Vagal Threshold Determination during Incremental Stepwise Exercise in Normoxia and Normobaric Hypoxia

Area of Science:

• Exercise Physiology
• Environmental Physiology
• Cardiovascular Regulation

Background:

• The vagal threshold (T_va) is a key indicator of exercise intensity and autonomic nervous system response.
• Understanding T_va under hypoxic conditions is crucial for optimizing athletic performance and acclimatization strategies.
• Limited data exists on the T_va response to varying degrees of normobaric hypoxia during incremental exercise.

Purpose of the Study:

• To determine the vagal threshold (T_va) during incremental exercise in normoxia and simulated normobaric hypoxia.
• To assess the impact of different fractions of inspired oxygen (FiO₂) on exercise intensity at T_va (PO_th).
• To evaluate changes in maximal heart rate reserve (%MHRR) relative to T_va under hypoxic conditions.

Main Methods:

• Twenty-eight healthy men (20-30 years) performed incremental cycling exercise.
• Exercise was conducted under three conditions: normoxia (FiO₂ 20.9%), simulated altitude ~1500m (FiO₂ 17.3%), and ~2500m (FiO₂ 15.3%).
• The vagal threshold (T_va) and power output at T_va (PO_th) were determined, alongside %MHRR.

Main Results:

• T_va occurred at 45.0 ± 5.6% MHRR in normoxia.
• PO_th significantly decreased with increasing hypoxia, particularly at ~2500m (FiO₂ 15.3%) compared to normoxia (p=0.007) and ~1500m (p=0.001).
• Normoxic %MHRR adjusted for hypoxia at ~2500m reduced to 39.9 ± 5.5%.

Conclusions:

• Normobaric hypoxia significantly reduces exercise intensity at the vagal threshold (PO_th).
• The effect is more pronounced at higher simulated altitudes (~2500m).
• These findings highlight the importance of adjusting exercise intensity prescriptions for athletes training in hypoxic environments.