Time course adaptations in cardiorespiratory fitness and fractional oxygen extraction with low-volume high-intensity interval training and moderate-intensity continuous training in people with type 2 diabetes

Maximal oxygen uptake (VO2max) in people with type 2 diabetes (T2D) is ~20% lower than in healthy individuals and they display an over reliance in fractional oxygen extraction, reflected by an increased rate of muscle deoxygenation (i.e., deoxygenated haemoglobin and myoglobin, [HHb+Mb]) during ramp incremental exercise (RI) (Gildea et al. 2016). Moderate-intensity continuous training (MICT) and low-volume high-intensity interval training (LVHIIT) improve VO2max in those with T2D (MacAnaney et al. 2011; Francois et al. 2017); however, the time course of changes in VO2max and [HHb+Mb] following MICT or LVHIIT are not known. The aim of this study was to compare VO2max and [HHb+Mb] responses pretraining and at 3, 6, 9 and 12 weeks of either MICT or LVHIIT in middle-aged individuals with T2D. Thirty-one participants with T2D (18 men, mean ± SD: age 53 ± 9 yr, body mass index 30.1 ± 4.5 kg.m-2) were randomly assigned to MICT (n=13, 50 min of moderate-intensity cycling) or LVHIIT (n=9, 10 x 1 min at ~90% maximal heart rate interspersed by 1 min of ‘unloaded' cycling) or to a non-exercising control group (n=9). Exercising groups trained 3 times/week and every 3 weeks exercise intensity was adjusted. Muscle deoxygenation was measured using near-infrared spectroscopy (NIRS) at the vastus lateralis muscle. Normalised [HHb+Mb] profiles ([HHb+Mb]%) were plotted as a function of relative work rate (%POpeak) using a bilinear model as proposed by Spencer et al. (2012). Breath-by-breath VO2 data were recorded using an online metabolic unit. Time point analyses of VO2max and [HHb+Mb] response were performed using a 2-way repeated-measures ANOVA. Values are mean ± SD. VO2max significantly increased after 3 weeks of MICT (+17%; P < 0.05) and LVHIIT (+8%; P < 0.05) with no further significant changes thereafter (total increases in VO2max after 12 weeks of MICT and LVHIIT were 24% and 13% respectively). After 3 weeks there was a significant decrease in the slope of the first linear component of the delta[HHb+Mb]% vs delta%POpeak profile (S1) in both the LVHIIT and MICT groups with no further changes observed thereafter. There were no changes in S1 or VO2max in the control group at any time point. LVHIIT and MICT induced similar magnitude of improvements in VO2max following a similar time course in participants with T2D. These improvements were, at least in part, due to a reduction in the over reliance on O2 extraction and improved microvascular O2 delivery.