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Thesis Format

Integrated Article


Doctor of Philosophy




Garland, S. Jayne


Maintenance of standing balance is vital to daily living. Traditionally, perspectives on maintaining balance are achieved through the study of visual, vestibular and somatosensory inputs on motor outputs. While evidence suggests cardiovascular regulation also functions to assist postural control during orthostatic stress, how the cardiovascular system responds to postural perturbations is not well established. This dissertation includes four studies that investigated the effects of postural perturbations on the cardiovascular system and skeletal muscle responses in healthy adults. All of the studies involved exposing standing participants to surface-translation perturbations at different velocities, with known or unknown timing, and occurred while spontaneously breathing and breathing at six breaths per minute in young and older adults. Beat-to-beat heart rate and systolic blood pressure were measured continuously, while cardiac baroreflex sensitivity was calculated using the sequence method analysis. The results of Study One illustrated that heart rate and systolic blood pressure response were modulated following a perturbation and the initial cardiac response scaled with perturbation intensity. Additionally, cardiac baroreflex engagement was found to be involved in heart rate recovery. The sensitivity of the cardiac baroreflex was increased post-perturbation but was not intensity dependent. In Study Two, anticipatory cardiovascular responses to perturbations were not observed when the timing of the perturbation was controlled by the participant and perceived state anxiety was correlated with the initial heart rate response post-perturbation. In Studies Three and Four, when breathing at six breaths per minute, the latency of muscle burst onset decreased in young and older adults and muscle burst amplitude decreased in the lower limb musculature in young adults. Slow breathing decreased systolic blood pressure but had no effect on heart rate or cardiac baroreflex sensitivity post-perturbation in older adults. Together, the findings provide evidence that cardiovascular modulation occurs when standing balance is perturbed and support the notion that the cardiovascular, respiratory and motor control systems interact in a complex manner during postural perturbations.

Summary for Lay Audience

The ability to regain control of standing balance is vital to limiting falls and injuries. It is known that standing balance requires the coordination of sensation and muscle activity. However, the importance of heart rate and blood pressure control during disruptions to standing balance is less well known. This thesis includes four studies that investigated the effects of balance disturbances on heart rate, blood pressure and skeletal muscle activity of the lower limb in healthy adults. Young and older participants stood on a treadmill which moved backwards causing them to fall forward. These balance disturbances were repeated with different treadmill movement speeds, with the timing of the treadmill movement either known or unknown, and with the participant either breathing normally (spontaneous breathing) or breathing slowly at six breaths per minute. Balance disturbances caused short-lasting effects on heart rate and blood pressure. These effects were similar regardless of whether the timing of the balance disturbance was known or not. Slow breathing can cause the muscle to respond earlier to a balance disturbance in young and older adults. Heart rate was not affected but blood pressure was elevated during balance disturbances while spontaneously breathing in older adults. This thesis provides evidence that heart rate and blood pressure responded to balance disturbances and supports the idea that the heart, lungs and muscles influence each other during postural perturbations.

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