## 4 Discussion  
This was the first systematic review and meta-analysis to examine the effect of foot wedges on thigh muscles' activity during squatting. The results of the meta-analysis showed a significant effect of the use of posterior wedges on thigh muscles' activity levels (\(p<0.001\)). Importantly, sub-group assessments showed that these effects were selective during squatting, as the posterior wedge increased the activity level of the vastus medialis and the anterior wedge altered the activity of the rectus femoris (\(p<0.05\)). A biomechanical concept called the muscle length-tension relationship may, in part, explain changes in muscle activation due to using various foot wedges. This relationship states that muscle activity is significantly influenced by changes in joint position, muscle length, muscle contraction speed, and muscle action line [5]. Squatting requires concurrent movement of the hip, knee, and ankle joints [42]. Using a posterior wedge during squatting creates ankle joint plantarflexion, which reduces the passive tension and activity level of the triceps surae muscle group [9], resulting in higher activity in the knee extensor muscles, which was generally the case in the selected studies. Therefore, reducing the activity of the calf muscles can be considered as a mechanism for increasing the activity of the thigh muscles [43]. Proximal kinematic changes may be induced by altering the ankle joint position during the squat movement, such as a more erect trunk position with reduced hip flexion. This would shift the center of mass (COM) further back [19], which increases the knee extensor torque and thigh muscle activity [19; 44]. Increased quadriceps muscle activity with the posterior wedge may also be due to a narrower level of support, which  
Figure 4: Forest plot for the sub-group effect of medial wedge on the activity level of thigh muscles [5; 18; 20]. Figure 5: Forest plot for the sub-group effect of lateral wedge on the activity level of thigh muscles [18; 20]. challenges stability [45]. Using the posterior wedge during squatting to induce higher activity levels of the vastus medialis, as observed in the present study, can be a useful option for rehabilitation or strength programs. For example, when the quadriceps muscle strength is impaired in people with patellofemoral pain syndrome [46], specific strengthening of this muscle can be done by using the posterior wedge during squats. Conversely, squat movements are widely used to lift loads in work environments [5], where the goal is to delay fatigue by reducing the activity of thigh muscles, in which case, the posterior wedge is not appropriate. As noted above, the alignment of the lower limbs and joint angles, especially the knee, directly influences the muscles' length-tension relationship, which can affect the level of muscle activity. The selected studies exclusively included healthy subjects with no documentation of remarkable abnormalities in the lower limbs, except for one study [17]. Although knee joint angles were reported, the influence of specific kinematics on changes in the muscle activation levels were not investigated. The selected studies used shallow [17; 18; 20], medium [5; 9; 10; 12; 14; 19], or deep squats [11; 13; 15; 16; 27; 32], and some of them controlled the posture of lower limbs during squatting between non-wedge and wedge conditions [5; 9; 10; 14; 17; 18; 19]. Therefore, caution should be made when relating the results of this meta-analysis with the muscle length-tension relationship. The results of the meta-analysis showed that the anterior wedge only had a significant effect on the activity level of the rectus femoris during squatting (\(p<0.05\)), as increased activity was reported in two studies [9; 13], although a third study reported the opposite finding [18]. As noted above, lower-limb muscle activity is dependent on the knee flexion angle [13] and postural stability [47].