Foot kinematics during walking measured using bone and surface mounted markers
Introduction
Rigid segment models of the foot have been used to reduce the complexity of the foot for experimental (Hunt et al., 2001, Simon et al., 2006; Leardini et al., 1999a, Leardini et al., 2006; Kitaoka et al., 2006) and clinical studies (Woodburn, et al., 2004, Rattanaprasert, 1999, Theologis et al., 2003, Khazzam et al., 2006, Tome et al., 2006). In these models there are two sources of error, (i) the simplification of the foot anatomy into rigid segments which in reality are not rigid, and (ii) skin movement artefact. With respect to the first source of error, a scientific approach to defining the most appropriate number and composition of foot segments has not been proposed and segment definitions are decided largely by limitations related to marker location and acquisition of quality kinematic data for each marker. The models have varying degrees of evidence supporting their reliability (Carson et al., 2001, Simon et al., 2006, Stebbins et al., 2006) but none has evidence of the degree to which each segment in the model accurately represents the kinematics of the underlying bones comprising each segment. Thus, the errors in experimental kinematic data due to violation of the rigid body assumption are unknown.
With respect to the second source of error, skin movement artefact, Westblad et al (2002) reported the only available data related to walking, quantifying skin movement artefact for movement of the calcaneus relative to the tibia. Root mean square errors over the stance phase were small at 2.5° (inversion/eversion), 1.7° (plantarflexion/dorsiflexion) and 2.8° (adduction/abduction). Wrbaskic and Dowling (2007) reported 2D marker displacement errors between the first metatarsal and heel during simulated quasi-static tasks, but these data (in millimeters) are difficult to relate to the more common approach of reporting in vivo kinematic data as degrees of movement between rigid foot segments. No study has reported 3D skin movement artefact for the mid- and forefoot segments during walking. One suggestion for minimising error in kinematic data is to locate markers on rigid plates attached to the skin surface rather than directly onto the skin (Leardini et al., 1999a, Leardini et al., 1999b; Manal et al., 2000, Yack et al., 2000, Houck et al., 2004, Holden et al., 1997, Benedetti et al., 1998). Whilst this eradicates relative movement between markers it does not necessarily reduce errors due to skin movement. Indeed, the inertia of the rigid plate might introduce further errors (Leardini et al., 2005). To date there has been no comparison of kinematic data derived from skin and plate based marker protocols for the foot.
The aim of this study was to compare the kinematic description provided by an experimental four segment rigid body foot model, to the kinematics of the foot bones comprising the four segments. The four segment foot model was represented using two different marker attachment protocols. The first involved attaching markers directly to the skin surface, and the second involved attaching markers to rigid plates which were mounted on the skin surface. Thus, the study would collect and compare three data sets describing foot kinematics from (i) bone anchored markers, (ii) markers mounted directly onto the skin surface and (iii) using markers attached to plates mounted onto the skin surface. The comparisons would enable the effect of violation of the rigid segment assumption in the four segment foot model combined with the effects of skin movement artefact to be quantified. This would enable recommendations to be made for use of either the skin or plate marker attachment protocol.
Section snippets
Data collection
Six male volunteers (mean age 38, range 28–55, mean weight 85 kg, range 71–110, mean height 180.5 cm, range 176–183) gave informed consent to participate in the study. The study was approved by the ethical committee of Huddinge University Hospital, Sweden.
For the skin and plate mounted marker protocols, a pen was used to indicate the site where each reflective marker should be attached (Table 1). In the skin condition, 9 mm reflective markers were attached directly to these sites. For the plate
Results
There were only small differences between the stance times in the three experimental conditions (Table 3) (maximum 0.02 s). There were no statistically significant differences in the timing of any of the ground reaction force data, but there were differences in the magnitude of the ground reaction forces (Table 4). There were statistically significant differences in the tibial kinematics between the three experimental conditions (Table 5). In the sagittal plane, the mean difference was 2.6° (SD
Comparison of walking in the three experimental sessions
Most studies investigating skin movement artefact use the same gait trials to compare data from skin mounted and bone anchored markers (Lafortune et al., 1992, Reinschmidt et al., 1997, Yack et al., 2000). However, in the foot, there is insufficient space to place bone pins into individual bones safely and have skin and plates mounted markers in situ at the same time. Furthermore, bone pins may tether the skin and thus interfere with the skin movement artefact. The only alternative is to
Conclusion
In the context of the inevitable differences due to the data being collected in three separate testing sessions, the data do not provide a clear answer as to whether a skin or plate mounted marker protocol is preferable. The differences between skin and plate protocols and the actual bone motion were not consistent between subjects, between joints nor between planes of motion. Therefore, if a researcher's choice of foot model and marker attachment method is made with the objective of minimising
Conflict of interest
All authors have been involved in the research and written publication.
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