New Findings What is the topic of this review? This review focuses on the elevated demand placed on the respiratory muscle during whole-body exercise induced hyperpnoea, its role in the neural modulation of cardiovascular control; at the level of both respiratory and locomotor skeletal muscle and considers the mechanisms responsible. What advances does it highlight? Recent evidence suggests there a sympathetic restraint of blood flow to locomotor muscles during near maximal exercise, the purpose may be to maintain blood pressure. It also appears that during submaximal exercise respiratory muscle blood flow may be also be reduced if a ventilatory load is high enough. Additionally, methodological advances (NIRS-ICG) have allowed the confirmation of the assumption that blood flow is diverted away from the respiratory muscles when the work of breathing is alleviated. It is known that the respiratory muscles have a significant rising oxygen demand in line with hyperpnoea during whole-body endurance exercise and are susceptible to fatigue; much in the same way as locomotor muscles are also known to be. The act of ventilation can be itself considered a form of exercise. The manipulation of respiratory load at near maximal exercise alters leg blood flow significantly, demonstrating a competitive relationship between different skeletal muscle vascular beds to adequately perfuse both sets of muscles with a finite cardiac output. In recent years the question has moved towards whether this effect exists during submaximal exercise and employing more direct measurements of respiratory muscle blood flow itself to confirm assumptions that uphold the concept. Evidence thus far has shown there is reciprocal effect on blood flow redistribution during ventilatory load manipulation observed at the respiratory muscles themselves and that the effect is observable during submaximal exercise, where active limb blood flow was reduced under conditions that simulated a high work of breathing. This has clinical applications for populations with respiratory disease and heart failure, where the work of breathing is remarkably high, even during submaximal efforts.
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