Changes in gastrocnemii activation at mid-to-late stance markedly affects the intact and anterior cruciate ligament deficient knee biomechanics and stability in gait.

Introduction: We aimed to quantify the sensitivity in biomechanical response and stability of the intact and anterior cruciate ligament deficient (ACL-D) joints at mid-to-late stance periods of gait to the alterations in activation of gastrocnemii (Gas) muscles.
Methods: A validated kinematics-driven musculoskeletal finite-element model of the lower extremity is used to compute knee joint response and stability under reported kinetics-kinematics of healthy subjects. Activation in Gas is altered under prescribed gait data at the mid-to-late stance of gait and associated changes in remaining muscle forces/contact forces/areas/ACL force and joint stability are computed in both intact and ACL-D joints.
Results: In the intact joint, the anterior-tibial-translation (ATT) as well as ACL and joint contact forces follow variations in Gas forces. Both the stability and ATT of an ACL-D joint are restored to the near-intact levels when the activity in Gas is reduced. Knee joint instability, excessive ATT as well as larger peak articular contact stresses with a posterior shift in contact areas are estimated under greater Gas forces.
Conclusions: ACL-D joint is unstable with ATT > 10 mm under larger activities in Gas. Gas is an ACL-antagonist while hamstrings and soleus are ACL-agonists. The near-intact joint stability and ATT of an ACL-D joint can be restored at a lower activation in Gas; or in other words, when activation in ACL-antagonist muscles drops compared with that in ACL-agonist muscles. Results could help analyze the gait of ACL-D copers and non-copers and provide better understanding towards improved preventive, diagnostic, and treatment approaches.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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