| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Department of Sport and Exercise Science, Carwyn James Building, The University of Wales Aberystwyth, Penglais Campus, Aberystwyth, Ceredigion SY23 3FD, UK2 Department of Exercise and Sport Science, Manchester Metropolitan University, Hassall Road, Alsager ST7 2HL, UK
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
450 ml blood would result in slower phase II O2 uptake 
kinetics, a lower 
and a reduced time to exhaustion during severe-intensity cycle exercise. Eleven healthy subjects (mean ±S.D. age 23 ± 6 years, body mass 77.2 ± 11.0 kg) completed ‘step’ exercise tests from unloaded cycling to a severe-intensity work rate (80% of the difference between the predetermined gas exchange threshold and the 
) on two occasions before, and 24 h following, the voluntary donation of 450 ml blood. Oxygen uptake was measured breath-by-breath, and 
kinetics estimated using non-linear regression techniques. The blood withdrawal resulted in a significant reduction in haemoglobin concentration (pre: 15.4 ± 0.9 versus post: 14.7 ± 1.3 g dl–1; 95% confidence limits (CL): –0.04, –1.38) and haematocrit (pre: 44 ± 2 versus post: 41 ± 3%; 95% CL: –1.3, –5.1). Compared to the control condition, blood withdrawal resulted in significant reductions in 
(pre: 3.79 ± 0.64 versus post: 3.64 ± 0.61 l min–1; 95% CL: –0.04, – 0.27) and time to exhaustion (pre: 375 ± 129 versus post: 321 ± 99 s; 95% CL: –24, –85). However, the kinetic parameters of the fundamental 
response, including the phase II time constant (pre: 29 ± 8 versus post: 30 ± 6 s; 95% CL: 5, –3), were not altered by blood withdrawal. The magnitude of the 
slow component was significantly reduced following blood donation owing to the lower 
attained. We conclude that a reduction in blood O2-carrying capacity, achieved through the withdrawal of 450 ml blood, results in a significant reduction in 
and exercise tolerance but has no effect on the fundamental phase of the 
on-kinetics during severe-intensity exercise.
(Received 21 November 2005;
accepted after revision 17 January 2006; first published online 23 January 2006)
Corresponding author A. M. Jones: School of Sport and Health Sciences, University of Exeter, St Luke's Campus, Exeter EX1 2LU, UK. Email: a.m.jones{at}exeter.ac.uk
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
measured during large muscle group exercise in humans (Saltin & Strange, 1992; Wagner, 1995). For example, enhancing haemoglobin concentration ([Hb]) and therefore the potential for convective O2 transport to muscle either through the (re)infusion of erythrocytes (Ekblom et al. 1972, 1976; Spriet et al. 1986) or through the administration of recombinant human erythropoietin (RhEPO; Ekblom & Berglund, 1991; Russell et al. 2002; Wilkerson et al. 2005) results in a proportional increase in 
in normal healthy subjects performing maximal cycle ergometer or treadmill exercise. The inspiration of hyperoxic gas during maximal exercise also tends to increase 
(Knight et al. 1993; Richardson et al. 1999). Similarly, reducing the potential for O2 delivery to muscle either by reducing [Hb] through the withdrawal of whole blood (Balke et al. 1954; Ekblom et al. 1972, 1976; Woodson et al. 1978; Schaffartzik et al. 1993) or by reducing the capillary-to-myocyte O2 pressure gradient in hypoxia (Richardson et al. 1999; Calbet et al. 2003) has been consistently shown to result in a reduction in 
.
In contrast to 
, the extent to which the dynamic adjustment of oxygen uptake 
to a step change in work rate (that is, the 
kinetics) is limited by muscle O2 delivery remains controversial (Tschakovsky & Hughson, 1999; Poole & Jones, 2005). During ‘moderate’ intensity exercise performed below the gas exchange threshold (GET), a steady-state 
is reached within 2–3 min in healthy young subjects, and it is generally considered that the principal control determinants and/or limitations to 
kinetics reside within the metabolic machinery of the muscle cells themselves (Whipp & Mahler, 1980; Grassi, 2003). During ‘heavy’ or ‘severe’ exercise performed above the GET, however, the time constant (
) describing the phase II rise in 
(which is believed to closely reflect the kinetics of O2 consumption in the active muscles; Grassi et al. 1996; Rossiter et al. 2002) is often longer than for exercise below the GET (for review, see Poole & Jones, 2005), and a secondary ‘slow’ component of 
emerges which ultimately elevates 
above the anticipated steady-state value (Whipp & Wasserman, 1972; Barstow & Molé, 1991). There are suggestions that both the slower phase II 
kinetics and the 
slow component observed during exercise above the GET are linked either directly or indirectly to impaired muscle O2 transport (Poole & Jones, 2005). For example, experimental interventions designed to impair muscle O2 delivery, such as hypoxic gas breathing (Springer et al. 1991; Hughson & Kowalchuk, 1995; Engelen et al. 1996) and altering the position of the exercising muscles relative to the heart to reduce perfusion pressure (MacDonald et al. 1998; Koga et al. 1999; Koppo & Bouckaert, 2005), typically result in slower phase II 
kinetics and/or a larger 
slow component. In contrast, the application of lower-body positive pressure to reduce muscle blood flow did not significantly alter 
kinetics during either moderate or heavy-intensity exercise (Williamson et al. 1996). Enhancing the potential for muscle O2 delivery through interventions such as the performance of prior high-intensity exercise, the inspiration of hyperoxic gas or the administration of RhEPO has not altered the phase II 
kinetics in the majority of studies (MacDonald et al. 1997; Burnley et al. 2000; Koppo & Bouckaert, 2001; Haseler et al. 2004; Wilkerson et al. 2005), but there are some notable exceptions (Connes et al. 2003; Tordi et al. 2003). The role of muscle O2 supply in limiting 
kinetics during exercise above the GET therefore remains somewhat unclear.
In human studies, an important factor when considering the influence of alterations in O2 delivery on 
kinetics is the exercise intensity under investigation. During ‘submaximal’ (i.e. moderate and heavy) exercise, enhancements or impairments in blood O2-carrying capacity achieved through a variety of interventions are likely to be offset, at least to some extent, by compensatory adjustments in heart rate (HR) and cardiac output 
, or by the redistribution of blood flow, with the effect that muscle O2 delivery and O2 uptake might not be appreciably different from the control condition (MacDonald et al. 2000). During near-maximal (i.e. severe) or supramaximal exercise, however, when 
will be set on a trajectory towards its peak (Poole et al. 1988; Özyener et al. 2001), the potential for these adjustments to compensate for alterations in blood O2-carrying capacity would be restricted (Bangsbo, 2000) and the impact of impaired or enhanced muscle O2 delivery on 
kinetics should be more clearly observed (Grassi et al. 2000).
The primary purpose of this study was therefore to investigate the influence of the voluntary donation of whole blood (450 ml) on 
kinetics during severe-intensity cycle ergometer exercise in healthy humans. A secondary purpose was to examine the effect of blood donation on the 
attained and exercise performance (as the time to exhaustion). We hypothesized that blood withdrawal would result in a slowing of the phase II 
kinetics, a reduction in the 
, and therefore a reduced time to exhaustion.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Subjects visited the laboratory on four occasions over a 2 week period to perform exercise tests on the cycle ergometer. The first visit was used to establish 
and to estimate the GET. On each of the subsequent visits, subjects completed a single bout of severe-intensity exercise to the limit of tolerance. The repeat performance of severe-intensity exercise to exhaustion is physically and psychologically demanding for subjects who are not highly trained. To increase confidence in the subjects' ability to give a maximal effort on each occasion, we therefore limited the number of exercise bouts to three: two bouts were completed on separate days before blood withdrawal, and one bout was performed 24 h following blood withdrawal. The two exercise bouts that were performed before blood withdrawal were completed within 7 days of the blood withdrawal procedure but were separated by at least 48 h. We studied the subjects 24 h following the withdrawal of blood to enable the restoration of total blood volume.
Participants
Eleven healthy volunteers (10 male, 1 female) provided written informed consent to participate in the present study, which was approved by the research ethics committees of both the University of Wales Aberystwyth and the Manchester Metropolitan University. All procedures conformed to the Declaration of Helsinki. The physical characteristics of the subjects were (mean ±S.D.) age 23 ± 6 years, body mass 77.2 ± 11.0 kg and height 1.77 ± 0.04 m. The subjects were physically active but not highly trained. Subjects were instructed to arrive at the laboratory at the same time of day (± 1 h) in a rested (no heavy exercise during the previous 24 h), well-hydrated state, having consumed no food, caffeine or alcohol during the previous 3 h.
Exercise testing
All testing was performed on an electronically-braked cycle ergometer (Lode Excalibur Sport, Groningen, The Netherlands) in a well-ventilated laboratory at a temperature of between 21 and 25°C. During the first visit to the laboratory, the ergometer was adjusted so that each subject was comfortable, and the settings were recorded and replicated during all subsequent exercise tests. Following measurement of height and body mass, subjects performed a ‘ramp’ exercise test to determine 
and GET. This test consisted of 3 min of pedalling at ‘0 W’, followed by a continuous ramped increase in work rate of 25 or 30 W min–1. Subjects were instructed to maintain a cadence of 90 ± 2 r.p.m. throughout the test. When the subject could no longer maintain 
80 r.p.m. despite strong verbal encouragement, the test was terminated; in all cases, the fall in cadence was precipitous. All gas exchange and ventilatory variables were averaged and displayed over 10 s intervals. The 
was determined as the highest 
measured over 30 s, and the GET was determined using the V-slope method (Beaver et al. 1986) as the first disproportionate increase in carbon dioxide output 
relative to 
. The work rate corresponding to 80% ‘
’ (i.e. 80% of the difference between GET and 
; severe exercise) was calculated using linear regression of 
versus work rate with account taken of the lag in 
relative to work rate that exists during ramp incremental exercise (Whipp et al. 1981).
On each of the three subsequent laboratory visits, the subjects performed a single severe-intensity exercise bout to exhaustion. In each case, the test began with 3 min of pedalling at ‘0 W’, after which the work rate was abruptly increased to the target work rate. Subjects were given strong verbal encouragement throughout the exercise test, and the time to exhaustion (defined as the same precipitous fall in pedal cadence described above) was recorded to the nearest second. Capillary blood samples were taken from a prewarmed fingertip immediately before and immediately following the exercise bout for the determination of blood lactate concentration (YSI 1500, Yellow Springs Instruments, OH, USA).
Measurements
The withdrawal of blood was performed by the National Health Service as part of the national blood donation service. The subjects lay supine on a bed before 450 ml of blood was drawn from the antecubital vein over a 15 min period.
On arrival at the laboratory and prior to the commencement of the ‘step’ exercise tests, the subjects rested for 15 min before blood samples from a prewarmed fingertip were collected into three 30 µl heparinized microhaematocrit tubes (Hawksley and Sons Ltd, Lancing, Sussex, England). These samples subsequently underwent microcentrifugation for 8 min for the determination of haematocrit (Hct; Hawksley 1560 Micro-haematocrit reader). In addition, blood was collected into microcuvettes (HemoCue AB, Ängelholm, Sweden) from which haemoglobin was determined (B-Hemoglobin photometer, HemoCue AB) in duplicate. The photometer was checked for accuracy and repeatability by analysing 54 Hemotrol control (HemoCue AB) samples of 8.0, 12.0 and 16.0 g dl–1. The mean values determined were 8.0, 12.0 and 16.0 g dl–1, respectively, and the coefficients of variation were 0.95, 1.00 and 0.74%, respectively.
Pulmonary gas exchange and ventilation were measured breath-by-breath, with subjects wearing a nose clip and breathing through a low-dead space (90 ml), low-resistance mouthpiece (0.75 mmHg l–1 s–1 at 15 l s–1) and impeller turbine assembly (Jaeger Triple V). The inspired and expired gas volume and gas concentration signals were continuously sampled at 100 Hz, the latter using paramagnetic (O2) and infrared (CO2) analysers (Jaeger Oxycon Pro, Hoechberg, Germany) via a capillary line connected to the mouthpiece. The gas analysers were calibrated before each test with gases of known concentration, and the turbine volume transducer was calibrated using a 3 l syringe (Hans Rudolph, Kansas City, MO, USA). The volume and concentration signals were time aligned by accounting for the delay in the capillary gas transit and the analyser rise time relative to the volume signal. Oxygen uptake, 
and expiratory ventilation 
were calculated using standard formulae (Beaver et al. 1973) and displayed breath by breath. Heart rate was measured every 5 s using short-range radio telemetry (Polar S610, Polar Electro Oy, Kempele, Finland).
The breath-by-breath data from the step exercise tests were used to estimate the 
kinetics. The data were first manually filtered to remove outlying breaths, defined as breaths deviating by more than three standard deviations from the preceding five breaths. The data were subsequently interpolated to provide second-by-second values and modelled using a modification of the procedure described by Rossiter et al. (2001). The first 20 s of data following the onset of exercise were removed from the analysis to isolate the phase II component of the response. The data were then modelled with a mono-exponential function of the form:
|
|
(t) is the 
at time t; 
(b) is the baseline 
measured in the 60 s preceding the transition in work rate; and A, TD and are the amplitude, time delay and the time constant of the primary (phase II) response, respectively. For each individual, the data sets from the control (i.e. pre-withdrawal) condition were then time aligned and averaged to maximize the confidence in the parameter estimates. The model fit was initially constrained to the first 2 min of exercise data (i.e. 20–120 s), with the following initial parameter estimates: A= 3 l min–1; = 25 s; and TD = 15 s. Subsequent iterations were then made by the fitting process to reduce the residual sum of squares until the criteria for convergence were satisfied (Rossiter et al. 2001; Burnley et al. 2005). This approach provides functionally identical parameter estimates to the ‘traditional’ triple exponential fitting procedure (M. Burnley, unpublished observations) whilst providing higher confidence in the parameters of the phase II response (Whipp & Rossiter, 2005). The 
slow component was defined as the difference between the 
at the asymptote of the fundamental (i.e. phase I + phase II) response as determined by the model and the mean 
measured over the last 30 s of exercise. We took this approach, as opposed to mathematically modelling the 
slow component with a second exponential term (e.g. Burnley et al. 2000, 2002), because the ‘time delay’ and ‘time constant’ associated with the 
slow component are ‘parameters of convenience’ without physiological equivalents (Whipp & Rossiter, 2005) and because we wished to maximize confidence in our estimation of the phase II . Heart rate kinetics were modelled using the same approach as that outlined above, except that the first 20 s of data were not removed from the analysis. Statistical analysis
Differences in the parameters of interest between the control and blood donation conditions were determined using paired-samples 95% confidence intervals. Unless otherwise stated, results are therefore reported as means ±S.D. followed by the paired-samples 95% confidence intervals of the differences between conditions, with intervals not including the null value indicating a statistically significant difference between conditions.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
of the subjects measured during the ramp test was 3.69 ± 0.62 l min–1 (47.8 ± 8.0 ml kg–1 min–1), and the peak work rate was 338 ± 40 W. The GET occurred at a mean 
of 1.86 l min–1 or 50 ± 5% 
, with a corresponding work rate at GET of 118 ± 30 W. The work rate equivalent to 80% was 269 ± 51 W.
Blood donation significantly reduced both resting [Hb] (pre: 15.4 ± 0.9 versus post: 14.7 ± 1.3 g dl–1; 95% confidence limits (CL): –0.04, –1.38) and resting Hct (pre: 44 ± 2 versus post: 41 ± 3%; 95% CL: –1.3, –5.1). Therefore, the experimental intervention was successful in reducing blood O2-carrying capacity (by 5%).
The influence of blood donation on the 
response to severe-intensity exercise is summarized in Table 1, with the group mean 
response and the response of a representative individual subject shown in Figs 1 and 2, respectively. Blood donation resulted in a significant reduction in the time to exhaustion (of 54 s or 14%), and this was associated with a significant reduction in the end-exercise (peak) 
attained during the test (of 4%; Table 1). However, there was no significant difference between the 
attained during the initial ramp incremental test and the 
attained during severe-intensity constant work rate exercise either before (95% CL: –0.04, 0.254) or after blood donation (95% CL: –0.201, 0.109). The reduction in the 
attained during severe-intensity exercise occurred in association with the reductions in [Hb] and Hct caused by the blood donation, but neither the change in [Hb] (r= 0.01; P= 0.99) nor the change in Hct (r= 0.23; P= 0.49) were significantly correlated with the change in 
. In contrast, both the change in [Hb] (r= 0.57; P= 0.069) and the change in Hct (r= 0.88; P < 0.001) were strongly correlated with the change in time to exhaustion.
|
|
|
on-kinetics, i.e. neither the phase II (pre: 29 ± 8 versus post: 30 ± 6 s) nor the amplitude of the fundamental 
response was altered as a consequence of blood donation (Table 1). The 95% confidence limits associated with the estimation of the phase II for severe exercise were 4 ± 2 s before donation and 5 ± 2 s after donation. The reduction in the end-exercise 
following blood withdrawal, despite the similar asymptotic amplitude of the fundamental 
response, resulted in a significant (22%) reduction in the size of the 
slow component (Table 1).
Blood donation did not significantly alter either the baseline (pre: 1.2 ± 0.6 versus post: 1.1 ± 0.5 mM; 95% CL: 0.3, –0.4) or end-exercise blood lactate responses (pre: 9.4 ± 1.3 versus post: 10.1 ± 1.5 mM; 95% CL: 1.4, –0.1). Blood donation also did not significantly alter either the for the HR kinetics (pre: 42 ± 18 versus post: 38 ± 9 s; 95% CL: 13, –22) or the absolute HR values at baseline, across the transient, or at the end of exercise (Fig. 3).
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
450 ml of whole blood, which caused a significant fall in [Hb] and therefore in blood O2-carrying capacity, resulted in a significant reduction in the time to exhaustion and the 
attained during severe-intensity exercise. In contradiction to our hypothesis, however, blood donation had no significant effect on the phase II 
kinetics. These latter results suggest either that bulk muscle O2 delivery was surplus to requirement in the control condition and that a small reduction in O2 delivery following blood withdrawal was not sufficient to impact on 
kinetics or that adjustments in 
or in its distribution were able to compensate for the reduced blood O2-carrying capacity so that muscle O2 delivery was preserved across the transient.
The withdrawal of 450 ml blood was successful in significantly reducing the [Hb] (by 5%) and the Hct (by 7%), and therefore in reducing the O2-carrying capacity of the blood by a similar fraction. These results cohere with previous studies which have examined the influence of the withdrawal of a similar volume of blood on [Hb] and Hct (Panebianco et al. 1995; Duda et al. 2003). A reduction in [Hb] has the potential to reduce muscle O2 availability in several ways. For the same muscle blood flow, arterial O2 content and therefore convective muscle O2 delivery will be reduced in proportion to the reduction in [Hb]. However, it has also been demonstrated that muscle O2 diffusing capacity (DO2) is reduced with lower [Hb], presumably as a function of alterations in the intracapillary spacing of erythrocytes or slower dissociation of O2 from Hb (Hogan et al. 1991; Schaffartzik et al. 1993). Indeed, Schaffartzik et al. (1993) reported that the reduction in 
they observed with a lowering of [Hb] in humans could be attributed to reductions in both muscle O2 delivery (64%) and DO2 (36%).
Influence of blood withdrawal on 
and exercise tolerance
The reduction in 
following blood withdrawal is consistent with an important role for muscle O2 delivery in limiting the 
during large muscle group exercise (Saltin & Strange, 1992; Wagner, 1995). It has been calculated that 
will be limited by O2 delivery during exercise which engages a muscle mass greater than 15 kg because the peripheral blood flow requirement will outstrip cardiac pumping capacity (Andersen & Saltin, 1985). Recent studies have confirmed that the attainment of 
during cycle exercise is linked to a levelling off of muscle O2 delivery, since stroke volume and 
begin to plateau well before exhaustion is reached (Gonzalez-Alonso & Calbet, 2003; Mortensen et al. 2005). There is also considerable evidence that alterations in arterial blood O2-carrying capacity and therefore the potential for O2 delivery to muscle (achieved through interventions such as blood withdrawal/re-infusion, plasma volume expansion, RhEPO treatment, and the inspiration of gas with altered fractional O2 content) has corresponding effects on the 
(Balke et al. 1954; Ekblom et al. 1972, 1976; Woodson et al. 1978; Spriet et al. 1986; Knight et al. 1993; Schaffartzik et al. 1993; Richardson et al. 1999; Calbet et al. 2003; Wilkerson et al. 2005). For example, it has been reported that a 7% increase in [Hb] resulting from 4 weeks of RhEPO treatment is associated with a 7% increase in 
(Ekblom & Berglund, 1991; Russell et al. 2002; Connes et al. 2003; Wilkerson et al. 2005). In the present study, a 5% reduction in [Hb] resulted in a 4% reduction in 
(Fig. 4A). A similar proportionality between [Hb] and 
has been shown to exist following the withdrawal of both similar and larger volumes of blood (Balke et al. 1954; Ekblom et al. 1972, 1976; Woodson et al. 1978; Kanstrup & Ekblom, 1982; Thomson et al. 1982; Celsing et al. 1986; Schaffartzik et al. 1993).
|
is an important determinant of endurance exercise performance (Bassett & Howley, 2000). Acute alterations in 
caused by interventions which either enhance or impair muscle O2 delivery have corresponding effects on exercise performance (Balke et al. 1954; Ekblom et al. 1972; Linnarsson et al. 1974; Knight et al. 1993; Schaffartzik et al. 1993; Hogan et al. 1999; Wilkerson et al. 2005). Consistent with these previous studies, we found that the reduction in 
following blood withdrawal was associated with a 14% reduction in the time to exhaustion. The mean difference in the time to exhaustion between the two exercise bouts which were completed before blood withdrawal was 3 s, suggesting that the reduction in time to exhaustion of 54 s following blood withdrawal was not a consequence of poor test reliability. During severe-intensity cycle exercise, which is defined as those work rates above the critical power (CP) but below the 
will continue to rise over time until the 
is attained, with fatigue occurring shortly thereafter (Poole et al. 1988; Wilkerson et al. 2004). Because the CP typically occurs at a work rate approximately halfway between the GET and 
in healthy young subjects (Pringle & Jones, 2002; Wilkerson et al. 2004), this means that there is a range of ‘submaximal’ work rates, ostensibly requiring 80–95% 
, at which 
will be attained if exercise is continued to volitional fatigue. A diminution of the 
would therefore be expected to result in a reduction in the time to exhaustion during severe-intensity exercise, as was indeed the case in the present study. A reduction in the buffering capacity of the blood with a reduction in total Hb might also have contributed, to some extent, to the reduction in exercise capacity following blood withdrawal.
Influence of blood withdrawal on 
on-kinetics
Despite reducing 
and exercise performance, blood withdrawal had no significant effect on the parameters of the 
on-kinetics in the fundamental phase of the response (with the mean phase II differing by just 1 s when the pre- and postwithdrawal responses were compared; Fig. 4B). These results share similarities with those of Nybo et al. (2001) who reported that hyperthermia reduced 
and time to exhaustion but did not alter 
kinetics during severe-intensity cycle exercise. Collectively, these results might suggest that 
kinetics during severe-intensity cycle exercise are not constrained by bulk muscle O2 delivery limitations. This interpretation would be consistent with a number of other studies which have reported that experimental manipulations designed to alter muscle O2 delivery had no discernible effect on 
kinetics during heavy- or severe-intensity cycle exercise. For example, Williamson et al. (1996) reported that the application of lower-body positive pressure to impede muscle blood flow did not influence 
kinetics during heavy-intensity cycle exercise, suggesting that bulk muscle blood flow was in excess of its requirement in the control condition. Moreover, interventions designed to improve muscle O2 delivery during heavy- or severe-intensity exercise, such as the performance of prior high-intensity exercise (which will theoretically enhance muscle vasodilatation and right-shift the O2–Hb dissociation curve), treatment with RhEPO, and the inspiration of hyperoxic gas mixtures, have not resulted in faster phase II 
kinetics in most human studies (MacDonald et al. 1997; Burnley et al. 2000, 2002; Koppo & Bouckaert, 2001; Haseler et al. 2004; Wilkerson et al. 2005; but see also Connes et al. 2003; Tordi et al. 2003).
In contrast to the work described above, other studies suggest that muscle O2 delivery can limit 
kinetics during heavy- and severe-intensity exercise in some circumstances. For example, in the isolated in situ canine gastrocnemius preparation, it was reported that fixing the muscle blood flow at the requisite ‘steady-state’ level across the rest-to-exercise transient resulted in a significant reduction of the phase II during contractions requiring 100% 
(Grassi et al. 2000) but not 60% 
(Grassi et al. 1998). Also, in humans, interventions designed to reduce either convective or diffusive O2 delivery to muscle can result in slower phase II 
kinetics during heavy-intensity exercise. A reduction in muscle perfusion pressure elicited either by performing cycle exercise in the supine compared to the upright position (MacDonald et al. 1998; Koga et al. 1999), or by performing arm-crank exercise above compared to below the level of the heart (Koppo & Bouckaert, 2005), results in slower phase II 
kinetics. The inspiration of hypoxic gas has been reported to have similar effects (Springer et al. 1991; Hughson & Kowalchuk, 1995; Engelen et al. 1996). It is unclear why some studies have demonstrated a slowing of phase II 
kinetics when muscle O2 availability is restricted while others (including the present study) have found no change; however, possible contributory factors include the exercise modality, the exercise intensity, and the extent to which the intervention altered either the convective or the diffusive component of muscle O2 transport. Previous studies removed more substantial volumes of blood from subjects (i.e. 700–800 ml), resulting in proportionally larger reductions in [Hb] and 
(Ekblom et al. 1976; Woodson et al. 1978; Kanstrup & Ekblom, 1982; Celsing et al. 1986; Schaffartzik et al. 1993). It is possible that the withdrawal of a larger volume of blood might have resulted in a significant slowing of the phase II 
kinetics in the present study.
In interpreting the results of this and previous studies, it should be appreciated that even if there is limited evidence for a bulk muscle O2 delivery limitation to 
kinetics, regional heterogeneities in blood flow relative to local metabolic rate might still be important in ‘setting’
kinetics in the control situation. This might be particularly true during heavy- and severe-intensity exercise, when an increasing proportion of muscle power will be produced by type II muscle fibres. There is recent evidence that type II muscle has both impaired blood flow kinetics (which will reduce the driving pressure for capillary-to-myocyte O2 exchange; Behnke et al. 2003) and an impaired ability to utilize the available O2 (Kindig et al. 2003). Therefore, it is not possible to dismiss the possibility that 
kinetics might be constrained ‘locally’ by O2 availability during high-intensity exercise, even if bulk muscle O2 delivery appears to be adequate. Moreover, even if the measured 
is unchanged, alterations in the phosphorylation potential and redox state will be required to drive oxidative metabolism in situations where there is a low muscle partial pressure of O2 (Wilson et al. 1977). This could result in a greater breakdown of phosphocreatine and a greater accumulation of ions (Pi, H+ and lactate–) which have been implicated in the fatigue process (Green, 1997).
We deliberately studied severe-intensity exercise (80% , corresponding to an initial metabolic requirement of 90% 
) in the present study because this would be expected to demand that a high fraction of the maximal 
be distributed to the exercising muscles from the onset of exercise (Bangsbo, 2000), thereby limiting the extent to which an increased 
or altered distribution of blood flow could compensate for the reduced O2-carrying capacity caused by the blood withdrawal. However, although the capacity for these cardiovascular changes to conspire to ‘preserve’
on-kinetics in the face of a reduced blood O2-carrying capacity would have been reduced at the exercise intensity studied, it could not be completely eliminated. In this respect, examination of the HR response across the on-transient is instructive. During severe-intensity exercise, an increased HR would be expected to be the predominant mechanism for any increase in 
(Mortensen et al. 2005). It was interesting to note, however, that neither the describing the dynamic response of HR nor the absolute HR values at discrete time points throughout the transient were significantly different before compared to after blood withdrawal (Fig. 3). These data suggest that muscle blood flow was not significantly altered across the exercise transient following blood withdrawal and that muscle O2 delivery was therefore reduced. However, HR tended to be higher by 2–3 beats min–1 across the transient following blood withdrawal and so we are unable to rule out the possibility that cardiovascular adjustments were able to compensate, at least to some extent, for the reduced [Hb].
For all work rates exceeding the GET, a secondary ‘slow component’ of 
appears to be superimposed on the fundamental 
response and this leads, after sufficient time has elapsed, to a ‘steady-state’ or end-exercise value which is greater than would be predicted for the work rate (Whipp & Wasserman, 1972; Barstow & Molé, 1991). Because the 
slow component reduces metabolic efficiency, is associated with a decline in intramuscular phosphocreatine concentration, presumably accelerates muscle glycogen utilization and, for severe exercise, sets 
on a trajectory towards its peak value, it has been linked to the fatigue process (Poole et al. 1994). However, in the present study, the reduced time to exhaustion during severe-intensity exercise following blood withdrawal was associated with a smaller
slow component. This can be explained by the fact that the asymptotic amplitude of the fundamental 
response was not altered with blood withdrawal whereas the 
was significantly reduced; since the 
will set the ceiling for the continued increase in 
during severe-intensity exercise, this left ‘less room’ for the 
slow component to develop (Fig. 2). Blood withdrawal therefore provides an interesting example of a situation in which the magnitude of the 
slow component is inversely related to exercise tolerance.
Conclusions
The withdrawal of 450 ml blood resulted in: (1) a significant reduction in [Hb], and thus O2-carrying capacity; (2) a significant reduction in 
; (3) a significant reduction in exercise tolerance; but (4) no significant change in the phase II of the 
on-kinetics. One interpretation of our data is that bulk muscle O2 delivery is in excess of metabolic requirements across the transient to severe-intensity cycle exercise in young healthy subjects, so that a small reduction in O2 delivery following blood withdrawal has no discernible influence on 
kinetics. However, adjustments the potential for compensatory cardiovascular adaptations would have been minimal at the severe-intensity work rate that was applied in the present study, we cannot rule out the possibility that an increase in 
or a redistribution of 
towards the exercising muscles was sufficient to at least partly offset the relatively small reduction in O2-carrying capacity achieved by the intervention. Finally, our results confirm the importance of blood O2-carrying capacity in the determination of 
and exercise tolerance during large muscle group activities.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Balke
B, Grillo
GP, Konecci
EB
&
Luft
UC (1954). Work capacity after blood donation. J Appl Physiol
7, 231–238.
Bangsbo J (2000). Muscle oxygen uptake in humans at onset of and during intense exercise. Acta Physiol Scand 168, 457–464.[CrossRef][Medline]
Barstow
TJ
&
Molé
PA (1991). Linear and nonlinear characteristics of oxygen uptake kinetics during heavy exercise. J Appl Physiol
71, 2099–2106.
Bassett DR & Howley ET (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med Sci Sports Exerc 32, 70–84.[Medline]
Beaver
WL, Wasserman
K
&
Whipp
BJ (1973). On-line computer analysis and breath-by-breath graphical display of exercise function tests. J Appl Physiol
34, 128–132.
Beaver
WL, Wasserman
K
&
Whipp
BJ (1986). A new method for detecting anaerobic threshold by gas exchange. J Appl Physiol
60, 2020–2027.
Behnke
BJ, McDonough
P, Padilla
DJ, Musch
TI
&
Poole
DC (2003). Oxygen exchange profile in rat muscles of contrasting fibre types. J Physiol
549, 597–605.
Burnley
M, Doust
JH, Ball
D
&
Jones
AM (2002). Effects of prior heavy exercise on 
kinetics during heavy exercise are related to changes in muscle activity. J Appl Physiol
93, 167–174.
Burnley M, Doust JH & Jones AM (2005). Effects of prior warm-up regime on severe-intensity cycling performance. Med Sci Sports Exerc 37, 838–845.[CrossRef][Medline]
Burnley
M, Jones
AM, Carter
H
&
Doust
JH (2000). Effect of prior heavy exercise on phase II pulmonary oxygen uptake kinetics and the slow component during heavy exercise in humans. J Appl Physiol
89, 1387–1396.
Calbet
JA, Boushel
R, Radegran
G, Sondergaard
H, Wagner
PD
&
Saltin
B (2003). Determinants of maximal oxygen uptake in severe acute hypoxia. Am J Physiol Regul Integr Comp Physiol
284, R291–R303.
Celsing
F, Nystrom
J, Pihlstedt
P, Werner
B
&
Ekblom
B (1986). Effect of long-term anemia and retransfusion on central circulation during exercise. J Appl Physiol
61, 1358–1362.
Connes P, Perrey S, Varray A, Prefaut C & Caillaud C (2003). Faster oxygen uptake kinetics at the onset of submaximal cycling exercise following 4 weeks recombinant human erythropoietin (r-HuEPO) treatment. Pflugers Arch 447, 231–238.[CrossRef][Medline]
Duda K, Zoladz JA, Majerczak J, Kolodziejski L & Konturek SJ (2003). The effect of exercise performed before and 24 hours after blood withdrawal on serum erythropoietin and growth hormone concentrations in humans. Int J Sports Med 24, 326–331.[CrossRef][Medline]
Ekblom B & Berglund B (1991). Effect of erythropoietin administration on maximal aerobic power. Scand J Med Sci Sports 1, 88–93.
Ekblom
B, Goldbarg
AN
&
Gullbring
B (1972). Response to exercise after blood loss and reinfusion. J Appl Physiol
33, 175–180.
Ekblom
B, Wilson
G
&
Astrand
P-O (1976). Central circulation during exercise after venesection and reinfusion of red blood cells. J Appl Physiol
40, 379–383.
Engelen
M, Porszasz
J, Riley
M, Wasserman
K, Maehara
K
&
Barstow
TJ (1996). Effects of hypoxic hypoxia on O2 uptake and heart rate kinetics during heavy exercise. J Appl Physiol
81, 2500–2508.
Gonzalez-Alonso
J
&
Calbet
JA (2003). Reductions in systemic and skeletal muscle blood flow and oxygen delivery limit maximal aerobic capacity in humans. Circulation
107, 824–830.
Grassi B (2003). Oxygen uptake kinetics: old and recent lessons from experiments on isolated muscle in situ. Eur J Appl Physiol 90, 242–249.[Medline]
Grassi
B, Gladden
LB, Samaja
M, Stary
CM
&
Hogan
MC (1998). Faster adjustment of O2 delivery does not affect 
on-kinetics in isolated in situ canine muscle. J Appl Physiol
85, 1394–1403.
Grassi
B, Hogan
MC, Kelley
KM, Aschenbach
WG, Hamann
JJ, Evans
RK, Patillo
RE
&
Gladden
LB (2000). Role of convective O2 delivery in determining 
on-kinetics in canine muscle contracting at peak 
. J Appl Physiol
89, 1293–1301.
Grassi
B, Poole
DC, Richardson
RS, Knight
DR, Erickson
BK
&
Wagner
PD (1996). Muscle O2 uptake kinetics in humans: implications for metabolic control. J Appl Physiol
80, 988–998.
Green HJ (1997). Mechanisms of muscle fatigue in intense exercise. J Sports Sci 15, 247–256.[CrossRef][Medline]
Haseler
LJ, Kindig
CA, Richardson
RS
&
Hogan
MC (2004). The role of oxygen in determining phosphocreatine onset kinetics in exercising humans. J Physiol
558, 985–992.
Hogan
MC, Bebout
DE
&
Wagner
PD (1991). Effect of hemoglobin concentration on maximal O2 uptake in canine gastrocnemius muscle in situ. J Appl Physiol
70, 1105–1112.
Hogan
MC, Richardson
RS
&
Haseler
LJ (1999). Human muscle performance and PCr hydrolysis with varied inspired oxygen fractions: a 31P-MRS study. J Appl Physiol
86, 1367–1373.
Hughson RL & Kowalchuk JM (1995). Kinetics of oxygen uptake for submaximal exercise in hyperoxia, normoxia, and hypoxia. Can J Appl Physiol 20, 198–210.[Medline]
Kanstrup
IL
&
Ekblom (1982). Acute hypervolemia, cardiac performance, and aerobic power during exercise. J Appl Physiol
52, 1186–1191.
Kindig
CA, Howlett
RA
&
Hogan
MC (2003). Effect of extracellular PO2 on the fall in intracellular PO2 in contracting single myocytes. J Appl Physiol
94, 1964–1970.
Knight
DR, Schaffartzik
W, Poole
DC, Hogan
MC, Bebout
DE
&
Wagner
PD (1993). Effects of hyperoxia on maximal leg O2 supply and utilization in men. J Appl Physiol
75, 2586–2594.
Koga
S, Shiojiri
T, Shibasaki
M, Kondo
N, Fukuba
Y
&
Barstow
TJ (1999). Kinetics of oxygen uptake during supine and upright heavy exercise. J Appl Physiol
87, 253–260.
Koppo
K
&
Bouckaert
J (2001). The effect of prior high-intensity cycling exercise on the 
kinetics during high-intensity cycling exercise is situated at the additional slow component. Int J Sports Med
22, 21–26.[CrossRef][Medline]
Koppo
K
&
Bouckaert
J (2005). Prior arm exercise speeds the 
kinetics during arm exercise above the heart level. Med Sci Sports Exerc
37, 613–619.[Medline]
Linnarsson
D, Karlsson
J, Fagraeus
L
&
Saltin
B (1974). Muscle metabolites and oxygen deficit with exercise in hypoxia and hyperoxia. J Appl Physiol
36, 399–402.
MacDonald
MJ, Pedersen
PK
&
Hughson
RL (1997). Acceleration of 
kinetics in heavy submaximal exercise by hyperoxia and prior high-intensity exercise. J Appl Physiol
83, 1318–1325.
MacDonald
MJ, Shoemaker
JK, Tschakovsky
ME
&
Hughson
RL (1998). Alveolar oxygen uptake and femoral artery blood flow dynamics in upright and supine leg exercise in humans. J Appl Physiol
85, 1622–1628.
MacDonald MJ, Tarnopolsky MA & Hughson RL (2000). Effect of hyperoxia and hypoxia on leg blood flow and pulmonary and leg oxygen uptake at the onset of kicking exercise. Can J Physiol Pharmacol 78, 67–74.[CrossRef][Medline]
Mortensen
SP, Dawson
EA, Yoshiga
CC, Dalsgaard
MK, Damsgaard
R, Secher
NH
&
Gonzalez-Alonso
J (2005). Limitations to systemic and locomotor limb muscle oxygen delivery and uptake during maximal exercise in humans. J Physiol
566, 273–285.
Nybo
L, Jensen
T, Nielsen
B
&
Gonzalez-Alonso
J (2001). Effects of marked hyperthermia with and without dehydration on 
kinetics during intense exercise. J Appl Physiol
90, 1057–1064.
Özyener
F, Rossiter
HB, Ward
SA
&
Whipp
BJ (2001). Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans. J Physiol
533, 891–902.
Panebianco RA, Stachenfeld N, Coplan NL & Gleim GW (1995). Effects of blood donation on exercise performance in competitive cyclists. Am Heart J 130, 838–840.[Medline]
Poole
DC, Barstow
TJ, Gaesser
GA, Willis
WT
&
Whipp
BJ (1994). 
slow component: physiological and functional significance. Med Sci Sports Exerc
26, 1354–1358.[Medline]
Poole
DC
&
Jones
AM (2005). Towards an understanding of the mechanistic bases of 
kinetics. In
Oxygen Uptake Kinetics in Sport, Exercise & Medicine, ed.
Jones
AM
&
Poole
DC, pp.
298–328. Routledge,
London.
Poole DC, Ward SA, Gardner GW & Whipp BJ (1988). Metabolic and respiratory profile of the upper limit for prolonged exercise in man. Ergonomics 31, 1265–1279.[Medline]
Pringle JS & Jones AM (2002). Maximal lactate steady state, critical power and EMG during cycling. Eur J Appl Physiol 88, 214–226.[CrossRef][Medline]
Richardson
RS, Grassi
B, Gavin
TP, Haseler
LJ, Tagore
K, Roca
J
&
Wagner
PD (1999). Evidence of O2 supply-dependent 
in the exercise-trained human quadriceps. J Appl Physiol
86, 1048–1053.
Rossiter
HB, Ward
SA, Kowalchuk
JM, Howe
FA, Griffiths
JR
&
Whipp
BJ (2001). Effects of prior exercise on oxygen uptake and phosphocreatine kinetics during high-intensity knee-extension exercise in humans. J Physiol
537, 291–303.
Rossiter
HB, Ward
SA, Kowalchuk
JM, Howe
FA, Griffiths
JR
&
Whipp
BJ (2002). Dynamic asymmetry of phosphocreatine concentration and O2 uptake between the on- and off-transients of moderate- and high-intensity exercise in humans. J Physiol
541, 991–1002.
Russell G, Gore CJ, Ashenden MJ, Parisotto R & Hahn AG (2002). Effects of prolonged low doses of recombinant human erythropoietin during submaximal and maximal exercise. Eur J Appl Physiol 86, 442–449.[CrossRef][Medline]
Saltin B & Strange S (1992). Maximal oxygen uptake: ‘old’ and ‘new’ arguments for a cardiovascular limitation. Med Sci Sports Exerc 24, 30–37.[Medline]
Schaffartzik
W, Barton
ED, Poole
DC, Tsukimoto
K, Hogan
MC, Bebout
DE
&
Wagner
PD (1993). Effect of reduced haemoglobin concentration on leg oxygen uptake during maximal exercise in humans. J Appl Physiol
75, 491–498.
Spriet
LL, Gledhill
N, Froese
AB
&
Wilkes
DL (1986). Effect of graded erythrocythemia on cardiovascular and metabolic responses to exercise. J Appl Physiol
61, 1942–1948.
Springer C, Barstow TJ, Wasserman K & Cooper DM (1991). Oxygen uptake and heart rate responses during hypoxic exercise in children and adults. Med Sci Sports Exerc 23, 71–79.[Medline]
Thomson
JM, Stone
JA, Ginsburg
AD
&
Hamilton
P (1982). O2 transport during exercise following blood reinfusion. J Appl Physiol
53, 1213–1219.
Tordi
N, Perrey
S, Harvey
A
&
Hughson
RL (2003). Oxygen uptake kinetics during two bouts of heavy cycling separated by fatiguing sprint exercise in humans. J Appl Physiol
94, 533–541.
Tschakovsky ME & Hughson RL (1999). Interaction of factors determin
response to severe-intensity exercise before (•) and after the donation of 
) 
and time to exhaustion are also shown; note the reduction in both the 
and the time to exhaustion following blood withdrawal.
response to severe-intensity exercise in a representative subject before (A) and after the donation of 
. Note that blood donation does not affect the asymptotic amplitude of the fundamental component of the response but that it reduces the end-exercise 
and therefore the time to exhaustion as well.
(upper panel)and the time constant for the phase II 
response during severe-intensity exercise (lower panel) 
but not 
kinetics. Filled circles represent data from the present study in which [Hb] was reduced following blood withdrawal, open circles represent data from