Interaction of factors determining oxygen uptake at the onset of exercise

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 (.V(O2)) 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 .V(O2) 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]; .V(O2) was measured breath-by-breath using a custom-designed turbine and a mass spectrometer system. 3. The .V(O2) kinetics of the second exercise bout were altered compared with the first such that (a) not only was the instantaneous rate of .V(O2) change (at a given level of .V(O2)) greater but the phase II tau 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 .V(O2) 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.

Muscle chemoreflex elevates muscle blood flow and O2 uptake at exercise onset in nonischemic human forearm

We tested the hypothesis that increases in forearm blood flow (FBF) during the adaptive phase at the onset of moderate exercise would allow a more rapid increase in muscle O2 uptake (VO2 mus). Fifteen subjects completed forearm exercise in control (Con) and leg occlusion (Occ) conditions. In Occ, exercise of ischemic calf muscles was performed before the onset of forearm exercise to activate the muscle chemoreflex evoking a 25-mmHg increase in mean arterial pressure that was sustained during forearm exercise. Eight subjects who increased FBF during Occ compared with Con in the adaptation phase by >30 ml/min were considered "responders." For the responders, a higher VO2 mus accompanied the higher FBF only during the adaptive phase of the Occ tests, whereas there was no difference in the baseline or steady-state FBF or VO2 mus between Occ and Con. Supplying more blood flow at the onset of exercise allowed a more rapid increase in VO2 mus supporting our hypothesis that, at least for this type of exercise, O2 supply might be limiting.

Muscle oxygen uptake and energy turnover during dynamic exercise at different contraction frequencies in humans

1. It has been established that pulmonary oxygen uptake is greater during cycle exercise in humans at high compared to low contraction frequencies. However, it is unclear whether this is due to more work being performed at the high frequencies and whether the energy turnover of the working muscles is higher. The present study tested the hypothesis that human skeletal muscle oxygen uptake and energy turnover are elevated during exercise at high compared to low contraction frequency when the total power output is the same. 2. Seven subjects performed single-leg dynamic knee-extensor exercise for 10 min at contraction frequencies of 60 and 100 r.p.m. where the total power output (comprising the sum of external and internal power output) was matched between frequencies (54 +/- 5 vs. 56 +/- 5 W; mean +/- S.E.M.). Muscle oxygen uptake was determined from measurements of thigh blood flow and femoral arterial - venous differences for oxygen content (a-v O(2) diff). Anaerobic energy turnover was estimated from measurements of lactate release and muscle lactate accumulation as well as muscle ATP and phosphocreatine (PCr) utilisation based on analysis of muscle biopsies obtained before and after each exercise bout. 3. Whilst a-v O(2) diff was the same between contraction frequencies during exercise, thigh blood flow was higher (P < 0.05) at 100 compared to 60 r.p.m. Thus, muscle V(O2) was higher (P < 0.05) during exercise at 100 r.p.m. Muscle V(O2) increased (P < 0.05) by 0.06 +/- 0.03 (12 %) and 0.09 +/- 0.03 l min(-1) (14 %) from the third minute to the end of exercise at 60 and 100 r.p.m., respectively, but there was no difference between the two frequencies. 4. Muscle PCr decreased by 8.1 +/- 1.7 and 9.1 +/- 2.0 mmol (kg wet wt)(-1), and muscle lactate increased to 6.8 +/- 2.1 and 9.8 +/- 2.5 mmol (kg wet wt)(-1) during exercise at 60 and 100 r.p.m., respectively. The total release of lactate during exercise was 48.7 +/- 8.8 and 64.3 +/- 10.6 mmol at 60 and 100 r.p.m. (not significant, NS). The total anaerobic ATP production was 47 +/- 8 and 61 +/- 12 mmol kg(-1), respectively (NS). 5. Muscle temperature increased (P < 0.05) from 35.8 +/- 0.3 to 38.2 +/- 0.2 degrees C at 60 r.p.m. and from 35.9 +/- 0.3 to 38.4 +/- 0.3 degrees C at 100 r.p.m. Between 1 and 7 min muscle temperature was higher (P < 0.05) at 100 compared to 60 r.p.m. 6. The estimated mean rate of energy turnover during exercise was higher (P < 0.05) at 100 compared to 60 r.p.m. (238 +/- 16 vs. 194 +/- 11 J s(-1)). Thus, mechanical efficiency was lower (P < 0.05) at 100 r.p.m. (24 +/- 2 %) compared to 60 r.p.m. (28 +/- 3 %). Correspondingly, efficiency expressed as work per mol ATP was lower (P < 0.05) at 100 than at 60 r.p.m. (22.5 +/- 2.1 vs. 26.5 +/- 2.5 J (mmol ATP)(-1)). 7. The present study showed that muscle oxygen uptake and energy turnover are elevated during dynamic contractions at a frequency of 100 compared with 60 r.p.m. It was also observed that muscle oxygen uptake increased as exercise progressed in a manner that was not solely related to the increase in muscle temperature and lactate accumulation.

Effect of L-NAME on oxygen uptake kinetics during heavy-intensity exercise in the horse

There is evidence that oxidative enzyme inertia plays a major role in limiting/setting the O(2) uptake (VO(2)) response at the transition to higher metabolic rates and also that nitric oxide (NO) competitively inhibits VO(2) within the electron transport chain. To investigate whether NO is important in setting the dynamic response of VO(2) at the onset of high-intensity (heavy-domain) running in horses, five geldings were run on a treadmill across speed transitions from 3 m/s to speeds corresponding to 80% of peak VO(2) with and without nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor (20 mg/kg; order randomized). L-NAME did not alter (both P > 0.05) baseline (3 m/s, 15.4 +/- 0.3 and 16.2 +/- 0.5 l/min for control and L-NAME, respectively) or end-exercise VO(2) (56.9 +/- 5.1 and 55.2 +/- 5.8 l/min for control and L-NAME, respectively). However, in the L-NAME trial, the primary on-kinetic response was significantly (P < 0.05) faster (i.e., reduced time constant, 27.0 +/- 2.7 and 18.7 +/- 3.0 s for control and L-NAME, respectively), despite no change in the gain of VO(2) (P > 0.05). The faster on-kinetic response was confirmed independent of modeling by reduced time to 50, 63, and 75% of overall VO(2) response (all P < 0.05). In addition, onset of the VO(2) slow component occurred earlier (124.6 +/- 11.2 and 65.0 +/- 6.6 s for control and L-NAME, respectively), and the magnitude of the O(2) deficit was attenuated (both P < 0.05) in the L-NAME compared with the control trial. Acceleration of the VO(2) kinetics by L-NAME suggests that NO inhibition of mitochondrial VO(2) may contribute, in part, to the intrinsic metabolic inertia evidenced at the transition to higher metabolic rates in the horse.

Regulation of oxygen consumption at exercise onset: is it really controversial?

The conflicting hypotheses on the limiting factors for skeletal muscle VO2 on-kinetics might be reconciled in a unifying scenario. Under "normal" conditions, during transitions to moderate intensity exercise, the limiting factor appears to be an inertia of oxidative metabolism. During transitions to exercise of higher metabolic intensity, O2 delivery could play a relatively minor but significant role as a limiting factor.

Regulation of oxygen consumption at the onset of exercise

Increased aerobic production of ATP at the onset of exercise could be limited by availability of metabolic substrates independent of O2, or interaction between O2 and metabolic substrates. We point out the importance of feedback control to match O2 supply to demand and discuss metabolic control at the onset of exercise.

Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans

1. The maximal oxygen uptake (V(O(2),peak)) during dynamic muscular exercise is commonly taken as a crucial determinant of the ability to sustain high-intensity exercise. Considerably less attention, however, has been given to the rate at which V(O(2)) increases to attain this maximum (or to its submaximal requirement), and even less to the kinetic features of the response following exercise. 2. Six, healthy, male volunteers (aged 22 to 58 years), each performed 13 exercise tests: initial ramp-incremental cycle ergometry to the limit of tolerance and subsequently, on different days, three bouts of square-wave exercise each at moderate, heavy, very heavy and severe intensities. Pulmonary gas exchange variables were determined breath by breath throughout exercise and recovery from the continuous monitoring of respired volumes (turbine) and gas concentrations (mass spectrometer). 3. For moderate exercise, the V(O(2)) kinetics were well described by a simple mono-exponential function, following a short cardiodynamic phase, with the on- and off-transients having similar time constants (tau(1)); i.e. tau(1,on) averaged 33 +/- 16 s (+/- S.D.) and tau(1,off) 29 +/- 6 s. 4. The on-transient V(O(2)) kinetics were more complex for heavy exercise. The inclusion of a second slow and delayed exponential component provided an adequate description of the response; i.e. tau(1,on) = 32 +/- 17 s and tau(2,on) = 170 +/- 49 s. The off-transient V(O(2)) kinetics, however, remained mono-exponential (tau(1,off) = 42 +/- 11 s). 5. For very heavy exercise, the on-transient V(O(2)) kinetics were also well described by a double exponential function (tau(1,on) = 34 +/- 11 s and tau(2,on) = 163 +/- 46 s). However, a double exponential, with no delay, was required to characterise the off-transient kinetics (i.e. tau(1,off) = 33 +/- 5 s and tau(2,off) = 460 +/- 123 s). 6. At the highest intensity (severe), the on-transient V(O(2)) kinetics reverted to a mono-exponential profile (tau(1,on) = 34 +/- 7 s), while the off-transient kinetics retained a two-component form (tau(1,off) = 35 +/- 11 s and tau(2,off) = 539 +/- 379 s). 7. We therefore conclude that the kinetics of V(O(2)) during dynamic muscular exercise are strikingly influenced by the exercise intensity, both with respect to model order and to dynamic asymmetries between the on- and off-transient responses.

Fall in intracellular PO(2) at the onset of contractions in Xenopus single skeletal muscle fibers

It remains uncertain whether the delayed onset of mitochondrial respiration on initiation of muscle contractions is related to O(2) availability. The purpose of this research was to measure the kinetics of the fall in intracellular PO(2) at the onset of a contractile work period in rested and previously worked single skeletal muscle fibers. Intact single skeletal muscle fibers (n = 11) from Xenopus laevis were dissected from the lumbrical muscle, injected with an O(2)-sensitive probe, mounted in a glass chamber, and perfused with Ringer solution (PO(2) = 32 +/- 4 Torr and pH = 7.0) at 20 degrees C. Intracellular PO(2) was measured in each fiber during a protocol consisting sequentially of 1-min rest; 3 min of tetanic contractions (1 contraction/2 s); 5-min rest; and, finally, a second 3-min contractile period identical to the first. Maximal force development and the fall in force (to 83 +/- 2 vs. 86 +/- 3% of maximal force development) in contractile periods 1 and 2, respectively, were not significantly different. The time delay (time before intracellular PO(2) began to decrease after the onset of contractions) was significantly greater (P < 0.01) in the first contractile period (13 +/- 3 s) compared with the second (5 +/- 2 s), as was the time to reach 50% of the contractile steady-state intracellular PO(2) (28 +/- 5 vs. 18 +/- 4 s, respectively). In Xenopus single skeletal muscle fibers, 1) the lengthy response time for the fall in intracellular PO(2) at the onset of contractions suggests that intracellular factors other than O(2) availability determine the on-kinetics of oxidative phosphorylation and 2) a prior contractile period results in more rapid on-kinetics.

The slow component of O(2) uptake is not accompanied by changes in muscle EMG during repeated bouts of heavy exercise in humans

1. We hypothesized that either the recruitment of additional muscle motor units and/or the progressive recruitment of less efficient fast-twitch muscle fibres was the predominant contributor to the additional oxygen uptake (VO2) observed during heavy exercise. Using surface electromyographic (EMG) techniques, we compared the VO2 response with the integrated EMG (iEMG) and mean power frequency (MPF) response of the vastus lateralis with the VO2 response during repeated bouts of moderate (below the lactate threshold, < LT) and heavy (above the lactate threshold, > LT) intensity cycle ergometer exercise. 2. Seven male subjects (age 29 +/- 7 years, mean +/- S.D.) performed three transitions to a work rate (WR) corresponding to 90 % LT and two transitions to a work rate that would elicit a VO2 corresponding to 50 % of the difference between peak VO2 and the LT (i.e. Delta50 %, > LT1 and > LT2). 3. The VO2 slow component was significantly reduced by prior heavy intensity exercise (> LT1, 410 +/- 196 ml min(-1); > LT2, 230 +/- 191 ml min-1). The time constant (tau), amplitude (A) and gain (DeltaVO2/DeltaWR) of the primary VO2 response (phase II) were not affected by prior heavy exercise when a three-component, exponential model was used to describe the V2 response. 4. Integrated EMG and MPF remained relatively constant and at the same level throughout both > LT1 and > LT2 exercise and therefore were not associated with the VO2 slow component. 5. These data are consistent with the view that the increased O2 cost (i.e. VO2 slow component) associated with performing heavy exercise is coupled with a progressive increase in ATP requirements of the already recruited motor units rather than to changes in the recruitment pattern of slow versus fast-twitch motor units. Further, the lack of speeding of the kinetics of the primary VO2 component with prior heavy exercise, thought to represent the initial muscle VO2 response, are inconsistent with O2 delivery being the limiting factor in V > O2 kinetics during heavy exercise.

Effect of muscle mass on V(O(2)) kinetics at the onset of work

The dependence of O(2) uptake (V(O(2))) kinetics on the muscle mass recruited under conditions when fiber and muscle recruitment patterns are similar following the onset of exercise has not been determined. We developed a motorized cycle ergometer that facilitated one-leg (1L) cycling in which the electromyographic (EMG) profile of the active muscles was not discernibly altered from that during two-leg (2L) cycling. Six subjects performed 1L and 2L exercise transitions from unloaded cycling to moderate [<ventilatory threshold (VT)] and heavy (>VT) exercise. The 1L condition yielded kinetics that was unchanged from the 2L condition [the phase 2 time constants (tau(1), in s) for <VT were as follows: 1L = 16.8+/-8.4 (SD), 2L = 18.4 +/- 8.1, P > 0.05; for >VT: 1L = 26.8 +/- 12.0; 2L = 27.8 +/- 16.1, P > 0.05]. The overall V(O(2)) kinetics (mean response time) was not significantly different for the two exercise conditions. However, the gain of the fast component (the amplitude/work rate) during the 1L exercise was significantly higher than that for the 2L exercise for both moderate and heavy work rates. The slow-component responses evident for heavy exercise were temporally and quantitatively unaffected by the 1L condition. These data demonstrate that, when leg muscle recruitment patterns are unchanged as assessed by EMG analysis, on-transient V(O(2)) kinetics for both moderate and heavy exercise are not dependent on the muscle mass recruited.

Peripheral circulatory factors limit rate of increase in muscle O(2) uptake at onset of heavy exercise

We used an exercise paradigm with repeated bouts of heavy forearm exercise to test the hypothesis that alterations in local acid-base environment that remain after the first exercise result in greater blood flow and O(2) delivery at the onset of the second bout of exercise. Two bouts of handgrip exercise at 75% peak workload were performed for 5 min, separated by 5 min of recovery. We continuously measured blood flow using Doppler ultrasound and sampled venous blood for O(2) content, PCO(2), pH, and lactate and potassium concentrations, and we calculated muscle O(2) uptake (VO(2)). Forearm blood flow was elevated before the second exercise compared with the first and remained higher during the first 30 s of exercise (234 +/- 18 vs. 187 +/- 4 ml/min, P < 0.05). Flow was not different at 5 min. Arteriovenous O(2) content difference was lower before the second bout (4.6 +/- 0.9 vs. 7.2 +/- 0.7 ml O(2)/dl) and higher by 30 s of exercise (11.2 +/- 0.7 vs. 10.8 +/- 0.7 ml O(2)/dl, P < 0. 05). Muscle VO(2) was unchanged before the start of exercise but was elevated during the first 30 s of the transition to the second exercise bout (26.0 +/- 2.1 vs. 20.0 +/- 0.9 ml/min, P < 0.05). Changes in venous blood PCO(2), pH, and lactate concentration were consistent with reduced reliance on anaerobic glycolysis at the onset of the second exercise bout. These data show that limitations of muscle blood flow can restrict the adaptation of oxidative metabolism at the onset of heavy muscular exertion.

Effects of prior heavy exercise on phase II pulmonary oxygen uptake kinetics during heavy exercise

We tested the hypothesis that heavy-exercise phase II oxygen uptake (VO(2)) kinetics could be speeded by prior heavy exercise. Ten subjects performed four protocols involving 6-min exercise bouts on a cycle ergometer separated by 6 min of recovery: 1) moderate followed by moderate exercise; 2) moderate followed by heavy exercise; 3) heavy followed by moderate exercise; and 4) heavy followed by heavy exercise. The VO(2) responses were modeled using two (moderate exercise) or three (heavy exercise) independent exponential terms. Neither moderate- nor heavy-intensity exercise had an effect on the VO(2) kinetic response to subsequent moderate exercise. Although heavy-intensity exercise significantly reduced the mean response time in the second heavy exercise bout (from 65.2 +/- 4.1 to 47.0 +/- 3.1 s; P < 0.05), it had no significant effect on either the amplitude or the time constant (from 23.9 +/- 1.9 to 25.3 +/- 2.9 s) of the VO(2) response in phase II. Instead, this "speeding" was due to a significant reduction in the amplitude of the VO(2) slow component. These results suggest phase II VO(2) kinetics are not speeded by prior heavy exercise.

Role of convective O(2) delivery in determining VO(2) on-kinetics in canine muscle contracting at peak VO(2)

A previous study (Grassi B, Gladden LB, Samaja M, Stary CM, and Hogan MC, J Appl Physiol 85: 1394-1403, 1998) showed that convective O(2) delivery to muscle did not limit O(2) uptake (VO(2)) on-kinetics during transitions from rest to contractions at approximately 60% of peak VO(2). The present study aimed to determine whether this finding is also true for transitions involving contractions of higher metabolic intensities. VO(2) on-kinetics were determined in isolated canine gastrocnemius muscles in situ (n = 5) during transitions from rest to 4 min of electrically stimulated isometric tetanic contractions corresponding to the muscle peak VO(2). Two conditions were compared: 1) spontaneous adjustment of muscle blood flow (Q) (Control) and 2) pump-perfused Q, adjusted approximately 15-30 s before contractions at a constant level corresponding to the steady-state value during contractions in Control (Fast O(2) Delivery). In Fast O(2) Delivery, adenosine was infused intra-arterially. Q was measured continuously in the popliteal vein; arterial and popliteal venous O(2) contents were measured at rest and at 5- to 7-s intervals during the transition. Muscle VO(2) was determined as Q times the arteriovenous blood O(2) content difference. The time to reach 63% of the VO(2) difference between resting baseline and steady-state values during contractions was 24.9 +/- 1.6 (SE) s in Control and 18.5 +/- 1.8 s in Fast O(2) Delivery (P < 0.05). Faster VO(2) on-kinetics in Fast O(2) Delivery was associated with an approximately 30% reduction in the calculated O(2) deficit and with less muscle fatigue. During transitions involving contractions at peak VO(2), convective O(2) delivery to muscle, together with an inertia of oxidative metabolism, contributes in determining the VO(2) on-kinetics.

Exercise testing in children: an alternative approach

In recent years there has been a call for new methods of evaluating the cardiorespiratory responses of children to exercise that complement their everyday exercise patterns. One potential method would be to use a sub-maximal, intermittent, pseudo-random binary sequence (PRBS) exercise test protocol to measure oxygen uptake kinetics (VO2 kinetics). Ten children of mean (SD) age 10.8 (+/- 1.5) years completed a 20 - 50 W cycle ergometer protocol of 17-min duration. An estimate of alveolar oxygen uptake (VO2) was calculated on a breath-by-breath basis. The VO2 kinetic parameters were expressed in the frequency domain as amplitude ratio and phase delay using standard Fourier techniques. Analysis was restricted to the frequency range 2.2 to 8.9 mHz. The mean (SD) amplitude ratio responses decreased from 10.33 (+/- 0.73) to 7.42 (+/- 0.99) ml min(-1) W(-1) and the mean phase delay increased from -26.78 degrees (+/- 6.37 degrees) to -81.93 degrees (+/- 10.45 degrees) over the frequency range 2.2-8.9 mHz. Significant correlations (p < 0.05) were found between chronological age and amplitude ratio (r = 0.68 and 0.62), and chronological age and phase delay (r = -0.62 and -0.69) at the frequencies of 2.2 and 4.4 mHz, respectively. No significant correlations were found between VO2 kinetics and stature or VO2 kinetics and body mass. The observations demonstrated the use of the PRBS technique to measure VO2 kinetics in the frequency domain in children. This approach may be a useful addition to the tests that are used to quantify the oxygen uptake responses to exercise in children.

Effects of PDH activation by dichloroacetate in human skeletal muscle during exercise in hypoxia

During the onset of exercise in hypoxia, the increased lactate accumulation is associated with a delayed activation of pyruvate dehydrogenase (PDH; Parolin ML, Spreit LL, Hultman E, Hollidge-Horvat MG, Jones NL, and Heigenhauser GJF. Am J Physiol Endocrinol Metab 278: E522-E534, 2000). The present study investigated whether activation of PDH with dichloroacetate (DCA) before exercise would reduce lactate accumulation during exercise in acute hypoxia by increasing oxidative phosphorylation. Six subjects cycled on two occasions for 15 min at 55% of their normoxic maximal oxygen uptake after a saline (control) or DCA infusion while breathing 11% O(2). Muscle biopsies of the vastus lateralis were taken at rest and after 1 and 15 min of exercise. DCA increased PDH activity at rest and at 1 min of exercise, resulting in increased acetyl-CoA concentration and acetylcarnitine concentration at rest and at 1 min. In the first minute of exercise, there was a trend toward a lower phosphocreatine (PCr) breakdown with DCA compared with control. Glycogenolysis was lower with DCA, resulting in reduced lactate concentration ([lactate]), despite similar phosphorylase a mole fractions and posttransformational regulators. During the subsequent 14 min of exercise, PDH activity was similar, whereas PCr breakdown and muscle [lactate] were reduced with DCA. Glycogenolysis was lower with DCA, despite similar mole fractions of phosphorylase a, and was due to reduced posttransformational regulators. The results from the present study support the hypothesis that lactate production is due in part to metabolic inertia and cannot solely be explained by an oxygen limitation, even under conditions of acute hypoxia.

Muscle oxygen kinetics at onset of intense dynamic exercise in humans

The present study examined the onset and the rate of rise of muscle oxidation during intense exercise in humans and whether oxygen availability limits muscle oxygen uptake in the initial phase of intense exercise. Six subjects performed 3 min of intense one-legged knee-extensor exercise [65.3 +/- 3.7 (means +/- SE) W]. The femoral arteriovenous blood mean transit time (MTT) and time from femoral artery to muscle microcirculation was determined to allow for an examination of the oxygen uptake at capillary level. MTT was 15.3 +/- 1.8 s immediately before exercise, 10.4 +/- 0.7 s after 6 s of exercise, and 4.7 +/- 0.5 s at the end of exercise. Arterial venous O(2) difference (a-v(diff) O(2)) of 18 +/- 5 ml/l before the exercise was unchanged after 2 s, but it increased (P < 0.05) after 6 s of exercise to 43 +/- 10 ml/l and reached 146 +/- 4 ml/l at the end of exercise. Thigh oxygen uptake increased (P < 0.05) from 32 +/- 8 to 102 +/- 28 ml/min after 6 s of exercise and to 789 +/- 88 ml/min at the end of exercise. The time to reach half-peak a-v(diff) O(2) and thigh oxygen uptake was 13 +/- 2 and 25 +/- 3 s, respectively. The difference between thigh oxygen delivery (blood flow x arterial oxygen content) and thigh oxygen uptake increased (P < 0.05) after 6 s and returned to preexercise level after 14 s. The present data suggest that, at the onset of exercise, oxygen uptake of the exercising muscles increases after a delay of only a few seconds, and oxygen extraction peaks after approximately 50 s of exercise. The limited oxygen utilization in the initial phase of intense exercise is not caused by insufficient oxygen availability.

Kinetics of oxygen uptake at the onset of exercise near or above peak oxygen uptake

We tested the hypothesis that kinetics of O(2) uptake (VO(2)) measured in the transition to exercise near or above peak VO(2) (VO(2 peak)) would be slower than those for subventilatory threshold exercise. Eight healthy young men exercised at approximately 57, approximately 96, and approximately 125% VO(2 peak). Data were fit by a two- or three-component exponential model and with a semilogarithmic transformation that tested the difference between required VO(2) and measured VO(2). With the exponential model, phase 2 kinetics appeared to be faster at 125% VO(2 peak) [time constant (tau(2)) = 16.3 +/- 8.8 (SE) s] than at 57% VO(2 peak) (tau(2) = 29. 4 +/- 4.0 s) but were not different from that at 96% VO(2 peak) exercise (tau(2) = 22.1 +/- 2.1 s). VO(2) at the completion of phase 2 was 77 and 80% VO(2 peak) in tests predicted to require 96 and 125% VO(2 peak). When VO(2) kinetics were calculated with the semilogarithmic model, the estimated tau(2) at 96% VO(2 peak) (49.7 +/- 5.1 s) and 125% VO(2 peak) (40.2 +/- 5.1 s) were slower than with the exponential model. These results are consistent with our hypothesis and with a model in which the cardiovascular system is compromised during very heavy exercise.

Muscle oxygen uptake in humans at onset of and during intense exercise

This review presents data on human muscle oxygen consumption in the initial phase of exercise as well as on muscle maximal oxygen uptake. It also discusses mechanistic limiting factors related to oxygen utilization at the onset of exercise and of maximal aerobic power of skeletal muscle. Direct measurements of oxygen utilization of a well-defined muscle show that contracting muscles utilize oxygen within a few seconds of exercise onset and that it takes some 45 s before oxygen extraction is maximal. The delayed oxygen utilization in the initial phase of intense exercise does not appear to be caused by insufficient oxygen availability. But it may rather be the result of a non-optimal distribution of blood flow in the exercising muscles and a limitation in the rate of oxygen extraction by the contracting muscle cells. The latter limitation does not appear to be caused by an insufficient activation of the enzyme pyruvate dehydrogenase. The maximal oxygen uptake of skeletal muscle is around 300-400 mL min-1 kg-1. This uptake rate corresponds to a TCA cycle rate of 4-5 mmol min-1 kg-1, which is of the same magnitude as the activity of oxyglutarate dehydrogenase and pyruvate dehydrogenase, suggesting that these enzymes may be rate limiting for oxygen uptake when an isolated muscle is exercising.

VO(2) kinetics of mild exercise are altered by RER

We propose that variations in fat and carbohydrate (CHO) oxidation by working muscle alter O(2) uptake (VO(2)) kinetics. This hypothesis provides two predictions: 1) the kinetics should comprise two exponential components, one fast and the other slow, and 2) their contribution should change with variations in fat and CHO oxidation, as predicted by steady-state respiratory exchange ratio (RER). The purpose of this study was to test these predictions by evaluating the VO(2) kinetic model: VO(2)(t) = alpha(R) + alpha(F)(1 - exp[(t - TD)/-tau(F)]) + alpha(C)(1 - exp[(t - TD)/-tau(C)]) for short-term, mild leg cycling in 38 women and 44 men, where VO(2)(t) describes the time course, alpha(R) is resting VO(2), t is time after onset of exercise, TD is time delay, alpha(F) and tau(F) are asymptote and time constant, respectively, for the fast (fat) oxidative term, and alpha(C) and tau(C) are the corresponding parameters for the slow (CHO) oxidative term. We found that 1) this biexponential model accurately described the VO(2) kinetics over a wide range of RERs, 2) the contribution of the fast (alpha(F), fat) component was inversely related to RER, whereas the slow (alpha(C), CHO) component was positively related to RER, and 3) this assignment of the fast and slow terms accurately predicted steady-state respiratory quotient and CO(2) output. Therefore, the kinetic model can quantify the dynamics of fat and CHO oxidation over the first 5-10 min of mild exercise in young adult men and women.