AN INVESTIGATION INTO THE DEFORMABLE CHARCATERISTICS OF THE HUMAN FOOT USING FLUOROSCOPIC IMAGING

Presented at NACOB 98:
North American Congress on Biomechanics
Canadian Society for Biomechanics - American Society of Biomechanics

University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998

AN INVESTIGATION INTO THE DEFORMABLE CHARCATERISTICS OF
THE HUMAN FOOT USING FLUOROSCOPIC IMAGING

N. Wrbaskic B.P.E. and J.J. Dowling Ph.D
Department of Kinesiology, McMaster University
Hamilton, ON, L8S 4K1

INTRODUCTION

Link-segment analysis has widely been used as a method to determine the underlying (internal) kinetics responsible for many human movements. For human movement such as walking, running, and jumping, these calculations are often intiated at the foot by using some form of external force measurement device such as a force-plate. Accurate kinematic information of the foot is therefore crucial in order that valid kinetic information of the foot and ankle may be determined. Since these kinetic data are subsequently used in the calculations for the energetics at the knee and hip, it is important to determine the true mechanics of the foot.

REVIEW AND THEORY

A method of validating link-segment calculations has often been the comparison of instantaneous segment power (ISP) with the rate of change of mechanical energy (RCE) for a particular segment. A good match between ISP and the RCE indicates that accurate forces and moments have been determined (Robertson & Winter, 1980). There has not been a good match between ISP and the RCE for the human foot, when modelled as a single rigid structure, during load bearing activities. It has been shown that the foot deforms when under load (Ker et al. 1986) and as a result, it is believed that the use of a rigid foot model does not accurately represent the true mechanics of the foot. The purpose of this investigation was 1) to determine the true motion of the bones of the foot when placed under load by using Fluoroscopic imaging, 2) to determine if surface markers can be used to reflect the true motion of the foot bones, and 3) if a multi-segment foot model is necessary in order to model its true behaviour, then to determine the efficacy of using a multi-component pressure sensing device, such as FSCAN, to obtain force information under each segment of the foot

PROCEDURES

A Fluoroscopic Imaging Scanner was used to gather real-time data of the motion of the bones of the foot during load bearing activities. Thirteen ball-bearing skin markers were placed on the medial side of the left foot in order to compare skin marker movement with actual bone movement (See Figure 1).

Figure 1: Skin marker placement in relation to bones as observed through Fluoroscopy.

An FSCAN pressure sensing device was taped to the top of a force-plate for the comparision of vertical ground reaction forces (Fy) and centres of pressure (CofP) between the two. Six healthy subjects were asked to perform the following three tasks: 1) jump off of their left foot, 2) lift themselves onto their toes of their left foot, and 3) only make contact with the ball of their foot with the FSCAN and force-plate. Skin marker and bone movement was digitized from video.

RESULTS

The results of the bone movement indicated that the foot could not be modelled as a single rigid structure but rather as several rigid structures linked together as depicted in Figure 2.

Figure 2. Rigid foot segments as observed through Fluoroscopy.

The contractile element and spring have been put into the model as a result of previous work done on the foot (Wrbaskic and Dowling, 1994). For the most part, the external skin markers do represent the movements of the bones. Since this foot model has at least two contact points with the ground, a single force-plate is insufficient as a source of external force measurement. Table 1 shows the comparison between the force-plate and FSCAN for measures of the vertical ground reaction force and the CofP in the saggital plane of the foot.

Average Force Average CofP

RMSerror 32.39 N 0.0380 m

%RMS 10.02 38.98

r 0.9952 0.9272

r ² 0.9904 0.8626

	Average Force	Average CofP
RMSerror	32.39 N	0.0380 m
%RMS	10.02	38.98
r	0.9952	0.9272
r ²	0.9904	0.8626

Table 1: Comparison of FSCAN and force-plate as two time series; the data was averaged across all trials; N = Newtons, m = metres.

DISCUSSION

Fluoroscopic imaging has unlocked some of the mysteries of the motions of internal body structures. This study has made use of this technology to develop a new foot model which better resembles true foot mechanics. Surface markers can be continued to be used as a method of gathering kinematic information for the foot. The potential for segmenting the resultant vertical ground reaction into components that underlie each foot segment is possible through the use of a multi-pressure sensing device such as FSCAN.

REFERENCES

Ker, R. F. et al The spring in the arch of the human foot. Nature; 325: 147-149, 1987

Robertson, D. G. E. and Winter, D. A. Mechanical energy generation, absorption, and transfer amongst segments during walking. J. Biomechanics; 13: 845-854, 1980.

Wrbaskic, N and Dowling J.J. Validation of ankle joint reaction forces and moments. proc. ISBS, pp 60, 1997.