# Walking in the Absence of Anterior Cruciate Ligament: The Role of the Quadriceps and Hamstrings
## 4 Discussion
### Ligament Forces Redistribution  
In the absence of the ACL, both the superficial and deep bundles of the MCL appeared overloaded in the load acceptance phase and even more so in the early swing phase. Specifically, the deep layer of the MCL exhibited the greatest increase in its tension. The posterior-lateral component of the fibrous capsule also had a mild increase during the load acceptance phase. In particular, Cap-Post-M, superficial MCL and deep MCL reached maximum tensions of 38 N, 94 N and 72 N, respectively, at the same time instant as the peak expected for an ACL in an intact knee. These increases in tension did not appear to be random but were well-calibrated so that when added together, they managed to produce the tension that was developed by ACL in the intact knee during the load acceptance phase (approximately 200 N). Hence, the prolonged recruitment of the superficial and deep MCLs in the first 20% of the cycle was probably required to compensate for the ACL deficiency in coping with an anterior tibial translation; instead, in the first half of the swing phase, the increase in MCL tension (superficial and deep) was even more consistent because it had to cope not only with the greater forward sliding of the tibia but also with its greater abduction. These results are in good agreement with the study of [1]. No relevant changes in the loading of the lateral components of the capsule (Caps-Ant-L and Caps-Post-L) or the LCL and PCL were observed when the ACL was sacrificed. Thus, it appeared that the MCL (superficial and deep) was the main passive stabilizer of the anterior translation of the tibia when the ACL was lacking. In the absence of the hannstrings, the main effects were observed again at the MCL components, where their recruitment along the whole swing phase was prolonged with a peak at late swing. However, in this condition, the whole capsule (Caps-Ant-L, Caps-Post-L and Caps-Post-M) also exhibited a peak of force at the end of the swing phase. The PCL, on the other hand, appeared completely relaxed in the late swing. This appeared to be the net consequence of the lack of restraint exerted by the hannstrings on the forward displacement of the tibia. No relevant changes in the LCL were observed with the hannstring removal during the late swing phase. The deactivation of the quadriceps when the ACL was removed produced a reduction in the MCL and deep MCL tensions. On the other hand, it produced early and long-lasting recruitment of the PCL during the stance phase. Noticeable was the sharp loading of the LCL in the second half of load acceptance when the knee was recovering from its initial yielding. If we consider that in the same phase, the PCL was also starting to be loaded and the knee exhibited abnormal adduction (see Figure 4b), we can recognize that the lack of the quadriceps made the knee more unstable, and this was demonstrated also by the frequent oscillations observed in almost all the variables in this condition (see the force in the Caps-Post-L in particular). However, the reduction in the quadriceps activity (which, over time, can lead to partial atrophy) would find its effectiveness in reducing the loads of all ligaments in general, with the exception of the PCL, which instead requires more consistent recruitment in the support phase. It could be concluded that in subjects with ACL removal or severe lesions, in conservative rehabilitation treatment, the training of hannstrings aimed at the maintenance of good muscle tone would be of fundamental importance either to compensate for the ligament deficit and limit the anterior sliding of the tibia and to reduce the risk of injury to the MCL (the deep component in particular), which would reach very high tensions acting against the anterior tibial translation and valgus deflection. Not with the same objectives, similar treatment is also to be provided to the quadriceps to avoid excessive weakening. Other clinical considerations can be advanced in relation to patients undergoing ACL reconstruction.