## 1 Introduction
### High-Heeled Gait  
Wearing high-heeled shoes is one of the factors that most affects general gait and posture characteristics [12]. The influence of high heels on gait has been studied in the scientific literature from many viewpoints, such as clinical [13], biomechanical [14; 15], energetic [16], kinetic [2; 17; 18] and kinematic [2; 19; 20].  
The use of high heels induces prejudicial effects on the musculoskeletal system [21], such as foot pain [22], lower back pain [23], ankle grains [24], and increases the probability of suffering degenerative osteoarthritis in the knee [25]. High heels also perturbate the synchronic interaction between the joints of the pelvis-leg system during gait, havinga noticeable effect on spatio-temporal parameters [26], joint kinematics [23; 26], muscle activity [27], energy consumption [16] and plantar foot pressures [28].  
From a kinematic point of view, many experimental techniques can be followed to analyze human joints [29], such as radiographic measurements [30], inertial sensors [31] or traditional goniometry [32; 33], but one of the most widely used in clinical, biomechanical and sports fields is optical motion tracking [34]. Such tracking techniques use markers stuck to the body to facilitate the identification of the anatomical points required by the system by their contrast so that movement can be later reproduced in a computer [35]. However, optical motion tracking systems normally involve several high-quality cameras (from 2 to 8) and licensed software is necessary to process footage. Clinics with low budgets might not be able to afford such systems, for example, those in underdeveloped countries. Therefore, finding low-cost systems that seek research sustainability and, therefore, pursue attaining one of the sustainable development goals (number 10--\"Reduce Inequalities\") [36] is of paramount interest.  
Kinematics can also be used to evaluate the function of healthy, pathological and operated joints [37] and, although the scientific literature about hip, knee and ankle joints when using high heels is ample, very few articles have studied metatarsophalangeal joints [18; 38; 39]. [PERSON] et al. [38] focused on this joint, but just studied the validity of a smartphone goniometer when compared with a traditional one, so all measures were statically taken. [PERSON] et al. [39] validated a portable system for kinematic and dynamic analysis of the foot during gait against a gold standard, but this system only covered the stance phase. [PERSON] et al. [18] are the only authors that seem to have studied the metatarsophalangeal joint when high heels are used, but they focused on a kinetic approach, centering in ground reaction forces; although they obtained data of angular variation in the joint, they did so just for 6.8 \(\pm\) 0.8 cm high heels. Therefore, as far as the authors know, no studies have investigated variations in the sagittal plane from a kinematic point of view while wearing different heights in the heel.  
This article presents a low-cost system capable of measuring angular variations in the sagittal plane in metatarsophalangeal and tibiotalar joints by means of a geometrical model and free video analysis software. A pilot study of healthy participants walking with high heels tested the effectiveness of the method, which has the potential to make gait analysis more affordable.