Rate of Information Transfer During an Open-loop
Weight-shifting Task

R. A. L. Liston & J. P. Sommer

School of Physiotherapy, Curtin University of Technology,
Burwood, Australia


In order to have an understanding of the processing necessary for motor control, it is important to investigate different forms of signal processing. An open-loop task was consequently developed as an adaptation of the Pro Balance MasterTM (BM) system to enable measurement of the rate of information transfer during a weight-shifting task.

Eight healthy volunteers aged between 25 and 38 years (mean = 29± 4 years) were recruited to participate as part of a pilot study to trial this novel method. The equipment consisted of a BM which measures centre of pressure and is able to provide visual input. A modification of the BM program was created to provide an open-loop task whereby participants would need to respond to visual input without the use of feedback. This task consisted of movement of a cursor on a screen in a random order of x and y directions at a range of input frequencies. Participants were required to match the movement of this cursor by shifting their body weight which registered as an additional cursor on the screen. A sampling frequency of 50 Hz was utilised.

Time series spectral analyses were performed on the data. Bivariate analyses of squared coherency and phase spectrum were examined by frequency in order to determine a suitable cut-off frequency. The rate of information transfer (bits/sec) was then determined using Shannon’s formula.

Test-retest reliability was examined using Friedman tests. Rate of information transfer was found to be reliable at all frequencies in the x direction of movement, but in the y direction measures from two out of a possible five frequencies varied significantly between days (p=0.04). Wilcoxon Signed Rank tests were used to investigate any differences between rates in the x and y directions and measures were found to be significantly different (p<0.05) at all except the highest frequency of movement.

In conclusion, this method of analysis is reliable for examining movements in the x but not y direction at a range of frequencies. The rate of information transfer is higher for movements in an x as opposed to a y direction. This may in part be due to the more functional nature of these movements as shifting weight side to side reflects components of gait and so is a well practised task. Movements in a y direction, however, may challenge the limits of stability and so represent a more difficult and less familiar task.