Abstract

This paper outlines the progress that has been

made in a study on human body dynamics during vibration

therapy. At the Toronto Rehabilitation Institute, vibration

therapy has been applied to spinal cord injured (SCI) patients

in an effort to maintain the patients' bone density. Some

clinical trials have not been successful, and thus a better

understanding of human responses to vibration therapy is

required to ascertain if and how it can be applied to maintain

bone density in SCI subjects. Experiments with SCI and

healthy subjects were conducted to determine the accelerations

present in the lower extremities during vibration therapy. The

results showed negligible differences between the responses of

SCI and healthy subjects, but considerable differences between

the responses of subjects with different body types. A

mathematical model of a standing subject was also developed,

and theoretical predictions using the model were found to

match experimental data reasonably.

Introduction

Doctors and rehabilitation therapists at the Lyndhurst

Centre, Toronto Rehabilitation Institute (TRI) have

conducted several clinical trials investigating the feasibility

of using vibration therapy to regulate bone density in the

lower extremities of spinal cord injured patients who have

little or no motor control below the waist. TRI has an

apparatus that secures the subject in a standing position with

the feet bearing most of the subject's weight. Vibrations are

then applied to the subject via the platform that they are

standing on. The TRI apparatus vibrates the subject in a

horizontal direction, unlike previous studies of this kind that

applied vertical vibrations [1]. Because the apparatus

vibrates the subject horizontally and constrains the body at

certain points, the propagation of vibrations through the

body may differ significantly from situations where the

induced vibrations are vertical. The purpose of this study is

to ascertain the degree to which differences in the type of

therapy applied to a patient affect the parameters that are

important in stimulating increases in bone density through

vibration therapy, and to determine how different subjects

will respond to vibration therapy.

A number of studies have been done to identify the most

important parameters in bone remodeling through

mechanical stimulation. Several studies conducted with

animal subjects in the 1980's demonstrated that the strain

magnitudes [2], strain rates [3], and strain distributions [4]

present in bones have large effects on bone density. These

parameters are functions of the forces present in the lower

extremities.

To examine the forces in the legs during different types

of vibration therapy, a lumped mass model has been

developed using physiological data from literature. In the

past, linear lumped mass models have been used to predict

human responses to both uni-directional vertical vibrations

[5] and multi-directional vibrations [6]. Experiments were

also conducted with healthy and spinal cord injured subjects

to determine the accelerations present in the lower

extremities during different types of vibration therapy. The

experimental results were analyzed to provide a better

understanding of the mechanical properties of the human

body during vibration therapy. These results were also used

to calibrate the model in order to produce more accurate

theoretical predictions.

Results

The experimental results showed behavior that was

generally as expected. The magnitude of the RMS

acceleration was typically shown to decrease from segment

to segment moving from the foot to the upper body, and the

high frequency content of the acceleration signals was

strongly attenuated by successive body segments in all of

the subjects.

When SCI and healthy subject data were compared, the

results generally showed negligible differences in the

mechanical properties of these two subject groups. RMS

acceleration ratios varied much more between subjects with

different body types than between healthy and SCI subjects.

All of the subjects displayed similar frequency responses,

regardless of body type or health status.

Almost completely uniformly, the data showed an

increase in the foot to tibia and foot to femur acceleration

ratio values when the subject was constrained, for both

healthy subjects and for SCI subject No. 2. The most

notable exceptions to this trend were the measured hip

accelerations. These accelerations were shown to decrease

when the constraining device was applied in most cases. In

contrast to the healthy subjects and SCI subject No. 2, SCI

subject No. 1 showed uniform decreases in acceleration ratio

values during constrained therapy.

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