## 4 Discussion  
Quantifying errors in optical motion capture analysis and its use with a multi-segment foot model has not been previously performed to our knowledge. With the four-segment model of [PERSON] and [PERSON] [11], four of the possible six intersegmental motions were examined. The other two could not be measured with the fluoroscopic RSA system and the current testing protocol since the field of view was too small to image the ankle joint or the hallux. Of the four intersegmental joint motions examined, there were no significant differences found between optical motion capture and fluoroscopic RSA for motions of the hindfoot and forefoot. But the MLA height-to-length ratio was significantly different at midstance and toe off. The intersegmental kinematics measured in this study with the multi-segment foot model agree with the results of [PERSON] and [PERSON] [11]. Early in the stance phase, the hindfoot tended to pronate and internally rotate. It then tended to supinate and externally rotate at the approach of toe off. This accompanied the MLA height-to-length ratio that showed an initially depressing arch early in the stance phase and a rising arch later in the stance phase. The forefoot in this study tended to pronate early in the stance and then supinate as toe off approached. This is all consistent with a pronated and flexible foot early in the stance phase and a supinate and rigid foot configuration later in the stance phase. A fluoroscopic RSA study by [PERSON] et al. [17] calculated kinematics within the foot slightly differently than the current study. Joint motions were reported between individual bones of the foot rather than between functional segments of the foot. The mean range of motion of the calcaneus with respect to the talus in the frontal plane was 8.9\({}^{\circ}\). This joint motion is a good approximation of the supination/pronation movement of the hindfoot tested in our current study, which had a mean range of motion of 8.05\({}^{\circ}\)[17]. The other joint motions from the [PERSON] et al. [17] study had too dissimilar definitions to serve as reliable comparisons. The MLA height-to-length ratio kinematics showed the smallest difference between optical motion capture and fluoroscopic RSA. The differences found were 0.00, 0.02, and 0.05, respectively, for heel strike, midstance, and toe off. Therefore, the measurement of the MLA during walking gait can be reliably performed using a multi-segment foot model and optical motion capture. In a clinical setting, the behaviour of the MLA is of diagnostic value for determining foot pathology. High-arched (pes cavus) individuals tend to have a more raised and rigid arch, which limits shock absorption in early stance [18]. [PERSON] et al. [19] found that high-arched individuals with more mobile arches displayed smaller initial loading forces as well as lower forces at the second vertical ground reaction force peak during the loading phase [19]. Flat-footed (pes planus) individuals tend to have difficulty with force production later in the stance phase. [PERSON] et al. [20] found that individuals with low arches had increased ground reaction forces peaks during two-foot drop landings when compared to normal feet [20]. Thus, quantifying the MLA height-to-length ratio could also be used to dynamically categorize the pathological foot type [18]. Hindfoot internal/external rotation showed the next smallest difference between optical motion capture and fluoroscopic RSA. Differences were found to be 1.33\({}^{\circ}\), 0.45\({}^{\circ}\), and 0.95\({}^{\circ}\) for heel strike, midstance, and toe off, respectively, with the greatest difference seen at heel strike. This may be due to increased muscle contraction around the ankle and subtalar joints causing the markers to move relative to the underlying bone. The hindfoot