Are oxygen uptake kinetics at the onset of exercise speeded up by local metabolic status in active muscles?

Effects of Cycling on Subsequent Running Performance, Stride Length, and Muscle Oxygen Saturation in Triathletes

Running performance is a determinant factor for victory in Sprint and Olympic distance triathlon. Previous cycling may impair running performance in triathlons, so brick training becomes an important part of training. Wearable technology that is used by triathletes can offer several metrics for optimising training in real-time. The aim of this study was to analyse the effect of previous cycling on subsequent running performance in a field test, while using kinematics metrics and SmO₂ provided by wearable devices that are potentially used by triathletes. Ten trained triathletes participated in a randomised crossover study, performing two trial sessions that were separated by seven days: the isolated run trial (IRT) and the bike-run trial (BRT). Running kinematics, physiological outcomes, and perceptual parameters were assessed before and after each running test. The running distance was significantly lower in the BRT when compared to the IRT, with a decrease in stride length of 0.1 m (p = 0.00) and higher %SmO₂ (p = 0.00) in spite of the maximal intensity of exercise. No effects were reported in vertical oscillation, ground contact time, running cadence, and average heart rate. These findings may only be relevant to 'moderate level' triathletes, but not to 'elite' ones. Triathletes might monitor their %SmO₂ and stride length during brick training and then compare it with isolated running to evaluate performance changes. Using wearable technology (near-infrared spectroscopy, accelerometry) for specific brick training may be a good option for triathletes.

Is Moderate Intensity Cycling Sufficient to Induce Cardiorespiratory and Biomechanical Modifications of Subsequent Running?

Walsh, JA, Dawber, JP, Lepers, R, Brown, M, and Stapley, PJ. Is moderate intensity cycling sufficient to induce cardiorespiratory and biomechanical modifications of subsequent running? J Strength Cond Res 31(4): 1078-1086, 2017-This study sought to determine whether prior moderate intensity cycling is sufficient to influence the cardiorespiratory and biomechanical responses during subsequent running. Cardiorespiratory and biomechanical variables measured after moderate intensity cycling were compared with control running at the same intensity. Eight highly trained, competitive triathletes completed 2 separate exercise tests; (a) a 10-minute control run (no prior cycling) and, (b) a 30-minute transition run (TR) (preceded by 20-minute of variable cadence cycling, i.e., run versus cycle-run). Respiratory, breathing frequency (fb), heart rate (HR), cost of running (Cr), rate constant, stride length, and stride frequency variables were recorded, normalized, and quantified at the mean response time (MRT), third minute, 10th minute (steady state), and overall for the control run (CR) and TR. Cost of running increased (p ≤ 0.05) at all respective times during the TR. The V[Combining Dot Above]E/V[Combining Dot Above]CO2 and respiratory exchange ratio (RER) were significantly (p < 0.01) elevated at the MRT and 10th minute of the TR. Furthermore, overall mean increases were recorded for Cr, V[Combining Dot Above]E, V[Combining Dot Above]E/V[Combining Dot Above]CO2, RER, fb (p < 0.01), and HR (p ≤ 0.05) during the TR. Rate constant values for oxygen uptake were significantly different between CR and TR (0.48 ± 0.04 vs. 0.89 ± 0.15; p < 0.01). Stride length decreased across all recorded points during the TR (p ≤ 0.05) and stride frequency increased at the MRT and 3 minutes (p < 0.01). The findings suggest that at moderate intensity, prior cycling influences the cardiorespiratory response during subsequent running. Furthermore, prior cycling seems to have a sustained effect on the Cr during subsequent running.

The VO2-on Kinetics in Constant Load Exercise Sub-Anaerobic Threshold Reflects Endothelial Function and Dysfunction in Muscle Microcirculation

To propose a test to evaluate endothelial function, based on VO2 on-transition kinetics in sub-anaerobic threshold (AT) constant load exercise, we tested healthy subjects and patients with ischemic-hypertensive cardiopathy by two cardiopulmonary tests on a cycle ergometer endowed with an electric motor to overcome initial inertia: a pre-test and, after at least 24 h, one 6 min constant load exercise at 90 % AT. We measured net phase 3 VO2-on kinetics and, by phase 2 time constant (τ), valued endothelial dysfunction. We found shorter τ in repeated tests, shorter time between first and second test, by persisting endothelium-dependent arteriolar vasodilatation and/or several other mechanisms. Reducing load to 80 % and 90 % AT did not produce significant changes in τ of healthy volunteers, while in heart patients an AT load of 70 %, compared to 80 % AT, shortened τ (=4.38±1.65 s, p=0.013). In heart patients, no correlation was found between NYHA class, ejection fraction (EF), and the two variables derived from incremental cycle cardio-pulmonary exercise, as well as between EF and τ; while NYHA class groups were well correlated with τ duration (r=0.92, p=0.0001). Doxazosin and tadalafil also significantly reduced τ. In conclusion, the O2 consumption kinetics during the on-transition of constant load exercise below the anaerobic threshold are highly sensitive to endothelial function in muscular microcirculation, and constitute a marker for the evaluation of endothelial dysfunction.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Energy expenditure combining strength and aerobic training

The combination of Strength Training (ST) with Aerobic Training (AT) exercises in the same training session, which commonly appears in literature as the concurrent training, is widely used in fitness and physical condition programs, especially when the aim is to increase the energy expenditure during and after training session. The aim of this study was to identify, through literature, whether the combination of exercises of the ST with exercises of the AT allows changes in body composition and energy expenditure during and after the training session. Chronic studies have showed a positive effect on body composition (decreased in relative body fat) when the ST are combined with AT. Similarly, the acute effects of the order of combining these two types of exercise does not seem to affect energy expenditure, measured by oxygen consumption (VO₂), during the training session and only change this expenditure in the first 15 minutes after the training session. In conclusion, we can say that the studies indicate that the combination of exercises of the ST with exercises of the AT has a positive effect on changes in body composition, and energy expenditure during and after training sessions.

Prior moderate and heavy exercise accelerate oxygen uptake and cardiac output kinetics in endurance athletes

Cardiorespiratory interactions at the onset of dynamic cycling exercise are modified by warm-up exercises. We tested the hypotheses that oxygen uptake (Vo(2)) and cardiac output (Q) kinetics would be accelerated at the onset of heavy and moderate cycling exercise by warm-up. Nine male endurance athletes (peak Vo(2): 60.5 +/- 3.2 ml.min(-1).kg(-1)) performed multiple rides of two different 36-min cycling protocols, involving 6-min bouts at moderate and heavy intensities. Breath-by-breath Vo(2) and beat-by-beat stroke volume (SV) and Q, estimated by Modelflow from the finger pulse, were measured simultaneously with kinetics quantified from the phase II time constant (tau(2)). One novel finding was that both moderate (M) and heavy (H) warm-up bouts accelerated phase II Vo(2) kinetics during a subsequent bout of heavy exercise (tau(2): after M = 22.5 +/- 2.7 s, after H = 22.1 +/- 2.9 vs. 26.2 +/- 3.2 s; P < 0.01). Q kinetics in heavy exercise were accelerated by both warm-up intensities (tau(2): M = 22.0 +/- 4.1 s, H = 23.8 +/- 5.6 s vs. 27.4 +/- 7.2 s; P < 0.05). During moderate exercise, prior heavy-intensity warm-up (one or two bouts) accelerated Vo(2) kinetics and elevated Q at exercise onset, with no changes in Q kinetics. A second novel finding was a significant overshoot in the estimate of SV from Modelflow in the first minutes of each moderate and heavy exercise bout. These findings suggest that the acceleration of Vo(2) kinetics during heavy exercise was enabled by the acceleration of Q kinetics, and that rapid increases in Q at the onset of moderate and heavy exercise might result, in part, from an overshoot of SV.

Effects of warm-up intensity on oxygen transport during supramaximal exercise in horses

OBJECTIVE

To determine whether warm-up exercise at different intensities alters kinetics and total contribution of aerobic power to total metabolic power in subsequent supramaximal exercise in horses.

ANIMALS

11 horses.

PROCEDURES

Horses ran at a sprint until fatigued at 115% of maximal oxygen consumption rate (VO(2max)), beginning at 10 minutes following each of 3 warm-up protocols: no warmup (NoWU), 1 minute at 70% VO(2max) (moderate-intensity warm-up [MoWU]), or 1 minute at 115% VO(2max) (high-intensity warm-up [HiWU]). Cardiopulmonary and blood gas variables were measured during exercise.

RESULTS

The VO(2) was significantly higher in HiWU and MoWU than in NoWU throughout the sprint exercise period. Blood lactate accumulation rate in the first 60 seconds was significantly lower in MoWU and HiWU than in NoWU. Specific cardiac output after 60 seconds of sprint exercise was not significantly different among the 3 protocols; however, the arterial mixed-venous oxygen concentration difference was significantly higher in HiWU than in NoWU primarily because of decreased mixed-venous saturation and tension. Run time to fatigue following MoWU was significantly greater than that with NoWU, and there was no difference in time to fatigue between MoWU and HiWU.

CONCLUSIONS AND CLINICAL RELEVANCE

HiWU and MoWU increased peak values for VO(2) and decreased blood lactate accumulation rate during the first minute of intense exercise, suggesting a greater use of aerobic than net anaerobic power during this period.

Control of oxygen uptake during exercise

Other than during sleep and contrived laboratory testing protocols, humans rarely exist in prolonged metabolic steady states; rather, they transition among different metabolic rates (V O2). The dynamic transition of V O2 (V O2 kinetics), initiated, for example, at exercise onset, provides a unique window into understanding metabolic control. This brief review presents the state-of-the art regarding control of V O2 kinetics within the context of a simple model that helps explain the work rate dependence of V O2 kinetics as well as the effects of environmental perturbations and disease. Insights emerging from application of novel approaches and technologies are integrated into established concepts to assess in what circumstances O2 supply might exert a commanding role over V O2 kinetics, and where it probably does not. The common presumption that capillary blood flow dynamics can be extrapolated accurately from upstream arterial measurements is challenged. From this challenge, new complexities emerge with respect to the relationships between O2 supply and flux across the capillary-myocyte interface and the marked dependence of these processes on muscle fiber type. Indeed, because of interfiber type differences in O2 supply relative to V O2, the presence of much lower O2 levels in the microcirculation supplying fast-twitch muscle fibers, and the demonstrated metabolic sensitivity of muscle to O2, it is possible that fiber type recruitment profiles (and changes thereof) might help explain the slowing of V O2 kinetics at higher work rates and in chronic diseases such as heart failure and diabetes.

New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle

In skeletal muscle, carnitine plays an essential role in the translocation of long-chain fatty-acids into the mitochondrial matrix for subsequent beta-oxidation, and in the regulation of the mitochondrial acetyl-CoA/CoASH ratio. Interest in these vital metabolic roles of carnitine in skeletal muscle appears to have waned over the past 25 years. However, recent research has shed new light on the importance of carnitine as a regulator of muscle fuel selection. It has been established that muscle free carnitine availability may be limiting to fat oxidation during high intensity submaximal exercise. Furthermore, increasing muscle total carnitine content in resting healthy humans (via insulin-mediated stimulation of muscle carnitine transport) reduces muscle glycolysis, increases glycogen storage and is accompanied by an apparent increase in fat oxidation. By increasing muscle pyruvate dehydrogenase complex (PDC) activity and acetylcarnitine content at rest, it has also been established that PDC flux and acetyl group availability limits aerobic ATP re-synthesis at the onset of exercise (the acetyl group deficit). Thus, carnitine plays a vital role in the regulation of muscle fuel metabolism. The demonstration that its availability can be readily manipulated in humans, and impacts on physiological function, will result in renewed business and scientific interest in this compound.

Effects of baseline metabolic rate on pulmonary O2 uptake on-kinetics during heavy-intensity exercise in humans

We hypothesised that initiating heavy-intensity exercise from an elevated baseline metabolic rate would result in slower Phase II O2 uptake V(O2) kinetics and a greater overall 'gain' in V(O2) per unit increase in work rate. Seven healthy males performed a series of like-transitions on a cycle ergometer: (1) from light exercise to 'moderate' exercise (80% of the gas exchange threshold, GET; L-->M); (2) from light exercise to 'heavy' exercise (40% of the difference between GET and V(O2) peak; L-->H); (3) from moderate exercise to heavy exercise (M-->H). The Phase II time constant (tau) was significantly (P<0.01) greater in the M-->H condition (48+/-11 s) compared to the L-->M and L-->H conditions (26+/-6 s versus 27+/-4 s, respectively). Moreover, the end-exercise 'gain' values were significantly different between the three conditions (L-->M, 8.1+/-0.7 mL min-1 W-1; L-->H, 9.7+/-0.4 mL min-1 W-1; M-->H, 10.7+/-0.7 mL min-1 W-1; P<0.05). This 'non-linearity' in the pulmonary V(O2) response to exercise might be attributed, at least in part, to differences in the metabolic properties of the muscle fibres recruited in the abrupt transition from a lower to a higher work rate.

Effects of prior heavy-intensity exercise during single-leg knee extension on v̇o2 kinetics and limb blood flow

The effects of prior heavy-intensity exercise on O2 uptake (V̇o2) kinetics of a second heavy exercise may be due to vasodilation (associated with metabolic acidosis) and improved muscle blood flow. This study examined the effect of prior heavy-intensity exercise on femoral artery blood flow (Qleg) and its relationship with V̇o2 kinetics. Five young subjects completed five to eight repeats of two 6-min bouts of heavy-intensity one-legged, knee-extension exercise separated by 6 min of loadless exercise. V̇o2 was measured breath by breath. Pulsed-wave Doppler ultrasound was used to measure Qleg. V̇o2 and blood flow velocity data were fit using a monoexponential model to identify phase II and phase III time periods and estimate the response amplitudes and time constants (τ). Phase II V̇o2 kinetics was speeded on the second heavy-intensity exercise [mean τ (SD), 29 ( 10 ) s to 24 ( 10 ) s, P < 0.05] with no change in the phase II (or phase III) amplitude. Qleg was elevated before the second exercise [1.55 (0.34) l/min to 1.90 (0.25) l/min, P < 0.05], but the amplitude and time course [τ, 25 ( 13 ) s to 35 ( 13 ) s] were not changed, such that throughout the transient the Qleg (and ΔQleg/ΔV̇o2) did not differ from the prior heavy exercise. Thus V̇o2 kinetics were accelerated on the second exercise, but the faster kinetics were not associated with changes in Qleg. Thus limb blood flow appears not to limit V̇o2 kinetics during single-leg heavy-intensity exercise nor to be the mechanism of the altered V̇o2 response after heavy-intensity prior exercise.

Kinetics of pulmonary $$\dot{V} \hbox{O}_{2}$$ and femoral artery blood flow and their relationship during repeated bouts of heavy exercise

The mechanism that alters the pulmonary VO2 response to heavy-intensity exercise following prior heavy exercise has been frequently ascribed to an improvement in pre-exercise blood flow (BF) or O(2) delivery. Interventions to improve O(2) delivery have rarely resulted in a similar enhancement of VO2. However, the actual limb blood flow and VO2 dynamics in the second bout of repeated exercise remain equivocal. Seven healthy female subjects (21-32 years) performed consecutive 6-min (separated by 6 min of 10 W exercise) bilateral knee extension (KE) exercise in a semisupine position at a work rate halfway between the lactate threshold (LT) and VO2peak. Femoral artery blood flow (FBF) was measured by Doppler ultrasound simultaneously with breath-by-breath VO2 each protocol being repeated at least four times for precise kinetic characterization. The effective time-constant (tau') of the VO2 response was reduced following prior exercise (bout 1: 61.0 +/-10.5 vs. bout 2: 51.6+/-9.0 s; mean +/- SD; P<0.05), which was a result of a reduced slow component (bout 1: 16.0+/-8.0 vs. bout 2: 12.5+/-6.7 %; P<0.05) and an unchanged 'primary' tau. FBF was consistently faster than VO2. However, there was no bout-effect on tau' FBF (bout 1: 28.2+/-12.0 vs. bout 2: 34.2+/-8.5 s). The relationship between the exercise-associated VO2 (i.e., deltaVO2) and Delta FBF was similar between bouts, with a tendency (N.S: P>0.05) for deltaVO2/deltaFBF to be increased during the transition to bout 2 rather than decreased, as hypothesized. The return of VO2 kinetics toward first order, therefore, was associated with an 'appropriate', not enhanced, BF to the working muscles. Whether a relative prior-hyperemia in bout 2 enables a more homogeneous intramuscular distribution of BF and/or metabolic response is unclear, however, these data are consistent with events more proximal to the exercise muscle in mediating the VO2 response during repeated heavy-intensity KE exercise.

Metabolic inertia in contracting skeletal muscle: a novel approach for pharmacological intervention in peripheral vascular disease

Peripheral vascular disease (PVD) is generally accepted to result in the failure of skeletal muscle blood flow to increase adequately at the onset of muscular work. There are currently no routine pharmacological interventions towards the treatment of PVD, however, recent Phase III trials in the USA have demonstrated the clinical potential of the phosphodiesterase III inhibitor Cilostazol for pain-free and maximal walking distances in patients with intermittent claudication. PVD is characterized by a marked reliance on oxygen-independent routes of ATP regeneration (phosphocreatine hydrolysis and glycolysis) in skeletal muscle during contraction and the rapid onset of muscular pain and fatigue. The accumulation of metabolic by-products of oxygen-independent ATP production (hydrogen and lactate ions and inorganic phosphate) has long been associated with an inhibition in contractile function in both healthy volunteers and PVD patients. Therefore, any strategy that could reduce the reliance upon ATP re-synthesis from oxygen-independent routes, and increase the contribution of oxygen-dependent (mitochondrial) ATP re-synthesis, particularly at the onset of exercise, might be expected to improve functional capacity and be of considerable therapeutic value. Historically, the increased contribution of oxygen-independent ATP re-synthesis to total ATP generation at the onset of exercise has been attributed to a lag in muscle blood flow limiting oxygen delivery during this period. However, recent evidence suggests that limited inertia is present at the level of oxygen delivery, whilst considerable inertia exists at the level of mitochondrial enzyme activation and substrate supply. In support of this latter hypothesis, we have reported on a number of occasions that activation of the pyruvate dehydrogenase complex, using pharmacological interventions, can markedly reduce the dependence on ATP re-synthesis from oxygen-independent routes at the onset of muscle contraction. This review will focus on these findings and will highlight the pyruvate dehydrogenase complex as a novel therapeutic target towards the treatment of peripheral vascular disease, or any other disease state where premature muscular fatigue is prevalent due to metabolite accumulation.

Nitric oxide synthase inhibition speeds oxygen uptake kinetics in horses during moderate domain running

Within the moderate exercise intensity domain, the speed of oxygen uptake (V(O(2))) kinetics at the transition to a higher metabolic rate is thought to be limited by an inertia of the oxidative machinery. Nitric oxide (NO)-induced inhibition of O(2) consumption within the electron transport chain may contribute to this inertia. This investigation tested the hypothesis that a reduction or removal of any such NO effect via infusion of N(omega)-nitro-L-arginine methyl ester (L-NAME; a NOS inhibitor) would speed V(O(2)) kinetics at the onset of moderate exercise. Five Thoroughbred geldings underwent four transitions to running speeds of 7 m sec(-1) (two control, C, 2 L-NAME [20 mg kg(-1)]) on an equine treadmill during which pulmonary gas exchange was determined using a bias flow system. Consistent with exercise in the moderate intensity domain, in none of the transitions was a V(O(2)) slow component elicited. The L-NAME treatment significantly accelerated V(O(2)) kinetics via a reduction of the primary amplitude time constant (C, 17.3 +/- 1.7; L-NAME, 11.8 +/- 1.5 sec, P < 0.05) concomitant with faster overall dynamics (i.e. T(50) and T(75) both P < 0.05) and a trend toward a decreased O(2) deficit (C, 6.4 +/- 0.7; L-NAME, 4.7 +/- 1.2 L; P = 0.06). These data support the notion that NO contributes prominently to the oxidative enzyme inertia and thus the speed of V(O(2)) kinetics at the onset of moderate intensity exercise in the horse.

VO(2) kinetics in heavy exercise is not altered by prior exercise with a different muscle group

We examined whether lactic acidemia-induced hyperemia at the onset of high-intensity leg exercise contributed to the speeding of pulmonary O(2) uptake (VO(2)) after prior heavy exercise of the same muscle group or a different muscle group (i.e., arm). Six healthy male subjects performed two protocols that consisted of two consecutive 6-min exercise bouts separated by a 6-min baseline at 0 W: 1) both bouts of heavy (work rate: 50% of lactate threshold to maximal VO(2)) leg cycling (L1-ex to L2-ex) and 2) heavy arm cranking followed by identical heavy leg cycling bout (A1-ex to A2-ex). Blood lactate concentrations before L1-ex, L2-ex, and A2-ex averaged 1.7 +/- 0.3, 5.6 +/- 0.9, and 6.7 +/- 1.4 meq/l, respectively. An "effective" time constant (tau) of VO(2) with the use of the monoexponential model in L2-ex (tau: 36.8 +/- 4.3 s) was significantly faster than that in L1-ex (tau: 52.3 +/- 8.2 s). Warm-up arm cranking did not facilitate the VO(2) kinetics for the following A2-ex [tau: 51.7 +/- 9.7 s]. The double-exponential model revealed no significant change of primary tau (phase II) VO(2) kinetics. Instead, the speeding seen in the effective tau during L2-ex was mainly due to a reduction of the VO(2) slow component. Near-infrared spectroscopy indicated that the degree of hyperemia in working leg muscles was significantly higher at the onset of L2-ex than A2-ex. In conclusion, facilitation of VO(2) kinetics during heavy exercise preceded by an intense warm-up exercise was caused principally by a reduction in the slow component, and it appears unlikely that this could be ascribed exclusively to systemic lactic acidosis.

Effects of prior contractions on muscle microvascular oxygen pressure at onset of subsequent contractions

In humans, pulmonary oxygen uptake (.V(O2)) kinetics may be speeded by prior exercise in the heavy domain. This "speeding" arises potentially as the result of an increased muscle O(2) delivery (.Q(O2)) and/or a more rapid elevation of oxidative phosphorylation. We adapted phosphorescence quenching techniques to determine the.Q(O2)-to-O(2) utilization (.Q(O2)/.V(O2)) characteristics via microvascular O(2) pressure (P(O2,m)) measurements across sequential bouts of contractions in rat spinotrapezius muscle. Spinotrapezius muscles from female Sprague-Dawley rats (n = 6) were electrically stimulated (1 Hz twitch, 3-5 V) for two 3 min bouts (ST(1) and ST(2)) separated by 10 min rest. P(O2,m) responses were analysed using an exponential + time delay (TD) model. There was no significant difference in baseline and DeltaP(O2,m) between ST(1) and ST(2) (28.5 +/- 2.6 vs. 27.9 +/- 2.4 mmHg, and 13.9 +/- 1.8 vs. 14.1 +/- 1.3 mmHg, respectively). The TD was reduced significantly in the second contraction bout (ST(1), 12.2 +/- 1.9; ST(2), 5.7 +/- 2.2 s, P < 0.05), whereas the time constant of the exponential P(O2,m) decrease was unchanged (ST(1), 16.3 +/- 2.6; ST(2), 17.6 +/- 2.7 s, P > 0.1). The shortened TD found in ST(2) led to a reduced time to reach 63 % of the final response of ST(2) compared to ST(1) (ST(1), 28.3 +/- 3.0; ST(2), 20.2 +/- 1.8 s, P < 0.05). The speeding of the overall response in the absence of an elevated P(O2,m) baseline (which had it occurred would indicate an elevated.Q(O2)/.V(O2) or muscle blood flow suggests that some intracellular process(es) (e.g. more rapid increase in oxidative phosphorylation) may be responsible for the increased speed of P(O2,m) kinetics after prior contractions under these conditions.

Oxygen uptake kinetics for moderate exercise are speeded in older humans by prior heavy exercise

This study examined the effect of heavy-intensity warm-up exercise on O(2) uptake (VO(2)) kinetics at the onset of moderate-intensity (80% ventilation threshold), constant-work rate exercise in eight older (65 +/- 2 yr) and seven younger adults (26 +/- 1 yr). Step increases in work rate from loadless cycling to moderate exercise (Mod(1)), heavy exercise, and moderate exercise (Mod(2)) were performed. Each exercise bout was 6 min in duration and separated by 6 min of loadless cycling. VO(2) kinetics were modeled from the onset of exercise by use of a two-component exponential model. Heart rate (HR) kinetics were modeled from the onset of exercise using a single exponential model. During Mod(1), the time constant (tau) for the predominant rise in VO(2) (tau VO(2)) was slower (P < 0.05) in the older adults (50 +/- 10 s) than in young adults (19 +/- 5 s). The older adults demonstrated a speeding (P < 0.05) of VO(2) kinetics when moderate-intensity exercise (Mod(2)) was preceded by high-intensity warm-up exercise (tau VO(2), 27 +/- 3 s), whereas young adults showed no speeding of VO(2) kinetics (tau VO(2), 17 +/- 3 s). In the older and younger adults, baseline HR preceding Mod(2) was elevated compared with Mod(1), but the tau for HR kinetics was slowed (P < 0.05) in Mod(2) only for the older adults. Prior heavy-intensity exercise in old, but not young, adults speeded VO(2) kinetics during Mod(2). Despite slowed HR kinetics in Mod(2) in the older adults, an elevated baseline HR before the onset of Mod(2) may have led to sufficient muscle perfusion and O(2) delivery. These results suggest that, when muscle blood flow and O(2) delivery are adequate, muscle O(2) consumption in both old and young adults is limited by intracellular processes within the exercising muscle.

Low intensity exercise in humans accelerates mitochondrial ATP production and pulmonary oxygen kinetics during subsequent more intense exercise

We undertook this study to determine whether low intensity exercise (55 % (O2,max) would significantly alter the metabolic and ventilatory responses observed during 10 min of subsequent moderate intensity exercise (75 % (O2,max). By executing this work, we hoped to further our understanding of the mechanisms that limit mitochondrial ATP production at the onset of exercise. Seven healthy human subjects performed 10 min of moderate intensity exercise in the presence and absence of 10 min of low intensity exercise, which preceded the moderate intensity exercise by 3 min. Muscle biopsy samples were obtained from the vastus lateralis at pre-determined time points and oxygen consumption kinetics were determined at rest and during low and moderate intensity exercise. Following low intensity exercise and 3 min of passive recovery, muscle lactate and acetylcarnitine concentrations were elevated above basal levels, but (O2) had returned to the resting rate. When moderate intensity exercise was preceded by low intensity exercise, there was a significant sparing of phosphocreatine (PCr, approximately 25 %, P < 0.05) and reductions in glucose 6-phosphate (G-6-P, approximately 50 %, P < 0.05) and lactate (approximately 50 %, P < 0.05) accumulation during the first minute of moderate intensity exercise. No differences were observed after 10 min of moderate intensity exercise. The (O2) on-kinetic response over the first minute of moderate intensity exercise was accelerated when preceded by low intensity exercise. Collectively, our results suggest the lag in the oxidative ATP delivery at the onset of moderate intensity exercise can be overcome by prior low intensity exercise. Furthermore, our findings support the view that this lag is at least in part attributable to a limitation in acetyl group delivery/availability at the onset of exercise, rather than delayed oxygen supply.

Effects of warm-up intensity on kinetics of oxygen consumption and carbon dioxide production during high-intensity exercise in horses

OBJECTIVE

To compare effects of low and high intensity warm-up exercise on oxygen consumption (VO2) and carbon dioxide production (VCO2) in horses.

ANIMALS

6 moderately conditioned adult Standard-breds.

PROCEDURES

Horses ran for 2 minutes at 115% of maximum oxygen consumption (VO2max), 5 minutes after each of the following periods: no warm-up (NoWU); 10 minutes at 50% of VO2max (LoWU); or 7 minutes at 50% VO2max followed by 45-second intervals at 80, 90, and 100% VO2max (HiWU). Oxygen consumption and VCO2 were measured during exercise, and kinetics of VO2 and VCO2 were calculated. Accumulated O2 deficit was also calculated.

RESULTS

For both warm-up trials, the time constant for the rapid exponential increase in VO2 was 30% lower than for NoWU. Similarly, the rate of increase in VCO2 was 23% faster in LoWU and HiWU than in NoWU. Peak values for VO2 achieved during the high-speed test were not significantly different among trials (LoWU, 150.2 +/- 3.2 ml/kg/min; HiWU, 151.2 +/- 4.2 ml/kg/min; NoWU, 145.1 +/- 4.1 ml/kg/min). However, accumulated O2 deficit (ml of O2 equivalents/kg) was significantly lower during LoWU (65.3 +/- 5.1) and HiWU (63.4 +/- 3.9) than during NoWU (82.1 +/- 7.3).

CONCLUSIONS AND CLINICAL RELEVANCE

Both the low- and high-intensity warm-up, completed 5 minutes before the start of high-intensity exercise, accelerated the kinetics of VO2 and VCO2 and decreased accumulated O2 deficit during 2 minutes of intense exertion in horses that were moderately conditioned.

Reliability of submaximal exercise tests in patients with COPD. Chronic obstructive pulmonary disease

Submaximal constant work rate exercise tests are often used to measure the efficacy of an exercise intervention, but the reliability of these tests in patients with chronic obstructive pulmonary disease (COPD) has not been established.

PURPOSE

To examine the reproducibility of submaximal exercise responses of COPD patients compared with those of healthy elderly subjects.

METHODS

Sixteen COPD patients and 15 healthy subjects performed four weekly submaximal exercise tests against two different constant work rates: 20 W and 50% of the peak work rate (PWR). Spirometry was performed before each exercise test. COPD patients and healthy subjects were: age 69 +/- 5 and 65 +/- 5 yr, body mass index 26.4 +/- 3.9 and 26.7 +/- 3.0 kg x m(-2), respectively.

RESULTS

Both groups had no change in minute ventilation (V(E)), oxygen uptake (VO2), breathlessness (RPB), and leg fatigue (RPLF) for either work rate over repeated measures (P > 0.05). At 50% PWR test-retest reliability coefficients for V(E) and VO2 ranged from r = 0.88 to r = 0.96 for COPD patients and from r = 0.72 to r = 0.97 for healthy subjects; for RPB and RPLF test-retest reliability ranged from r = 0.76 to r = 0.89 for COPD patients and from r = 0.70 to r = 0.91 for healthy subjects. Intrasubject mean absolute differences for repeated measures of V(E), VO2, RPB, or RPLF were low and there were no group differences (P > 0.05). Percent error for V(E) and VO2 ranged from 6 +/- 3 to 9 +/- 7%, and for RPB and RPLF ranged from 19 +/- 18 to 68 +/- 65% for both groups at each work rate.

CONCLUSIONS

Submaximal exercise responses were reliable over a 1-month period, and measures of the physiological and psychophysical responses of COPD patients were as reliable as those of healthy subjects.

Oxygen uptake kinetics during exercise

The characteristics of oxygen uptake (VO2) kinetics differ with exercise intensity. When exercise is performed at a given work rate which is below lactate threshold (LT), VO2 increases exponentially to a steady-state level. Neither the slope of the increase in VO2 with respect to work rate nor the time constant of VO2 responses has been found to be a function of work rate within this domain, indicating a linear dynamic relationship between the VO2 and the work rate. However, some factors, such as physical training, age and pathological conditions can alter the VO2 kinetic responses at the onset of exercise. Regarding the control mechanism for exercise VO2 kinetics, 2 opposing hypotheses have been proposed. One of them suggests that the rate of the increase in VO2 at the onset of exercise is limited by the capacity of oxygen delivery to active muscle. The other suggests that the ability of the oxygen utilisation in exercising muscle acts as the rate-limiting step. This issue is still being debated. When exercise is performed at a work rate above LT, the VO2 kinetics become more complex. An additional component is developed after a few minutes of exercise. The slow component either delays the attainment of the steady-state VO2 or drives the VO2 to the maximum level, depending on exercise intensity. The magnitude of this slow component also depends on the duration of the exercise. The possible causes for the slow component of VO2 during heavy exercise include: (i) increases in blood lactate levels; (ii) increases in plasma epinephrine (adrenaline) levels; (iii) increased ventilatory work; (iv) elevation of body temperature; and (v) recruitment of type IIb fibres. Since 86% of the VO2 slow component is attributed to the exercising limbs, the major contributor is likely within the exercising muscle itself. During high intensity exercise an increase in the recruitment of low-efficiency type IIb fibres (the fibres involved in the slow component) can cause an increase in the oxygen cost of exercise. A change in the pattern of motor unit recruitment, and thus less activation of type IIb fibres, may also account for a large part of the reduction in the slow component of VO2 observed after physical training.

Interaction of factors determining oxygen uptake at the onset of exercise

Considerable debate surrounds the issue of whether the rate of adaptation of skeletal muscle O2 consumption (QO2) at the onset of exercise is limited by 1) the inertia of intrinsic cellular metabolic signals and enzyme activation or 2) the availability of O2 to the mitochondria, as determined by an extrinsic inertia of convective and diffusive O2 transport mechanisms. This review critically examines evidence for both hypotheses and clarifies important limitations in the experimental and theoretical approaches to this issue. A review of biochemical evidence suggests that a given respiratory rate is a function of the net drive of phosphorylation potential and redox potential and cellular mitochondrial PO2 (PmitoO2). Changes in both phosphorylation and redox potential are determined by intrinsic metabolic inertia. PmitoO2 is determined by the extrinsic inertia of both convective and diffusive O2 transport mechanisms during the adaptation to exercise and the rate of mitochondrial O2 utilization. In a number of exercise conditions, PmitoO2 appears to be within a range capable of modulating muscle metabolism. Within this context, adjustments in the phosphate energy state of the cell would serve as a cytosolic "transducer," linking ATP consumption with mitochondrial ATP production and, therefore, O2 consumption. The availability of reducing equivalents and O2 would modulate the rate of adaptation of QO2.

Delayed vagal withdrawal slows circulatory but not oxygen uptake responses at work increase

The effect of delayed vagal activity withdrawal on cardiorespiratory responses at an increase in workload was examined. Eleven volunteers (21 +/- 3 yr, 66 +/- 4 kg) performed cycle ergometer exercise at a work rate corresponding to 80% of ventilatory threshold after 3 min of unloaded cycling. Facial stimulation was given by applying a vinyl bag filled with cold water (3-5 degrees C) to the face 1 min before to 1 min after the increase in workload (S2 trial) or no stimulation was given (Nr trial). Oxygen uptake (VO2), heart rate (HR), and cardiac output (Q) were continuously recorded in four transitions for each trial. Data were averaged for each subject and trial. Mean response time (MRT, sum of delay and time constant) was calculated with monoexponential fitting. Facial stimulation induced acute bradycardia (-10 +/- 5 beats/min in S2 trial). The MRT of HR and Q was significantly longer in the S2 trials (46 +/- 35 and 37 +/- 23 s) than in the Nr trials (26 +/- 18 and 28 +/- 19 s, respectively), but no significant change in VO2 MRT was shown (36 +/- vs. 38 +/- 12 s). These findings suggest that increased vagal activity delays the central circulatory responses, which does not alter the VO2 kinetics at the onset of stepwise increase in workload. The maintenance of VO2 kinetics during acute bradycardia may either reflect the fact that some intramuscular processes (such as oxidative enzyme inertia) limit VO2 kinetics or alternatively that increased sympathetic vasoconstriction at some remote site defends exercising muscle blood flow.

Substrate availability limits human skeletal muscle oxidative ATP regeneration at the onset of ischemic exercise

We have demonstrated previously that dichloroacetate can attenuate skeletal muscle fatigue by up to 35% in a canine model of peripheral ischemia (Timmons, J.A., S.M. Poucher, D. Constantin-Teodosiu, V. Worrall, I.A. Macdonald, and P.L. Greenhaff. 1996. J. Clin. Invest. 97:879-883). This was thought to be a consequence of dichloroacetate increasing acetyl group availability early during contraction. In this study we characterized the metabolic effects of dichloroacetate in a human model of peripheral muscle ischemia. On two separate occasions (control-saline or dichloroacetate infusion), nine subjects performed 8 min of single-leg knee extension exercise at an intensity aimed at achieving volitional exhaustion in approximately 8 min. During exercise each subject's lower limbs were exposed to 50 mmHg of positive pressure, which reduces blood flow by approximately 20%. Dichloroacetate increased resting muscle pyruvate dehydrogenase complex activation status by threefold and elevated acetylcarnitine concentration by fivefold. After 3 min of exercise, phosphocreatine degradation and lactate accumulation were both reduced by approximately 50% after dichloroacetate pretreatment, when compared with control conditions. However, after 8 min of exercise no differences existed between treatments. Therefore, it would appear that dichloroacetate can delay the accumulation of metabolites which lead to the development of skeletal muscle fatigue during ischemia but does not alter the metabolic profile when a maximal effort is approached.