This study described and compared the three-dimensional kinematics of the calcaneus, knee, and hip joints in elementary school children with and without flexible flatfeet. During the stance phase of the gait, the pattern of calcaneal movement was similar between the two groups, and was in agreement with the pattern reported in previous kinematic studies [8
]. The maximum dorsiflexion angle found in this study was around 8°, which was comparable to the data reported by Ganley and Powers found in adults [25
]. With or without flexible flatfeet, children in our study showed a maximum calcaneal eversion angle of 3° to 4.5° during the stance phase. Levinger et al. reported a similar maximum calcaneal eversion angle in adults [8
]. A larger calcaneal eversion angle between 10° to 15° was observed in both adults and children with flatfeet [15
] and with normal arch [26
]. Differences in data collection methods and joint angle definitions would partly contribute to these discrepancies.
Excessive calcaneal eversion is suggested as an important factor related to lower extremity kinematic deviations in subjects with flexible flatfeet [8
]. Despite children of the flexible flatfoot group showed a significantly larger calcaneal eversion angle in quiet standing (Table ), similar calcaneal movement during walking was noted for both groups (Table ). The discrepancies in calcaneal angular changes during quiet standing and walking might be a result of different loading conditions (relaxed standing with most muscles quiet vs. walking with active muscle control) and diverse coupling movement patterns of the whole lower extremity kinematic chain.
Our results were supported by findings of Twomey et al. that kinematic differences in the foot between children with normal and low arched feet were small [29
]. In adults, Hunt and Smith found no statistical difference in the total range of calcaneal frontal plane movement between normal and flexible flatfoot groups [30
]; whereas Kernozek and Ricard [31
], and Williams et al. [32
] demonstrated that a lower arch height led to greater calcaneal eversion excursion and maximum calcaneal eversion angle. Levinger and colleagues also identified a trend of larger rearfoot eversion in adults with flexible flatfeet [8
]. These inconsistent findings might be a result of subject variations, such as different age groups and severity of flatfoot, and differences in the definition and classification of flatfoot, and in methods of motion measurement [8
]. In addition, researchers should not overlook the contribution of midfoot and forefoot joints to the presence of flexible flatfoot [1
]. The collapsed medial arch during weight bearing could also be a result of a pronated midfoot or forefoot, which was not measured as part of this study.
With the tight connection between tibia and talus under a weight bearing condition, it has been proposed that the presentation of flexible flatfoot could be associated with abnormal knee and/or hip rotation in the transverse plane, which might lead to lower extremity problems, such as patellofemoral pain [34
]. Despite similar sagittal and frontal plane knee joint movement in the two groups, we found that children with flexible flatfoot exhibited a trend towards increased knee internal rotation in the transverse plane throughout the stance phase (Figure , Table ). Therefore, possible future injuries associated with this alteration should be carefully monitored in this population.
No previous studies have compared hip joint kinematics between subjects with and without flatfoot conditions. This is the first study to identify that children with flexible flatfeet have a tendency towards greater hip flexion, adduction and less hip internal rotation during the stance phase of gait (Figure ). At the instance of heel strike, the hip exhibited a similar rotational position in the transverse plane for both groups of subjects; as the gait progressed, children with flexible flatfeet demonstrated less overall movement of internal hip rotation during the weight bearing phase. The altered knee and hip kinematics might be a compensatory mechanism for excessive knee joint internal rotation. Whether changes of hip and knee kinematic performance were unique in our subject group, and whether these alterations linked to any clinical symptoms should be examined further.
There were a few limitations of this study. We used a skin-based method of sensor placement, which usually causes minor measurement errors because of the skin movement. These measurement errors might contribute to larger SEMs for some parameters such as maximum calcaneal eversion, thus resulting in our insignificant findings. Although the measurement method including the kinematic model used in this study, has been reported previously in adults, it has not been validated in children. Whether or not factors such as different anthropometric proportions in children jeopardized our measurement accuracy should be considered. The screening method for flexible flatfoot in this study was clinical observation combined with the measurement of navicular height. Although this method is not as accurate as the imaging techniques such as radiography, it is practical and commonly used in clinical practice. Lastly, the alignment and movement of the midfoot and forefoot, which could possibly impact our findings, were not measured in this study. An investigation taking account of midfoot and forefoot structure may need to be considered in future studies.