Time course adaptations in oxygen uptake kinetics during heavy-intensity exercise initiated from an elevated baseline subsequent to a 12 Week high-intensity interval or moderate-intensity continuous training in type 2 diabetes

The dynamic response of pulmonary oxygen uptake (V̇O2) at the onset of heavy- intensity exercise initiated from an elevated baseline (work-to-work) is constrained in middle-aged individuals with uncomplicated type 2 diabetes (T2D) (Gildea et al. 2018b) attributed to progressive limitations of O2 delivery and/or utilization. Both low-volume, high-intensity interval training (LVHIIT) and moderate-intensity continuous training (MICT) have been shown to be effective at accelerating V̇O2 kinetics during stepwise transitions from rest to moderate- (Gildea et al. 2018a), and heavy-intensity exercise (Nevin et al. 2018) in middle-aged individuals with T2D. However, the effects of such protocols on V̇O2 kinetics during heavy-intensity work-to-work cycling in individuals with T2D are unknown. Purpose: To assess the rates of adjustment of V̇O2p and muscle deoxygenation (i.e., deoxygenated haemoglobin and myoglobin, [HHb+Mb]) during the on-transition to heavy-intensity work-to-work cycling before training and at weeks 3, 6, 9 and 12 of MICT and LVHIIT. Methods: Twenty eight middle-aged individuals with T2D (17 men, 11 women; mean ± SD; age: 53 ± 10 yr, body mass index: 29.8 ± 4.4 kg.m-2) were randomly assigned to MICT (n=11, 50 min of moderate-intensity cycling), LVHIIT (n=8, 10 x 1 min cycling 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 with intensity adjusted every 3 weeks. V̇O2 kinetics was calculated from continuously measured breath-by-breath V̇O2 data, and the rate of muscle deoxygenation continuously measured by near-infrared spectroscopy at the vastus lateralis muscle. Time point analysis of V̇O2 and [HHb+Mb] responses were performed using a two-way ANOVA with repeated-measures, and Tukey post-hoc tests performed when significant differences presented. Results: The pre-training time constant of the V̇O2 primary phase (τV̇O2P), the amplitude of the V̇O2 slow component (V̇O2scA) and V̇O2 mean response time (MRT), decreased (P<0.05) by a similar magnitude at wk 3 of training in both the MICT (48±10 to 30±8 s; 0.18±0.08 to 0.10±0.05 L.min-1 and 80±19 to 61±13 s, respectively) and LVHIIT (43±13 to 27±6 s; 0.20±0.07 to 0.14±0.06 L.min-1 and 73±17 to 60±13 s, respectively) groups with no further changes thereafter. No changes were reported in the control group. The pre-training parameter estimates of Δ[HHb+Mb] remained unchanged throughout the interventions. Conclusion: The enhanced V̇O2 kinetics responses during work-to-work transitions consequent to both MICT and LVHIIT in T2D is likely attributed to a combination of training-induced changes in skeletal muscle properties, motor unit recruitment patterns and improvements in O2 delivery relative to utilization during exercise.