The role of impact loading on artery adaptations and the effect of muscle unloading on local blood supply and exercise tolerance

Kurzfassung

Background: Evidence suggests that muscle disuse and exercise training have direct vascular (de-) conditioning effects that can modify cardiovascular risk. The underlying mechanisms, however, are to date not entirely understood. It is clear that particularly arteries adapt to changes of mechanical stress that may predominantly act on endothelial cells (ECs) or vascular smooth muscle cells (VSMCs). Nonetheless, not all potential sources for mechanical stress have been considered yet as previous research basically investigated the conditioning effects of intrinsic hemodynamic forces. Given the gravitational environment on earth, the present thesis sought therefore to investigate the impact of gravity-induced impact loading, provoking mass accelerations, on arterial adaptations in conditions associated with exercise training and muscle disuse. Moreover, the relationship between local arterial blood flow, muscle perfusion and dynamic exercise tolerance after prolonged local muscle disuse constitutes a major topic of the present thesis, as to date no study has ever elaborated on this. Methods: Two clinical interventional studies have been carried out. Within the scope of the HEP-study, 11 healthy male subjects wore a new orthotic device (HEPHAISTOS) for 56 days to unilaterally unload the calf muscles without changing the impact of gravitational loading. The EVE-study has been conducted to study the effects of whole body vibrations (WBV) if combined with conventional resistive exercise. 26 healthy male subjects were recruited and assigned to either a resistive exercise group or a resistive vibration exercise group that performed a 6 week training intervention. Major endpoint measurements for both studies encompassed structural and functional arterial parameters as measured with sonography. In addition, soleus muscle morphology and tissue oxygenation have been assessed in the HEP study using soleus muscle biopsies and near infrared spectroscopy, respectively. Results: Muscle unloading with the HEPHAISTOS orthosis led to distinct decreases of superficial femoral artery (SFA) calibers, while wall thickness and endothelial function remained unaffected. Although muscle size and arterial calibers significantly decreased, functional exercise blood flow, tissue oxygenation and exercise tolerance did not change after the intervention. During the EVE-study, SFA calibers increased significantly and carotid artery wall thickness decreased significantly in response to resistive exercise while superposition of WBV did not reveal an additional effect. Conclusion: Both studies highlight the importance of muscle work and hence the importance of intrinsic hemodynamic forces for arterial adaptations. However, gravity-induced impact loading seems to have a direct conditioning effect on arterial wall thickness and on endothelial function, thereby modulating parameters for cardiovascular risk. In addition, functional muscle perfusion seems to remain unaffected after prolonged muscle disuse and possibly as a consequence of this, dynamic exercise tolerance does not change.