###### Abstract  
Gait analysis with optical motion capture typically treats the foot as a single segment, which can measure clinically useful kinematics but is insufficient to measure the kinematics of joint motions within the foot. This study hypothesizes that a four-segment foot model, tracking the hindfoot, midfoot, forefoot, and hallux, can accurately measure intrinsic foot kinematics when validated against the gold standard of fluoroscopic X-ray radiostereometric analysis (RSA) during walking gait. Ten healthy volunteers were tested, with the left foot tracked during the stance phase from heel strike to toe off. The results indicated that the height-to-length ratio of the medial longitudinal arch (MLA) and the transverse plane motion of the hindfoot were the most reliable kinematic measures, showing the best agreement between the optical motion capture and RSA methods. In contrast, the frontal plane motions of the hindfoot and forefoot showed the greatest differences, though these were not statistically significant at \(p<0.05\). These findings demonstrate that the multi-segment foot model is a valid method for measuring intrinsic foot kinematics in a clinical setting, providing a reliable alternative to more invasive techniques.  
gait; foot kinematics; multi-segment model; biomechanics; radiostereometric analysis; fluoroscopy +
Footnote †: journal: Applied sciences  
1  
model used in this study was developed by [PERSON] and [PERSON] [11]. This model tracks the foot as four connected segments: the hindfoot (calaneous), midfoot (three cuneiforms, nauicular, and cuboid), forefoot (metatarsals), and hallux [11].  
Optical motion capture tracks reflective markers attached at various anatomical landmarks on the body. Three markers per rigid segment can track the six-degree-of-freedom pose of the segment [7; 8; 9]. While optical motion capture has the advantage of allowing the patient great freedom of movement, its limitation is that the reflective markers must be attached to the skin or clothing. Therefore, there is always an amount of relative motion between the markers and the underlying bones [12]. Multi-segment foot models require more reflective markers to be attached to the foot than the usual three. How much more kinematic error arises due to these extra markers is unknown, and the effect on the intersegmental measurements within the foot is unknown. This limits the clinical adoption of multi-segment foot models [4; 5; 6; 7; 8; 9; 10].  
A more accurate method for tracking bone kinematics is available with bi-planar fluoroscopic radiostereometric analysis (RSA) [12; 13; 14; 15]. X-ray fluoroscopy allows for the direct and simultaneous visualization of the skin-mounted markers of the optical system and the bones themselves during dynamic, weight-bearing activity. Bi-planar fluoroscopy images the bones of interest from two different angles. The RSA method determines the six-degree-of-freedom pose of each bone with respect to time [12; 13; 14; 15; 16]. The accuracy of fluoroscopic RSA has been demonstrated to be superior to optical motion capture in the foot [14; 15].  
In the current study, the kinematics of the joints of the foot are measured using the multi-segment foot model with optical motion capture and bi-planar fluoroscopic RSA simultaneously. The purpose of this study is to validate the intersegment kinematics as measured with the multi-segment foot model and quantify the errors arising from relative motion between the reflective markers and the underlying bones. This is performed in healthy volunteers during a fully weight-bearing walking gait.