Oxygen uptake as related to work rate increment during cycle ergometer exercise

Summary We postulated that the commonly observed constant linear relationship between and work rate during cycle ergometry to exhaustion is fortuitous and not due to an unchanging cost of external work. Therefore we measured continuously in 10 healthy men during such exercise while varying the rate of work incrementation and analyzed by linear regression techniques the relationship between and work rate ( / wr). After excluding the first and last portions of each test we found the mean ±SD of the / wr in ml · min^–1· W^–1 to be 11.2±0.15, 10.2±0.16, and 8.8±0.15 for the 15, 30, and 60 W·min^–1 tests, respectively, expressed as ml·J^–1 the values were 0.187±0.0025, 0.170±0.0027 and 0.147±0.0025. The slopes of the lower halves of the 15 and 30 W·min^–1 tests were 9.9±0.2 ml·min^–1·W^–1 similar to the values for aerobic work reported by others. However the upper halves of the 15, 30, and 60 W·min^–1 tests demonstrated significant differences: 12.4±0.36 vs 10.5±0.31 vs 8.7±0.23 ml·min^–1·W^–1 respectively. We postulate that these systematic differences are due to two opposing influences: 1) the fraction of energy from anaerobic sources is larger in the brief 60 W·min^–1 tests and 2) the increased energy requirement per W of heavy work is evident especially in the long 15 W·min^–1 tests.     

Effects of prolonged cycle ergometer exercise on maximal muscle power and oxygen uptake in humans

Summary The mechanical power (W_tot, W·kg^–1) developed during ten revolutions of all-out periods of cycle ergometer exercise (4–9 s) was measured every 5–6 min in six subjects from rest or from a baseline of constant aerobic exercise [50%–80% of maximal oxygen uptake (VO_2max)] of 20–40 min duration. The oxygen uptake [VO₂ (W·kg^–1, 1 ml O₂ = 20.9 J)] and venous blood lactate concentration ([la]_b, mM) were also measured every 15 s and 2 min, respectively. During the first all-out period, W_tot decreased linearly with the intensity of the priming exercise (W_tot = 11.9–0.25·VO₂). After the first all-out period (i greater than 5–6 min), and if the exercise intensity was less than 60% VO_2max, W_tot, VO₂ and [la]_b remained constant until the end of the exercise. For exercise intensities greater than 60% VO_2max, VO₂ and [la]_b showed continuous upward drifts and W_tot continued decreasing. Under these conditions, the rate of decrease of W_tot was linearly related to the rate of increase of V [(d W_tot/dt) (W·kg^–1·s^–1) = 5.0·10^–5 –0.20·(d VO₂/dt) (W·kg^–1·s^–1)] and this was linearly related to the rate of increase of [la]_b [(d VO₂/dt) (W·kg^–1·s^–1) = 2.310^–4 + 5.910^–5·(d [la]_b/dt) (mM·s^–1)]. These findings would suggest that the decrease of W_tot during the first all-out period was due to the decay of phosphocreatine concentration in the exercising muscles occurring at the onset of exercise and the slow drifts of VO₂ (upwards) and of W_tot (downwards) during intense exercise at constant W_tot could be attributed to the continuous accumulation of lactate in the blood (and in the working muscles).     

Neuromuscular fatigue during prolonged pedalling exercise at different pedalling rates

The purpose of this study was to estimate the differences in neuromuscular fatigue among prolonged pedalling exercises performed at different pedalling rates at a given exercise intensity. The integrated electromyogram (iEMG) slope defined by the changes in iEMG as a function of time during exercise was adopted as the measurement for estimating neuromuscular fatigue. The results of this experiment showed that the relationship between pedalling rate and the means of the iEMG slopes for eight subjects was a quadratic curve and the mean value at 70 rpm [1.56 (SD 0.65) V·min^–1] was significantly smaller (P < 0.01) than that at 50 and 60 rpm [2.25 (SD 0.54), and 2.22 (SD 0.68), respectively]. On the other hand, the mean value of oxygen consumption obtained simultaneously showed a tendency to increase linearly with the increase in pedalling rate, and the values at 70 and 80 rpm were significantly higher than those at 40 and 50 rpm. In conclusion, it was demonstrated that the degree of neuromuscular fatigue estimated by the iEMG changes for five periods of prolonged pedalling exercise at a given exercise intensity was different among the different pedalling rates, and that the pedalling rate at which minimal neuromuscular fatigue was obtained was not coincident with the rate at which the minimal oxygen consumption was obtained, but was coincident with the rate which most subjects preferred. These findings would suggest that the reason why most people prefer a relative higher pedalling rate, even though higher oxygen consumption is required, is closely related to the development of neuromuscular fatigue in the working muscles.     

Dynamics of oxygen uptake following exercise onset in rat skeletal muscle

Technical limitations have precluded measurement of the V(O(2)) profile within contracting muscle (mV(O(2))) and hence it is not known to what extent V(O(2)) dynamics measured across limbs in humans or muscles in the dog are influenced by transit delays between the muscle microvasculature and venous effluent. Measurements of capillary red blood cell flux and microvascular P(O(2)) (P(O(2)m)) were combined to resolve the time course of mV(O(2)) across the rest-stimulation transient (1 Hz, twitch contractions). mV(O(2)) began to rise at the onset of contractions in a close to monoexponential fashion (time constant, J = 23.2 +/- 1.0 sec) and reached it's steady-state value at 4.5-fold above baseline. Using computer simulation in healthy and disease conditions (diabetes and chronic heart failure), our findings suggest that: (1) mV(O(2)) increases essentially immediately (< 2 sec) following exercise onset; (2) within healthy muscle the J blood flow (thus O(2) delivery, J Q(O(2)m)) is faster than JmV(O(2)) such that oxygen delivery is not limiting, and 3) a faster P(O(2)m) fall to a P(O(2)m) value below steady-state values within muscle from diseased animals is consistent with a relatively sluggish Q(O(2)m) response compared to that of mV(O(2)).     

Inverse relationship between exercise economy and oxidative capacity in muscle

An inverse relationship has been shown between running and cycling exercise economy and maximum oxygen uptake The purposes were: 1) determine the relationship between walking economy and and 2) determine the relationship between muscle metabolic economy and muscle oxidative capacity and fiber type. Subjects were 77 premenopausal normal weight women. Walking economy was measured at 3 mph and during graded treadmill test. Muscle oxidative phosphorylation rate (OxPhos), and muscle metabolic economy (force/ATP) were measured in calf muscle using ³¹P MRS during isometric plantar flexion at 70 and 100% of maximum force, (HI) and (MI) respectively. Muscle fiber type and citrate synthase activity were determined in the lateral gastrocnemius. Significant inverse relationships (r from –0.28 to –0.74) were observed between oxidative metabolism measures and exercise economy (walking and muscle). Type IIa fiber distribution was inversely related to all measures of exercise economy (r from –0.51 to –0.64) and citrate synthase activity was inversely related to muscle metabolic economy at MI (r=–0.56). In addition, Type IIa fiber distribution and citrate synthase activity were positively related to and muscle OxPhos at HI and MI (r from 0.49 to 0.70). Type I fiber distribution was not related to any measure of exercise economy or oxidative capacity. Our results support the concept that exercise economy and oxidative capacity are inversely related. We have demonstrated this inverse relationship in women both by indirect calorimetry during walking and in muscle tissue by ³¹P MRS.     

Postural effect on cardiac output,oxygen uptake and lactate during cycle exercise of varying intensity

Owing to changes in cardiac output, blood volume distribution and the efficacy of the muscle pump, oxygen supply may differ during upright and supine cycle exercise. In the present study we measured, in parallel, circulatory (heart rate, stroke volume, blood pressure) and metabolic parameters (oxygen uptake, lactic acid concentration [1a]) during incremental-exercise tests and at constant power levels ranging from mild to severe exercise. In supine position, cardiac output exceeded the upright values by 1.0-1.5 1 · min^–1 during rest, light ([la] < 2 mmol · 1^–1) and moderate ([la] =2–4 mmol · 1^–1) exercise. At higher exercise intensities the cardiac output in an upright subject approached and eventually slightly exceeded the supine values. For both rest-exercise transitions and large-amplitude steps (W 140 W) the cardiac output kinetics was significantly faster in upright cycling. The metabolic parameters (VO₂ and [la]) showed no simple relationship to the circulatory data. In light to moderate exercise they were unaffected by body position. Only in severe exercise, when cardiac output differences became minimal, could significant influences be observed: with supine body posture, [la] started to rise earlier and maximal power (W=23 W) and exercise duration (64 s) were significantly reduced. However, the maximal [la] value after exercise was identical in both positions. The present findings generally show advantages of upright cycling only for severe exercise. With lower workloads the less effective muscle pump in the supine position appears to be compensated for by the improved central circulatory conditions and local vasodilatation.     

Effects of moderate hyperoxia on oxygen consumption during submaximal and maximal exercise

The present study examined the effect of hyperoxia on oxygen uptake (V˙O₂) and on maximal oxygen uptake (V˙O_2max) during incremental exercise (IE) and constant work rate exercise (CWRE). Ten subjects performed IE on a bicycle ergometer under normoxic and hyperoxic conditions (30% oxygen). They also performed four 12-min bouts of CWRE at 40, 55, 70 and 85% of normoxic V˙O_2max (ex1, ex2, ex3 and ex4, respectively) in normoxia and in hyperoxia. V˙O_2max was significantly improved by 15.0 (15.2)% under hyperoxia, while performance (maximum workload, W _max) was improved by only +4.5 (3.0)%. During IE, the slope of the linear regression relating V˙O₂ to work rate was significantly steeper in hyperoxia than in normoxia [10.80 (0.88) vs 10.06 (0.66) ml·min^–1·W^–1]. During CWRE, we found a higher V˙O₂ at ex1, ex2, ex3 and ex4, and a higher V˙O₂ slow component at ex4 under hyperoxia. We have shown that breathing hyperoxic gas increases V˙O_2max, but to an extent that is difficult to explain by an increase in oxygen supply alone. Changes in metabolic response, fibre type recruitment and V˙O₂ of non-exercising tissue could explain the additional V˙O₂ for a given submaximal work rate under hyperoxia. Electronic Publication     

Relationship between cardiac output and oxygen uptake at the onset of exercise

Summary The purpose of the present study was to assess the relationship between the rapidity of increased gas exchange (i.e. oxygen uptake ) and increased cardiac output ( ) during the transient phase following the onset of exercise. Five healthy male subjects performed multiple rest-exercise or light exercise (25 W)-exercise transitions on an electrically braked ergometer at exercise intensities of 50, 75, or 100 W for 6 min, respectively. Each transition was performed at least eight times for each load in random order. The was obtained by a breath-by-breath method, and was measured by an impedance method during normal breathing, using an ensemble average. On transitions from rest to exercise, rapidly increased during phase I with time constants of 6.8–7.3 s. The also showed a similar rapid increment with time constants of 6.0–6.8 s with an apparent increase in stroke volume (SV). In this phase I, increased to about 29.7%–34.1% of the steady-state value and increased to about 58.3%–87.0%. Thereafter, some 20 s after the onset of exercise a mono-exponential increase to steady-state occurred both in and with time constants of 26.7–32.3 and 23.7–34.4 s, respectively. The insignificant difference between and time constants in phase I and the abrupt increase in both and SV at the onset of exercise from rest provided further evidence for a cardiodynamic contribution to following the onset of exercise from rest.     

A model for studying the distortion of muscle oxygen uptake patterns by circulation parameters

Summary The transmission of muscle oxygen uptake patterns to the pulmonary site is a basically nonlinear process during unsteady state exercise. We were mainly interested in three questions concerning the dynamic relationship between power input and pulmonary output: 1. To what extent can linear system analysis be applied? 2. What is the relative influence of muscle on pulmonary as compared to other parameters such as muscle perfusion kinetics? 3. To what extent does pulmonary reflect muscle ? Investigations were performed by means of a mathematical model including a muscle compartment and two serial, flow-varying time delays. The non-exercising parts of the body were. incorporated as one term for perfusion and one for . Parameters were adjusted so as to represent a reference state of aerobic exercise while monofrequent sinusoidal changes in aerobic metabolism were used as forcing signals. The following answers were derived from the simulations: 1. Non-linear distortions of the signals are negligible provided that analyses are not driven too far into the higher frequency range (periods shorter than about 1 min). 2. Variations of muscle kinetics have greater effects on pulmonary than changes of perfusion kinetics or venous volume. This finding applies irrespective of whether or not pulmonary closely reflects muscle 3. Small differences in the time constants for muscle perfusion and muscle are a major prerequisite if pulmonary , kinetics are to be taken as correct estimates of muscle kinetics. High basal muscle perfusion, small perfusion changes and small venous volumes between muscle and lungs are further factors reducing dynamic distortions of the muscle signal.     

Attenuated relationship between cardiac output and oxygen uptake during high-intensity exercise

Aim: Recent findings have challenged the belief that the cardiac output (CO) and oxygen consumption (VO₂) relationship is linear from rest to maximal exercise. The purpose of this study was to determine the CO and stroke volume (SV) response to a range of exercise intensities, 40–100% of VO_2max, during cycling. Methods: Ten well‐trained cyclists performed a series of discontinuous exercise bouts to determine the CO and SV vs. VO₂ responses. Results: The rate of increase in CO, relative to VO₂, during exercise from 40 to 70% of VO_2max was 4.4 ± 1.4 L L^?1. During exercise at 70–100% of VO_2max, the rate of increase in CO was reduced to 2.1 ± 0.9 L L^?1 (P = 0.01). Stroke volume during exercise at 80–100% of VO_2max was reduced by 7% when compared to exercise at 50–70% of VO_2max (134 ± 5 vs. 143 ± 5 mL per beat, P = 0.02). Whole body arterial‐venous O₂ difference increased significantly as intensity increased. Conclusion: The observation that the rate of increase in CO is reduced as exercise intensity increases suggests that cardiovascular performance displays signs of compromised function before maximal VO₂ is reached.     

The influence of circulatory difference on muscle oxygenation and fatigue during intermittent static dorsiflexion

This study aimed to examine the influences of circulatory difference on the utilization of O₂ and the progression of fatigue in the tibialis anterior (TA) muscle during dorsiflexion exercise, with reference to different body postures. The subjects performed intermittent static dorsiflexion at 50% of maximal voluntary contraction (MVC) up to exhaustion with the right leg either up or down relative to the heart. These exercises were performed with and without occluding muscle blood flow. Simultaneously with the surface electromyogram (EMG) measurement, total hemoglobin volume change and tissue oxygenation (S _tO₂) of TA were measured using near-infrared spectroscopy (NIRS). When the subjects performed an exhaustive intermittent dorsiflexion exercise at 50% MVC, the endurance time decreased in the leg up position. Also, the progression of fatigue in TA detected using EMG signals (i.e. integrated EMG and mean power frequency of EMG) was faster with the leg elevated. The NIRS data indicated a lower blood volume and S _tO₂ with the leg up than with the leg down during the exercise, which suggests that the deficit in the O₂ supply to exercising muscles demand was more apparent in the leg up position. However, these differences in EMG and NIRS data disappeared when the blood flow was restricted in both positions. From these results it is concluded that the difference in exercising muscle oxygenation between two different body postures influenced the progression of muscle fatigue and caused the difference in endurance performance.     

Force kinetics and oxygen consumption during bicycle ergometer work in racing cyclists and reference-group

Summary The forces acting on the right crank of a bicycle ergometer were measured in 18 male subjects (6 racing cyclists, 8 students of physical education, 4 long distance runners) during an incremental exercise test. Oxygen consumption ( ) was simultaneously determined by means of a breath-by-breath method.Differences in peak values of the typical force record might indicate a different force distribution during each cranking cycle. When compared to the reference group, the racing cyclists showed peak values that were significantly lower at all levels of work load. Oxygen consumption during the initial 20 min of the test was found to be significantly lower in the cyclist group (cyclists: 37.2±3.2 l, reference group: 41.1±3.9 l). These results suggest that a different force distribution during a crank revolution might lead to an improved gross efficiency in the cyclist group. The findings might be due to different fractions of ST-fibres in the exercising muscle.     

The relationship between anaerobic threshold and electromyographic fatigue threshold in college women

Summary The purpose of this study was to investigate the relationship between anaerobic threshold (Th_an) and muscle fatigue threshold (EMG_FT) as estimated from electromyographic (EMG) data taken from the quadriceps muscles (vastus lateralis) during exercise on a cycle ergometer. The subjects in this study were 20 female college students, including highly trained endurance athletes and untrained sedentary individuals, whose fitness levels derived from their maximal oxygen consumption ranged from 24.9 to 62.2 ml · kg^–1·min^–1. The rate of increase in integrated EMG (iEMG) activity as a function of time (iEMG slope) was calculated at each of four constant power outputs (350, 300, 250, 200 W), sufficiently high to bring about muscle fatigue. The iEMG slopes so obtained were plotted against the exercise intensities imposed, resulting in linear plots which were extrapolated to zero slope to give an intercept on the power axis which was in turn interpreted as the highest exercise intensity sustainable without electromyographic evidence of neuromuscular fatigue (EMGF_FT). The Th_an was estimated from gas exchange parameters during an incremental exercise test on the same cycle ergometer. The mean results indicated that oxygen uptake (VO₂) at Th_an was 1.391·min^–1, SD 0.44 andVO₂ at EMG_FT was 1.33 1·min^–1, SD 0.57. There was no significant difference between these mean values (P>0.05) and there was a highly significant correlation betweenVO₂ at Th_an andVO₂ at EMG_FT (r=0.823,P<0.01). These data supported the concept of Th_an on the basis that Than was associated with the highest exercise intensity that could be sustained without evidence of neuromuscular fatigue and thus suggested that EMG_FT may provide an attractive alternative to the measurement of Th_an.     

Oxygen uptake kinetics during severe exercise: a comparison between young and older men

This study examined the relationship between the slow component of oxygen uptake (VO2) kinetics and muscle electromyography (EMG) during severe exercise in nine young (21.7+/-0.9 yr) and nine older (71.6+/-0.8 yr) men. Oxygen uptake (VO2) and surface EMG activity of the left vastus lateralis muscle were measured during a 7-min square-wave bout of severe exercise on a cycle ergometer. The absolute amplitude of the VO2 slow component was greater and occurred approximately 60 s earlier in the young compared to older subjects. However, the rate of increase in the slow component, expressed as a percentage of the total VO2 response per unit time, was not different between young and older subjects (young: 4.8+/-0.5%.min(-1); older: 4.9+/-0.6%.min(-1)). The mean power frequency (MPF) of the EMG increased significantly during the slow component phase of exercise by 6.4+/-1.0% in the young and by 5.4+/-0.7% in the older group and this rise was not significantly different between the two groups. These results indicate that normal ageing may not alter the VO2 slow component (measured as the rate of increase in VO2) and that this finding may be related to similar muscle fibre recruitment patterns in the two groups during severe-intensity exercise.     

Relationship between myoelectric and mechanical manifestations of fatigue in the quadriceps femoris muscle group

Fatigue is commonly defined as “the failure to maintain the required force”. As such, it may be argued that the use of electromyographic (EMG) power spectral statistics to monitor muscle fatigue is inappropriate, because, during the maintenance of a submaximal force of contraction, EMG changes are readily observable in the absence of any decline in the muscle's mechanical output. However, it is possible that the EMG changes reflect the changing metabolic status of the muscle and hence its inability to generate its normal maximal force. The present study sought to examine whether the decline in EMG median frequency, which occurs during the maintenance of a submaximal force, is correlated with a reduction in the muscle's maximum force-generating capacity. The maximum voluntary contraction (MVC) of the knee extensors in ten young, healthy subjects was determined. On five separate occasions, randomly assigned forces of 20, 30, 40, 50 and 60% MVC were held to the limit of endurance. At intervals throughout the sustained contractions, subjects were required to rapidly generate an MVC for 1–2 s, then return to the fixed submaxial target force. Surface EMG signals were recorded throughout the contractions from the rectus lemons and vastus lateralis muscles, from which the power spectrum median frequency was calculated. Regression analysis revealed highly significant relationships between the rate of decline in MF and the rate of decline in MVC, and between each of these parameters and endurance time to fatigue (P = 0.0001, in each case). It is concluded that the decline in MF can be used to monitor fatigue, where fatigue is defined as the inability to generate the maximum force that can be produced by the muscle in its fresh state.     

Negative accumulated oxygen deficit during heavy and very heavy intensity cycle ergometry in humans

The concept of the accumulated O₂ deficit (AOD) assumes that the O₂ deficit increases monotonically with increasing work rate (WR), to plateau at the maximum AOD, and is based on linear extrapolation of the relationship between measured steady-state oxygen uptake (O₂) and WR for moderate exercise. However, for high WRs, the measured O₂ increases above that expected from such linear extrapolation, reflecting the superimposition of a "slow component" on the fundamental O₂ mono-exponential kinetics. We were therefore interested in determining the effect of the O₂ slow component on the computed AOD. Ten subjects [31 (12) years] performed square-wave cycle ergometry of moderate (40%, 60%, 80% and 90% ), heavy (40%), very heavy (80%) and severe (110% O_{2 peak}) intensities for 10–15 min, where is the estimated lactate threshold and is the WR difference between and O_{2 peak}. O₂ was determined breath-by-breath. Projected "steady-state" O₂ values were determined from sub- tests. The measured O₂ exceeded the projected value after ~3 min for both heavy and very heavy intensity exercise. This led to the AOD actually becoming negative. Thus, for heavy exercise, while the AOD was positive [0.63 (0.41) l] at 5 min, it was negative by 10 min [–0.61 (1.05) l], and more so by 15 min [–1.70 (1.64) l]. For the very heavy WRs, the AOD was [0.42 (0.67) l] by 5 min and reached –2.68 (2.09) l at exhaustion. For severe exercise, however, the AOD at exhaustion was positive in each case: +1.69 (0.39) l. We therefore conclude that the assumptions underlying the computation of the AOD are invalid for heavy and very heavy cycle ergometry (at least). Physiological inferences, such as the "anaerobic work capacity", are therefore prone to misinterpretation.     

The highest intensity and the shortest duration permitting attainment of maximal oxygen uptake during cycling: effects of different methods and aerobic fitness level

The aims of this study were: (1) to verify the validity of previous proposed models to estimate the lowest exercise duration (T _LOW) and the highest intensity (I _HIGH) at which VO₂max is reached (2) to test the hypothesis that parameters involved in these models, and hence the validity of these models are affected by aerobic training status. Thirteen cyclists (EC), eleven runners (ER) and ten untrained (U) subjects performed several cycle-ergometer exercise tests to fatigue in order to determine and estimate T _LOW (ET _LOW) and I _HIGH (EI _HIGH). The relationship between the time to achieved VO₂max and time to exhaustion (T _lim) was used to estimate ET _LOW. EI _HIGH was estimated using the critical power model. I _HIGH was assumed as the highest intensity at which VO₂ was equal or higher than the average of VO₂max values minus one typical error. T _LOW was considered T _lim associated with I _HIGH. No differences were found in T _LOW between ER (170 ± 31 s) and U (209 ± 29 s), however, both showed higher values than EC (117 ± 29 s). I _HIGH was similar between U (269 ± 73 W) and ER (319 ± 50 W), and both were lower than EC (451 ± 33 W). EI _HIGH was similar and significantly correlated with I_HIGH only in U (r = 0.87) and ER (r = 0.62). ET _LOW and T _LOW were different only for U and not significantly correlated in all groups. These data suggest that the aerobic training status affects the validity of the proposed models for estimating I _HIGH.     

Oxygen uptake kinetics during high-intensity arm and leg exercise

The purpose of the present study was to examine the oxygen uptake kinetics during heavy arm exercise using appropriate modelling techniques, and to compare the responses to those observed during heavy leg exercise at the same relative intensity. We hypothesised that any differences in the response might be related to differences in muscle fibre composition that are known to exist between the upper and lower body musculature. To test this, ten subjects completed several bouts of constant-load cycling and arm cranking exercise at 90% of the mode specific V(O(2)) peak. There was no difference in plasma [lactate] at the end of arm and leg exercise. The time constant of the fast component response was significantly longer in arm exercise compared to leg exercise (mean+/-S.D., 48+/-12 vs. 21+/-5 sec; P < 0.01), while the fast component gain was significantly greater in arm exercise (12.1+/-1.0 vs. 9.2+/-0.5 ml min(-1) W(-1); P < 0.01). The V(O(2)) slow component emerged later in arm exercise (126+/-27 vs. 95+/-20 sec; P < 0.01) and, in relative terms, increased more per unit time (5.5 vs. 4.4% min(-1); P < 0.01). These differences between arm crank and leg cycle exercise are consistent with a greater and/or earlier recruitment of type II muscle fibres during arm crank exercise.     

Tricarboxylic acid cycle intermediates accumulate at the onset of intense exercise in man but are not essential for the increase in muscle oxygen uptake

It was proposed that a contraction-induced increase in tricarboxylic acid cycle intermediates (TCAI) is obligatory for the increase in muscle oxygen uptake at the start of exercise. To test this hypothesis, we measured changes in muscle TCAI during the initial seconds of intense exercise and used dichloroacetate (DCA) in an attempt to alter the level of TCAI. Five men performed strenuous leg kicking exercise (64±8 W) under noninfused control (CON) and DCA-supplemented conditions; biopsies (vastus lateralis) were obtained at rest and after 5, 15, and 180 s of exercise. In CON, the total concentration of three measured TCAI (ΣTCAI: citrate, malate, and fumarate) increased (p<0.05) by 71% during the first 15 s of exercise. The ΣTCAI was lower (p<0.05) in DCA than in CON at rest [0.18±0.02 vs 0.64±0.09 mmol kg⁻¹ dry weight (d.w.)], after 5 s (0.30±0.07 vs 0.85±0.14 mmol kg⁻¹ d.w.), and 15 s of exercise (0.60±0.07 vs 1.09±0.16 mmol kg⁻¹ d.w.), but not different after 3 min (3.12±0.53 vs 3.23±0.55 mmol kg⁻¹ d.w.). Despite differences in the level of muscle TCAI, muscle phosphocreatine degradation was similar in DCA and CON during the first 15 s of exercise (17.5±3.3 vs 25.6±4.1 mmol kg⁻¹ d.w.). Taken together with our previous observation that DCA does not alter muscle oxygen uptake during the initial phase of intense leg kicking exercise (Bangsbo et al. Am J Physiol 282:R273–R280, 2002), the present data suggest that muscle TCAI accumulate during the initial seconds of exercise; however, this increase is not essential for the contraction-induced increase in mitochondrial respiration.