Influence of the fitting window on the O2 uptake kinetics at the onset of moderate intensity exercise

The O2 uptake (V'O2) data at the onset of an exercise are usually fitted with a mono-exponential function, after removal of the data pertaining to a conventional initial time period (ΔTr) lasting ~20s. We performed a thorough quantitative analysis on the effects of removing data pertaining to different ΔTr, aiming at identifying an objective method to establish the appropriate ΔTr. Breath-by-breath O2 uptake responses, acquired on 25 healthy adults performing a step moderate-intensity exercise, and 104 simulated bi-exponential responses, were analyzed. For all the responses, the kinetic parameters of a mono-exponential function, and the corresponding Asymptotic Standard Errors (ASE), were estimated by non-linear regression, removing the data pertaining to progressively longer initial periods (1s each) up to 60s. Four methods to establish objectively ΔTr were compared. The minimum estimated tau was obtained for ΔTr≅35s in both the V'O2 and simulated data, that was about 30% lower compared to that obtained for ΔTr≅0s. The average ASE values remained quite constant up to ΔTr≅35s, thereafter they increased remarkably. The tau used to generate the simulated response fell within the confidence intervals of the estimated tau in ~85% of cases for ΔTr=20s ("20s-w" method); this percentage increased to ~92% of cases when ΔTr was established according to both the minimum tau and its narrowest confidence interval ("Mixed" method). In conclusion, the effects of removing the V'O2 data pertaining to different ΔTr are remarkable. The "Mixed" method provided estimated parameters close to those used to generate the simulated responses and is thus endorsed.

Effect of Various Intensities of Short-Term Interval Training on Oxygen Uptake Kinetics

Oxygen (O2) uptake kinetics reflect the rate at which an individual’s oxygen consumption (VO2) changes to meet a new metabolic demand, such as an increase or decrease in exercise intensity. O2 uptake kinetics can indicate cardiovascular and metabolic fitness and are described by the time variable, τ (tau), of the exponential equation describing the change in VO2. With training τ decreases, indicating a more rapid increase in VO2 to meet the new metabolic demand. Interval training at supramaximal (>100% VO2max) intensities has been shown to elicit similar improvements in O2 uptake kinetics to traditional endurance training. This study looked to determine the optimal intensity of interval training for eliciting improvements in O2 uptake kinetics. Fifteen recreationally active individuals (males: n=9, age = 23.3±3.3 years, VO2max = 44.2±6.5 ml O2•min-1•kg-1; females: n=6, age = 21.5±0.7 years, VO2max = 39.7±5.4 ml O2•min-1•kg-1) participated in 12 training sessions over 4 weeks. To measure O2 uptake kinetics subjects completed three step transitions from loadless (~25W) to low work-rate (~80W) cycling, prior and following training. Each subject was randomly assigned to one of three intensities - high-intensity interval training (HIIT ~120% VO2max), moderate-intensity interval training (MIIT ~90% VO2max), and low-intensity interval training (LIIT ~65% VO2max). Each session consisted of 8-12 intervals of 1-minute duration with 1-minute recovery on a stationary bicycle at the prescribed relative intensity. No significant differences between groups were observed in changes in τ (Δτ: LIIT: 3.1±8.3s; MIIT: -0.59±12.4s; HIIT: 6.2±6.0s).

Inorganic nitrate supplementation improves muscle oxygenation, O2 uptake kinetics, and exercise tolerance at high but not low pedal rates

We tested the hypothesis that inorganic nitrate (NO3−) supplementation would improve muscle oxygenation, pulmonary oxygen uptake (V̇o2) kinetics, and exercise tolerance (Tlim) to a greater extent when cycling at high compared with low pedal rates. In a randomized, placebo-controlled cross-over study, seven subjects (mean ± SD, age 21 ± 2 yr, body mass 86 ± 10 kg) completed severe-intensity step cycle tests at pedal cadences of 35 rpm and 115 rpm during separate nine-day supplementation periods with NO3−-rich beetroot juice (BR) (providing 8.4 mmol NO3−/day) and placebo (PLA). Compared with PLA, plasma nitrite concentration increased 178% with BR ( P < 0.01). There were no significant differences in muscle oxyhemoglobin concentration ([O2Hb]), phase II V̇o2 kinetics, or Tlim between BR and PLA when cycling at 35 rpm ( P > 0.05). However, when cycling at 115 rpm, muscle [O2Hb] was higher at baseline and throughout exercise, phase II V̇o2 kinetics was faster (47 ± 16 s vs. 61 ± 25 s; P < 0.05), and Tlim was greater (362 ± 137 s vs. 297 ± 79 s; P < 0.05) with BR compared with PLA. These results suggest that short-term BR supplementation can increase muscle oxygenation, expedite the adjustment of oxidative metabolism, and enhance exercise tolerance when cycling at a high, but not a low, pedal cadence in healthy recreationally active subjects. These findings support recent observations that NO3− supplementation may be particularly effective at improving physiological and functional responses in type II muscle fibers.