## 4 Discussion  
We recorded and analysed the electromyographic activity of lower limb and lumbar muscles during static body-weight wall-squat exercises executed with knees flexed at 45\({}^{\circ}\). The results highlight that muscle activity can be finely modulated by manipulating three different exercise variables: the distance (\(d\)) of the ankles from the wall; the voluntary shift \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & \multicolumn{4}{c}{\(d_{1}\)} & \multicolumn{4}{c}{\(d_{2}\)} \\ \hline  & **S** & & **P** & & **S** & & **P** \\ \hline  & **R** & **F** & **R** & **F** & **R** & **F** & **R** & **F** \\ \hline ES & 11.5 (5.7) & 16.4 (6.0) & 8.8 (4.2) & 8.4 (3.6) & 17.9 (5.9) & 23.3 (7.1) & 7.8 (3.7) & 9.0 (4.3) \\ GM & 3.2 (1.4) & 4.1 (1.7) & 2.8 (1.1) & 2.9 (1.0) & 4.5 (2.0) & 5.8 (2.0) & 3.1 (1.2) & 3.2 (1.4) \\ VM & 22.7 (9.2) & 14.0 (6.8) & 21.9 (9.4) & 15.2 (7.7) & 20.1 (7.1) & 11.9 (4.9) & 23.3 (8.1) & 16.3 (7.4) \\ VL & 25.5 (7.6) & 16.1 (8.0) & 25.5 (9.3) & 16.9 (7.3) & 24.0 (8.8) & 13.4 (5.9) & 26.7 (8.9) & 18.9 (7.8) \\ BF & 3.0 (1.5) & 3.0 (1.3) & 2.9 (1.3) & 2.9 (1.5) & 3.6 (1.9) & 6.2 (3.1) & 3.0 (1.4) & 4.0 (2.1) \\ ST & 5.0 (2.0) & 6.6 (2.4) & 3.2 (1.2) & 3.0 (1.2) & 4.7 (1.9) & 10.4 (3.4) & 3.2 (1.2) & 4.6 (2.0) \\ TA & 44.1 (14.8) & 3.9 (2.0) & 41.2 (13.6) & 3.8 (1.8) & 32.1 (10.6) & 4.3 (2.1) & 33.1 (9.2) & 4.3 (1.8) \\ GL & 4.3 (2.1) & 7.4 (3.3) & 3.9 (1.6) & 6.4 (2.8) & 3.9 (1.2) & 16.9 (6.8) & 4.3 (1.4) & 16.9 (6.6) \\ \hline \end{tabular}
\end{table} 
Table 4: Muscle activation levels (expressed as a percentage of the peak electromyographic amplitude at MVIC) recorded during the SBWS exercise executed with knees flexed at 45\({}^{\circ}\) and in two different conditions for each of the three exercise variables: (1) horizontal distance \(d\) of the ankles from the wall (\(d_{1}=0.5\cdot l_{\rm thigh}\) and \(d_{2}=l_{\rm thigh}\)); (2) scapular (S) or pelvic (P) location of C\({}_{\rm wall}\); (3) rearfoot (R) or forefoot (F) location of C\({}_{\rm GR}\). Data reported as mean (standard deviation). ES = erector spinae, GM = gluteus maximus, VM = vastus medialis, VL = vastus lateralis, BF = biceps femoris, ST = semitendinosus, GL = gastrocnemius lateralis, TA = tibialis anterior; S: C\({}_{\rm wall}\) at the scapular zone, P: C\({}_{\rm wall}\) at the pelvic zone, R: C\({}_{\rm GR}\) at rearfoot, F: C\({}_{\rm GR}\) at forefoot. between the scapular and pelvic zone of the centre of pressure (\(\rm C_{wall}\)) of the force exerted by the wall on the back; the voluntary shift between the rearfoot and forefoot of the centre of pressure (\(\rm C_{GR}\)) of the ground reaction force. This study highlights that the VM and VL activity can be considerably increased (up to nearly 100%) by the shift in \(\rm C_{GR}\) from forefoot to rearfoot, independently of the values of the other variables. The effect size of the \(\rm C_{GR}\) variable on the quadriceps (\(0.80\leq\rm n_{p}^{2}\leq 0.84\)) is at least two times higher than that of the other variables and the variables tested by [PERSON] during the dynamic wall-squat exercise [7]. The VM and VL reach levels of activation up to about 25% MVIC, which can be considered effective in the initial phase of a quadriceps strengthening intervention for knee rehabilitating patients. In the following phase, the demand of knee-extensor muscle torque and the quadriceps activity can be progressively enhanced by gradually increasing the knee flexion angle from the value (45\({}^{\circ}\)) selected in this study [14]. However, a parallel increase in the tibiofemoral and patellofemoral compressive forces also occurs. Actually, a knee flexion angle of 45\({}^{\circ}\) allows the minimisation of the tibiofemoral and patellofemoral compressive forces in absence of ACL-loading tibiofemoral shear forces, which may occur in the final 40-45\({}^{\circ}\) range of knee extension [9; 14; 22; 23; 24; 25].