Purpose: 

A relationship has been recently demonstrated between blood pressure (BP) variation and postural sway during quiet stance, and a physiological model proposed for an adaptive compensatory cardio-postural interaction as part of an integrated orthostatic reflex control system (Blaber A.P. et al., Respir. Physiol. Neurobiol. (2009), doi:10.1016/j.resp.2009.04.007).

Methods and Study Design: 

To further validate this model, we investigated simultaneous changes of medio-lateral center of pressure coordinates (M/L COP), filtered electromyographic activity (EMG) of muscles essential to postural stance along with continuous non-invasive BP data. The study was approved by the Office of Research and Ethics for Simon Fraser University. Five young males between the ages of 19 to 26 volunteered to participate. Participants were seated for 20 minutes and then asked to stand with eyes closed (blindfolded) in a quiet stance for 5 minutes. Postural sway was characterized by the time-varying position of the centre-of-pressure (COP). Surface EMG (transdermal differential recording), BP (photoplethysmography), and COP (force platform) were recorded with a sampling rate of 1000 Hz for the entire period of the test. Data from last 4 minutes was used for analysis purposes.

Results: 

The data was resampled at 15Hz and Discrete Wavelet Transform (DWT) was applied using the Daubechies 5 (db5) wavelet to decompose the signals (EMG, BP and COP) at the 7^th and 8^th scale to extract the approximation signal in the low frequency range (~ 0.11 Hz). The approximation signal revealed several regions with significant correlations (r>0.5, P<0.0001) between the EMG and BP and EMG and COP in the last 4 minutes of quiet stance.

Conclusions and Significance of Findings: 

The presence of correlation in the low frequency (~0.11Hz) region indicates a possible adaptive functional relationship between M/L COP sway, postural muscle EMG activity and BP variations during quiet stance.