The foot is one of the most complex organs within the body, and not only serves as a mode of transportation in our daily lives but also plays a crucial role in maintaining balance. It forms a vital component of a closed-chain system in which the lower limb provides support to the body. Consequently, any deviation in foot posture and stability can affect adjacent segments more proximally. ¹

Kinematic studies are often required to objectively evaluate human movements during various activities, including gait analysis, clinical research, sports management, footwear, and orthopedics. ² Regardless, the administration of these assessments remains highly dependent on the evaluators’ subjectivity. The goal of kinematic analysis is to obtain quantifiable data to help analyze the patient’s condition and changes during different moments, that is, changes in pre-and/or post-management. Gait analysis is a rigorous examination, requiring advanced medical tools and systems to perform an accurate analysis. Technical challenges with interpretation and setup, accompanied by expensive equipment and software, can restrict their application in both clinical and research settings. ³ To mitigate these issues associated with three-dimensional motion capture systems, new low-cost motion analysis applications have been emerging over the past couple of years. ^{4, 5}

One of these novel applications is Kinovea, which was first created in 2009 by a non-profit collaboration between several researchers, athletes, coaches, and programmers worldwide. Kinovea is an open-access application that allows physicians worldwide to conduct gait analysis with available technology. This application allows examiners to measure angles, distances, coordinates, and spatial-temporal parameters. Measurements can be taken from multiple perspectives, as the software can perform calibration in non-perpendicular planes of the camera-object line. ³ Within the realms of medicine, Kinovea has been used in many fields of research, medication or clinical analysis and sports.

Although Kinovea is a user-friendly, portable, and relatively cost-free tool, it is not without its flaws. Prior research has yielded inconclusive findings regarding its reliability, ranging from poor to excellent, with emphasis on the significance of proper configuration before application. ^{6– 8}

This study aimed to assess the reliability and applicability of a simple, cost-effective, user-friendly, and objective dynamic foot posture evaluation using the Kinovea software. By evaluating the inter-rater reliability of the Kinovea, an assessment of dynamic foot posture, represented by measurements of ankle inclination, medial longitudinal arch, calcaneal angle, and MTP 1 (metatarsophalangeal) angle at each gait phase will be conducted.

The study recruited 16 participants, consisting of equal numbers of males and females, without any alteration in gait patterns. There were no missing data in the examiners’ results. Video recording of patients were taken for all participants at varying speeds at 2, 3, and 4 kmph. Video analysis was conducted by two different examiners to compare the accuracy and reliability when observing MLA, MTP 1, CA and IA as shown in Figure 3.

As mentioned above, the point estimate of ICC will be interpreted as excellent (>0.9), good (0.76-0.9), moderate (0.5-0.75) and poor (<0.50). ⁹ The table below represents the mean angle measured, standard deviation of each examiner as well as the ICC results, 95%CI and p-value.

Table 1 presents the results obtained when the participants were asked to walk on the treadmill at 2 km/h. An inter-rater reliability of >0.9. was observed in both MLA and MTP 1 parameters, indicating excellent correlation, while CA and AI showed good correlation at 0.893 and 0.888 ICC, respectively. As seen in Tables 2 and 3, these observations remained consistent throughout the examination, as the same results were observed at 3 km/h and 4 km/h, with improvements in the AI ICC at 4 km/h, to a value of 0.927 from 0.889 at 3 km/h.

The human foot comprises a complex arrangement of 26 bones, making the assessment of interosseous angles extremely difficult, especially during dynamic processes such as walking. In particular, MTP 1 and MLA angles play essential roles as early indicators of foot pathologies. Restriction of the MTP 1 joint motion, such as in the hallux rigidus, affects propulsion during toe-off. This causes foot inversion and lateral transfer of stress, and can ultimately disrupt stride length. MLA changes are associated with early signs of foot pronation and hallux valgus. Both these pathologies have been shown to increase lumbopelvic muscle activation, causing low back pain and other dysfunctions. ^{10, 11}

Previous research has been conducted to analyze the inter-reliability of Kinovea to measure its consistency in large joints, such as the knee, ankle, hip, and pelvis. ^{2, 3, 6, 12, 13} However, there is a lack of evidence in the reliability of its use in smaller joints such as the foot. ^{2, 12, 13} The purpose of this study is to examine the consistency among different raters, and reliability of Kinovea as an objective, quantitative, cost-efficient, and straightforward, utilizing a 2D video setup, motion analysis software in analyzing the smaller joints of the human foot. To the best of our knowledge, this study is one of the first to compare the inter-rater reliability of Kinovea to measure dynamic foot posture at varying speeds during a gait cycle.

Kinovea allows for an objective and quantitative dynamic foot posture analysis using free software. The requirement to conduct the examination is relatively simple and allows for repeated measurements, which allows in-depth examination where three-dimensional motion capture systems might not be readily available. However, the primary drawbacks in reliability studies for Kinovea ^® in the medical literature are the lack of a standardized video protocol and consistent marker placement to allow homogenized and standardized examination. ^{12, 14}

The primary outcome of this study was to compare the inter-class reliability of the Kinovea to evaluate various dynamic foot posture parameters between examiners. Four parameters (MLA, MTP 1, CA, and AI) were measured during different gait phases by two independent examiners, and the ICC results were analyzed by a third examiner for statistical significance. All measurements had either excellent or good ICC between examiners, indicating that both examiners were able to consistently measure parameters reliably. These findings were especially true for MLA and MTP where ICC values of >0.9 were consistently observed at 2 kmph up to 4 kmph and for AI at 4 kmph, while “good” correlations were seen in when CA and AI were measured throughout 2 kmph to 3 kmph. These findings imply that Kinovea can be used in various settings, including various speeds, as long as the measurements are taken with the same parameters regardless of the examiner.

The high ICCs observed may be ascribed to the effective standardization of marker placement and sufficient training sessions undertaken by the raters, thereby minimizing errors. An accurate, user-friendly, and cost-effective tool is essential in any medical profession. Because Kinovea is free to download applications, it has virtually no cost to use, except for the camera, light source, and sticker marker. 3D motion analysis systems are dependable in medical contexts but pose challenges owing to their inconvenience, high cost, complexity, technical proficiency, and expensive equipment. In 2014, Macleod et al. estimated that the cost of conducting 3D gait studies could reach 2,000 USD, and the cost of building and constructing a movement laboratory could be as high as 300,000 USD. Given the advancements in technology, this value can be even higher. ¹⁵ As such, dynamic foot posture analysis with Kinovea proved to be a reliable alternative, which is more cost-friendly and can be applied more widely.

This study, conducted on 32 specimens, showed that Kinovea is a reliable and applicable software for examining and analyzing dynamic foot posture parameters at varying speeds. It was observed that reliability was obtained and maintained with “good” result, and even “excellent” in most of the parameters measured. In conclusion, Kinovea as a free software can be used as a reliable alternative for gait evaluation in clinical settings, where 3D motion analyzers might not be readily available.

This study was approved by the Research Ethics Committee of Universitas Indonesia. All methods were carried out in accordance with relevant guidelines and regulations (ethical approval reference number: KET-345/UN2.F1/ETIK/PPM.00.02/2023, March 23, 2023). Written informed consent was obtained from all study participants before the start of the study.