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

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.

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