John M. Kowalchuk

Dynamic asymmetry of phosphocreatine concentration and O2 uptake between the on- and off-transients of moderate- and high-intensity exercise in humans

The on‐ and off‐transient (i.e. phase II) responses of pulmonary oxygen uptake (V̇O2) to moderate‐intensity exercise (i.e. below the lactate threshold, θL) in humans has been shown to conform to both mono‐exponentiality and ‘on‐off’ symmetry, consistent with a system manifesting linear control dynamics. However above θL the V̇O2 kinetics have been shown to be more complex: during high‐intensity exercise neither mono‐exponentiality nor ‘on‐off’ symmetry have been shown to appropriately characterise the V̇O2 response. Muscle [phosphocreatine] ([PCr]) responses to exercise, however, have been proposed to be dynamically linear with respect to work rate, and to demonstrate ‘on‐off’ symmetry at all work intenisties. We were therefore interested in examining the kinetic characteristics of the V̇O2 and [PCr] responses to moderate‐ and high‐intensity knee‐extensor exercise in order to improve our understanding of the factors involved in the putative phosphate‐linked control of muscle oxygen consumption. We estimated the dynamics of intramuscular [PCr] simultaneously with those of V̇O2 in nine healthy males who performed repeated bouts of both moderate‐ and high‐intensity square‐wave, knee‐extension exercise for 6 min, inside a whole‐body magnetic resonance spectroscopy (MRS) system. A transmit‐receive surface coil placed under the right quadriceps muscle allowed estimation of intramuscular [PCr]; V̇O2 was measured breath‐by‐breath using a custom‐designed turbine and a mass spectrometer system. For moderate exercise, the kinetics were well described by a simple mono‐exponential function (following a short cardiodynamic phase for V̇O2,), with time constants (τ) averaging: τV̇O2,on 35 ± 14 s (±s.d.), τ[PCr]on 33 ± 12 s, τV̇O2,off 50 ± 13 s and τ[PCr]off 51 ± 13 s. The kinetics for both V̇O2 and [PCr] were more complex for high‐intensity exercise. The fundamental phase expressing average τ values of τV̇O2,on 39 ± 4 s, τ[PCr]on 38 ± 11 s, τV̇O2,off 51 ± 6 s and τ[PCr]off 47 ± 11 s. An associated slow component was expressed in the on‐transient only for both V̇O2 and [PCr], and averaged 15.3 ± 5.4 and 13.9 ± 9.1 % of the fundamental amplitudes for V̇O2 and [PCr], respectively. In conclusion, the τ values of the fundamental component of [PCr] and V̇O2 dynamics cohere to within 10 %, during both the on‐ and off‐transients to a constant‐load work rate of both moderate‐ and high‐intensity exercise. On average, ≈90 % of the magnitude of the V̇O2 slow component during high‐intensity exercise is reflected within the exercising muscle by its [PCr] response.

Effects of prior exercise on oxygen uptake and phosphocreatine kinetics during high‐intensity knee‐extension exercise in humans

1 A prior bout of high‐intensity square‐wave exercise can increase the temporal adaptation of pulmonary oxygen uptake (V̇O2) to a subsequent bout of high‐intensity exercise. The mechanisms controlling this adaptation, however, are poorly understood. 2 We therefore determined the dynamics of intramuscular [phosphocreatine] ([PCr]) simultaneously with those of V̇O2 in seven males who performed two consecutive bouts of high‐intensity square‐wave, knee‐extensor exercise in the prone position for 6 min with a 6 min rest interval. A magnetic resonance spectroscopy (MRS) transmit‐receive surface coil under the quadriceps muscle allowed estimation of [PCr]; V̇O2 was measured breath‐by‐breath using a custom‐designed turbine and a mass spectrometer system. 3 The V̇O2 kinetics of the second exercise bout were altered compared with the first such that (a) not only was the instantaneous rate of V̇O2 change (at a given level of V̇O2) greater but the phase II τ was also reduced ‐ averaging 46.6 ± 6.0 s (bout 1) and 40.7 ± 8.4 s (bout 2) (mean ±s.d.) and (b) the magnitude of the later slow component was reduced. 4 This was associated with a reduction of, on average, 16.1 % in the total exercise‐induced [PCr] decrement over the 6 min of the exercise, of which 4.0 % was due to a reduction in the slow component of [PCr]. There was no discernable alteration in the initial rate of [PCr] change. The prior exercise, therefore, changed the multi‐compartment behaviour towards that of functionally first‐order dynamics. 5 These observations demonstrate that the V̇O2 responses relative to the work rate input for high‐intensity exercise are non‐linear, as are, it appears, the putative phosphate‐linked controllers for which [PCr] serves as a surrogate.

Prior heavy exercise elevates pyruvate dehydrogenase activity and speeds O2 uptake kinetics during subsequent moderate‐intensity exercise in healthy young adults

The adaptation of pulmonary oxygen uptake during the transition to moderate‐intensity exercise (Mod) is faster following a prior bout of heavy‐intensity exercise. In the present study we examined the activation of pyruvate dehydrogenase (PDHa) during Mod both with and without prior heavy‐intensity exercise. Subjects (n= 9) performed a Mod1–heavy‐intensity–Mod2 exercise protocol preceded by 20 W baseline. Breath‐by‐breath kinetics and near‐infrared spectroscopy‐derived muscle oxygenation were measured continuously, and muscle biopsy samples were taken at specific times during the transition to Mod. In Mod1, PDHa increased from baseline (1.08 ± 0.2 mmol min−1 (kg wet wt)−1) to 30 s (2.05 ± 0.2 mmol min−1 (kg wet wt)−1), with no additional change at 6 min exercise (2.07 ± 0.3 mmol min−1 (kg wet wt)−1). In Mod2, PDHa was already elevated at baseline (1.88 ± 0.3 mmol min−1 (kg wet wt)−1) and was greater than in Mod1, and did not change at 30 s (1.96 ± 0.2 mmol min−1 (kg wet wt)−1) but increased at 6 min exercise (2.70 ± 0.3 mmol min−1 (kg wet wt)−1). The time constant of was lower in Mod2 (19 ± 2 s) than Mod1 (24 ± 3 s). Phosphocreatine (PCr) breakdown from baseline to 30 s was greater (P < 0.05) in Mod1 (13.6 ± 6.7 mmol (kg dry wt)−1) than Mod2 (6.5 ± 6.2 mmol (kg dry wt)−1) but total PCr breakdown was similar between conditions (Mod1, 14.8 ± 7.4 mmol (kg dry wt)−1; Mod2, 20.1 ± 8.0 mmol (kg dry wt)−1). Both oxyhaemoglobin and total haemoglobin were elevated prior to and throughout Mod2 compared with Mod1. In conclusion, the greater PDHa at baseline prior to Mod2 compared with Mod1 may have contributed in part to the faster kinetics in Mod2. That oxyhaemoglobin and total haemoglobin were elevated prior to Mod2 suggests that greater muscle perfusion may also have contributed to the observed faster kinetics. These findings are consistent with metabolic inertia, via delayed activation of PDH, in part limiting the adaptation of pulmonary and muscle O2 consumption during the normal transition to exercise.

The effect of resistive breathing on leg muscle oxygenation using near-infrared spectroscopy during exercise in men

The effect of added respiratory work on leg muscle oxygenation during constant‐load cycle ergometry was examined in six healthy adults. Exercise was initiated from a baseline of 20 W and increased to a power output corresponding to 90% of the estimated lactate threshold (moderate exercise) and to a power output yielding a tolerance limit of 11.8 min (± 1.4, S.D.) (heavy exercise). Ventilation and pulmonary gas exchange were measured breath‐by‐breath. Profiles of leg muscle oxygenation were determined throughout the protocol using near‐infrared (NIR) spectroscopy (Hamamatsu NIRO 500) with optodes aligned midway along the vastus lateralis of the dominant leg. Four conditions were tested: (i) control (Con) where the subjects breathed spontaneously throughout, (ii) controlled breathing (Con Br) where breathing frequency and tidal volume were matched to the Con profile, (iii) increased work of breathing (Resist Br) in which a resistance of 7 cmH2O l−1 s−1 was inserted into the mouthpiece assembly, and (iv) partial leg blood flow occlusion (Leg Occl), where muscle perfusion was reduced by inflating a pressure cuff (∼90 mmHg) around the upper right thigh. During Resist Br and Leg Occl, subjects controlled their breathing pattern to reproduce the ventilatory profile of Con. An ∼3 min period with respiratory resistance or pressure cuff was introduced ∼4 min after exercise onset. NIR spectroscopy data for reduced haemoglobin‐myoglobin (Δ[Hb]) were extracted from the continuous display at specific times prior to, during and after removal of the resistance or pressure cuff. While the Δ[Hb] increased during moderate‐ and heavy‐intensity exercise, there was no additional increase in Δ[Hb] with Resist Br. In contrast, Δ[Hb] increased further with Leg Occl, reflecting increased muscle O2 extraction during the period of reduced muscle blood flow. In conclusion, increasing the work of breathing did not increase leg muscle deoxygenation during heavy exercise. Assuming that leg muscle O2 consumption did not decrease, this implies that leg blood flow was not reduced consequent to a redistribution of flow away from the working leg muscle.

OXYGEN UPTAKE KINETICS OF OLDER HUMANS ARE SLOWED WITH AGE BUT ARE UNAFFECTED BY HYPEROXIA

Cross‐sectional studies have compared the oxygen uptake (VO2) kinetics during the on‐transient of moderate intensity exercise in older and younger adults. The slower values in the older adults may have been due to an age‐related reduction in the capacity for O2 transport or alternatively a reduced intramuscular oxidative capacity. We studied: (1) the effects of ageing on VO2 kinetics in older adults on two occasions 9 years apart, and (2) the effect of hyperoxia on VO2 kinetics at the second test time. After a 9 year period, follow‐up testing was undertaken on seven older adults (78 ± 5 years, mean ±s.d.). They each performed six repeats of 6 min bouts of constant‐load cycle exercise from loadless cycling to 80 % of their ventilatory threshold. They breathed one of two gas mixtures (euoxia: inspired O2 fraction, FI,O2, 0·21; hyperoxia: FI,O2, 0·70) on different trials determined on a random basis. Breath‐by‐breath VO2 data were time aligned and ensemble averaged. VO2 kinetics, modelled with a single exponential from phase 2 onset (+20 s) to steady state and described by the exponential time constant ([tau]) were compared with data collected from the same adults 9 years earlier. One‐way repeated measures analysis of variance revealed that [tau] was slowed significantly with age (from 30 ± 8 to 46 ± 10 s), but was unaffected by hyperoxia (43 ± 15 s). We concluded that: (1) in older adults studied longitudinally over a 9 year period, the on‐transient VO2 kinetics are slowed, in agreement with, but to a greater extent, than from cross‐sectional data; and (2) the phase 2 time constant ([tau]) for these older adults was not accelerated by hyperoxic breathing. Thus the expected hyperoxia‐induced increase in the capacity for O2 transport was not associated with faster on‐transient VO2 kinetics suggesting either that O2 transport may not limit VO2 kinetics during the 8th decade, or that O2 transport was not improved with hyperoxia.

The effect of hypoxia on pulmonary O2 uptake, leg blood flow and muscle deoxygenation during single-leg knee-extension exercise.

The effect of hypoxic breathing on pulmonary O2 uptake (VO2p), leg blood flow (LBF) and O2 delivery and deoxygenation of the vastus lateralis muscle was examined during constant‐load single‐leg knee‐extension exercise. Seven subjects (24 ± 4 years; mean ±s.d.) performed two transitions from unloaded to moderate‐intensity exercise (21 W) under normoxic and hypoxic (PETO2= 60 mmHg) conditions. Breath‐by‐breath VO2p and beat‐by‐beat femoral artery mean blood velocity (MBV) were measured by mass spectrometer and volume turbine and Doppler ultrasound (VingMed, CFM 750), respectively. Deoxy‐(HHb), oxy‐, and total haemoglobin/myoglobin were measured continuously by near‐infrared spectroscopy (NIRS; Hamamatsu NIRO‐300). VO2p data were filtered and averaged to 5 s bins at 20, 40, 60, 120, 180 and 300 s. MBV data were filtered and averaged to 2 s bins (1 contraction cycle). LBF was calculated for each contraction cycle and averaged to 5 s bins at 20, 40, 60, 120, 180 and 300 s. VO2p was significantly lower in hypoxia throughout the period of 20, 40, 60 and 120 s of the exercise on‐transient. LBF (l min−1) was approximately 35% higher (P > 0.05) in hypoxia during the on‐transient and steady‐state of KE exercise, resulting in a similar leg O2 delivery in hypoxia and normoxia. Local muscle deoxygenation (HHb) was similar in hypoxia and normoxia. These results suggest that factors other than O2 delivery, possibly the diffusion of O2, were responsible for the lower O2 uptake during the exercise on‐transient in hypoxia.

Determinants of oxygen uptake kinetics in older humans following single-limb endurance exercise training.

We hypothesised that the observed acceleration in the kinetics of exercise on‐transient oxygen uptake (V̇O2) of five older humans (77 ± 7 years (mean ± S.D.) following 9 weeks of single‐leg endurance exercise training was due to adaptations at the level of the muscle cell. Prior to, and following training, subjects performed constant‐load single‐limb knee extension exercise. Following training V̇O2 kinetics (phase 2, τ) were accelerated in the trained leg (week 0, 92 ± 44 s; week 9, 48 ± 22 s) and unchanged in the untrained leg (week 0, 104 ± 43 s; week 9, 126 ± 35 s). The kinetics of mean blood velocity in the femoral artery were faster than the kinetics of V̇O2, but were unchanged in both the trained (week 0, 19 ± 10 s; week 9, 26 ± 11 s) and untrained leg (week 0, 20 ± 18 s; week 9, 18 ± 10 s). Maximal citrate synthase activity, measured from biopsies of the vastus lateralis muscle, increased (P < 0.05) in the trained leg (week 0, 6.7 ± 2.0 μmol (g wet wt)−1 min−1; week 9, 11.4 ± 3.6 μmol (g wet wt)−1 min−1) but was unchanged in the untrained leg (week 0, 5.9 ± 0.5 μmol (g wet wt)−1 min−1; week 9, 7.9 ± 1.9 μmol (g wet wt)−1 min−1). These data suggest that the acceleration of V̇O2 kinetics was due to an improved rate of O2 utilisation by the muscle, but was not a result of increased O2 delivery.

Time course and mechanisms of adaptations in cardiorespiratory fitness with endurance training in older and young men

The time-course and mechanisms of adaptation of cardiorespiratory fitness were examined in 8 older (O) (68 +/- 7 yr old) and 8 young (Y) (23 +/- 5 yr old) men pretraining and at 3, 6, 9, and 12 wk of training. Training was performed on a cycle ergometer three times per week for 45 min at approximately 70% of maximal oxygen uptake (Vo(2 max)). Vo(2 max) increased within 3 wk with further increases observed posttraining in both O (+31%) and Y (+18%), (P < 0.05). Maximal cardiac output (Q(max), open-circuit acetylene) and stroke volume were higher in O and Y after 3 wk with further increases after 9 wk of training (P < 0.05). Maximal arterial-venous oxygen difference (a-vO(2 diff)) was higher at weeks 3 and 6 and posttraining compared with pretraining in O and Y (P < 0.05). In O, approximately 69% of the increase in Vo(2 max) from pre- to posttraining was explained by an increased Q(max) with the remaining approximately 31% explained by a widened a-vO(2 diff). This proportion of Q and a-vO(2 diff) contributions to the increase in Vo(2 max) was consistent throughout testing in O. In Y, 56% of the pre- to posttraining increase in Vo(2 max) was attributed to a greater Q(max) and 44% to a widened a-vO(2 diff). Early adaptations (first 3 wk) mainly relied on a widened maximal a-vO(2 diff) (approximately 66%) whereas further increases in Vo(2 max) were exclusively explained by a greater Q(max). In conclusion, with short-term training O and Y significantly increased their Vo(2 max); however, the proportion of Vo(2 max) increase explained by Q(max) and maximal a-vO(2 diff) throughout training showed a different pattern by age group.

A Comparison of Modelling Techniques used to Characterise Oxygen Uptake Kinetics During the on‐Transient of Exercise

We compared estimates for the phase 2 time constant (τ) of oxygen uptake (V̇O2) during moderate‐ and heavy‐intensity exercise, and the slow component of V̇O2 during heavy‐intensity exercise using previously published exponential models. Estimates for τ and the slow component were different (P < 0.05) among models. For moderate‐intensity exercise, a two‐component exponential model, or a mono‐exponential model fitted from 20 s to 3 min were best. For heavy‐intensity exercise, a three‐component model fitted throughout the entire 6 min bout of exercise, or a two‐component model fitted from 20 s were best. When the time delays for the two‐ and three‐component models were equal the best statistical fit was obtained; however, this model produced an inappropriately low ΔV̇O2/ΔWR (WR, work rate) for the projected phase 2 steady state, and the estimate of phase 2 τ was shortened compared with other models. The slow component was quantified as the difference between V̇O2 at end‐exercise (6 min) and at 3 min (ΔV̇O2 (6‐3 min); 259 ml min−1), and also using the phase 3 amplitude terms (truncated to end‐exercise) from exponential fits (409‐833 ml min−1). Onset of the slow component was identified by the phase 3 time delay parameter as being of delayed onset ∼2 min (vs. arbitrary 3 min). Using this delay ΔV̇O2 (6‐2 min) was ∼400 ml min−1. Use of valid consistent methods to estimate τ and the slow component in exercise are needed to advance physiological understanding.

Influence of phase I duration on phase II VO2 kinetics parameter estimates in older and young adults.

Older adults (O) may have a longer phase I pulmonary O(2) uptake kinetics (Vo(2)(p)) than young adults (Y); this may affect parameter estimates of phase II Vo(2)(p). Therefore, we sought to: 1) experimentally estimate the duration of phase I Vo(2)(p) (EE phase I) in O and Y subjects during moderate-intensity exercise transitions; 2) examine the effects of selected phase I durations (i.e., different start times for modeling phase II) on parameter estimates of the phase II Vo(2)(p) response; and 3) thereby determine whether slower phase II kinetics in O subjects represent a physiological difference or a by-product of fitting strategy. Vo(2)(p) was measured breath-by-breath in 19 O (68 ± 6 yr; mean ± SD) and 19 Y (24 ± 5 yr) using a volume turbine and mass spectrometer. Phase I Vo(2)(p) was longer in O (31 ± 4 s) than Y (20 ± 7 s) (P < 0.05). In O, phase II τVo(2)(p) was larger (P < 0.05) when fitting started at 15 s (49 ± 12 s) compared with fits starting at the individual EE phase I (43 ± 12 s), 25 s (42 ± 10 s), 35 s (42 ± 12 s), and 45 s (45 ± 15 s). In Y, τVo(2)(p) was not affected by the time at which phase II Vo(2)(p) fitting started (τVo(2)(p) = 31 ± 7 s, 29 ± 9 s, 30 ± 10 s, 32 ± 11 s, and 30 ± 8 s for fittings starting at 15 s, 25 s, 35 s, 45 s, and EE phase I, respectively). Fitting from EE phase I, 25 s, or 35 s resulted in the smallest CI τVo(2)(p) in both O and Y. Thus, fitting phase II Vo(2)(p) from (but not constrained to) 25 s or 35 s provides consistent estimates of Vo(2)(p) kinetics parameters in Y and O, despite the longer phase I Vo(2)(p) in O.