Training intensity relative to ventilatory thresholds determines cardiorespiratory fitness improvements in sedentary adults with obesity

The aim of the present study was to study if training intensity relative to ventilatory thresholds (VTs) determines the improvements in cardiorespiratory fitness (CRF) in middle-aged sedentary individuals with obesity. Before and after 16-weeks of HIIT (43-min alternating bouts at 70/90% of HR_MAX), oxygen consumption ( $$ O₂) and heart rate (HR) at ventilatory threshold 1 ( $$ O_2VT1, HR_VT1), ventilatory threshold 2 ( $$ O_2VT2, HR_VT2) and at maximal effort ( $$ O_2MAX, HR_MAX) were assessed during a graded cycle-ergometer exercise test. Retrospectively, participants were divided into two groups based on whether training intensities were under (UNDER; n = 39) or over (OVER; n = 37) VT₁ and VT₂. At baseline, age, body composition, $$ O_2VT1, $$ O_2VT2, and HR_MAX were similar in both groups. However, $$ O_2MAX was higher in OVER (P = 0.033), whereas HR_VT1 and HR_VT2 were higher in UNDER (P < 0.05). $$ O_2MAX (9.0%) and HR_MAX (2.2%) improved similarly in both groups. $$ O_2VT1 and $$ O_2VT2 improved with training in both groups (P < 0.001) but the improvement was larger in OVER versus UNDER in $$ O_2VT1 (P = 0.013) and tended to be higher in $$ O_2VT2 (P = 0.068). HR_VT1 increased only in OVER (P < 0.001), whereas HR_VT2 did not change in any group (P = 0.248). A 16-week programme of HIIT improves $$ O_2MAX similarly in individuals training at intensities over or under their VTs. However, individuals training over their VTs showed a larger improvements in $$ O_2VT1 expanding exercise workloads fuelled by oxidative metabolism.