Oxygen uptake kinetics during treadmill running across exercise intensity domains

The purpose of the present study was to examine comprehensively the kinetics of oxygen uptake ( ) during treadmill running across the moderate, heavy and severe exercise intensity domains. Nine subjects [mean (SD age, 27 (7) years; mass, 69.8 (9.0) kg; maximum , , 4,137 (697) ml·min^–1] performed a series of "square-wave" rest-to-exercise transitions of 6 min duration at running speeds equivalent to 80% and 100% of the at lactate threshold (LT; moderate exercise); and at 20%, 40%, 60%, 80% and 100% of the difference between the at LT and (Δ, heavy and severe exercise). Critical velocity (CV) was also determined using four maximal treadmill runs designed to result in exhaustion in 2–15 min. The response was modelled using non-linear regression techniques. As expected, the amplitude of the primary component increased with exercise intensity [from 1,868 (136) ml·min^–1 at 80% LT to 3,296 (218) ml·min^–1 at 100% Δ, P<0.05]. However, there was a non-significant trend for the "gain" of the primary component to decrease as exercise intensity increased [181 (7) ml·kg^–1·km^–1 at 80% LT to 160 (6) ml·kg^–1·km^–1 at 100% Δ]. The time constant of the primary component was not different between supra-LT running speeds (mean value range = 17.9–19.1 s), but was significantly shorter during the 80% LT trial [12.7 (1.4) s, P<0.05]. The slow component increased with exercise intensity from 139 (39) ml·min^–1 at 20% Δ to 487 (57) ml·min^–1 at 80% Δ (P<0.05), but decreased to 317 (84) ml·min^–1 during the 100% Δ trial (P<0.05). During both the 80% Δ and 100% Δ trials, the at the end of exercise reached [4,152 (242) ml·min^–1 and 4,154 (114) ml·min^–1, respectively]. Our results suggest that the "gain" of the primary component is not constant as exercise intensity increases across the moderate, heavy and severe domains of treadmill running. These intensity-dependent changes in the amplitudes and kinetics of the response profiles may be associated with the changing patterns of muscle fibre recruitment that occur as exercise intensity increases. Electronic Publication     

Relationship of exercise test variables to cycling performance in an Ironman triathlon

The purpose of this study was, firstly, to investigate the intensity of exercise performanceof highly trained ultra-endurance triathletes during the cycling portion of an Ironman triathlon, and, secondly, to examine the anaerobic threshold and its relationship to this performance. Following a peak oxygen consumption (VO_2peak) test on a cycle ergometer to determine the heart rate (HR_Th,vent) and power output (PO_Th,vent) at the ventilatory threshold (Th_vent), 11 highly trained male triathletes [mean (SEM) age 35.8 (1.6) years, body fat 11.7 (1.2)%. VO_2peak 67.5 (1.0) ml·kg^–1·min^–1] who were participating in an Ironman triathlon, in random order: (1) cycled at their PO_Th,vent (Bi_Th,vent) until they were exhausted, and (2) cycled for 5 h at a self-selected intensity (Bi_SSI). Cycling power output (PO), oxygen uptake (VO₂), heart rate (HR) and blood lactate concentration ([La^–]_b) were recorded at regular intervals during these trials, while performance HR was recorded during the cycling phase of the Ironman triathlon. Significantly greater (P<0.05) values were attained during Bi_Th,vent than during Bi_SSI for PO [274 (9) compared to 188 (9) W], VO₂ [3.61 (0.15) compared to 2.64 (0.09) l·min^–1], and [La^–]_b [6.7 (0.8) compared to 2.8 (0.4) mmol·l^–1]. Moreover, mean HR during the Ironman triathlon cycle phase [146.3 (2.4) beats·min^–1; n=7] was significantly greater than mean HR during Bi_SSI [130 (4) beats·min^–1], and significantly less than mean HR during Bi_Th,vent [159 (3) beats·min^–1; all P<0.05]. However, HR during the cycle portion of the Ironman triathlon was highly related to (r=0.873; P<0.05) and not significantly different to HR_Th,vent [150 (4) beats·min^–1]. These data suggest that ultra-endurance triathletes cycle during the Ironman triathlon at a HR intensity that approximates to HR_Th,vent, but at a PO that is significantly below PO_Th,vent. Electronic Publication     

Oxygen uptake kinetics during horizontal and uphill treadmill running in humans

The aim of this study was to examine the effect of increasing the ratio of concentric to eccentric muscle activation on oxygen uptake (V˙O₂) kinetics during treadmill running. Nine subjects [2 women; mean (SD) age 29 (7) years, height 1.77 (0.07) m, body mass 73.0 (7.5) kg] completed incremental treadmill tests to exhaustion at 0% and 10% gradients to establish the gradient-specific ventilatory threshold (VT) and maximal oxygen uptake (V˙O_2max). Subsequently, the subjects performed repeated moderate intensity (80% of gradient-specific VT) and heavy intensity (50% of the difference between the gradient specific VT and V˙O_2max) square-wave runs with the treadmill gradient set at 0% and 10%. For moderate intensity exercise, there were no significant differences between treadmill gradients for V˙O₂ kinetics. For heavy intensity exercise, the amplitude of the primary component of V˙O₂ was not significantly different between 0% and 10% treadmill gradients [mean (SEM) 2,940 (196) compared to 2,869 (156) ml·min^–1, respectively], but the amplitude of the V˙O₂ slow component was significantly greater at the 10% gradient [283 (43) compared to 397 (37) ml·min^–1; P<0.05]. These results indicate that the muscle contraction regimen (i.e. the relative contribution of concentric and eccentric muscle action) significantly influences the amplitude of the V˙O₂ slow component. Electronic Publication     

Serum lipoprotein cholesterols in older oarsmen

We evaluated effects of age and rowing on concentrations of lipids and lipoprotein cholesterols in the blood. Maximal oxygen uptake (VO_2max), and concentrations of total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) were measured in 17 oarsmen [mean (SD)] [age 64 (4) years, body mass 69 (6) kg] and in sedentary men [age 65 (3) years, body mass 70 (7) kg] who were matched on the basis of body size. Also the variables were obtained from young oarsmen [age 22 (2) years, body mass 70 (4) kg] and young sedentary men [age 22 (3) years, body mass 69 (7) kg]. The percentage body fat of the older oarsmen was lower than that of the older sedentary men [18 (4)% compared to 23 (4)%, P<0.05], but it was similar to that of the young sedentary men [17 (4)%]. Although older oarsmen possessed a lower VO_2max than the young oarsmen [3.0 (0.4) l·min^–1 compared to 4.1 (0.3) l·min^–1, P<0.01], they showed a VO_2max similar to that of the young sedentary men [3.1 (0.5) l·min^–1] but a higher value than obtained from the older sedentary men [2.2 (0.3) l·min^–1, P<0.05]. Although the indices of risk factors for coronary artery disease in the older oarsmen were higher than those in the young oarsmen [LDL-C/HDL-C 1.7 (0.2) compared to 1.3 (0.4), TC/HDL-C 3.1 (0.2) compared to 2.6(0.4), P<0.05], they were lower than those in both the older [2.1 (0.3), 3.6 (0.3), P<0.05] and the young sedentary men [2.1 (0.4), 3.5 (0.4), P<0.05]. The results suggest that rowing is an appropriate type of exercise for the promotion of health. Electronic Publication     

Cardiorespiratory responses to exercise in acute hypoxia, hyperoxia and normoxia

There is a prevailing hypothesis that an acute change in the fraction of oxygen in inspired air (F _IO₂) has no effect on maximal cardiac output ( ), although maximal oxygen uptake ( ) and exercise performance do vary along with F _IO₂. We tested this hypothesis in six endurance athletes during progressive cycle ergometer exercise in conditions of hypoxia (F _IO₂=0.150), normoxia (F _IO₂=0.209) and hyperoxia (F _IO₂=0.320). As expected, decreased in hypoxia [mean (SD) 3.58 (0.45) l·min^–1, P<0.05] and increased in hyperoxia [5.17 (0.34) l·min^–1, P<0.05] in comparison with normoxia [4.55 (0.32) l·min^–1]. Similarly, maximal power ( ) decreased in hypoxia [334 (41) W, P<0.05] and tended to increase in hyperoxia [404 (58) W] in comparison with normoxia [383 (46) W]. Contrary to the hypothesis, was 25.99 (3.37) l·min^–1 in hypoxia (P<0.05 compared to normoxia and hyperoxia), 28.51 (2.36) l·min^–1 in normoxia and 30.13 (2.06) l·min^–1 in hyperoxia. Our results can be interpreted to indicate that (1) the reduction in in acute hypoxia is explained both by the narrowing of the arterio-venous oxygen difference and reduced , (2) reduced in acute hypoxia may be beneficial by preventing a further decrease in pulmonary and peripheral oxygen diffusion, and (3) reduced and in acute hypoxia may be the result rather than the cause of the reduced and skeletal muscle recruitment, thus supporting the existence of a central governor. Electronic Publication     

Effect of water ingestion on cardiovascular and thermal responses to prolonged cycling and running in humans: a comparison

The aim of this investigation was to examine the effect of water ingestion on physiological responses to prolonged cycling (CYC) and running (RUN). A group of 11 men with mean (SEM) maximal oxygen uptake (V˙O_2max) 48.5 (1.8) ml·kg^–1·min^–1 on a cycle-ergometer and 52.1 (2.2) ml·kg^–1·min^–1 on a treadmill (P<0.01) exercised for 90 min on four occasions, twice on each ergometer, at 60% of mode specific V˙O_2max. No fluid was taken (D) in one trial on each ergometer, whereas 60% of fluid losses were replaced by drinking water in the other trial (W). In CYC, water ingestion attenuated the change in cardiac output ( ) and the reduction in stroke volume (ΔSV) [ΔSV: –22.7 (3.8) in D, –10.7 (2.9) ml·beat^–1 in W, P<0.01; : –1.9 (0.5) in D, –0.2 (0.4) l·min^–1 in W at 85 min, P<0.01], but did not affect rectal temperature [T _re at 90 min: 38.8 (0.1)°C in D, 38.7 (0.1)°C in W]. In contrast, fluid replacement reduced hyperthermia in RUN [T _re at 90 min: 39.6 (0.2) in D, 39.1 (0.2)°C in W, P<0.01], and this was linked with a higher skin blood flow [RUN-W 88.9 (8.5), RUN-D 70.7 (8.4)%, P<0.05]. The and ΔSV were also attenuated with water ingestion in this mode of exercise (P<0.05). It is concluded that water ingestion improves physiological function in both cycling and running, but that the underlying mechanism is different in the two modes of exercise. Electronic Publication     

Comparison of cardio-locomotor synchronization during running and cycling

By comparing the characteristics of cardiac-locomotor synchronization (CLS) in running and cycling individuals, we tested whether the characteristics of CLS occurring during rhythmic exercise adhere to the central origin hypothesis, which postulates a direct interaction between cardiovascular centers in the brain and the pattern generator in the spinal cord. Ten healthy subjects performed both exercises at the same intensity (150 beats·min⁻¹) and cadence (150 steps·min⁻¹ during running and 75 rpm during cycling), while electrocardiograms and electromyograms from the right vastus lateralis muscle were monitored continuously. An examination of the occurrence of heart beats with respect to the locomotor phase revealed that, in running subjects, CLS exists for relatively prolonged periods at specific phases, whereas, in cycling subjects, it occurs intermittently and is not phase-specific [maximum duration of CLS: 113.6 (66.5) and 58.0 (29.3) s (P<0.05), respectively]. Determining the probability of CLS by chance as a function of its duration, we also found that, during running, CLS likely results from entrainment, whereas, during cycling, it results from chance, occurring when the cardiac rhythm approached the locomotor rhythm. Our result indicated that the duration of muscle contraction during cycling [317.0 (18.1) ms] was significantly longer than during running [205.6 (20.2) ms]. These results indicated that the difference in the CLS characteristics between running and cycling might be influenced by differences in peripheral inputs between exercise modes. Electronic Publication     

Heart rate as an indicator of the intensity of physical activity in human adolescents

The aims of this study were, in a group of adolescents, firstly to identify the absolute heart rates (HR) and the percentages of maximal heart rates (HR_max) corresponding to 40%, 60% and 80% of peak oxygen uptake ( ), secondly to identify absolute and relative ( ) oxygen uptakes ( ) corresponding to HR of 120, 140 and 160 beats·min^–1, and thirdly to examine a possible effect of fatness and fitness on the relationship between HR and . The subjects were 127 (60 boys, 67 girls) adolescents with a mean age of 14.8 (SD 0.3) years. The HR and were measured by means of an incremental exercise test to exhaustion. Linear regressions were performed for the and relationships using absolute and relative (%HR_max, ) data for each individual. From these regressions, target HR and were computed. Average target HR corresponding to 40%, 60% and 80% of were: 119 (SD 9), 145 (SD 9), 171 (SD 8), and 120 (SD 10), 146 (SD 8), 172 (SD 8) beats·min^–1 for boys and girls, respectively. Average corresponding to HR of 120, 140 and 160 beats·min^–1 were: 22 (SD 5), 30 (SD 5), 38 (SD 6) and 18 (SD 4), 24 (SD 4), 31 (SD 4) mlO₂·kg^–1·min^–1for boys and girls, respectively. An analysis of covariance showed a significant fitness effect (P<0.001) for predicted at all HR studied. The results suggest that the use of absolute HR to define exercise intensity levels when assessing young people's physical activity using HR monitoring detracts from the validity of the interpretation of the data. Electronic Publication     

Saliva flow and composition in humans exposed to acute altitude hypoxia

Summary The effects of acute hypoxia (2 days at 4350 m) on whole saliva flow and composition were studied on 12 sea-level natives, at rest and following a maximal exercise. Exercise, performed in normoxia and hypoxia, did not induce variations in saliva flow rate, saliva potassium or α-amylase concentrations. In contrast, acute hypoxia did lead to an increase in mean saliva flow rate both at rest (0.63 ml·min⁻¹ to 0.93 ml·min⁻¹,P<0.01) and after exercise (0.56 ml·min⁻¹ to 1.06 ml·min⁻¹,P<0.05) and a decrease in mean saliva potassium concentration at rest (20.8 mmol·1⁻¹ to 14.7 mmol·1⁻¹,P<0.01) as well as after exercise (21.7 mmol·1⁻¹ to 16.5 mmol·1⁻¹,P<0.05). This effect might be the consequence of a hypoxia-induced stimulation of the parasympathetic nervous system.     

Maximal oxygen uptake during field running does not exceed that measured during treadmill exercise

Modern ergometric equipment enables the simulation of laboratory maximal oxygen uptake (V˙O_2max) testing in the field. Therefore, it was investigated whether the improved event specificity on the track might lead to higher V˙O_2max measurements in running. Identical protocols were used on the treadmill and on the track (speed was indicated by a computer-driven flashing light system). Ambulatory measurements of gas exchange were carried out throughout both tests, which were executed in randomized order. There were no significant differences (P=0.71) in V˙O_2max between treadmill [4.65 (0.51) ml·min^–1] and field tests [4.63 (0.55) ml·min^–1]. However, the test duration differed significantly (P<0.001) by approximately 5%: treadmill 691 (39) s; field test 727 (42) s. With the exception of maximum heart rate (HR_max; significantly higher in the field with P=0.02) all criteria for the degree of effort were similar between the two tests. However, the difference in HR_max at less than 2 beats·min^–1, was practically negligible. Submaximal measurements of oxygen uptake and minute ventilation were significantly higher on the treadmill (P<0.001 for both parameters). In summary, field tests with incremental running protocols do not result in higher V˙O_2max measurements compared to laboratory treadmill exercise. A better running economy on the track results in higher maximal velocities and longer exercise durations being sustained. The determination of V˙O_2max is not a reasonable application for ambulatory gas exchange measurements because laboratory values are not surpassed. Electronic Publication     

Effect of a 1 year combined aerobic- and weight-training exercise programme on aerobic capacity and ventilatory threshold in patients suffering from coronary artery disease

Weight-training is recommended as a complement to conventional aerobic-training for most low to moderate risk patients suffering from coronary artery disease (CAD). The purpose of this study was to evaluate the effect of a 1 year exercise programme combining weight- and aerobic-training on peak oxygen uptake (VO_2,peak) and ventilatory threshold (VT). We studied 40 men suffering CAD who were divided into three groups: 14 subjects to weight-training plus aerobic-training [mean (SD] [combined exercise group, age 55 (10) years], 14 to aerobic-training only [aerobic-training group, age 57 (11) years], and 12 to a control group [standard care, age 57 (11) years]. A symptom-limited graded exercise test using the standard Bruce protocol was performed using a 12-lead electrocardiogram, and gas analysis techniques. Muscle strength was determined only in the combined exercise group using the one-repetition maximum method on each of eight weight exercises. Arm and leg strength increased by 21.9% and 27.8% respectively (P<0.0001) from pre to post-tests. The VO_2,peak did not differ between the combined and aerobic-training groups but their adjusted means were greater than those of the control group [39 (1.8) and 35.3 (1.8) compared to 26.2 (2.7) ml·kg^–1·min^–1 (P<0.001)]. The oxygen uptake at VT was higher in the combined group [24.7 (1.4) ml·kg^–1·min^–1] compared to aerobic [18.7 (1.4) ml·kg^–1·min^–1] and control [13.6 (1.7) ml·kg^–1min^–1] groups (P<0.001). Similar results were found for exercise tolerance (treadmill time to peak and at VT). Combined exercise training increased the VT more than aerobic-training alone. Combined exercise training did not improve the VO_2,peak or the functional capacity more than aerobic-training alone. Electronic Publication     

The effect of prolonged submaximal exercise on gas exchange kinetics and ventilation during heavy exercise in humans

This study compared ventilation, gas exchange (oxygen uptake, V̇O₂) and the surface electromyogram (EMG) activity of four major lower limb muscles during heavy exercise before (Pre-Ex) and after (Post-Ex) a sustained 90-min cycling exercise at 60% V̇O_2peak. The 90-min exercise was incorporated under the hypothesis that sustained exercise would alter substrate availability in the second exercise bout causing differences in fibre recruitment patterns, gas exchange and ventilation. Nine trained male subjects [V̇O_2peak=60.2 (1.7) ml·kg⁻¹·min⁻¹] completed two identical 6-min bouts of cycling performed at high intensity [~90% V̇O_2peak; 307 (6) W, mean (SE)]. Ventilation and gas exchange were measured breath-by-breath and the EMG was recorded during the last 12 s of each minute of the two 6-min bouts. EMG signals were analysed to determine integrated EMG (iEMG) and mean power frequency (MPF). V̇O₂ at min 3 and min 6 in Post-Ex were significantly higher (i.e., +201 and 141 ml·min⁻¹, respectively, P<0.05) than in Pre-Ex but there was a ~25% decrease of the slow component, taken as the difference between min 6 and min 3 [187 (27) vs 249 (35) ml·min⁻¹, respectively, P<0.05]. The greater whole-body V̇O₂ after 3 min of exercise in Post-Ex was not accompanied by clear alterations in the iEMG and MPF of the examined leg muscles. Ventilation and heart rate were elevated (~12–16 l·min⁻¹ and ~10 beats·min⁻¹, respectively, P<0.05) as were the ratios V̇ _E/O₂ and V̇ _E/V̇CO₂ in the Post-Ex tests. It was concluded that the V̇O₂ and ventilation responses to high-intensity exercise can be altered following prolonged moderate intensity exercise in terms of increased amplitude without associated major changes in either iEMG or MPF values among conditions.     

Shivering endurance and fatigue during cold water immersion in humans

An important component of survival time during cold exposure is shivering endurance. Nine male and three female healthy and fit subjects [mean (SD) age 24.8 (6.3) years, body mass 71.7 (13.2) kg, height 1.75 (0.10) m, body fat 22.7 (7.4)%] were immersed to the upper chest level in cold water for periods ranging from 105 to 388 min on two occasions to test a prediction of shivering endurance. The water was cooled from 20 to 8°C during the first 15 min of immersion and subsequently rewarmed (<20°C) to elicit a near constant submaximal shivering response. The data were divided according to moderate (M) and high (H) levels of shivering intensity. Respective mean total immersion times were 250 (75) and 199 (80) min (P=0.086) at different average shivering intensities of 61 (10) and 69 (8)% relative to maximal shivering (P<0.001). Blood plasma glucose concentration increased during the immersion [from 3.44 (0.54) pre- to 3.94 (0.60) mmol·l^–1 post-immersion (P=0.037)] and levels were higher during M (P=0.012). When compared to a model prediction of shivering endurance, shivering activity continued well beyond the predicted endurance times in 18 out of the 24 trials. The average rates of oxygen consumption over the entire immersion period were lower (P=0.002) during M [0.93 (0.20) l·min^–1] compared to H [1.05 (0.21) l·min^–1), and while these rates did not change during the last 90 min of immersion, there was an increase in fat oxidation. There were no trial differences in the average esophageal (T _es) and mean skin temperatures during the entire immersion period (36.0 and 18.0°C, respectively), yet T _es decreased (P=0.003) approximately 0.4°C during the last 90 min of immersion. When the shivering intensity was normalized to account for this decrease, a significant downward trend of approximately 17%·h^–1 in the normalized shivering intensity was found after the predicted end of shivering endurance. These results suggest that shivering drive, and not shivering intensity per se, decreased during the latter stages of the immersion. Underlying mechanisms such as fatigue and habituation for this diminishing cold sensitivity are discussed. Electronic Publication     

Beta-endorphin immunoreactivity during high-intensity exercise with and without opiate blockade

Nine highly fit men [mean (SE) maximum oxygen uptake, : 63.9 (1.7) ml·kg^–1·min^–1; age 27.6 (1.6) years] were studied during two treadmill exercise trials to determine plasma β-endorphin immunoreactivity during intense exercise (80% ). A double-blind experimental design was used, and subjects performed the two exercise trials in counterbalanced order. Exercise trials were 30 min in duration and were conducted 7 days apart. One exercise trial was undertaken following administration of naloxone (1.2 mg; 3 cm³) and the other after receiving a placebo (0.9% NaCl saline; 3 cm³). Prior to each experimental trial, a flexible catheter was placed into an antecubital vein and baseline blood samples were collected. Thereafter, each subject received either a naloxone or placebo bolus injection. Blood samples were also collected after 10, 20 and 30 min of continuous exercise. β-Endorphin was higher (P<0.05) during exercise when compared to pre-exercise in both trials. However, no statistically significant difference was found (P>0.05) between exercise time points within either experimental trial. β-endorphin immunoreactivity was greater (P<0.05) in the naloxone than in the placebo trial during each exercise sampling time point [10 min: 63.7 (3.9) pg·ml^–1 vs 78.7 (3.8) pg·ml^–1; 20 min: 68.7 (4.1) pg·ml^–1 vs 83.8 (4.3) pg·ml^–1; 30 min: 71.0 (4.3) pg·ml^–1 vs 82.5 (3.2) pg·ml^–1]. These data suggest that intense exercise induces significant increases in β-endorphin that are maintained over time during steady-rate exercise. Exercise and naloxone had an interactive effect on β-endorphin release that warrants further investigation. Electronic Publication     

Effect of prior metabolic rate on the kinetics of oxygen uptake during moderate-intensity exercise

Pulmonary oxygen uptake ( ) dynamics during moderate-intensity exercise are often assumed to be dynamically linear (i.e. neither the gain nor the time constant (τ) of the response varies as a function of work rate). However, faster, slower and unchanged kinetics have been reported during work-to-work transitions compared to rest-to-work transitions, all within the moderate-intensity domain. In an attempt to resolve these discrepancies and to improve the confidence of the parameter estimation, we determined the response dynamics using the averaged response to repeated exercise bouts in seven healthy male volunteers. Each subject initially performed a ramp-incremental exercise test for the estimation of the lactate threshold ( ). They then performed an average of four repetitions of each of three constant-work-rate (WR) tests: (1) between 20 W and a work rate of 50% (WR50) between 20 W and 90% (step 1→2), (2) between WR50 and 90% (step 2→3), and (3) between 20 W and 90% (step 1→3); 6 min was spent at each work rate increment and decrement. Parameters of the kinetic response of phase II were established by non-linear least-squares fitting techniques. The kinetics of were significantly slower at the upper reaches of the moderate-intensity domain (step 2→3) compared to steps 1→2 and 1→3 [group mean (SD) phase II τ: step 1→2 25.3 (4.9) s, step 2→3 40.0 (7.4) s and step 1→3 32.2 (6.9) s]. The off-transient values of τ were not significantly different from each other: 36.8 (16.3) s, 38.9 (11.6) s and 30.8 (5.7) s for steps 1→2, 2→3 and 1→3, respectively. Surprisingly, the on-transient gain (G, ) was also found to vary among the three steps [G=10.56 (0.42) ml·min^–1·W^–1, 11.85 (0.64) ml·min^–1·W^–1 and 11.23 (0.52) ml·min^–1·W^–1 for steps 1→2, 2→3 and 1→3, respectively]; the off-transient G did not vary significantly and was close to that for the on-transient step 1→3 in all cases. Our results do not support a dynamically linear system model of during cycle ergometer exercise even in the moderate-intensity domain. The greater oxygen deficit per unit power increment in the higher reaches of the moderate-intensity domain necessitates a greater transient lactate contribution to the energy transfer, or a greater phosphocreatine breakdown, or possibly both. Electronic Publication     

Vastus medialis muscle oxygenation trends during a simulated 20-km cycle time trial

In this study we examined the oxygenation trend of the vastus medialis muscle during sustained high-intensity exercise. Ten cyclists performed an incremental cycle ergometer test to voluntary exhaustion [mean (SD) maximum oxygen uptake 4.29 (0.63) l·min^–1; relative to body mass 60.8 (2.4) ml·kg^–1·min^–1] and a simulated 20-km time trial (20TT) on a wind-loaded roller system using their own bicycle (group time = 23–31 min) in two separate sessions. Cardiorespiratory responses were monitored using an automated metabolic cart and a wireless heart rate monitor. Tissue absorbency, which was used as an index of muscle oxygenation, was recorded simultaneously from the vastus medialis using near-infrared spectroscopy. Group mean values for oxygen uptake, ventilation, heart rate, respiratory exchange ratio, power output, and rating of perceived exhaustion were significantly (P≤0.05) higher during the incremental test compared to the 20TT [4.29 (0.63) l·min^–1 vs 4.01 (0.55) l·min^–1, 120.4 (26) l·min^–1 vs 97.6 (16.1) l·min^–1, 195 (8) beats·min^–1 vs 177 (9) beats·min^–1, 1.15 (0.06) vs 0.93 (0.06), 330.1 (31) W vs 307.2 (24.5) W, and 19 (1.5) vs 16 (1.7), respectively]. Oxygen uptake and heart rate during the 20TT corresponded to 93.5% and 90.7%, respectively, of the maximal values observed during the incremental test. Comparison of the muscle oxygenation trends between the two tests indicated a significantly greater degree of deoxygenation during the 20TT [–699 (250) mV vs –439 (273) mV; P≤0.05] and a significant delay in the recovery oxygenation from the 20TT. The mismatching of whole-body oxygen uptake and localised tissue oxygenation between the two tests could be due to differences in muscle temperature, pH, localised blood flow and motor unit recruitment patterns between the two tests. Electronic Publication     

Oxygen uptake kinetics during severe intensity running and cycling

The purpose of this study was to investigate the effect of exercise mode on the characteristics of the oxygen uptake ( V̇O₂) response to exercise within the severe intensity domain. Twelve participants each performed a treadmill running test and a cycle ergometer test to fatigue at intensities selected to elicit a mode-specific V̇O₂max and to cause fatigue in ~5 min. The tests were at 234 (30) m·min⁻¹ and 251 (59) W, and times to fatigue were 297 (15) s and 298 (14) s, respectively. The overall rapidity of the V̇O₂response was influenced by exercise mode [ V̇O₂max was achieved after 115 (20) s in running versus 207 (36) s in cycling; p<0.01]. V̇O₂ responses were fit to a three-phase exponential model. The time constant of the primary phase was faster in treadmill tests than in cycle ergometer tests [14 (6) s versus 25 (4) s; p<0.01], and the amplitude of the primary phase was greater in running than in cycling when it was expressed in absolute terms [2327 (393) ml·min⁻¹ versus 2036 (301) ml·min⁻¹; p=0.02] but not when it was expressed as a percentage of the total increase in V̇O₂ [86 (6)% versus 82 (6)%; p=0.09]. When quantified as the difference between the end-exercise V̇O₂ and the V̇O₂ at 2 min, the amplitude of the slow component was ~40% smaller in running [177 (92) ml·min⁻¹ versus 299 (153) ml min⁻¹; p=0.03]. It is concluded that exercise modality affects the characteristics of the V̇O₂ response at equivalent intensities in the severe domain.     

Gas exchange responses to continuous incremental cycle ergometry exercise in primary pulmonary hypertension in humans

In patients suffering from primary pulmonary hypertension (PPH), a raised pulmonary vascular resistance may limit the ability to increase pulmonary blood flow as work rate increases. We hypothesised that oxygen uptake (V˙O₂) may not rise appropriately with increasing work rate during incremental cardiopulmonary exercise tests. Nine PPH patients and nine normal subjects performed symptom-limited maximal continuous incremental cycle ergometry exercise. Mean peak V˙O₂ [1.00 (SD 0.22) compared to 2.58 (SD 0.64) l · min⁻¹] and mean V˙O₂ at lactic acidosis threshold [LAT, 0.73 (SD 0.17) compared to 1.46 (SD 0.21 · l) ml · min⁻¹] were much lower in patients than in normal subjects (both P < 0.01, two-way ANOVA with Tukey test). The mean rate of change of V˙O₂ with increasing work rate above the LAT [5.9 (SD 2.1) compared to 9.4 (SD 1.3) ml · min⁻¹ · W⁻¹, P < 0.01)] was also much lower in patients than in normal subjects [apparent δ efficiency 60.3 (SD 38.8)% in patients compared to 31.0 (SD 4.9)% in normal subjects]. The patients displayed lower mean values of end-tidal partial pressure of carbon dioxide than the normal subjects at peak exercise [29.7 (SD 6.8) compared to 42.4 (SD 5.8) mmHg, P < 0.01] and mean oxyhaemoglobin saturation [89.1 (SD 4.1) compared to 93.6 (SD 1.8)%, P < 0.05]. Mean ventilatory equivalents for CO₂ [49.3 (SD 11.4) compared to 35.0 (SD 7.3), P < 0.05] and O₂ [44.2 (SD 10.7) compared to 29.9 (SD 5.1), P < 0.05] were greater in patients than normal subjects. The sub-normal slopes for the V˙O₂-work-rate relationship above the LAT indicated severe impairment of the circulatory response to exercise in patients with PPH. The ventilatory abnormalities in PPH suggested that the lung had become an inefficient gas exchange organ because of impaired perfusion of the ventilated lung. Accepted: 17 April 2000     

Methodological aspects of maximal lactate steady state—implications for performance testing

The maximal lactate steady state (MLSS) is the highest blood lactate concentration (BLC) that can be identified as maintaining a steady-state during a prolonged submaximal constant workload. Comparative interpretation of published data about MLSS is complicated by the fact that different methods of testing have been utilized. Thus, three methods, corresponding to the time course of changes in BLC incurred during either 30 min (MLSS I) or 20 min (MLSS II and III) of constant submaximal workload exercise, were compared in 26 male subjects [mean (SD) age 24.6 (5.6) years, height 181.6 (4.9) cm, body mass 74.4 (5.2) kg]. MLSS I [5.1 (1.3) mmol·l^-1], II [4.9 (1.3) mmol·l^-1], and III [4.3 (1.3) mmol·l^-1] were different (P<0.01). The workload corresponding to MLSS III [244.8 (44.0) W] was lower (P<0.01) than that at MLSS I [254.0 (40.8) W] and II [251.9 (40.4) W]. No difference could be confirmed between the workloads established for MLSS I and MLSS II. The differences between MLSS I, MLSS II, and MLSS III and corresponding workloads reflect insufficient contribution to lactate kinetics by testing procedures that depend strongly upon the time course of changes in BLC during the initial 20–25 min of constant-workload exercise. Based on the present findings, constant-load tests lasting at least 30 min and a BLC increase of no more than 1.0 mmol·l^-1 after the 10th testing minute appear to be the most reasonable with respect to valid testing results. Electronic Publication     

Muscle blood flow and flow heterogeneity during exercise studied with positron emission tomography in humans

Blood flow is the main regulator of skeletal muscle's oxygen supply, and several studies have shown heterogeneous blood flow among and within muscles. However, it remains unclear whether exercise changes the heterogeneity of flow in exercising human skeletal muscle. Muscle blood flow and spatial flow heterogeneity were measured simultaneously in exercising and in the contralateral resting quadriceps femoris (QF) muscle in eight healthy men using H¹⁵ ₂O and positron emission tomography. The relative dispersion (standard deviation/mean) of blood flow was calculated as an index of spatial flow heterogeneity. Average muscle blood flow in QF was 29 (10) ml · (kg muscle)⁻¹ · min⁻¹ at rest and 146 (54) ml · (kg muscle)⁻¹ · min⁻¹ during exercise (P=0.008 for the difference). Blood flow was significantly (P < 0.001) higher in the vastus medialis and the vastus intermedius than in the vastus lateralis and the rectus femoris, both in the resting and the exercising legs. Flow was more homogeneous in the exercising vastus medialis and more heterogeneous (P < 0.001) in the exercising vastus lateralis (P=0.01) than in the resting contralateral muscle. Flow was more homogeneous (P < 0.001) in those exercising muscles in which flow was highest (vastus intermedius and vastus medialis) as compared to muscles with the lowest flow (vastus lateralis and the rectus femoris). These data demonstrate that muscle blood flow varies among different muscles in humans both at rest and during exercise. Muscle perfusion is spatially heterogeneous at rest and during exercise, but responses to exercise are different depending on the muscle. Accepted: 16 June 2000