Chloe Gustafson, 

Title:

Role of III/IV muscle afferents during exercise at sea-level and high-altitude

Abstract:

Group III/IV muscle afferents are nerve endings that sit on and within skeletal muscle fibers. Their job is to relay information to the brain regarding the state of the muscle during rest and exercise. This feedback can be partially blocked by administering fentanyl into the lower spine. Using this technique, it has been well-established that group III/IV afferents are necessary for appropriate breathing and oxygenation of blood during exercise. In some disease states that cause local and systemic hypoxia, e.g. pulmonary disease, group III/IV afferents become highly sensitive and make exercise intolerable. The objective of this project was to isolate the role of group III/IV muscle afferents on breathing, blood oxygenation, and blood pressure during exercise in healthy humans in normoxic and hypoxic environments. Our secondary objective was to test how group III/IV afferents contribute to exercise performance.

Fourteen healthy participants completed experimental (spinal fentanyl) and control (saline) conditions during exercise at sea-level (340m) and following 8 days at high-altitude (3800m). Participants completed identical cycling sessions for each condition, consisting of three 3-min stages at ~45, 60 and 75% of sea-level maximal aerobic power. Participants completed a 5-km time trial at both altitudes.

Group III/IV afferent suppression decreased breathing, blood oxygenation, and blood pressure responses to exercise; the magnitude of these effects was similar at sea-level and high-altitude indicating hypoxia per se does not sensitize this reflex. Time-trial performance was 15% slower at high-altitude and there was no effect of feedback suppression at either altitude.