Maximal lactate steady state,respiratory compensation threshold and critical power

Critical power (CP) and the second ventilatory threshold (VT₂) are presumed to indicate the power corresponding to maximal lactate steady state (MLSS). The aim of this study was to investigate the use of CP and VT₂ as indicators of MLSS. Eleven male trained subjects [mean (SD) age 23 (2.9) years] performed an incremental test (25 W·min⁻¹) to determine maximal oxygen uptake (V˙O_2max), maximal aerobic power (MAP) and the first and second ventilatory thresholds (VT₁ and VT₂) associated with break points in minute ventilation (V˙_E), carbon dioxide production (V˙CO₂), V˙_E/V˙CO₂ and V˙_E/V˙O₂ relationships. Exhaustion tests at 90%, 95%, 100% and 110% of V˙O_2max and several 30-min constant work rates were performed in order to determine CP and MLSS, respectively. MAP and V˙O_2max values were 344 (29) W and 53.4 (3.7) ml·min⁻¹·kg⁻¹, respectively. CP [278 (22) W; 85.4 (4.8)% V˙O_2max] and VT₂ power output [286 (28) W; 85.3 (5.6)% V˙O_2max] were not significantly different (p=0.96) but were higher (p<0.05) than the MLSS work rate [239 (21) W; 74.3 (4.0)% V˙O_2max] and VT₁ power output [159 (23) W; 52.9 (6.9)% V˙O_2max]. MLSS work rate was significantly correlated (p<0.05) with those noted at VT₁ and VT₂ (r=0.74 and r=0.93, respectively). VT₂ overestimated MLSS by 10.9 (6.3)% V˙O_2max which was significantly higher than VT₁ [+21.4 (5.6)% V˙O_2max; p<0.01]. CP calculated from a given range of exhaustion times does not correspond to MLSS. Electronic Publication     

Peak anaerobic power in patients with COPD: gender related differences

The aim of the study was to investigate peak anaerobic power during all-out exercise in patients with COPD. Twenty patients (ten women, ten men) [FEV₁ = 50.5 (7.6)% of predicted] and 11 healthy subjects (six women, five men) performed: (1) three maximal sprints on a cycle ergometer to measure peak anaerobic power (P _max) and optimal velocity (V _opt), (2) assessment of whole-body composition by dual-energy X-ray absorptiometry (DEXA) and (3) assessment of mean habitual daily energy expenditure (MHDEE). P _max was 30% lower in COPD than in healthy subjects [22.9 (7.1) vs. 32.8 (5.6) W kg⁻¹ _{legs FFM}, P < 0.001]. Nevertheless, V _opt was similar in both series. In COPD, P _max was lower in women than in men [21.4 (7.7) vs. 23.8(6.4) W kg⁻¹ _{legs FFM}, P < 0.05]. V _opt was lower in women than in COPD men [72.6 (11.3) vs. 89.3 (13.8) rpm, P < 0.05]. MHDEE was lower in COPD than in healthy subjects [8019 (1254) vs. 9093 (1660) kJ day⁻¹]. In COPD, MHDEE was lower in women than in men (P < 0.001). This study demonstrates that in COPD patients, the decrease in peak anaerobic power could play a role in their specific muscular dysfunction. Considerable differences were observed in peripheral muscle function, body composition and MHDEE between women and men. The skeletal muscle of women and men may therefore adapt to COPD in different ways.     

Influence of moderate hypoxia on tolerance to high-intensity exercise

It remains uncertain as how the reduction in systemic oxygen transport limits high-intensity exercise tolerance. 11 participants (5 males; age 35 ± 10 years; peak [(V)\dot]\textO₂ max {\dot{V}\text{O}}_{2} \max 3.5 ± 0.4 L min⁻¹) performed cycle ergometry to the limit of tolerance: (1) a ramp test to determine ventilatory threshold (VT) and peak [(V)\dot]\textO₂ {\dot{V}\text{O}}_{2} ; (2) three to four constant-load tests in order to model the linear P–t ⁻¹ relationship for estimation of intercept (critical power; CP) and slope (AWC). All tests were performed in a random order under moderate hypoxia (FiO₂ = 0.15) and normoxia. The linearity of the P–t ⁻¹ relationship was retained under hypoxia, with a systematic reduction in CP (220 ± 25 W vs. 190 ± 28 W; P < 0.01) but no significant difference in AWC (11.7 ± 5.5 kJ vs. 12.1 ± 4.4 kJ; P > 0.05). However, large individual variations in the change of the latter were observed (−36 to +66%). A significant relationship was found between the % change in CP (r = 0.80, P < 0.01) and both peak [(V)\dot]\textO₂ {\dot{V}\text{O}}_{2} (CP: r = −0.65, P < 0.05) and VT values recorded under normoxia (CP: r = −0.65, P < 0.05). The present study demonstrates the aerobic nature of the intercept of the P–t ⁻¹ relationship, i.e. CP. However, the extreme within-individual changes in AWC do not support the original assumption that AWC reflects a finite energy store. Lower hypoxia-induced decrements in CP were observed in aerobically fitter participants. This study also demonstrates the greater ability these participants have to exercise at supra-CP but close to CP workloads under moderate hypoxia.     

Similarity in physiological and perceived exertion responses to exercise at continuous and intermittent critical power

The purpose of this study was to compare the physiological responses [oxygen uptake (VO₂), heart rate (HR) and blood lactate concentrations ([BLa])] and the rating of perceived exertion (RPE) response until exhaustion (TTE) at the continuous (CP_c) and intermittent (CP_i) critical power workloads. Ten moderately active men (25.5 ± 4.2 years, 74.1 ± 8.0 kg, 177.6 ± 4.9 cm) participated in this study. The incremental test was applied to determine the highest values of oxygen uptake (VO_2max), heart rate (HR_max), blood lactate concentrations ([BLa_max]), and maximal aerobic power (MAP). Continuous and intermittent exhaustive predictive trials were performed randomly. The hyperbolic relation between power and time was used to estimate CP_c and CP_i. CP_i was derived from predictive trial results at an effort and recovery ratio of 30:30 s. Exercise at CP_c and CP_i as well as the physiological and RPE responses were measured until exhaustion. The values of physiological variables during CP_c and CP_i did not differ in either TTE test and were lower than the VO_2max, HR_max and [BLa_max] values. RPE was maximal at the end of exercise at CP_c and CP_i. There was a high correlation between VO_2max (L min⁻¹) and CP_c and CP_i intensities (r ≥ 0.90) and between MAP, CP_c and CP_i (r ≥ 0.95). Similar physiological and RPE responses were found at CP_c and CP_i for the times analyzed.     

Determination of critical power in trained rowers using a three-minute all-out rowing test

The purpose of this study was to determine whether the hyperbolic relationship between power output and time to exhaustion (work − time and power − [1/time] models) could be estimated from a modified version of a three-minute all-out rowing test (3-min RT), and to investigate the test–retest reliability of the 3-min RT. Eighteen male rowers volunteered to participate in this study and underwent an incremental exercise test (IRT), three constant-work rate tests to establish the critical power (CP) and the curvature constant (W′), and two 3-min RTs against a fixed resistance to estimate the end-test power (EP) and work-done-above-EP (WEP) on a rowing ergometer. Peak ( [(V)\dot]\textO _{2 \textpeak} ) \left( {\dot{V}{\text{O}}_{{ 2 {\text{peak}}}} } \right) and maximal ( [(V)\dot]\textO_2max ) \left( {\dot{V}{\text{O}}_{2\max } } \right) oxygen uptakes were calculated as the highest 30 s average achieved during the 3-min RT and IRT tests. The results showed that EP and WEP determinations, based on the 3-min RT, have moderate reproducibility (P = 0.002). EP (269 ± 39 W) was significantly correlated with CP (work − time, 272 ± 30 W; power − [1/time], 276 ± 32 W) (P = 0.000), with no significant differences observed between the EP and CP values (P = 0.474). However, WEP did not significantly correlate with W′ (P = 0.254), and was significantly higher than the W′ values. There was a significant correlation between the [(V)\dot]\textO _{2 \textpeak} \dot{V}{\text{O}}_{{ 2 {\text{peak}}}} (60 ± 3 ml kg⁻¹ min⁻¹) and [(V)\dot]\textO_2max \dot{V}{\text{O}}_{2\max } (61 ± 4 ml kg⁻¹ min⁻¹) (P = 0.003). These results indicate that the 3-min RT has moderate reliability, and is able to appropriately estimate the aerobic capacity in rowers, particularly for the CP and [(V)\dot]\textO_2max \dot{V}{\text{O}}_{2\max } parameters.     

Blood lactate response to overtraining in male endurance athletes

Many physiological markers vary similarly during training and overtraining. This is the case for the blood lactate concentration ([La⁻]_b), since a right shift of the lactate curve is to be expected in both conditions. We examined the possibility of separating the changes in training from those of overtraining by dividing [La⁻]_b by the rating of perceived exertion ([La⁻]_b/RPE) or by converting [La⁻]_b into a percentage of the peak blood lactate concentration ([La⁻]_b,peak). Ten experienced endurance athletes increased their usual amount of training by 100% within 4 weeks. An incremental test and a time trial were performed before (baseline) and after this period of overtraining, and after 2 weeks of recovery (REC). The [La⁻]_b and RPE were measured during the recovery of each stage of the incremental test. We diagnosed overtraining in seven athletes, using both physiological and psychological criteria. We found a decrease in mean [La⁻]_b,peak from baseline to REC [9.64 (SD 1.17), 8.16 (SD 1.31) and 7.69 (SD 1.84) mmol · l⁻¹, for the three tests, respectively; P < 0.05] and a right shift of the lactate curve. Above 90% of maximal aerobic speed (MAS) there was a decrease of mean [La⁻]_b/RPE from baseline to REC [at 100% of MAS of 105.41 (SD 17.48), 84.61 (SD 12.56) and 81.03 (SD 22.64) arbitrary units, in the three tests, respectively; P < 0.05), but no difference in RPE, its variability accounting for less than 25% of the variability of [La⁻]_b/RPE (r=0.49). Consequently, [La⁻]_b/RPE provides little additional information compared to [La⁻]_b alone. Expressing [La⁻]_b as a %[La⁻]_b,peak resulted in a suppression of the right shift of the lactate curve, suggesting it was primarily the consequence of a decreased production of lactate by the muscle. Since the right shift of the curve induced by optimal training is a result of improved lactate utilization, the main difference between the two conditions is the decrease of [La⁻]_b,peak during overtraining. We propose retaining it as a marker of overtraining for long duration events, and repeating its measurement after a sufficient period of rest to make the distinction with overreaching. Accepted: 26 September 2000     

Critical power in adolescents: physiological bases and assessment using all-out exercise

This study examined whether critical power (CP) in adolescents: (1) provides a landmark for maximal steady-state exercise; and (2) can be determined using ‘all-out’ exercise. Nine active 14–15 year olds (6 females, 3 males) performed five cycling tests: (1) a ramp test to determine [(V)\dot]\textO_2 \textpeak \dot{V}{\text{O}}_{{2\,{\text{peak}}}} ; (2) up to four constant power output tests to determine CP; (3–4) constant power output exercise 10% above and 10% below CP; and (5) a 3 min all-out cycle test to establish the end power (EP) at 90 and 180 s of exercise. All participants completed 30 min of exercise below CP and were characterized by steady-state blood lactate and [(V)\dot]\textO₂ {\dot{V}\text{O}}_{2} profiles. In contrast, time to exhaustion during exercise above CP was 15.0 ± 7.0 min and characterized by an inexorable rise in blood lactate and a rise, stabilization (~91% [(V)\dot]\textO_2 \textpeak {\dot{V}\text{O}}_{{2\,{\text{peak}}}} ) and fall in [(V)\dot]\textO₂ {\dot{V}\text{O}}_{2} (~82% [(V)\dot]\textO_2 \textpeak {\dot{V}\text{O}}_{{2\,{\text{peak}}}} ) prior to exhaustion. Eight out of nine participants completed the 3 min test and their EPs at 90 s (148 ± 29 W) and 180 s (146 ± 30 W) were not different from CP (146 ± 27 W) (P = 0.98). The typical error of estimates for establishing CP using EP at 90 s or 180 s of the 3 min test were 25 W (19.7% CV) and 25 W (19.6% CV), respectively. CP in active adolescence provides a valid landmark for maximal steady-state exercise, although its estimation on an individual level using the 3 min all-out test may be of limited value.     

Maximal voluntary hyperpnoea increases blood lactate concentration during exercise

Ventilatory work during heavy endurance exercise has not been thought to influence systemic lactate concentration. We evaluated the effect of maximal isocapnic volitional hyperpnoea upon arterialised venous blood lactate concentration ([lac⁻]_B) during leg cycling exercise at maximum lactate steady state (MLSS). Seven healthy males performed a lactate minimum test to estimate MLSS, which was then resolved using separate 30 min constant power tests (MLSS=207±8 W, mean ± SEM). Thereafter, a 30 min control trial at MLSS was performed. In a further experimental trial, the control trial was mimicked except that from 20 to 28 min maximal isocapnic volitional hyperpnoea was superimposed on exercise. Over 20–28 min minute ventilation, oxygen uptake, and heart rate during the control and experimental trials were 87.3±2.4 and 168.3±7.0 l min⁻¹ (P<0.01), the latter being comparable to that achieved in the maximal phase of the lactate minimum test (171.9±6.8 l min⁻¹), 3.46±0.20 and 3.83 ± 0.20 l min⁻¹ (P<0.01), and 158.5±2.7 and 166.8±2.7 beats min⁻¹ (P<0.05), respectively. From 20 to 30 min of the experimental trial [lac⁻]_B increased from 3.7±0.2 to 4.7±0.3 mmol l⁻¹ (P<0.05). The partial pressure of carbon dioxide in arterialised venous blood increased approximately 3 mmHg during volitional hyperpnoea, which may have attenuated the [lac⁻]_B increase. These results show that, during heavy exercise, respiratory muscle work may affect [lac⁻]_B. We speculate that the changes observed were related to the altered lactate turnover in respiratory muscles, locomotor muscles, or both.     

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.     

The effect of endurance training on the ventilatory response to exercise in elite cyclists

The purpose of this study was to investigate the effects of endurance training on the ventilatory response to acute incremental exercise in elite cyclists. Fifteen male elite cyclists [mean (SD) age 24.3 (3.3) years, height 179 (6) cm, body mass 71.1 (7.6) kg, maximal oxygen consumption (V˙O_2max) 69 (7) ml · min⁻¹ · kg⁻¹] underwent two exercise tests on a cycle ergometer. The first test was assessed in December, 6 weeks before the beginning of the cycling season. The second test was performed in June, in the middle of the season. During this period the subjects were expected to be in a highly endurance-trained state. The ventilatory response was assessed during an incremental exercise test (20 W · min⁻¹). Oxygen consumption (V˙O₂), carbon dioxide production (V˙CO₂), minute ventilation (V˙ _E), and heart rate (HR) were assessed at the following points during the test: at workloads of 200 W, 250 W, 300 W, 350 W, 400 W and at the subject's maximal workload, at a respiratory exchange ratio (R) of 1, and at the ventilatory threshold (Th_vent) determined using the V-slope-method. Post-training, the mean (SD) V˙O_2max was increased from the pre-training level of 69 (7) ml · min⁻¹ · kg⁻¹ (range 61.4–78.6) to 78 (6) ml · min⁻¹ · kg⁻¹ (range 70.5–86.3). The mean post-training V˙O₂ was significantly higher than the pre training value (P < 0.01) at all work rates, at Th_vent and at R=1. V˙O₂ was also higher at all work rates except for 200 W and 250 W. V˙ _E was significantly higher at Th_vent and R=1. Training had no effect on HR at all workloads examined. An explanation for the higher V˙O₂ cost for the same work rate may be that in the endurance-trained state, the adaptation to an exercise stimulus with higher intensity is faster than for the less-trained state. Another explanation may be that at the same work rate, in the less-endurance-trained state power is generated using a significantly higher anaerobic input. The results of this study suggest the following practical recommendations for training management in elite cyclists: (1) the V˙O₂ for a subject at the same work rate may be an indicator of the endurance-trained state (i.e., the higher the V˙O₂, the higher the endurance-trained capacity), and (2) the need for multiple exercise tests for determining the HR at Th_vent during a cycling season is doubtful since at Th_vent this parameter does not differ much following endurance training. Accepted: 19 October 1999     

Systemic oxygen extraction during incremental exercise in patients with severe chronic obstructive pulmonary disease

To determine if decreased systemic oxygen (O₂) extraction contributes to the exercise limit in severe chronic obstructive pulmonary disease (COPD), 40 consecutive incremental cycle ergometer exercise tests performed by such patients, from which a “log-log” lactate threshold (LT) was identified, were compared to those of 8 patients with left ventricular failure (LVF) and 10 normal controls. Pulmonary gas exchange and minute ventilation were measured continuously and arterial blood gas tensions, pH, and lactate concentrations were sampled each minute. Cardiac output (Q˙ _c) was measured by first-pass radionuclide ventriculography. The systemic O₂ extraction ratio (O₂ER) was calculated as arterial − mixed venous O₂ content difference (C _aO₂ − C _vO₂)/C _aO₂. Peak exercise O₂ uptake (V˙O_2peak) was markedly reduced in both COPD and LVF [41 (3) and 42 (3)% predicted, respectively], compared to controls [89 (2)% predicted, P < 0.0001 for each]. Similarly, the LT occurred at a low percentage of predicted maximal oxygen consumption in both COPD and LVF [25 (2) and 27 (3)%] compared to normals [46 (3)%, P < 0.0001 for each]. The systemic O₂ER at peak exercise was severely reduced in COPD [0.36 (0.02)] compared to the other groups [P < 0.0001 for each], for whom it was nearly identical [0.58 (0.03) vs 0.63 (0.04), LVF vs control, P > 0.05]. In the COPD group, an early LT correlated with reduced systemic O₂ER at peak exercise (r = 0.64, P < 0.0001), but not with any index of systemic O₂ delivery. These data suggest that lactic acidemia during exercise in patients with severe COPD is better related to abnormal systemic O₂ extraction than to its delivery and contributes to the exercise limit. Accepted: 10 March 1998     

Critical power of knee extension exercises does not depend upon maximal strength

A possible dependence of critical power (CP) and the Y-intercept of the work/exhaustion time relationship (Y _intercept) on maximal muscular strength of the same muscle group has been studied in nine endurance-trained subjects, seven gymnasts, and seven weight-lifters. CP was calculated as being equal to the slope of the linear relationship between exhaustion time and the work performed at exhaustion on a knee extension ergometer. Y _intercept was equal to the intercept between this relationship and the work axis. The muscular strength of the knee was evaluated by measuring the torques exerted on a Biodex knee isokinetic dynamometer at four angular velocities: 0° · s⁻¹ (T₀), 90° · s⁻¹ (T₉₀), 180° · s⁻¹ (T₁₈₀) and 240° · s⁻¹ (T₂₄₀). The results of the present study do not support the hypothesis that CP depends upon maximal strength. Indeed, CP was not correlated with T₀, T₉₀, T₁₈₀ or T₂₄₀ (|r| < 0.01). Y _intercept was significantly and positively correlated only with T₉₀. Accepted: 1 November 1999     

Influence of respiratory pressure support on hemodynamics and exercise tolerance in patients with COPD

Inspiratory pressure support (IPS) plus positive end-expiratory pressure (PEEP) ventilation might potentially interfere with the “central” hemodynamic adjustments to exercise in patients with chronic obstructive pulmonary disease (COPD). Twenty-one non- or mildly-hypoxemic males (FEV₁ = 40.1 ± 10.7% predicted) were randomly assigned to IPS (16 cmH₂O) + PEEP (5 cmH₂O) or spontaneous ventilation during constant-work rate (70–80% peak) exercise tests to the limit of tolerance (T _lim). Heart rate (HR), stroke volume (SV), and cardiac output (CO) were monitored by transthoracic cardioimpedance (Physioflow™, Manatec, France). Oxyhemoglobin saturation was assessed by pulse oximetry (SpO₂). At similar SpO₂, IPS₁₆ + PEEP₅ was associated with heterogeneous cardiovascular effects compared with the control trial. Therefore, 11 patients (Group A) showed stable or increased Δ “isotime” – rest SV [5 (0–29) mL], lower ΔHR but similar ΔCO. On the other hand, ΔSV [−10 (−15 to −3) mL] and ΔHR were both lower with IPS₁₆ + PEEP₅ in Group B (N = 10), thereby reducing ΔCO (p < 0.05). Group B showed higher resting lung volumes, and T _lim improved with IPS₁₆ + PEEP₅ only in Group A [51 (−60 to 486) vs. 115 (−210 to 909) s, respectively; p < 0.05]. We conclude that IPS₁₆ + PEEP₅ may improve SV and exercise tolerance in selected patients with advanced COPD. Impaired SV and CO responses, associated with a lack of enhancement in exercise capacity, were found in a sub-group of patients who were particularly hyperinflated at rest.     

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

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

Endothelial function in aorta segments of apolipoprotein E-deficient mice before development of atherosclerotic lesions

Acetylcholine (ACh)-induced relaxation declines in apolipoprotein E-deficient (apoE^−/−) mouse aortas, but only after atherosclerotic plaque formation. This study investigated intracellular calcium concentrations [Ca²⁺]_i and changes in phenylephrine-induced contractions as index of baseline nitric oxide (NO) bioavailability before plaque development. Isometric contractions of thoracic aorta rings of young (4 months) apoE^−/− and C57BL/6J (WT) mice were evoked by phenylephrine (3 × 10⁻⁹–3 × 10⁻⁵ M) in the presence and absence of endothelial cells (ECs) or NO synthase (NOS) inhibitors. [Ca²⁺]_i (Fura-2 AM) and endothelium-dependent relaxation were measured at baseline and after ACh stimulation. Segments of apoE^−/− mice were significantly more sensitive and developed more tension than WT segments in response to phenylephrine. The differences disappeared after NOS inhibition or EC removal or upon increasing [Ca²⁺]_i in apoE^−/− strips with 10⁻⁶ M cyclopiazonic acid or 10⁻⁷ M Ca²⁺-ionophore A23187. Expression of endothelial NOS (eNOS) mRNA was similar in apoE^−/− and WT aorta segments. Basal [Ca²⁺]_i was significantly lower in apoE^−/− than in WT strips. Relaxation by ACh (3 × 10⁻⁹–10⁻⁵ M) was time- and dose-dependently related to [Ca²⁺]_i, but neither ACh-induced relaxation nor Ca²⁺ mobilization were diminished in apoE^−/− strips. In conclusion, basal, but not ACh-induced NO bioavailability, was compromised in lesion-free aorta of apoE^−/− mice. Decreased basal NO bioavailability was not related to lower eNOS expression, but most likely related to lower basal [Ca²⁺]_i. These findings further point to important differences between basal and stimulated eNOS activity.     

Kick frequency affects the energy cost of aquatic locomotion in elite monofin swimmers

The aim of this study was to evaluate the influence of kick frequency (K _F) on the energy cost of aquatic locomotion in elite monofin (Mf) swimmers at the surface. Eight subjects of international calibre (4 females, 4 males) were requested to perform in a 50-m outdoor swimming pool: (1) a continuous multi-stage incremental test to determine maximal physiological responses and (2) a submaximal exercise composed of five constant Mf-swimming tests (600-m exercise, 5-min rest) at an intensity corresponding to 90% of the velocity at the maximal oxygen uptake ( [(V)\dot]\textO_2max \dot{V}{\text{O}}_{2\max } ). The first submaximal Mf-swimming test was systematically conducted at a freely chosen K _F (FCK_F) and the other tests were performed at FCK_F − 15%, FCK_F − 10%, FCK_F + 10% and FCK_F + 20% in a random order. No significant effect of K _F on ventilation, heart rate and blood lactate concentration was observed throughout the submaximal Mf-swimming tests. However, mean values in Ec or fraction of [(V)\dot]\textO_2max \dot{V}{\text{O}}_{2\max } were significantly lower during the FCK_F + 10% condition as compared to those observed during the FCK_F − 15% (−11.5 and −9.6%, respectively, P < 0.05) and FCK_F − 10% (−10.4 and −9.3%, respectively, P < 0.05) conditions. In conclusion, the lack of significant differences between FCKF + 10% and FCK_F or FCK_F + 20% does not allow to identify a specific trend, but suggests the occurrence of an energetically optimal K _F close to that freely chosen by the Mf swimmers. Variations in muscle activity level and active drag have been hypothesized to explain the observed differences in Ec consecutive to the selection of various K _F.     

Comparison of cardiac output determined by different rebreathing methods at rest and at peak exercise

Several rebreathing methods are available for cardiac output (Q _T) measurement. The aims of this study were threefold: first, to compare values for resting Q _T produced by the equilibrium-CO₂, exponential-CO₂ and inert gas-N₂O rebreathing methods and, second, to evaluate the reproducibility of these three methods at rest. The third aim was to assess the agreement between estimates of peak exercise Q _T derived from the exponential and inert gas rebreathing methods. A total of 18 healthy subjects visited the exercise laboratory on different days. Repeated measures of Q _T, measured in a seated position, were separated by a 5 min rest period. Twelve participants performed an incremental exercise test to determine peak oxygen consumption. Two more exercise tests were used to measure Q _T at peak exercise using the exponential and inert gas rebreathing methods. The exponential method produced significantly higher estimates at rest (averaging 10.9 l min⁻¹) compared with the equilibrium method (averaging 6.6 l min⁻¹) and the inert gas rebreathing method (averaging 5.1 l min⁻¹; P < 0.01). All methods were highly reproducible with the exponential method having the largest coefficient of variation (5.3%). At peak exercise, there were non-significant differences between the exponential and inert gas rebreathing methods (P = 0.14). The limits of agreement were −0.49 to 0.79 l min⁻¹. Due to the ability to evaluate the degree of gas mixing and to estimate intra-pulmonary shunt, we believe that the inert gas rebreathing method has the potential to measure Q _T more precisely than either of the CO₂ rebreathing methods used in this study. At peak exercise, the exponential and inert gas rebreathing methods both showed acceptable limits of agreement.     

Effects of endurance training on intracellular calcium concentration in T lymphocytes

The purpose of the present study was to determine whether 12 months of endurance training reduced [Ca²⁺]i in T helper (CD4⁺) lymphocytes in trained (TR) men compared to untrained (UT). Fourteen trained (Ironman triathletes) and nine untrained (sedentary) men volunteered for the study. The TR group averaged 12 km of swimming, 300 km of cycling and 60 km of running per week during the year. Resting blood samples were taken from TR (VO_2peak 64 ± 2 ml kg⁻¹ min⁻¹) and UT (VO_2peak 42 ± 2 ml kg⁻¹ min⁻¹) subjects every 4 weeks for 52 weeks (October 1, 1999–October 1, 2000). Leukocyte concentration was measured using a full blood count. Unstimulated CD4⁺ lymphocytes were separated and analysed for changes in free ([Ca²⁺]i) and total ([Ca²⁺]t) calcium using flow cytometry. There were no significant differences in leukocyte concentration between UT and TR groups. There were significant differences between TR and UT in [Ca²⁺]i (October B and November), and [Ca²⁺]t (January and March). There were also significant sequential monthly changes in both [Ca²⁺]i and [Ca²⁺]t for TR and UT groups during the study. Significant increases in [Ca²⁺]i and [Ca²⁺]t during summer (January and March) for both TR and UT groups suggest an increase in intracellular signalling during hot weather. [Ca²⁺]i and [Ca²⁺]t were significantly lower in TR lymphocytes during November and March, suggesting that endurance training during warmer months may decrease [Ca²⁺]i through altered intracellular signalling, possibly to maintain lymphocyte function during heat stress.     

Local critical power is an index of local endurance

The hypothesis that critical power (CP) is significantly lower than the maximal aerobic power of the knee extensors has been tested in nine endurance-trained subjects, seven gymnasts and seven weight lifters. CP was calculated as being equal to the slope of the linear relationship between exhaustion time and work performed at exhaustion on a knee-extension ergometer. CP was compared with the power output at the end of a progressive knee-extension exercise (P _peak) and the power outputs corresponding to exhaustion times equal to 4 (P _4 min), 6 (P _6 min), 8 (P _8 min) and 10 min (P _10 min), calculated according to the linear relationship between work and exhaustion time. The hypothesis that CP corresponds to a steady state in metabolic and physiological parameters was tested in the gymnasts and the weight lifters by comparing CP with the fatigue thresholds of the integrated electromyogram (iEMG_FT), lactate level (La_FT), oxygen uptake (V˙O_2FT) and heart rate (HR_FT). The results of the present study demonstrate that the value of CP of a local exercise cannot be considered as the equivalent of the maximal aerobic power for general exercises. The values of P _4 min, P _6 min, P _8 min, P _10 min and P _peak were significantly higher than CP, and corresponded to 138, 126, 119, 115 and 151% CP, respectively. The results of the present study indicate that CP can be considered as an index of muscular endurance. Indeed, La_FT, iEMG _FT, V˙O_2FT and HR_FT were not significantly different from CP. All of these fatigue thresholds were significantly correlated with CP (r > 0.92). Moreover, the highest coefficient of correlation (r=0.71; P < 0.01) between the percentage of maximal aerobic power in cycling that corresponds to a blood lactate concentration of 4 mmol · l⁻¹ (OBLA%) and the different local aerobic indices was observed with CP. Received: 22 February 1999 / Accepted: 16 June 1999     

Breathing pattern and exercise endurance time after exhausting cycling or breathing

The aim of the present study was to investigate whether the changes in breathing pattern that frequently occur towards the end of exhaustive exercise (i.e., increased breathing frequency, f _b, with or without decreased tidal volume) may be caused by the respiratory work itself rather than by leg muscle work. Eight healthy, trained subjects performed the following three sessions in random order: (A) two sequential cycling endurance tests at 78% peak O₂ consumption (V˙O_2peak) to exhaustion (A1, A2); (B) isolated, isocapnic hyperpnea (B1) at a minute ventilation (V˙ _E) and an exercise duration similar to that attained during a preliminary cycling endurance test at 78% V˙O_2peak, followed by a cycling endurance test at 78% V˙O_2peak (B2); (C) isolated, isocapnic hyperpnea (C1) at a V˙ _E at least 20% higher than that of the preliminary cycling test and the same exercise duration as the preliminary cycling test, followed by a cycling endurance test at 78% V˙O_2peak (C2). Neither of the two isocapnic hyperventilation tasks (B1 or C1) affected either the breathing pattern or the endurance times of the subsequent cycling tests. Only cycling test A2 was significantly shorter [mean (SD) 26.5 (8.3) min] than tests A1 [41.0 (9.0) min], B2 [41.9 (6.0) min], and C2 [42.0 (7.5) min]. In addition, compared to test A1, only the breathing pattern of test A2 was significantly different [i.e., V˙ _E: +10.5 (7.6) l min⁻¹, and f _b: +12.1 (8.5) breaths min⁻¹], in contrast to the breathing patterns of cycling tests B2 [V˙ _E: −2.5 (6.2) l min⁻¹, f _b: +0.2 (3.6) breaths min⁻¹] and C2 [V˙ _E: −3.0 (7.0) l min⁻¹, f _b: +0.6 (6.1) breaths min⁻¹]. In summary, these results suggest that the changes in breathing pattern that occur towards the end of an exhaustive exercise test are a result of changes in the leg muscles rather than in the respiratory muscles themselves. Accepted: 7 October 1999