Although breath hold time ( BHT ) increased after apnea training in the NBH group by ~ 46 % , CO2 sensitivity slopes were not different among experimental conditions and groups ( 2.8 0.3 , 2.9 0.4 L min ( 1 ) mmHg ( 1 ) in the CBH and 2.7 0.5 , 2.7 0.3 L min ( 1 ) mmHg ( 1 ) in the NBH during the second and third visit , respectively ) .