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

This study focuses on the determination of the vagal threshold (T(va)) during exercise with increasing intensity in normoxia and normobaric hypoxia. The experimental protocol was performed by 28 healthy men aged 20 to 30 years. It included three stages of exercise on a bicycle ergometer with a fraction of inspired oxygen (FiO(2)) 20.9% (normoxia), 17.3% (simulated altitude ~1500 m), and 15.3% (~2500 m) at intensity associated with 20% to 70% of the maximal heart rate reserve (MHRR) set in normoxia. T(va) level in normoxia was determined at exercise intensity corresponding with (M ± SD) 45.0 ± 5.6% of MHRR. Power output at T(va) (PO(th)), representing threshold exercise intensity, decreased with increasing degree of hypoxia (normoxia: 114 ± 29 W; FiO(2) = 17.3%: 110 ± 27 W; FiO(2) = 15.3%: 96 ± 32 W). Significant changes in PO(th) were observed with FiO(2) = 15.3% compared to normoxia (p = 0.007) and FiO(2) = 17.3% (p = 0.001). Consequentially, normoxic %MHRR adjusted for hypoxia with FiO(2) = 15.3% was reduced to 39.9 ± 5.5%. Considering the convenient altitude for exercise in hypoxia, PO(th) did not differ excessively between normoxic conditions and the simulated altitude of ~1500 m, while more substantial decline of PO(th) occurred at the simulated altitude of ~2500 m compared to the other two conditions.