Physiological insights from gravity-free ballistocardiography

Terrestrial ballistocardiographic (BCG) measurements are typically performed in only one or two axes because of the coupling between the subject and the ground. An appropriate physiological interpretation of these BCG signals therefore assumes that the information in the unmeasured axis is either understood, or able to be ignored. BCG signals from measurements in microgravity can be made in all three axes and permit examination of these assumptions. Such microgravity measurements show that lung volume significantly affects the BCG signals, predominately in the head-to-foot direction. Further, the maximum accelerations recorded following systole are poorly captured by coronal plane measurements as the greatest displacements occur in the sagittal plane. These results suggest a need to carefully consider the influence of the motion in the unmeasured plane when interpreting terrestrial BCG signals.