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 (VO2) 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). 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-, and heavy-intensity exercise in middle-aged individuals with T2D.
Purpose
To assess the rates of adjustment of V̇O2 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 × 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 time constant of the V̇O2 primary phase (τV̇O2P), the amplitude of the V̇O2 slow component (V̇O2sc A) and V̇O2 mean response time (MRT), decreased (P<0.05) by a similar magnitude at week 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 parameter estimates of Δ[HHb+Mb] remained unchanged throughout the interventions.
Conclusion
V̇O2 kinetics during work-to-work transitions were greatly improved as a consequence of a time efficient LVHIIT in T2D within a short period of time. Whilst the underlying mechanisms appear less clear, likely attributions include training-induced changes in skeletal muscle properties, motor unit recruitment patterns and improvements in O2 delivery relative to utilization during exercise. Importantly, faster V̇O2 kinetics are related to improvements in exercise tolerance that may increase the willingness to participate in exercise and thus result in an increased quality of life in this clinical population.