The effects of whole body vibration on peripheral cardiovascular function
Gholoum, Mahmoud S. M. A.
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Exposure to acute bouts of whole body vibration (WBV), which can be employed as a novel form of exercise, has been reported to increase local skeletal muscle blood flow. However, the mechanism for this effect remains unclear. Therefore, this research aimed to explore the mechanism that would explain the effect of vibration on the peripheral cardiovascular function. Initially, the aim was to investigate the potential mechanism of the effect of WBV on the systemic blood flow, since there are currently no studies reporting any systemic effects of WBV on blood flow. The results did not demonstrate any systemic effects on blood flow (i.e. forearm blood flow) in response to acute unloaded and loaded squats with WBV. It was concluded that it was difficult to identify the effects of vibration on systemic cardiovascular function because, most likely due to the higher exercise intensity, skeletal muscle activation resulted in a decrement in blood flow from a distal site (i.e. forearm) to the main site (i.e. lower limb). Through the development of experimental methods involving applying vibration passively to the lower limbs, which avoids any influence of direct skeletal muscle activation and focuses solely on the mechanism inducing effects, it was demonstrated that ankle systolic blood pressure and ankle brachial pressure index substantially decreased in the post-vibration period. It was concluded that vibration has a direct effect on the peripheral cardiovascular function via increased vasodilatation; however, the mechanism underlying this effect remained unresolved. The effects of different durations of passive vibration on the peripheral circulation were also investigated and the results demonstrated that a longer duration of passive vibration (i.e. 8 minutes) resulted in a significantly higher lower leg blood flow during the recovery period than a shorter duration (i.e. 1, 2 and 4 minutes) of passive vibration. These data provide evidence for a greater effect of WBV occurring with a longer duration on the peripheral cardiovascular function, caused by the vasodilatation response throughout the recovery period. However, there might be a minimum effect of skeletal muscle activation occurring with a longer duration of passive vibration that leads to a direct response to localised heating. Furthermore, the thesis attempted to distinguish the effects of passive vibration on skeletal muscle activation from those on the peripheral vascular system. An experiment was designed in which passive vibration was applied with and without circulatory occlusion, to examine whether there was any underlying skeletal muscle activation. It was found that vibration with intact circulation produces more heat than the control, no vibration and occlusion, and occlusion plus vibration conditions. These effects were reflected by the higher skin temperature observed during exposure to vibration, and continuing into recovery. These data provide evidence that passive vibration does not appear to induce an increase in muscle activity. The data also suggest that the mechanism of the rise in skin temperature in response to passive vibration exposure is due to a vasodilatation that occurred in the lower limb via inducing an increase in shear stress at the blood vessels wall and led to an increase in circulating blood flow during exposure that continues into recovery. Overall, the results obtained demonstrate that vasodilatation occurs during and after vibration exposure and appears to be a process that is independent of skeletal muscle activation. It is postulated that the stimulus is a direct effect on the blood vessels via inducing an increase in shear stress that results in an increased vasodilatation, thereby increasing blood flow. Hence, these observations demonstrate that vibration stimulus has a direct effect on the muscle vascular bed as a primary effect and that there is no carry over effect into the systemic circulation. Thus, the results of this thesis indicate that vibration induced enhancement in the peripheral circulation could be using as a training stimulus and also could have a beneficial effect in assisting recovery routines from exertion.