Causality in the cardio-postural interactions during quiet stance

Cardiovascular and postural systems are understood to interact with each other to maintain equilibrium. Prolonged standing is known to induce postural sway, pre-syncope symptoms and muscle fatigue. Prior work has shown the presence of interactions between the cardiovascular and postural systems during quiet stance. The cause-and-effect relation between the representative signals of the two systems remains to be established. This preliminary work presents a study to identify the causal relationship between the blood pressure waveform (BP) and resultant center of pressure (COPr) as well as between systolic arterial pressure (SAP) and COPr signals during quiet stance. A 5 minute sit-to-stand experiment was conducted for 7 healthy young participants to acquire data for the SAP and COP signals. The statistical framework of Granger causality was applied to test for the bidirectional causal relation between the BP-COPr and SAP-COPr signal pairs. The algorithm computed the F-statistic and critical value from an F distribution for null hypothesis of no causality between the signals. Data were extracted for a window of 15 seconds length, translated with an overlap of 5 seconds over the last 4 minutes of the quiet stance phase to obtain 22 to 23 time windows. Data for subjects rejected the null hypothesis for both BPCOPr and SAP-COPr signal pairs. The results from this study suggest that during quiet stance bidirectional interaction exist between BP and COPr as well as between SAP and COPr.