## 4 Discussion  
The present study investigated the effect of shoe heel height and concurrent cognitive demands on postural balance performance in women. First, it was hypothesized that postural balance performance would be impaired due to heel height. The results on the CoP sway magnitude showed that balance performance decreased as heel height increased, thus confirming our first hypothesis. It was further hypothesized that the interference of concurrent cognitive demands on postural balance performance would be more pronounced during quiet standing in shoes with an increased heel height. The results did not show a significant systematic effect of cognitive load on balance performance, nor an interaction between cognitive load and balance performance at different heel heights, rejecting our second hypothesis. It has been shown that an increase in heel height increases postural instability and induces a number of kinematic changes that place significant neuromuscular demands on the system. The main kinematic changes in the lower limbs when standing in high heels (i.e., typically over 4 cm) include lumbar flattening, posterior pelvic tilt and a significant decrease in the range of motion of the pelvis, knee and ankle joints [1; 2; 3]. Heel elevation induces an increase in electromyographic activity of the lower limb muscles, particularly the calf muscles, which may increase lower limb stiffness to cope with the possible threat to postural stability [11; 12], indicating a greater neuromuscular effort to balance control [13]. Furthermore, the magnitude and velocity of CoP oscillation either in anteroposterior or mediolateral direction increased with increasing heel height, indicating a significant compromise in postural balance [6; 8; 9]. We found a significantly greater CoP path length, up to 1.05 times greater in the medium- and high-heeled shoes compared to the low-heeled shoes. The increased CoP path length was associated with greater anteroposterior sway magnitude, up to 75%, while mediolateral CoP sway was unaffected by heel height. In agreement with our results, [PERSON] et al. and [PERSON] et al. [6; 7] also reported that the increase in heel height mainly affected the CoP range in the anteroposterior direction, with no changes in the mediolateral direction. Taking these results together, we can argue that the greater neuromuscular demand of balance control with increased heel height is regulated with anteroposterior adjustments of the CoP. From a biomechanical point of view, the increased heel in shoes mainly reduces the mediolateral base of support and thus limits the possibility of mediolateral adjustments. Everyday life is abundant with situations where cognitive and motor tasks occur simultaneously, and because cognitive resources are required for postural and motor control [15; 50], there is an interaction between cognitive and motor tasks. This interaction between the two tasks referred to as dual-tasking interference has been shown to result in the diversion of attentional resources to the secondary cognitive tasks [15; 33], thus rendering locomotor performance vulnerable. It has been reported that balance safety is prioritized at the expense of cognitive task performance [35; 36], referred to as the \"posture-first strategy\" [35]. Both cognitive tasks were considered as perceptually challenging. The participants performed the first task by recalling the exact order of the 10-digit string with an average of 5 \(\pm\) 2 errors, while in the sentence completion task, their performance was ~92% successful (mean score 5.5 out of the maximum score of 6 per sentence). These scores show that both cognitive tasks demanded close to maximum performance from the participants, and we can argue that important cognitive resources were used.