Short communicationThe effect of marker size on three-dimensional motion analysis of the foot
Introduction
Marker-based three-dimensional motion analysis is based on reflective markers that usually are skin-mounted and captured by light-emitting cameras. The available marker sizes can vary in their diameter.
Based on best practice experiences, small markers are recommended for small segments and movements, such as hand or finger motions, while larger markers are used for gait analysis. Arguably, the larger the marker, the more pixels can be processed to determine the location of the marker [1]. However, a larger marker might affect further calculations, because the center of the marker moves away from the desired anatomical landmark. A larger marker size might also increase its susceptibility to differences in skin rigidity [2]. In addition, large markers might hamper the subjects’ natural motion, or increase the risk of falling off by accidental collisions.
In foot biomechanics, the size of used markers ranges from 4 to 16 mm [3,4], as summarized recently [5]. However, despite this variation, there has been little quantitative analysis of the effect of marker size on the captured marker trajectories and kinematic parameters. Therefore, this paper has two objectives: First, to investigate a possible impact of skin rigidity on differences in marker trajectories. Second, to ascertain the effect of marker size on the calculation of kinematic parameters that reflect foot biomechanics.
Section snippets
Methods
The study was approved by the institutional ethics committee (V-287-17-FE-Füße-02072018) and conducted in accordance with the Declaration of Helsinki. Seven males and ten females (27.0 ± 4.7 years; 172.9 ± 10.16 cm; 70.7 ± 18.8 kg) gave informed consent and participated in this study. The number of participants ranges between studies of a comparable nature [4,6].
Five anatomical landmarks of the right leg were selected (Fig. 1B): posterior calcaneus (CAL), first and fifth metatarsal head (MET1,
Results
There were significant differences in the oscillation frequency between the anatomical landmarks (p < 0.001, Welch test). The results of the Dunnet-T3 post-hoc analyses are summarized in Fig. 1A. The differences in the oscillation frequency were not reflected by the spatial relation of the marker trajectories. Table 1 compares the RMSE of the spatial relation in x-, y-, and z-direction. The differences between the two marker sizes ranged between 2.51 ± 0.92 mm and 7.54 ± 8.18 mm. The effect of
Discussion
This study investigated the effects of marker size on three-dimensional estimates of foot kinematics. The results did not show significant differences between the two marker sizes, but practical aspects favor the application of small markers.
We expected that greater skin rigidity would result in considerable differences in the trajectories of both marker sizes. However, despite significant variation in skin motion, the mean differences were all within a comparable range. Presumably, the
Declaration of Competing Interest
None.
CRediT authorship contribution statement
Florian Ebrecht: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. Freddy Sichting: Conceptualization, Methodology, Software, Writing - review & editing, Supervision.
Acknowledgments
The Saxon State government finances this study out of the State budget approved by the Saxon State Parliament. The authors thank Lisa Peterson for proofreading of the manuscript. The authors also thank Franziska Karl, Judith Helzer and Richard Parker for support during data collection. A special thank goes to Nicolai Kram.
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