The specific role of gravitational accelerations for arterial adaptations.

It is mostly agreed that arterial adaptations occur, among others, in response to changes in mechanical stimuli. Models like bed rest, spinal cord injury, or limb suspension have been applied to study vascular adaptations to unloading in humans. However, these models cannot distinguish the role of muscle contractions and the role of gravitational accelerations for arterial adaptation. The HEPHAISTOS orthosis allows normal ambulation, while it significantly reduces force generation in the lower leg muscles. Eleven subjects wore HEPHAISTOS unilaterally for 56 days and were followed up for another 4 wk. Arterial diameters, intima media thickness (IMT), flow-mediated dilation (FMD), and resting blood flow (BF(rest)) were measured using high-frequency ultrasonography. Arterial adaptations were investigated in the superficial femoral artery (SFA), the brachial artery (BA), and the carotid artery (CA). Mean SFA resting diameter was decreased from 6.57 mm (SD = 0.74 mm) at baseline to 5.77 mm (SD = 0.87 mm) at the end of the intervention (P < 0.001), whereas SFA wall-to-lumen ratio, SFA BF(rest), and SFA FMD remained unaffected throughout the study. The application of HEPHAISTOS had no effect on structure and function of the systemic control sites, the BA, and the CA. Our findings highlight the importance of muscular contractions for arterial diameter adaptations. Moreover, we propose that FMD and wall-to-lumen ratio are unaffected by ambulating with the HEPHAISTOS orthosis, which is suggestive of habitual acceleration profiles in the lower leg constituting an important stimulus for the maintenance of FMD and wall-to-lumen ratio.