Elsevier

Gait & Posture

Volume 82, October 2020, Pages 217-219
Gait & Posture

Short communication
The effect of marker size on three-dimensional motion analysis of the foot

https://doi.org/10.1016/j.gaitpost.2020.09.007Get rights and content

Highlights

  • Marker size does not have an impact on calculations of foot kinematics.

  • Larger markers appear to fall off more frequently.

  • Small markers should be used for motion analysis of the foot.

Abstract

Background

In the field of three-dimensional motion analysis of the foot, there is little agreement on the preferred size of markers to record kinematic parameters. Although currently applied marker sizes show a considerable range, there has been no detailed investigation of the effect of marker size on the calculation of foot kinematics in the current literature.

Research question

The objective of this research was to determine whether marker size impacts essential parameters that describe foot biomechanics.

Methods

Seventeen subjects participated in this randomized repeatability study. All participants had to walk on a treadmill twice to test two sets of markers (set A: small marker, 9.5 mm, 1 g; set B: large marker, 14 mm, 2 g). Three-dimensional motion capturing was used to record the trajectories of the markers. The spatial relation of the markers, as well as vertical motion of the navicular bone and the angle of the medial longitudinal arch were calculated based on the marker trajectories. In addition to motion capturing, skin rigidity was quantified by applying an oscillatory shear force to the skin. Analysis of variance, root-mean-square error calculations and linear fit methods were applied to evaluate effects of marker size on the calculation of foot kinematics and the impact of skin rigidity.

Results

The estimated foot kinematics appeared to be unaffected by the size of the markers. Further, there was no evidence that skin rigidity influenced the error of the marker trajectories. Interestingly, the large markers fell off more frequently.

Significance

The findings will be of interest to those who use marker-based three-dimensional motion capturing, especially to analyze foot biomechanics. Although the calculation of kinematic parameters appears to be unaffected by marker size, practical aspects, like accidental marker loss, favor the application of small markers.

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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