# Walking in the Absence of Anterior Cruciate Ligament: The Role of the Quadriceps and Hamstrings
###### Abstract  
The actual need for surgical reconstruction of an injured anterior cruciate ligament (ACL) is still a matter of debate. The aim of this study was to quantify and analyze the knee kinematics and the ligament force redistribution resulting from the ACL sacrifice and the consequent effects of changing the quadriceps and hamstring activity during walking. A three-dimensional musculoskeletal model was used to simulate the behavior of the knee joint during the gait cycle, in different testing conditions: an ACL-deficient knee, an ACL-deficient knee with hamstrings inactivated and an ACL-deficient knee with quadriceps inactivated. Our results showed that in the ACL-deficient knee, the medial collateral ligament (MCL) was the main passive stabilizer of the anterior translation of the tibia. Furthermore, the compensation effects produced by the hamstrings and quadriceps in the ACL-deficient knee during walking were quantified and analyzed. In the case of ACL sacrifice, reducing the quadriceps activity would allow for relieving all ligaments in general, except for the posterior cruciate ligament, which required more consistent recruitment in the support phase. Meanwhile, hamstring activity was crucial not only to compensate for the absence of ACL in limiting the anterior tibial displacement but also to reduce the risk of MCL injury. knee joint ligaments; ACL deficiency; muscular compensation; musculoskeletal modelling; dynamic simulation; walking; knee joint biomechanics Article  
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The occurrence of coping with an ACL-deficient knee relies mostly on a balance between functional needs and psychological attitude, but the anatomical substratum of this clearing skill depends on several biomechanical adjustments. Early biomechanical studies [8] underlined the possibility that the hamstrings can compensate for ACL deficiency. The external knee joint moment was shown to shift from flexor to extensor in the early phase of the stride cycle, and this effect was attributed to the hamstring's increased activity. Quite a wide review of the interaction between muscle forces and ACL loading was performed by [9]. In their investigation, the authors stated that not only can the muscles spanning the knee joint affect the loads on the knee's internal structure but also non-knee-spanning muscles have to be considered in that they can change the ground reaction force and can transfer moments and forces to other anatomical segments due to the phenomenon of dynamic coupling. However, the most relevant effects on ACL loading are commonly attributed to the quadriceps, which act as an antagonist of the ligament, increasing the ACL load, and to the hamstrings, which act as an agonist to reduce the ACL load. However, their role is tightly dependent on the kinematic condition since the direction of their force and their relative moment arm at the knee depends on the joint angle [9]. Furthermore, in physiological conditions, there are specific phases along the stride cycle in which the two muscle groups are activated, and this makes it difficult to understand how they influence the tension of the ligaments. The effects of different muscle contractions on the knee ligaments' tension during walking were thoroughly analyzed in previous work [10] through the use of a musculoskeletal model that was specifically developed for that purpose. The results showed that the quadriceps, acting at the load acceptance phase and the late stance-early swing phase, are mainly responsible for the ACL tension in that, when removed, the load on this ligament decreases considerably at the load acceptance phase and reduces to close to zero at late stance-early swing phase. In contrast, the removal of the hamstrings has no relevant effect in the mentioned phases but instead results in an increase in ACL tension in the late swing phase when these muscles are active in physiological conditions.