Oxygen Uptake and Heart Rate Kinetics during Moderate and Heavy Work-to-Work Transitions

Abstract

The time course of phase II oxygen uptake (VO2) is slower during step transitions from an elevated exercising baseline (∼45% ventilatory threshold, VT) compared with resting or very low-level intensity baselines. The slowing in VO2 during the on-transient from an elevated exercising baseline at ∼45% VT is likely due to an attenuated heart rate (HR) response. However, the time course of VO and HR during work-to-work transitions in the moderate (between 45 and 100% VT) and heavy (above VT) exercise intensity domain is unknown. PURPOSE: The purpose of this study was to investigate VO2 and HR on-transient responses from elevated exercising baselines at moderate and heavy exercise intensities. We hypothesized 1) that the time course of phase II VO2 would not differ during work-to-work transitions from moderate and heavy intensity exercising baselines, 2) that a shift in the relative contribution of phase II and phase III responses would be observed during progressively heavier exercise, and 3) that the overall time course (mean response time, MRT) of VO2 would be directly related to the MRT of HR. METHODS: Following determination of VT and peak VO2, 13 trained male cyclists (mean± SD; age 25 ± 3.7 years; height 1.82 ± 0.07 m; weight 78.9 ± 8.0 kg; peak VO2 4.48 ± 0.46 L/min) completed work-to-work transitions representing moderate (40–50 and 50–60% VO2 reserve) and heavy (70–80 and 80–90% VO2reserve) exercise intensities. The time course and amplitude response of VO and HR was determined using a first or second-order non-linear exponential model. Comparisons were made with ANOVA and p <0.05 was considered significant. RESULTS: The time course of phase II VO2 during work-to-work transitions from 40–50(21 ± 10 s), 50–60 (22 ± 16 s), 70–80 (19 ± 4 s), and 80–90% VO2 reserve (20 ± 10 s) were not significantly different (p >0.05). As well, the time course of phase II HR during work-to-work transitions from 40–50 (23 ± 12 s), 50–60 (22 ± 12 s), 70–80 (21 ± 15 s), and 80–90% VO2 reserve (26 ± 21 s) were not significantly different (p >0.05). There was a non-significant trend (F = 2.45, p = 0.079) for a reduction in the phase II VO amplitude response during heavy work-to-work transitions, with a similar significant effect observed in HR. The MRT of HR was significantly correlated with VO2 where MRT VO2 = 0.61 (MRT HR) + 29.3 (r = 0.73, p <0.001) CONCLUSION: VO and HR respond in a linear fashion during work-to-work transitions from moderate and heavy intensity exercising baselines. The significant relationship between the MRT of HR and VO2 suggests that the time course of VO2 is HR and thus, O2 delivery dependent.