Vibration transmission through the human spine during physical activity
Osteoporosis causes bone to become fragile. Pharmacological treatments of osteoporosis are burdened with adverse effects and increase bone mineral density (BMD) only between 1% and 15% depending on the drug and time used. Thus non pharmacological treatments are needed to complement pharmacological ones. Physical activity is a non pharmacological treatment of osteoporosis and is essential for maintaining bone health at any age. However, physical activities have been identified to produce a modest improvement of spinal strength or just preserve it. In addition, it is not known how much exercise is optimal and safe for people with spinal osteoporosis. Most research employs conflicting definitions of physical activity and measure the effect of exercise on BMD alone instead of combining it with measurements of three dimensional bone strength. There is the need to offer a technique to measure the effect of physical activity on the overall strength of the spine, not only on its bone mineral content. Vibration transmissibility is a measurement of the mechanical response of a system to vibration expressed as stiffness or damping, thus offering a variable that represents structural strength. It can be employed to measure the mechanical response of the human spine during physical activity by attaching inertial sensors over the spine. However, it has not been employed to characterize the way vibration is transmitted through the osteoporotic spine during physical activity. Understanding the effects of osteoporosis and ageing on vibration transmission is important since such effects are related to the stiffness of the spine and thus very likely to the incidence of vertebral fractures. It is also often recommended that fast walking is beneficial to the bone, yet it is not known if fast walking affects the mechanical response of the spine of people with osteoporosis. The aims of this study were (1) to evaluate the feasibility of employing inertial sensors and a skin correction method to measure vibration transmission through the spine during physical activity (2) to characterize the transmission of vibration in the lumbar and thoracic spines of people with and without osteoporosis during physical activities, (3) to characterize the effect of osteoporosis on vibration transmissibility at levels of the thoracic spine which are known to fracture and (4) to investigate the effects of fast walking on vibration transmissibility. 100 young and healthy and older volunteers with and without osteoporosis were recruited. Participants were asked to perform straight walking, stair negotiation and turning while having inertial sensors attached to the skin over the spinous process of the first sacral (S1), twelfth (T12), eighth (T8) and first thoracic vertebrae (T1). Vibration transmissibility was calculated as the square root of the acceleration of the output (T12 for the lumbar and T1 for the thoracic spine) over the input (S1 for the lumbar and T12 for the thoracic spine) in the frequency spectrum. Vibration transmissibility was corrected for the movement of the skin-sensor interface and for the inclination of the sensor over the spine of every subject. All physical activities were performed at self selected normal and fast walking speeds. Lumbar and thoracic curvatures were determined with an electromagnetic device and BMD was measured through quantitative ultrasound. Skin measurement of transmission of vertical vibration is feasible with the inertial sensors and correction method presented. Vibration transmissibility through the human spine is significantly different between dissimilar physical activities and frequency dependent. Ageing significantly alters the vibration transmissibility of the spine. Osteoporosis has a minimal effect on vibration transmissibility of the spine. The effect of ageing and osteoporosis are frequency dependent. Older lumbar spines may receive greater stimulation than young and healthy ones, whereas older thoracic spines may receive lower stimulation during fast walking. There are significant differences in vibration transmissibility between lumbar and thoracic spines. A percentage of vibration transmission of the lumbar and thoracic spines is determined by their curvatures. This thesis has provided a technique that future research can employ to correlate vibration transmissibility with mechanotransduction signals in bone as well as volumetric bone health measurements and the risk of vertebral fractures. Until then it will be possible to prescribe physical activity taking into account individual capabilities, bone strength and differences in mechanical response between lumbar and thoracic sections.