer, and the medial hamstring was used with more refined control while performing the stabilizer task. While the medial gastrocnemius was activated in a principally forwards direction for both tasks, the lateral gastrocnemius was principally activated in a backwards direction when subjects performed the stabilizer task. This suggests that neuromuscular control is specific to limb task, even though the external forces are nearly identical. Second, the neuromuscular control of the ACL deficient individuals was compared to uninjured individuals using the standing target-matching task. ACL deficient individuals demonstrated reduced neuromuscular control of the muscles about the knee for both limb tasks in comparison to uninjured individuals matched for age, sex, and activity level. ACL deficient individuals activated their lateral hamstrings primarily while pushing their foot in the forwards direction while performing the stabilizer task, which was the opposite direction of control subjects or when the ACL deficient individuals performed the mobilizer task. Voluntary neuromuscular control was significantly affected in the ACL deficient subjects, and the lateral hamstrings muscle may be activated to brace the knee when subjects are not paying attention to their limb (acting as the stabilizer). Thirdly, the muscle morphology and neuromuscular control of a sample of ACL deficient individuals who were predicted to compensate well after ACL injury (potential coper) was compared to individuals that were predicted to not compensate well after ACL injury (non-copers). Both groups demonstrated quadriceps atrophy, but the non-copers used reduced neuromuscular control of the muscles about the knee while the potential copers used symmetrical neuromuscular control. Findings suggest that the potential to compensate following ACL injury is dependent more on how the individual uses their muscle rather than the amount of quadriceps atrophy. Fourthly, the effect of graft harvest on muscle morphology was made using a pre-post ACL reconstruction comparison between a semitendinosus-gracilis (STG) tendon autograft group and a cadaveric allograft group. The autograft group demonstrated synergistic muscle hypertrophy, while the autograft group remained symmetrical morphologically following surgery. Muscle morphology changes in the autograft group indicate that the muscles remaining following graft harvest hypertrophy to compensate for the functional loss of the STG. Finally, a long-term follow-up of morphology was performed on individuals that underwent STG graft harvest 6 years prior in comparison to both pre-operatively and at the time of return-to-sports. STG tendon regeneration occurred in two out of three participants; however the third had fatty inclusions within both STG muscles indicating that this muscle was no longer functional. Graft harvest technique may have played a role in the amount of regeneration of the STG muscles. In summary, this series of studies establishes a new tool to examine standing and weight-bearing neuromuscular control and examines the neuromuscular and morphological response to ACL injury and reconstruction in both the short and long term.