## 4 Discussion
\begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Heel Strike** & **Midstance** & **Toe Off** \\ \hline Hindfoot & & & \\ Supination/pronation (\({}^{\circ}\)) & \(-\)4.45 & \(-\)5.12 & \(-\)6.70 \\ Internal/external rotation (\({}^{\circ}\)) & \(-\)1.33 & 0.45 & \(-\)0.95 \\ Forefoot (\({}^{\circ}\)) & 7.68 & \(-\)1.81 & \(-\)13.63 \\ MLA height-to-length ratio & 0.00 & 0.02 & 0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Difference in the angles between optical motion capture and fluoroscopic RSA for all foot joint motions and the normalized MLA height-to-length ratio. marker cluster is placed adjacent to the peroneus longus and brevis tendon pathways. It is perhaps the tendon motion that partially influences the marker cluster positioning. It is interesting to note that hindfoot internal/external rotation is the least variable difference between test subjects [21]. Hindfoot supination/pronation motion had differences between optical motion capture and fluoroscopic RSA of 4.45\({}^{\circ}\), 5.12\({}^{\circ}\), and 6.70\({}^{\circ}\) for heel strike, midstance, and toe off, respectively. There was also a greater range of 15.80\({}^{\circ}\) measured with optical motion capture than for fluoroscopic RSA, 8.05\({}^{\circ}\). This is consistent with the literature, which found that frontal plane markers and bone markers had poor agreement. Three studies found greater errors in skin-mounted markers relative to the underlying bone in the frontal area of the knee during running [22], in the ankle [17; 23; 24], and in the foot [23; 24]. These studies addressed the same issue of validation as in the current study and found that while the range of motion was larger with optical techniques, the mean measurements were in relatively good agreement [17; 22; 23]. The greatest difference between optical motion capture and fluoroscopy was in the supination/pronation motion of the forefoot. The differences seen between motion capture and fluoroscopy were 7.68\({}^{\circ}\), 1.81\({}^{\circ}\), and 13.63\({}^{\circ}\) for heel strike, midstance, and toe off, respectively, with the greatest difference at toe off. The tibialis anterior muscle is primarily responsible for dorsi-flexing the foot at toe off. Its tendon inserts on the dorsal surface of the first cuneiform, close to the midfoot marker cluster. Therefore, contraction of the tibialis anterior may be influencing the position of this cluster [21]. The range for this joint was 18.27\({}^{\circ}\) as measured by optical motion capture versus 10.18\({}^{\circ}\) for fluoroscopic RSA. One difficulty using a multi-segment foot model is that significantly more markers must be placed on the foot than when the foot is tracked as a single rigid segment. Five marker clusters are placed on the foot with the current model, with three on the medial side of the foot. With small feet, these clusters can be too close together for reliable tracking. This study had limitations. The sample size was small due to the difficulty of collecting and analysing bi-planar fluoroscopic RSA data. Work is currently underway in our facility to increase this initial sample size. Future studies should examine the influence of pes cavus and pes planus foot types since the intersegmental motion within the foot is also very likely to be different with these foot pathologies, as the MLA behaviour has been shown to be. Validating the multi-segment foot model during walking and running in shoes should also be performed. Shoes may restrict the intersegmental motion of the foot segments and control the rise and fall of the MLA.