"The effects of varied seating posture conditions on cardiovascular responses in healthy women were studied during exposure to whole-body vibration (WBV). After an assessment of upper-body fitness, 14 healthy young women were exposed to 3, 4.5, and 6 Hz (at 0.9gr.m.s. in the vertical direction) on a vibration simulator for 3 separate days. Each day the protocol consisted of 6 min of baseline without WBV (control), 8 min of WBV exposure to a single frequency either ‘with' or ‘without' backrest, 4 min of recovery, followed by 8 min of WBV at the same frequency with opposite backrest condition, and 4 min of recovery. Cardiac output was estimated by using the carbon dioxide rebreathing technique during three conditions: (1) control; and before completion of WBV session of sitting (2) ‘with'; and (3) ‘without' backrest during exposure to WBV. Statistical analysis revealed that pulmonary oxygen uptake responses were not influenced by the change in frequency but only by the posture. Although cardiac output reached equal levels during sitting ‘without' backrest at 3 and 4.5 Hz (P>0.05), the heart rate difference was significant between these frequencies (P<0.05), suggesting the role of stroke volume in maintaining cardiac output. As heart rate responses were not significant at 4.5 and 6 Hz (P>0.05), the greater increase in cardiac output (P<0.05) during sitting ‘with' backrest compared to the control condition, can be attributed to decrease in stroke volume only. Based on the cardiovascular responses, it is speculated that the muscle pump in the legs is inactive during WBV exposure in the seated posture, thereby affecting stroke volume, especially at the spinal resonance frequency of 4.5 Hz. Overall, this study demonstrates that subjects exposed to WBV will experience greater metabolic responses compared to sitting without WBV, and these physiological responses during WBV resemble that of ‘light' physical work."

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