The influence of blood sampling site on lactate concentration during submaximal exercise at 4 mmol · 1−1 lactate level

This study examined lactate concentration during incremental and submaximal treadmill exercise at work rates corresponding to 4 mmol· 1^–1 lactate concentration, determined by fingertip (OBLAI) and venous blood (OBLA2). Initially, eight subjects performed a 4-min incremental exercise test until exhaustion. On two other occasions, seven of the subjects undertook submaximal exercise tests (30 min) at work rates corresponding to OBLA1 and OBLA2. Blood was simultaneously obtained from both sites at rest and at the end of each exercise stage during the incremental exercise, and at 5, 10, 20 and 30 min during the submaximal exercise and 5 min into recovery. Fingertip blood lactate concentrations were significantly higher (P<0.05) than venous blood at rest, throughout the incremental exercise, consistently during exercise at OBLA1 and OBLA2, and into recovery. Data also revealed an exercise intensity-dependent lactate difference between the two sampling sites during both exercise protocols. Exercise at OBLA1 did not result in a progressive increase in lactate level nor exhaustion, and the lactate value at the end of 30 min corresponded to the predetermined value. However, exercise at OBLA2 resulted in a significantly higher (P<0.05) lactate level than OBLA1, the lactate concentration at the end of 30 min was substantially higher than the predetermined value (P<0.05) and exhaustion was evident. It is concluded that the lactate concentration value during incremental and submaximal exercise (at 4 mmol·l^–1 OBLA) is dependent on the blood sampling site. This finding should be considered in studies concerned with the determination of OBLA.     

Effect of hypoxia on arterial and venous blood levels of oxygen,carbon dioxide,hydrogen ions and lactate during incremental forearm exercise

Summary The purpose of the present study was to investigate whether, in humans, hypoxia results in an elevated lactate production from exercising skeletal muscle. Under conditions of both hypoxia [inspired oxygen fraction (F_IO₂): 11.10%] and normoxia (F_IO₂: 20.94%), incremental exercise of a forearm was performed. The exercise intensity was increased every minute by 1.6 kg·m·min^–1 until exhaustion. During the incremental exercise the partial pressure of oxygen (PO₂) and carbon dioxide (PCO₂), oxygen saturation (SO₂), pH and lactate concentration [HLa] of five subjects, were measured repeatedly in blood from the brachial artery and deep veins from muscles in the forearm of both the active and inactive sides. The hypoxia (arterial SO₂ approximately 70%) resulted in (1) the difference in [HLa] in venous blood from active muscle (values during exercise — resting value) often being more than twice that for normoxia, (2) a significantly greater difference in venous-arterial (v-a) [HLa] for the exercising muscle compared to normoxia, and (3) a difference in v-a [HLa] for non-exercising muscle that was slightly negative during normoxia and more so with hypoxia. These studies suggest that lower O₂ availability to the exercising muscle results in increased lactate production.     

Blood lactate accumulation in intermittent supramaximal exercise

Summary Blood lactate accumulation rate and oxygen consumption have been studied in six trained male runners, aged 20 to 30 years. Subjects ran on a treadmill at a rate representing 172±5% for four 45 s sessions, separated by 9 min rest periods. Oxygen consumption was measured throughout. Blood lactate was determined in samples taken from the ear and was measured at the end of each exercise session, and two, five and nine minutes later. After the fourth exercise session, the same measurements were made every five min for 30 min. 4 subjects repeated a single exercise of the same type, duration and intensity and the same measurements were taken. With repetitive intermittent exercise, gradual increases in blood lactate concentration ([LA]b) occurred, whereas its rate of accumulation ([LA]b) decreased. The amount of oxygen consumed during each 45 s exercise session remained unchanged for a given subject. After cessation of intermittent exercise, the half-time of blood lactate was 26 min, whereas it was only 15 min after a single exercise session. values, on the other hand, returned to normal after 15 to 20 min. All other conditions being equal, the gradual decrease in [LA]b during intermittent exercise could be explained if the lactate produced during the first exercise session is used during the second period, and/or if the diffusion space of lactate increases. The diffusion space seems to be multicompartmental on the basis of half-time values noted for [LA]b after intermittent exercise, compared with those noted after a single exercise session. The distinction between the rapid return to normal values and the more gradual return to normal blood lactate levels confirms that there is no simple and direct relationship between oxygen debt and the accumulation of blood lactate after muscular exercise. In practical terms, these results show that the calorific equivalent of lactic acid defined by Margaria et al. (1963) cannot be used in the case of intermittent exercise of supramaximal intensity.     

Effect of exercise duration during incremental exercise on the determination of anaerobic threshold and the onset of blood lactate accumulation

Summary To determine the effect of the duration of incremental exercise on the point at which arterial blood lactate concentration (HLa) increases above the resting value (anaerobic threshold: AT) and on the point at which HLa reaches a constant value of 4 mM (onset of blood lactate accumulation: OBLA), eight male students performed two different kinds of incremental exercise. A comparison of arterial HLa and venous HLa was made under both conditions of incremental exercise. The incremental bicycle exercise tests consisted of 25 W increase every minute (1-min test) and every 4 min (4-min test). At maximal exercise, there were no significant differences in either gas exchange parameters or HLa values for the two kinds of incremental exercise. However, the peak workloads attained during the two exercises were significantly different (P<0.01). At OBLA and AT, there were no significant differences in gas exchange parameters during the 1-min and 4-min tests except for the workload (at OBLAP<0.01; at ATP<0.05). When venous blood HLa was used instead of arterial HLa for a 4-min test, AT was not significantly different from that obtained by arterial HLa, but OBLA was significantly different from that obtained by arterial HLa (P<0.05). On the other hand, for the 1-min test, venous HLa values yielded significantly higher AT and OBLA compared with those obtained using arterial HLa (P<0.01).It was concluded that when arterial blood was used, there was no effect of duration of workload increase in an incremental exercise test on the determination of the AT and OBLA expressed in . On the other hand, when venous HLa was used instead of arterial blood, these points might be overestimated when a fast increase in workload, such as the 1-min test, is used.     

Prolonged stage duration during incremental cycle exercise: effects on the lactate threshold and onset of blood lactate accumulation

The aim of this study was to investigate whether increasing the duration of workloads from 3 min to 8 min during incremental exercise would influence workload (W), oxygen consumption ( ) and heart rate (HR) at the lactate threshold (LT) and the onset of blood lactate accumulation(OBLA). Two groups of six male cyclists were assigned to a well-trained (WT) and recreational (REC) group on the basis of their performance in a maximal incremental ramp test. Each subject then performed two incremental lactate tests (EXT) consisting of six workloads of either 3 min (EXT_3-min) or 8 min (EXT_8-min) duration. At the completion of each workload whole capillary blood samples were obtained for the determination of blood lactate (BLa) concentration (mM). Power output (Watts, W), HR and were averaged in the final minute of each workload as well as in the third minute of the EXT_8-min. The workload, HR and at the LT and OBLA were subsequently determined from the data of EXT_3-min and EXT_8-min. The results demonstrate that workload and , but not HR, at the LT and OBLA were higher in the WT cyclists. At the same time, the workload at the LT obtained from the results of the EXT_3-min was significantly (P<0.05) higher then the value obtained in the EXT_8-min in the WT subjects but not the REC subjects. However, the workload, and HR at the OBLA, together with the and HR at the LT were not significantly different when calculated from data obtained from EXT_3-min or EXT_8-min. The data obtained in this study suggest that incremental exercise protocols using workloads of duration longer than 3 min have the effect of increasing the workload at the LT in well-trained cyclists. However, the OBLA determined in exercise tests using stage increments of either 3 min or 8 min is similar in cyclists of different training status. Electronic Publication     

Comparison of incremental and steady state tests of endurance training

Summary To compare the results obtained by incremental or constant work load exercises in the evaluation of endurance conditionning, a 20-week training programme was performed by 9 healthy human subjects on the bicycle ergometer for 1 h a day, 4 days a week, at 70–80% . Before and at the end of the training programme, (1) the blood lactate response to a progressive incremental exercise (18 W increments every 2nd min until exhaustion) was used to determine the aerobic and anaerobic thresholds (AeT and AnT respectively). On a different day, (2) blood lactate concentrations were measured during two sessions of constant work load exercises of 20 min duration corresponding to the relative intensities of AeT (1st session) and AnT (2nd session) levels obtained before training. A muscle biopsy was obtained from vastus lateralis at the end of these sessions to determine muscle lactate. AeT and AnT, when expressed as % , increased with training by 17% (p<0.01) and 9% (p<0.05) respectively. Constant workload exercise performed at AeT intensity was linked before training (60% ) to a blood lactate steady state (4.8±1.4 mmol·l^–1) whereas, after training, AeT intensity (73% ) led to a blood lactate accumulation of up to 6.6±1.7 mmol·l^–1 without significant modification of muscle lactate (7.6±3.1 and 8.2±2.8 mmol·kg^–1 wet weight respectively). It is concluded that increase in AeT with training may reflect transient changes linked to lower early blood lactate accumulation during incremental exercise. Nevertheless, the results obtained at the end of the constant work load exercises were assumed to be independant of these changes, the occurrence of blood lactate accumulation being postulated to reflect a decreased removal from the blood linked to a higher relative work intensity. So, the use of incremental exercise is an incomplete procedure when evaluating endurance training effects.     

两种不同运动方式血乳酸浓度对血管内溶血的影响

目的 观察运动中血乳酸浓度的变化对血管内溶血的影响。方法 受试者为大学体育系学生10名健康男子，通过功率自行车，进行渐增负荷和稳定负荷两种不同运动。最大负荷运动从1kp的强度开始，每1分钟增加0．25kp，直到力竭为止。稳定负荷运动为最大运动负荷时的最大心率的50％（50％HRmax)，进行30分钟的稳定负荷运动。在运动前、运动结束后即刻及运动后恢复期30分钟，由肘静脉采血。血液中指标有血乳酸、Ht和Hp。结果 最大负荷运动结束后血乳酸浓度明显高于运动前水平，运动结束1小时后血乳酸浓度恢复到运动前水平，可是在稳定负荷运动结束后却没有变化。此外，最大负荷运动结束后即刻Hp浓度下降，但没有显著性差异。结论 虽运动中血乳酸引起血pH下降，但并未引起溶血发生。     

Changes in the concentrations of Na+, K+ and Cl− in secretion from the skin during progressive increase in exercise intensity

Summary A simple method for sampling skin secretion in 1-min periods was developed for investigating the effects of progressive increases in exercise intensity on Na⁺, K⁺ and CI^– secretions from the skin of the forearm. Ten healthy male subjects performed exercise consisting of eight stepwise increases in intensity from 50 to 225 W, with a 25-W increase at each step. Exercise at each step was for 3 min followed by a 1-min recovery period. Samples of blood and skin secretion were taken during the recovery period. Significant positive correlations were found between the mean concentrations of Na⁺ and Cl^– and between those of K⁺ and Cl^– in the skin secretion. The concentrations of electrolytes in the skin secretion also showed significant correlations with the blood lactate concentrations. The inflection points for secretions of Na⁺, K⁺ and Cl^– were 4.04, 3.61 and 3.83 mmol · l^– of blood lactate; 64.42, 61.96 and 62.14% of maximal oxygen consumption ( ); and exercise intensities of 123.01, 117.65 and 125.07 W, respectively. No significant differences were observed between the value of 67.27% of or 134.00W at the onset of blood lactate accumulation (OBLA) and the inflection points. From these results we concluded that changes in electrolyte concentrations in skin secretion during incremental exercise according to this protocol were closely related with the change in the blood lactate concentration, and that the inflection points for electrolytes may have been near the exercise intensity at OBLA.     

Lactate concentration differences in plasma,whole blood,capillary finger blood and erythrocytes during submaximal graded exercise in humans

Summary The aim of the study was to investigate the distribution of lactate in plasma, whole blood, erythrocytes, and capillary finger blood, before and during submaximal exercise. Ten healthy male subjects performed submaximal graded cycle ergometer exercise for 20–25 min. Venous blood samples and capillary finger blood samples were taken before exercise and every 5th min during exercise for lactate determination. The plasma lactate concentration was significantly higher (P<0.001, approximately 50%) than in the erythrocytes. This difference was not altered by the venous blood lactate concentration or exercise intensity. A significant difference (P<0.01) in lactate concentration was also found between capillary whole blood and venous whole blood. It was concluded that direct comparisons between lactate in capillary finger blood, venous whole blood and plasma could not be made.The study was performed at the Department of Clinical Physiology, University Hospital, S-75185 Uppsala, Sweden     

Lactate in blood,mixed skeletal muscle,and FT or ST fibres during cycle exercise in man

The relationship between muscle and blood lactate levels during progressively step-wise incrementing cycle exercise has been investigated in 10 male subjects. Steps between power outputs during exercise were 50 W and each stage, from loadless pedalling until voluntary exhaustion, lasted 4 min. Blood samples and biopsies (m. vastus lateralis) were taken for lactate determination at each power output beginning with the exercise intensity perceived by the subject as being “rather moderate”. The ratio muscle: blood lactate was greater than one at all power outputs and increased most markedly at the power output closest to that eliciting 4 mmol × I^-1 blood lactate (W_OBLA). At W_OBLA. blood lactate was positively correlated to muscle lactate concentrations which covaried widely among subjects (mean 8.3. range 4.5–14.4 mmol × kg^-l wet weight). Muscle fibres from the W_OBLA biopsy in 6 subjects were dissected out and identified as fast twitch (FT) or slow twitch (ST). No significant difference in lactate concentration was observed between pools of FT or ST fibres.     

Cerebral oxygenation during intermittent supramaximal exercise

This study examined cerebral deoxygenation during intermittent supramaximal exercise in six healthy male subjects (age: 27.2 +/- 0.6 years (mean +/- S.E.). The subjects performed seven times exercise at an intensity corresponding to 150% of maximal oxygen uptake (VO2max) on cycle ergometer (30 s exercise/15 s rest). Cerebral oxygenation was measured by near-infrared spectroscopy (NIRS). The peak blood lactate concentration after exercise was 15.3 +/- 0.2 mmol/l. Cerebral oxygenation increased in first repetition compared with at rest (+ 5.7 +/- 0.6 microM; P < 0.05), but then decreased with time. Thus, in the last repetition cerebral oxygenation was - 8.5 +/- 0.4 microM (P < 0.05). There was no significant change in arterial oxygen saturation (99.6 +/- at rest, 98.4 +/- 0.2 at the final set of intermittent exercise), and there was no correlated change in end-tidal CO2 concentration with cerebral oxygenation (P > 0.05). These findings suggest that the fatigue resulting from dynamic severe exercise related to a decrease in the cerebral oxygenation level.     

The use of critical power as a determinant for establishing the onset of blood lactate accumulation

Eight highly trained male kayakers were studied to determine the relationship between critical power (CP) and the onset of blood lactate accumulation (OBLA). Four exercise sessions of 90 s, 240 s, 600 s, and 1200 s were used to identify the CP of each kayaker. Each individual CP was obtained from the line of best fit (LBF_CP) obtained from the progressive work output/time relationships. The OBLA was identified by the 4 mmol·l^–1 blood lactate concentration and the work output at this level was determined using a lactate curve test. This consisted of paddling at 50 W for 5 min after which a 1-min rest was taken during which a 25-l blood sample was taken to analyse for lactate. Exercise was increased by 50 W every 5 min until exhaustion, with the blood sample being taken in the 1-min rest period. The exercise intensity at the OBLA for each subject was then calculated and this was compared to the exercise intensity at the LBF_CP. The intensity at LBF_CP was found to be significantly higher (t=2.115, P<0.05) than that at the OBLA of 4 mmol·1^–1. These results were further confirmed by significant differences being obtained in blood lactate concentration (t=8.063, P<0.05) and heart rate values (t=2.90, P<0.05) obtained from the exercise intensity at LBF_CP over a 20-min period and that of the anaerobic threshold (Th_an) parameters obtained from the lactate/heart rate curve. These differences suggest that CP and Th_an are different physiological events and that athletes have utilised either one or the other methods for monitoring training and its effects.     

Temperature-induced changes in metabolic and hormonal responses to intensive dynamic exercise

Seven male subjects performed intensive cycle exercise to exhaustion at subnormal muscle temperature (T_m, 29 ± 2.8 °C). Exercise at exactly the same rate of exercise and duration (370 ± 34 W, 1.5 ± 0.15 min) was then repeated with normal T_m (35 ± 0.9 °C). During exercise both the arterial (a) and femoral venous (fv) contents of oxygen were significantly higher at subnormal than at normal T_m, because of the higher haemoglobin concentration, but the a-fv oxygen difference was the same in the two situations. The rate of increase in lactate concentration in both arterial and venous blood during exercise was the same in the two situations. During exercise the plasma concentrations of adrenaline and noradrealine in arterial and venous blood were significantly higher at subnormal than at normal T_m. At rest and after exercise the calf blood flow was significantly reduced at subnormal T_m At the end of exercise the concentrations of glucose-6-phosphate and lactate in the muscle were significantly higher at subnormal T_m than in the muscle of normal temperature. These findings suggest that there was a greater increase in glycolysis in the muscle of subnormal temperature during exercise, possibly as a result of impaired work efficiency and/or reduced blood flow in the cold muscle.     

The effect of different blood sampling sites and analyses on the relationship~ between exercise intensity and 4.0 mmol ·1− blood lactate concentration

Summary The aim of the study was to examine whether the difference in lactate concentration in different blood fractions is of practical importance when using blood lactate as a test variable of aerobic endurance capacity. Ten male firefighters performed submaximally graded exercise on a cycle ergometer for 20–25 min. Venous and capillary blood samples were taken every 5 min for determination of haematocrit and lactate concentrations in plasma, venous and capillary blood. At the same time, expired air was collected in Douglas bags for determination of the oxygen consumption. A lactate concentration of 4.0 mmol·1^–1 was used as the reference value toi compare the oxygen consumption and exercise intensity when different types of blood specimen and sampling sites were used for lactate analysis. At this concentration the exercise intensity was 17% lower (P<0.01) when plasma lactate was compared toi venous blood lactate, and 12% lower (P<0.05) when capillary blood lactate was used. Similar discrepancies were seen in oxygen consumption. The results illustrated the importance of standardizing sampling and handling of blood specimens for lactate determination to enable direct comparisons to be made among results obtained in different studies.The study was performed at the Department of Clinical Physiology University Hospital, S-75185 Uppsala, Sweden     

Gentle exercise with a previously inactive muscle group hastens the decline of blood lactate concentration after strenuous exercise

Summary The aim of this study was to elucidate the mechanism by which the disappearance of blood lactate following severe exercise is enhanced during active recovery in comparison with recovery at rest. Rates of decline of arterialised venous blood lactate concentrations in man after maximal one-leg exercise were compared during four different modes of recovery: passive (PR), exercise of the muscles involved in the initial exercise (SL), exercise of the corresponding muscles in the hitherto-inactive leg (OL), or exercise of one arm (RA). Recovery exercise workloads were each 40% of the onset of blood lactate accumulation (OBLA) for the limb used. In comparison with PR, SL and OL accelerated the fall in blood lactate to similar extents whereas RA was without effect. The first-order rate constant (min^–1) for decline of arterialised venous blood lactate concentration after the intense exercise was 0.027 (0.003) in PR, 0.058 (0.025) in SL, 0.034 (0.002) in OL, and in RA was 0.028 (0.002) [mean (SEM),n = 6 subjects]. Preliminary studies had shown that RA in isolation elevated blood lactate whereas SL and OL did not. Thus, with appropriate workloads, exercise of either hitherto active or passive muscles enhanced blood lactate decline during recovery from intense exercise. This suggests that the effect resulted principally from the uptake and utilisation of lactate in the circulation by those exercising muscles rather than from increased transport of lactate to other sites of clearance by sustained high blood flow through the previously active muscles.     

Reproducibility of the blood lactate threshold, 4 mmol·l<Superscript>–1</Superscript> marker,heart rate and ratings of perceived exertion during incremental treadmill exercise in humans

The aim of this study was to investigate the reproducibility of blood lactate measurements, heart rate (HR) and ratings of perceived exertion (RPE) during treadmill exercise at speeds corresponding to the lactate threshold (v _Th,la-) and a fixed blood lactate concentration of 4 mmol·l^–1(v _la-_,4). Possible differences in reproducibility related to fitness levels were also investigated. A group of 20 men [mean (SD)] [age 20.5 (1.4) years] and 16 women [age 21.2 (0.9) years] took part in the study. The subjects performed two identical incremental exercise tests consisting of at least six 4 min stages. Blood lactate concentrations, HR and RPE were recorded at the end of each stage. Limits of agreement (LoA), correlation coefficients and 95% confidence intervals for the mean difference between tests were employed to investigate the level of agreement and reproducibility of blood lactate concentration, HR and RPE. For the group as a whole, the sample correlation coefficient for speed at v _Th,la- was r=0.88, and was r=0.92 for the speed at v _la-_,4. At v _Th,la -, the correlation coefficients for the moderately fit and unfit were r=0.94 and r=0.36, respectively, and at v _la-_,4 r=0.93 and r=0.68, respectively. The LoA for the moderately fit group indicated that a change of 1.62 km·h^–1 in v _Th,la- would be necessary to be considered a change in training status. For HR and RPE, relationships between the tests were generally poor. The LoA suggested that changes in scores must be unacceptably large. These findings cast doubt on the sensitivity of testing for change of blood lactate concentration, HR and RPE in this population. Electronic Publication     

Day-to-day changes in oxygen uptake kinetics at the onset of exercise during strenuous endurance training

Summary The aim of this study was to assess the effect of strenuous endurance training on day-to-day changes in oxygen uptake (VO₂) on-kinetics (time constant) at the onset.of exercise. Four healthy men participated in strenuous training, for 30 min·day^–1, 6 days·week^–1 for 3 weeks. The VO₂ was measured breath-by-breath every day except Sunday at exercise intensities corresponding to the lactate threshold (LT) and the onset of blood lactate accumulation (OBLA) which were obtained before training. Furthermore, an incremental exercise test was performed to determine LT, OBLA and maximal oxygen uptake (VO_2max) before and after the training period and every weekend. The 30-min heavy endurance training was performed on a cycle ergometer 5 days·week^–1 for 3 weeks. Another six men served as the control group. After training, significant reductions of the VO₂ time constant for exercise at the pretraining LT exercise intensity (P<0.05) and at OBLA exercise intensity (P<0.01) were observed, whereas the VO₂ time constants in the control group did not change significantly. A high correlation between the decrease in the VO₂ time constant and training day was observed in exercise at the pretraining LT exercise intensity (r=–0.76; P<0.001) as well as in the OBLA exercise intensity (r= –0.91; P<0.001). A significant reduction in the blood lactate concentration during submaximal exercise and in the heart rate on-kinetics was observed in the training group. Furthermore, VO₂ at LT, VO₂ at OBLA and VO_2max increased significantly after training (P<0.05) but such was not the case in the control group. These findings indicated that within a few weeks of training a rapidly improved VO₂ on-kinetics may be observed. This may be explained. by some effect of blood lactate during exercise on VO₂ on-kinetics, together with significantly improved cardiovascular kinetics at the onset of exercise.     

Anaerobic threshold and maximal steady-state blood lactate in prepubertal boys

Summary To elucidate further the special nature of anaerobic threshold in children, 11 boys, mean age 12.1 years (range 11.4–12.5 years), were investigated during treadmill running. Oxygen uptake, including maximal oxygen uptake (VO_2max), ventilation and the ventilatory anaerobic threshold were determined during incremental exercise, with determination of maximal blood lactate following exercise. Within 2 weeks following this test four runs of 16-min duration were performed at a constant speed, starting with a speed corresponding to about 75% ofVO_2max and increasing it during the next run by 0.5 or 1.0 km·h^–1 according to the blood lactate concentrations in the previous run, in order to determine maximal steady-state blood lactate concentration. Blood lactate was determined at the end of every 4-min period. Anaerobic threshold was calculated from the increase in concentration of blood lactate obtained at the end of the runs at constant speed. The mean maximal steady-state blood lactate concentration was 5.0 mmol · 1^–1 corresponding to 88% of the aerobic power, whereas the mean value of the conventional anaerobic threshold was only 2.6 mmol · 1^–1, which corresponded to 78% of theVO_2max. The correlations between the parameters of anaerobic threshold, ventilatory anaerobic threshold and maximal steady-state blood lactate were only poor. Our results demonstrated that, in the children tested, the point at which a steeper increase in lactate concentrations during progressive work occurred did not correspond to the true anaerobic threshold, i.e. the exercise intensity above which a continuous increase in lactate concentration occurs at a constant exercise intensity.     

Changes in venous blood content from active and inactive hindlimb during isotonic exercise

Summary Venous blood samples were obtained from either exercising (n=9) or nonexercising (n=8) hindlimb during a progressive isotonic exercise in rabbits anesthetized with urethane and chloralose. Each experimental session consisted of 5-min non-exercise periods alternated with 6-min exercise periods, followed by a 10-min postexercise period. During each exercise period, stimulation of the distal stump of the right sciatic nerve at 1 Hz induced plantar flexions which lifted loads comparable to 2, 5, 8, 30, or 50% of an afterload at which only an isometric tension developed. Free-flowing venous blood samples were obtained before the first exercise period, during the last minute of each exercise period, and 10 min following the last exercise session. Increases in [Na⁺], [K⁺] and lactate concentration were obtained in blood from active limbs. Only lactate concentration increased in blood from non-exercising limbs, while [K⁺] decreased slightly. Inferences concerning the vascular volume response to this protocol would be quite different depending on the blood sampling site. Changes in blood from inactive tissue, further, may indicate only saturation of homeostatic mechanisms which normally compensate for vascular volume alterations initiated in active tissue.     

Utilization of blood-borne and intramuscular substrates during continuous and intermittent exercise in man.

1. Substrate utilization in the legs during bicycle exercise was studied in five subjects when performing intermittent intense exercise (15 sec work--15 sec rest) as well as continuous exercise during 60 min, with an almost identical average power output and oxygen uptake in both situations. 2. Muscle biopsies were obtained from vastus lateralis at rest, during, and after exercise in order to determine intramuscular lipid and carbohydrate utilization. The contribution from blood-borne substrates to total oxidative metabolism was determined by arterial-femoral venous (a-fv) differences for oxygen, FFA, glucose, and lactate and leg blood flow. 3. Intermittent and continuous exercise revealed a similar glycogen depletion and the intramuscular lactate accumulation was rather small. A similar uptake of blood-borne substrate (FFA, glucose) was found in both situations whereas a release of lactate only was observed in intermittent exercise. 4. ATP and CP levels oscillated between work and rest periods in intermittent exercise but were not resynthesized to resting levels at the end of the rest periods. The mainly aerobic energy release during each work period in intermittent exercise is partly caused by myoglobin functioning as an oxygen store; this factor was calculated to be more important than ATP and CP or lactate level oscillations. 5. The metabolic response to intermittent exercise was found to be similar to that found in continuous exercise with approximately the same average power output and oxygen uptake. This indicates that some factor in the intermediary metabolism, for instance citrate, functions as a regulator retarding glycolysis and favouring lipid utilization and an aerobic energy release in intermittent exercise.