Test Description; Equipment Description - biodex BioSway Operation Manual

APPENDIX A
balance SyStem clInIcal teSt of SenSory InteGratIon and
balance (ctSIb) Sway Index equatIon

teSt deScrIptIon

The objective of this test is to quantitatively determine a score defining a patient's ability to
maintain a stable vertical posture while positioned on a stationary platform. The patient is posi-
tioned on a stable platform and instructed to try to maintain a stable vertical posture under a
variety of sensatory conditions, eyes open, eyes closed, and vision partially obscured.

equIpment deScrIptIon

Sway information is collected by positioning the patient on a static force plate and then sam-
pling and recording patient movement. The system employs a series of strain gauges to deter-
mine variation in the subject's resultant center of pressure (COP). The center of pressure is the
patient's center of gravity projection on the platform resulting from sway angle and the patient
height. Data is sampled at the rate of 20Hz. Each recorded sample consists of a (X, Y) coordinate.
What is displayed is the sway angle derived from the position of the COG from zero and the
height of the patients COG taken as .55 times the patient height.
This data is recorded for later analysis and also displayed, in real time on, an LCD display
observable by the patient. The resultant movement results in a "spaghetti plot" as shown below.
This plot indicates patient movement from one sample to the next.
Essentially, the database consists of an array of (X, Y) coordinates defining the calculated COP.
The data can be interpreted as an ordered series of sequential vectors from point to point.
For example: (X0, Y0)
(X1, Y1)
(X2, Y2)
..........
..........
(Xn, Yn)
The "Score" is defined as the standard deviation of position over the length of the test. The
Standard deviation is interpreted to be the absolute vector length deviation from the mean vec-
tor end point. Basically, all vectors (X,Y) coordinates are summed and divided by the number of
samples, to obtain a vector sum which represents the position of the mean.
N-1
σx = 1/N σ Xn
n=0
N-1
σy = 1/N Σ Yn
n=0
and
N-1 2
φ = sqrt(Σ((Xn-σx)+(Yn-σy) )
n=0
N
2
— A-1 — SETUP AND POSITIONING