*2.3. Instrumentation; Assessment of the IMTT and PPH*

Three-dimensional degrees of the angular deviation of the IMTT bone movement; dorsiflexion/plantar flexion, adduction/abduction, and inversion/eversion at the first MCJ; dorsiflexion/plantar flexion, adduction/abduction, and inversion/eversion of PPH bone; and dorsiflexion/plantarflexion, adduction/abduction and inversion/eversion at the first MPJ, with and without wearing high-heeled shoes were studied during static and push-off phases using a 6Space Fastrak® (Polhemus, Inc., Colchester, VT, USA) (Figure 1). This system had a 120 Hz sampling rate and a excellent validity and reliability (*r* > 0.99) [38] with ICC = 0.88–0.99 and 0.95–0.99 (SEM = 0.7–0.8 mm), respectively [40], according to previous studies; the equipment contained two sensors that send an electromagnetic field with 6◦ of freedom relative to the electromagnetic emitter. A 30-foot serial cable connected the sensors and emitter to a receiver module that collected, filtered, and managed all the signal's information.

**Figure 1.** Polhemus device. From left to the right: sensors, receiver module, emitter module.

The signal from each sensor was captured by the receiver module. It was digitally transformed by the software to generate spatial orientation data from each sensor. The system showed a 8-mm static accuracy relative to the sensor position and 0.15◦ regarding sensor orientation; the error was 1.6% [38,41]. The relation between range vs. resolution in orientation aspect was 0.3 m to 1.16 m; in addition, the relation between range vs. orientation aspect was 0.3 to 0.0038◦.

In this study, the electromagnetic emitter was 96 cm high at the midway point on a 6-m raised walkway. There were no metallic elements near the electromagnetic device or in the subject's walking path to avoid possible interference [42]. One of the sensors was placed at the medial aspect of the head of the IMTT bone (Figure 2), and the second one was located along the medial aspect of the PPH (Figure 3) on the right foot; the Polhemus Fastrack® (Polhemus, Inc., Colchester, VT, USA) did not measure angles but did measure bone displacements. Thus, we assessed the mobility of the PPH regardless of IMTT mobility to determine which areas were affected by the high heels. Both sensors were attached with tape and secured to the skin with an auto-adhesive bandage (Figure 4); this medial location was selected according to a protocol devised by Welsh et al. [43] to minimal overlying soft tissue of extensor hallucis longus tendon's excursion [41,44] All cables were fixed via straps to the thighs with a belt.

**Figure 2.** Sensor one. Location placed on the medial head of first metatarsal bone.

**Figure 3.** Sensor two. Location placed on the border of the proximal phalanx of the hallux.

**Figure 4.** Fastening of the sensors. White bandages are auto-adhesive fixatives.
