## 2 Materials and Methods
### Equipment and Procedure  
The hardware used for measuring purposes consisted of two digital cameras (model GE A1050) of 10.1 megapixels, two extensible tripods capable of reaching a height of 20 cm, a USB 2.0 cable of 1 m length, and a laptop ACER Intel Core i3-370M (2.40 Ghz) with 4 GB of RAM. Cameras recorded the walks of the participants in Quick Time Motion JPEG format, which was later exported to the laptop, using the USB 2.0 cable. Software required for the procedure was ArcSoft (provided with the cameras), which allowed the PC to recognize the camera as an external storage drive; the free software Tracker, which is built on the Java Open-Source Physics (OSP) platform [40], was used to import the JPEG videos and analyze the motion. First, the spatio-temporal variables were measured. Considering that the mean height of the three women who participated in our experiment was 1.60 m, a 6 m track length (T\({}_{\text{L}}\)) was set (Figure 2). Each participant did three walks and maintained a pace of approximately 1 m/s with each heel height along the track. The values obtained for the gait cycle length (L\({}_{\text{C}}\)), time spent to do a full cycle (T\({}_{\text{C}}\)) and speed (Sc), were recorded. The experimental design to measure the kinematic variables was based on recording each individual's full gait cycle under barefoot conditions and wearing high heels of 2, 6 and 10 cm. The procedure followed to collect data from this study was divided into several steps:  
* Step 1. Filling location selection and conditioning: A sports center with daylight was selected to avoid any shadow projections that could affect the quality of recordings. * Step 2. Mounting recording cameras: Cameras were mounted on the horizontal plane at a height of 12 cm and were placed 32 cm away from where the track began. They were oriented toward the sagittal plane. One was placed in front of the other and they were separated by 4 m. The objective was to record both the medial and lateral foot sides at the same time as the participants moved along the line between them. * Step 3. Fixing markers on the participants: Markers were made of adhesive cloth. Two sizes were used. Those with a 2 cm diameter were placed on tarsals and metataralsals, while those with a 1 cm diameter signaled phalanges. They were all stuck onto a thin, white sock that tightly fitted to the feet (Figure 3). Markers stuck onto a sock  
Figure 2: Spatio-temporal variables: T\({}_{\text{L}}\): track length; L\({}_{\text{C}}\): gait cycle length. Adapted from [41]. instead of directly on the skin may induce one to think of losses of accuracy. However, the use of markers stuck onto pieces of cloth, such as suits or gloves, are common in motion capture works, and in some cases, they are even stuck onto the shoes, as [PERSON] et al. [21] did. In our case, an elastic rubber was placed on top of the sock (Figure 3) to ensure its tight fit to the legs, and after each walk, checks were made to see if the sock was in exactly the same position as it was at the beginning. The proposed method would also work with markers stuck onto the skin, so any possible bias, if they exist, can be easily avoided. On the lateral foot side, markers were placed on the lateral malleolus, 5 cm over the lateral malleolus, the calcaneus lower part, the head and base of the fifth metatarsal and the distal phalange of the fifth toe. On the medial foot side, markers were placed on the medial malleolus, 5 cm over the medial malleolus, the calcaneus lower part, the head and base of the first metatarsal and the distal phalange of the hallux. As shown, the 2 cm high heel shoe was a slipper with wide open spaces around the metatarsal that tightly fitted around the instep, while the shoes selected for 6 and 10 cm high heels were ankle-strap wedges. * Step 4.