## 4 Discussion
Contrary to the free squat, either an ankle plantar flexion or dorsiflexion torque is needed to hold a static wall-squat position, depending on the point of application of the ground reaction force (\(x_{\text{GR}}\) at forefoot or rearfoot). In the free squat, the ankle plantar flexion torque is always need, as the weight of the body should be uniformly distributed between the rearfoot and forefoot (\(x_{\text{GR}}\) at midfoot) and the ground reaction is vertical (\(R_{\text{GR}}=0\)) [7]. Interestingly, the wall squat constitutes a bodyweight closed-kinetic-chain (CKC) exercise that might enable an effective strengthening of the ankle dorsiflexor muscles. CKC exercises are frequently used in rehabilitation as weight-bearing, which is a typical condition of CKC exercises, stabilizes the joints and protects ligaments [13; 14]. The muscle torque at the knee can be of extension or flexion type, depending on the selected variants. Specifically, the wall squat performed in the last \(30^{\circ}\) of knee extension, leaning against the wall with only the scapular zone (\(y_{\text{wall}}=y_{\text{shoulder}}\)), and placing the feet away from the wall, constitutes a CKC exercise for strengthening of the knee flexor muscles (\(\tau_{\text{knee}}<0\)). Conversely, the knee-extension muscle torque developed for \(y_{\text{wall}}=y_{\text{hip}}\), \(\theta_{\text{knee}}\sim 80^{\circ}\), and \(d=50\) cm reaches a value as high as 250 Nm, which nearly coincides with the peak torque potential of the knee extensor muscles [15; 18]. However, this wall-squat setup entails an extreme body position, with the hips nearly in full flexion (\(\theta_{\text{hip}}=60^{\circ}\)), 26 cm above the ground, and 22 cm below the level of the knees. In this condition (i.e., hip below the knee level and \(y_{\text{wall}}=y_{\text{hip}}\)) the quadriceps torque is maximized because the wall force \(\overset{\rightarrow}{F}_{\text{wall}}\) yields a knee-flexion torque, which adds to the knee-flexion torque developed by weight of the body (more precisely, by the weight of the portion of the body above the knee joints). This study highlights that low levels (about 60 N) of ACL-loading TF shear force (\(\varphi_{\text{TF, shear}}\)) may be developed in the final \(45^{\circ}\) of knee extension when \(x_{\text{GR}}=x_{\text{ankle}}\). However, these values of \(\varphi_{\text{TF, shear}}\) have been calculated assuming that a net knee-extension muscle torque (\(\tau_{\text{knee}}>0\)) is produced only by the quadriceps, in absence of hamstring cocontraction. Thus, as explained in the methods section, they should be considered as the maximum theoretical limit of ACL-loading TF shear force that might occur during the bodyweight wall-squat exercise. Nevertheless, due to the reduced equilibrium and stability demand induced by wall support and absence of external resistance, low cocontraction hamstring activity is expected when \(\tau_{\text{knee}}>0\) and \(\theta_{\text{knee}}<45\). This study could have important practical applications in training and rehabilitation interventions. With the use of Figures 3-5, trainers and physiotherapists might readily manage the wall-squat exercise conditions (\(d\), \(\theta_{\text{knee}}\), \(y_{\text{wall}}\), and \(x_{\text{GR}}\)) to finely modulate the lower-limb joint torques up to the their maximum values reported in Table 1. Interestingly, contrary to the free squat, the joint toques can be regulated, to a large extent, independently from one another. Figure 6 clearly highlights that anterior tibial pull forces can be avoided, even near full knee extension, in specific exercise conditions. Therefore, if adequately managed, then the wall squat exercise can be safely used in rehabilitation interventions after anterior cruciate ligament injury or reconstruction. The main limitation of this study stems from the lack of information related to the mechanical loading directly acting on the ACL.