# Loads on the Knee Joint Ligaments during Stair Climbing
## 4 Discussion
The hamstring activity during this phase is used to produce the extensor moment at the hip which, according to a previous publication [6], is required in this phase. The tibiofemoral force, which is the result of all internal and external forces applied to the knee, showed a distal-proximal component with two peaks during the stance phase (the second higher than the first one), a minimum at toe off, and a slow rise to a plateau during the swing phase, which could be mainly due to the activity of BFsh, ST, and RF. This pattern was in perfect agreement with those obtained through the direct measurement of forces in sensorised prostheses during walking up stairs [22; 26]. The difference in the force amplitude (about three times body weight in the reported publications, about four times body weight in our study) would probably depend on the different subjects considered: operation by knee arthroplasty, in case of the direct measurement, and one representative subject of a healthy population in our case. This makes us confident that our predictions of muscle force and ligament tension were reasonably accurate. To our knowledge, this is the first study trying to quantify the load supported by the knee ligaments during stair climbing. A unique study in the literature dealing with the ligament loads during this task was conducted in vivo by [PERSON] et al. [29] and it focused only on the ACL. The cited authors reported that the ACL reached the maximum strain in correspondence with a knee-flexion angle of about 20\({}^{\circ}\), which is in accordance with our results (see Figures 3c and 7). As to role of the ligaments, our data show that the PCL supports the highest loads (about 300 N) followed by the deep fibres of the MCL (about 250 N). Both ligaments have a peak at mid swing, where the knee was maximally flexed (almost 90\({}^{\circ}\)) and the ST and the BFsh were contracted to brake the forward movement of the shank. Actually, during the swing phase, the RF was also activated, but probably because of the high flexion degree it had no relevant effect in the direction tangential to the tibial plateaux. Hence, a net posteriorly-directed force was applied to the proximal tibia, loading the PCL. Moreover, during the swing phase the gastrocnemius antigonist of ACL) were relaxed. In fact, in this phase the tibialis anterior was activated for producing the ankle dorsiflexion (visible in Figure 3c) needed to avoid contact between the foot and the steps. The PCL force showed another peak occurring just after initial contact, where the knee was still flexed and was extending because of quadriceps contraction, but having a peak of the SM force at the same time. It is evident that the backwards-directed SM force is responsible for the backwards displacement of the tibia, and consequently for PCL loading in this phase. In fact, the force applied by the patellar tendon was likely to be perpendicular to the tibial plateaux, and not pulling forward as when the knee was extended. Actually, when the knee extension increases, ligament tension switches from the PCL (that completely relaxes at 20% of the gait cycle) to the ACL that quite suddenly becomes tensioned and remains loaded for the remining stance phase, in which the VM and VL are activated and exert an anteriorly-directed force via the patellar tendon. Moreover, ACL recruitment is coherent with the gastrocnemius activity, which act as an ACL antagonist. Important contributions to the knee-joint stability during the stance phase are provided by all components of the capsule (although with different patterns) and by both the superficial and deep components of the MCL. During the swing phase and early stance, the LCL and MCL appear involved as well, consistently with the relatively high degree of knee flexion. Their tension contributes to sustaining the weight and inertia forces of the leg and foot when they are suspended off the ground, and to providing stability at the early support phase when the foot is placed on the upper step.