Effect of posture on high-intensity constant-load cycling performance in men and women

The time sustained during a graded cycle exercise is ~10% longer in an upright compared with a supine posture. However, during constant-load cycling this effect is unknown. Therefore, we tested the postural effect on the performance of high-intensity constant-load cycling. Twenty-two active subjects (11 men, 11 women) performed two graded tests (one upright, one supine), and of those 22, 10 subjects (5 men, 5 women) performed three high-intensity constant-load tests (one upright, two supine). To test the postural effect on performance at the same absolute intensity, during the upright and one of the supine constant-load tests subjects cycled at 80% of the peak power output achieved during the upright graded test. To test the postural effect on performance at the same relative intensities, during the second supine test subjects cycled at 80% of the peak power output achieved during the supine graded test. Exercise time on the graded and absolute intensity constant-load tests for all subjects was greater (P<0.05) in the upright compared with supine posture (17.9±3.5 vs. 16.1±3.1 min for graded; 13.2±8.7 vs. 5.2±1.9 min for constant-load). This postural effect at the same absolute intensity was larger in men (19.4±8.5 upright vs. 6.6±1.6 supine, P<0.001) than women (7.1±2 upright vs. 3.9±1.4 supine, P>0.05) and it was correlated (P<0.05) with both the difference in between positions during the first minute of exercise (r=0.67) and the height of the subjects (r=0.72). In conclusion, there is a very large postural effect on performance during constant-load cycling exercise and this effect is significantly larger in men than women.     

Cerebral oxygenation is reduced during hyperthermic exercise in humans

Aim: Cerebral mitochondrial oxygen tension (P_mitoO₂) is elevated during moderate exercise, while it is reduced when exercise becomes strenuous, reflecting an elevated cerebral metabolic rate for oxygen (CMRO₂) combined with hyperventilation-induced attenuation of cerebral blood flow (CBF). Heat stress challenges exercise capacity as expressed by increased rating of perceived exertion (RPE). Methods: This study evaluated the effect of heat stress during exercise on P_mitoO₂ calculated based on a Kety-Schmidt-determined CBF and the arterial-to-jugular venous oxygen differences in eight males [27 ± 6 years (mean ± SD) and maximal oxygen uptake (VO_2max) 63 ± 6 mL kg⁻¹ min⁻¹]. Results: The CBF, CMRO₂ and P_mitoO₂ remained stable during 1 h of moderate cycling (170 ± 11 W, ∼50% of VO_2max, RPE 9–12) in normothermia (core temperature of 37.8 ± 0.4 °C). In contrast, when hyperthermia was provoked by dressing the subjects in watertight clothing during exercise (core temperature 39.5 ± 0.2 °C), P_mitoO₂ declined by 4.8 ± 3.8 mmHg (P < 0.05 compared to normothermia) because CMRO₂ increased by 8 ± 7% at the same time as CBF was reduced by 15 ± 13% (P < 0.05). During exercise with heat stress, RPE increased to 19 (19–20; P < 0.05); the RPE correlated inversely with P_mitoO₂ (r² = 0.42, P < 0.05). Conclusion: These data indicate that strenuous exercise in the heat lowers cerebral P_mitoO₂, and that exercise capacity in this condition may be dependent on maintained cerebral oxygenation.     

The effect of acute hypoxia on left ventricular function during exercise

The effect of acute hypoxia on the human left ventricular function during exercise was evaluated by 2D and Doppler echocardiography on 11 healthy male college students. Each subject completed 6-min moderate intensity (100 W) supine cycling exercises in normoxia and hypoxia, respectively. The concentration of inspired O₂ was adjusted to keep arterial hemoglobin O₂ concentration (SpO₂) at 88–92% during hypoxia. Doppler indices obtained were compared between normoxia and hypoxia. The left ventricular myocardial diastolic function was increased during exercise in hypoxia compared with normoxia. The peak velocity of early filling wave increased at rest (P < 0.05) and during exercise (P < 0.05 at second minute, and P < 0.01 at sixth minute) in hypoxia. The heart rate (P < 0.01) and cardiac output (P < 0.001) were elevated markedly at rest during hypoxia. The left ventricular systolic function variables, such as stroke volume, ejection fraction, and end-systolic volume were relatively unaltered during hypoxia compared with normoxia. The results suggest that acute hypoxia increases the left ventricular myocardial diastolic function during moderate intensity supine cycling exercise without affecting the systolic function.     

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 sustainability of VO2max: effect of decreasing the workload

The study examined the maintenance of VO_2max using VO_2max as the controlling variable instead of power. Therefore, ten subjects performed three exhaustive cycling exercise bouts: (1) an incremental test to determine VO_2max and the minimal power at VO_2max (PVO_max), (2) a constant-power test at PVO_max and (3) a variable-power test (VPT) during which power was varied to control VO₂ at VO_2max. Stroke volume (SV) was measured by impedance in each test and the stroke volume reserve was calculated as the difference between the maximal and the average 5-s SV. Average power during VPT was significantly lower than PVO_max (238 ± 79 vs. 305 ± 86 W; p < 0.0001). All subjects, regardless of their VO_2max values and/or their ability to achieve a VO_2max plateau during incremental test, were able to sustain VO_2max for a significantly longer time during VPT compared to constant-power test (CPT) (958 ± 368 s vs. 136 ± 81 s; p < 0.0001). Time to exhaustion at VO_2max during VPT was correlated with the power drop in the first quarter of the time to exhaustion at VO_2max (r = 0.71; p < 0.02) and with the stroke volume reserve (r = 0.70, p = 0.02) but was not correlated with VO_2max. This protocol, using VO_2max rather than power as the controlling variable, demonstrates that the maintenance of exercise at VO_2max can exceed 15 min independent of the VO_2max value, suggesting that the ability to sustain exercise at VO_2max has different limiting factors than those related to the VO_2max value.     

Pathophysiological adaptations to walking and cycling in primary pulmonary hypertension

Exercise tolerance inversely correlates with the severity of the disease in patients with idiopathic pulmonary arterial hypertension (IPAH). Cycling and walking protocols are commonly utilized in the evaluation of exercise intolerance in IPAH, but little information exists on possible differences in ventilatory and gas exchange adaptations to these exercise modalities. In a group of patients with moderate to severe IPAH (n = 13), we studied the ventilatory, cardiovascular and gas exchange adaptations to maximal incremental walking (W) and maximal incremental cycling (C). During W, compared to C, the ventilatory equivalents for CO₂ output (V′_E/V′CO₂) were significantly higher either expressed as the rate of increment (56 ± 5 vs. 45 ± 3; P < 0.0001) or as the absolute values at anaerobic threshold (AT) and at peak exercise. At AT, the increase in V′_E/V′CO₂ during W was associated with a significant lower value of end-tidal carbon dioxide. At peak W, compared to peak C, dyspnea sensation was higher and arterial oxygen saturation (SpO₂) was lower (87 ± 2 vs. 91 ± 2, P < 0.001). In patients with IPAH the physiologic information obtained with W are different from those obtained with C. Tolerance to W exercise is limited by high ventilatory response and dyspnea sensation. W should be used to assess the degree of lung gas exchange inefficiency and arterial O₂ desaturation during exercise.     

A low carbohydrate diet affects autonomic modulation during heavy but not moderate exercise

The aim of this study was to examine the effects of low carbohydrate (CHO) availability on heart rate variability (HRV) responses during moderate and severe exercise intensities until exhaustion. Six healthy males (age, 26.5 ± 6.7 years; body mass, 78.4 ± 7.7 kg; body fat %, 11.3 ± 4.5%; [(V)\dot] \textO_{2 max} , \dot{V} {\text{O}}_{{2{ \max }}} , 39.5 ± 6.6 mL kg⁻¹ min⁻¹) volunteered for this study. All tests were performed in the morning, after 8–12 h overnight fasting, at a moderate intensity corresponding to 50% of the difference between the first (LT₁) and second (LT₂) lactate breakpoints and at a severe intensity corresponding to 25% of the difference between the maximal power output and LT₂. Forty-eight hours before each experimental session, the subjects performed a 90-min cycling exercise followed by 5-min rest periods and subsequent 1-min cycling bouts at 125% [(V)\dot] \textO_{2 max} \dot{V} {\text{O}}_{{2{ \max }}} (with 1-min rest periods) until exhaustion, in order to deplete muscle glycogen. A diet providing 10% (CHO_low) or 65% (CHO_control) of energy as carbohydrates was consumed for the following 2 days until the experimental test. The Poicaré plots (standard deviations 1 and 2: SD1 and SD2, respectively) and spectral autoregressive model (low frequency LF, and high frequency HF) were applied to obtain HRV parameters. The CHO availability had no effect on the HRV parameters or ventilation during moderate-intensity exercise. However, the SD1 and SD2 parameters were significantly higher in CHO_low than in CHO_control, as taken at exhaustion during the severe-intensity exercise (P < 0.05). The HF and LF frequencies (ms²) were also significantly higher in CHO_low than in CHO_control (P < 0.05). In addition, ventilation measured at the 5 and 10-min was higher in CHO_low (62.5 ± 4.4 and 74.8 ± 6.5 L min⁻¹, respectively, P < 0.05) than in CHO_control (70.0 ± 3.6 and 79.6 ± 5.1 L min⁻¹, respectively; P < 0.05) during the severe-intensity exercise. These results suggest that the CHO availability alters the HRV parameters during severe-, but not moderate-, intensity exercise, and this was associated with an increase in ventilation volume.     

Difference in human cardiovascular response between upright and supine recovery from upright cycle exercise

Cardiovascular responses were examined in seven healthy male subjects during 10 min of recovery in the upright or supine position following 5 min of upright cycle exercise at 80% peak oxygen uptake. An initial rapid decrease in heart rate (f _c) during the early phase of recovery followed by much slower decrease was observed for both the upright and supine positions. The average f _c at the 10th min of recovery was significantly lower (P < 0.05) in the supine position than in the upright position, while they were both significantly greater than the corresponding pre-exercise levels (each P < 0.05). Accordingly, the amplitude of the high frequency (HF) component of R-R interval variability (by spectrum analysis) in both positions was reduced with a decrease in mean R-R interval, the relationship being expressed by a regression line – mean R-R interval = 0.006 × HF amplitude + 0.570 (r = 0.905, n = 28, P < 0.001). These results would suggest that the slower reduction in f _c following the initial rapid reduction in both positions is partly attributable to a retardation in the restoration of the activity of the cardiac parasympathetic nervous system. Post-exercise upright stroke volume (SV, by impedance cardiography) decreased gradually to just below the pre-exercise level, whereas post-exercise supine SV increased markedly to a level similar to that at rest before exercise. The resultant cardiac output (Q˙ _c) and the total peripheral vascular resistance (TPR) in the upright and supine positions returned gradually to their respective pre-exercise levels in the corresponding positions. At the 10th min of recovery, both average SV and Q˙ _c were significantly greater (each P < 0.005) in the supine than in the upright position, while average TPR was significantly lower (P < 0.05) in the supine than in the upright position. In contrast, immediately after exercise, mean blood pressure dropped markedly in both the supine and upright positions, and their levels at the 10th min of recovery were similar. Therefore we concluded that arterial blood pressure is maintained relatively constant through various compensatory mechanisms associated with f _c, SV, Q˙ _c, and TPR during rest and recovery in different body positions. Accepted: 4 September 1999     

Exercise performance and \dot{V}{\text{O}}_{ 2} kinetics during upright and recumbent high-intensity cycling exercise

This study investigated cycling performance and oxygen uptake ( [(V)\dot]\textO ₂ )( \dot{V}{\text{O}}_{ 2} ) kinetics between upright and two commonly used recumbent (R) postures, 65°R and 30°R. On three occasions, ten young active males performed three bouts of high-intensity constant-load (85% peak-workload achieved during a graded test) cycling in one of the three randomly assigned postures (upright, 65°R or 30°R). The first bout was performed to fatigue and second and third bouts were limited to 7 min. A subset of seven subjects performed a final constant-load test to failure in the supine posture. Exercise time to failure was not altered when the body inclination was lowered from the upright (13.1 ± 4.5 min) to 65°R (10.5 ± 2.7 min) and 30°R (11.5 ± 4.6 min) postures; but it was significantly shorter in the supine posture (5.8 ± 2.1 min) when compared with the three inclined postures. Resulting kinetic parameters from a tri-exponential analysis of breath-by-breath [(V)\dot]\textO ₂ \dot{V}{\text{O}}_{ 2} data during the first 7 min of exercise were also not different between the three inclined postures. However, inert gas rebreathing analysis of cardiac output revealed a greater cardiac output and stroke volume in both recumbent postures compared with the upright posture at 30 s into the exercise. These data suggest that increased cardiac function may counteract the reduction of hydrostatic pressure from upright ~25 mmHg; to 65°R ~22 mmHg; and 30°R ~18 mmHg such that perfusion of active muscle presumably remains largely unchanged, and also therefore, [(V)\dot]\textO ₂ \dot{V}{\text{O}}_{ 2} kinetics and performance during high-intensity cycling.     

Effect of body tilt angle on fatigue and EMG activities in lower limbs during cycling

This study compared the rate of fatigue and lower limb EMG activities during high-intensity constant-load cycling in upright and supine postures. Eleven active males performed seven cycling exercise tests: one upright graded test, four fatigue tests (two upright, two supine) and two EMG tests (one upright, one supine). During the fatigue tests participants initially performed a 10 s all-out effort followed by a constant-load test with 10 s all-out bouts interspersed every minute. The load for the initial two fatigue tests was 80% of the peak power (PP) achieved during the graded test and these continued until failure. The remaining two fatigue tests were performed at 20% PP and were limited to the times achieved during the 80% PP tests. During the EMG tests subjects performed a 10 s all-out effort followed by a constant-load test to failure at 80% PP. Normalised EMG activities (% maximum, NEMG) were assessed in five lower limb muscles. Maximum power and maximum EMG activity prior to each fatigue and EMG test were unaffected by posture. The rate of fatigue at 80% PP was significantly higher during supine compared with upright posture (−68 ± 14 vs. −26 ± 6 W min⁻¹, respectively, P < 0.05) and the divergence of the fatigue responses occurred by the second minute of exercise. NEMG responses were significantly higher in the supine posture by 1–4 min of exercise. Results show that fatigue is significantly greater during supine compared with upright high-intensity cycling and this effect is accompanied by a reduced activation of musculature that is active during cycling.     

Alterations in neuromuscular function and perceptual responses following acute eccentric cycling exercise

Previous investigators have reported velocity-dependent strength loss for single-joint actions following acute eccentric exercise. The extent to which velocity influences recovery of multi-joint function is not well documented. Our main purpose was to compare alterations in maximal cycling power produced across a range of pedaling rates following eccentric exercise. An additional purpose was to determine the extent to which changes in rating of perceived exertion (RPE) associated with submaximal cycling reflect changes in maximal cycling power. Eighteen cyclists performed baseline trials of maximal and submaximal single-leg concentric cycling immediately before and 24 and 48 h after acute submaximal single-leg eccentric (151 ± 32 W, 487 ± 107 s) and concentric (148 ± 21 W, 488 ± 79 s) cycling trials. Maximum cycling power (apex of power–pedaling rate relationship; P _max) was assessed using inertial-load cycling, and powers produced at 65, 110 and 155 rpm were also analyzed. Compared to baseline, P _max was reduced (11–13%) at 24–48 h in the eccentric leg (P < 0.001). Power produced at 65, 110 and 155 rpm was reduced by similar relative magnitudes (11–15%) at 24–48 h in the eccentric leg. RPE increased (15–18%) at 24–48 h in the eccentric leg (P < 0.001). Magnitudes of relative changes in RPE did not differ from those for P _max. There were no alterations in the concentric leg. Our results indicated a global, rather than velocity-specific, reduction in neuromuscular function. Such a global reduction does not support the notion of fiber-type specific damage from eccentric exercise. The similar relative changes in RPE and P _max suggest that increased exertion may reflect the need to recruit additional motor units to produce the same submaximal power.     

Cardiac response to exercise in normal-weight and obese, Hispanic men and women: implications for exercise prescription

Aim: The effects of obesity on cardiac function during incremental exercise to peak oxygen consumption (VO_2peak) have not been previously described. The purpose of this study was to compare submaximal and maximal cardiac function during exercise in normal‐weight and obese adults. Methods: Normal‐weight (n = 20; means ± SE: age = 21.9 ± 0.5 years; BMI = 21.8 ± 0.4 kg m^?2) and obese (n = 15; means ± SE: age = 25.1 ± 5.2 years; BMI = 34.1 ± 01.0 kg m^?2) participants were assessed for body composition, VO_2peak and cardiac variables (thoracic bioimpedance analysis) at rest and at heart rates (HR) of 110, 130, 150 and 170 beats min^?1 and maximal HR during incremental cycling exercise to exhaustion. Differences between groups were assessed with mixed‐model ancova with repeated measures. Cardiac variables were statistically indexed for body surface area and resting HR. VO₂ and arteriovenous oxygen difference (a‐vO₂) were statistically indexed for fat‐free mass and resting HR. Results: Significant main effects for group indicated obese participants had higher cardiac output (Q) index and stroke volume (SV) index but lower ejection fraction (EF) and a‐vO₂ index during incremental exercise to exhaustion compared with their normal‐weight peers, despite similar submaximal and maximal VO₂ and absolute power outputs (P < 0.05). Conclusions: Our findings suggest that although Q index and SV index were higher in obese, young adults, EF and a‐vO₂ index were significantly lower when compared to matched, normal‐weight adults.     

Isometric strength training lowers the O2 cost of cycling during moderate-intensity exercise

The effect of maximal voluntary isometric strength training of knee extensor muscles on pulmonary V′O₂ on-kinetics, the O₂ cost of cycling and peak oxygen uptake (V′O_2peak) in humans was studied. Seven healthy males (mean ± SD, age 22.3 ± 2.0 years, body weight 75.0 ± 9.2 kg, V′O_2peak 49.5 ± 3.8 ml kg^?1 min^?1) performed maximal isometric strength training lasting 7 weeks (4 sessions per week). Force during maximal voluntary contraction (MVC) increased by 15 % (P < 0.001) after 1 week of training, and by 19 % (P < 0.001) after 7 weeks of training. This increase in MVC was accompanied by no significant changes in the time constant of the V′O₂ on-kinetics during 6 min of moderate and heavy cycling intensities. Strength training resulted in a significant decrease (by ~7 %; P < 0.02) in the amplitude of the fundamental component of the V′O₂ on-kinetics, and therefore in a lower O₂ cost of cycling during moderate cycling intensity. The amplitude of the slow component of V′O₂ on-kinetics during heavy cycling intensity did not change with training. Training had no effect on the V′O_2peak, whereas the maximal power output reached at V′O_2peak was slightly but significantly increased (P < 0.05). Isometric strength training rapidly (i.e., after 1 week) decreases the O₂ cost of cycling during moderate-intensity exercise, whereas it does not affect the amplitude of the slow component of the V′O₂ on-kinetics during heavy-intensity exercise. Isometric strength training can have beneficial effects on performance during endurance events.     

Effectiveness of low-frequency vibration recovery method on blood lactate removal,muscle contractile properties and on time to exhaustion during cycling at <Emphasis Type="Italic">V</Emphasis>O<Subscript>2max</Subscript> power output

The aim of the study was to determine the effectiveness of low-frequency vibration recovery (LFV-rec) on blood lactate removal, muscle contractile properties, and on time to exhaustion during cycling at maximal oxygen uptake power output (pVO_2max). Twelve active males carried out three experimental sessions. In session 1, participant’s maximal oxygen uptake (VO_2max) and pVO_2max were determined, and in sessions 2 and 3, the participants performed a fatiguing exercise (2 min of cycling at pVO_2max) and then a 15 min recovery period using one of two different methods: LFV-rec which consisted on sitting with feet on the vibratory platform (20 Hz; 4 mm) and passive recovery (P-rec), sitting without vibration stimulus. After that, participants performed an all-out exercise test on cycle ergometer at pVO_2max. In the recovery period, variables such as heart rate (HR), blood lactate concentration [Lac], and tensiomyographic parameters (D _m: maximal radial displacement; T _s: time of contraction maintenance, and T _r: relaxation time) were measured. In an all-out exercise test, mean time to exhaustion (TTE), total distance covered (TD), mean cycling velocity (V _m), and maximal HR (HR_max) were also assessed. The results showed no effect of recovery strategy on any of the assessed variables; nevertheless, higher values, although not significant, were observed in TTE, TD, and V _m after LFV-rec intervention. In conclusion, LFV-rec strategy applied during 15 min after short and intense exercise does not seem to be effective on blood lactate removal, muscle contractile properties, and on time to exhaustion during cycling at pVO_2max.     

Influence of age on blood pressure recovery after maximal effort ergometer exercise in non-athletic adult males

This study investigated whether age influences blood pressure recovery after maximal exercise in adult males. Forty healthy, non-athletic adult males (20 young, aged 22 ± 3.46 years and 20 older, aged 48 ± 6.91 years) participated in the study. Subjects performed a maximal-effort ergometer exercise test. Peak oxygen uptake (VO_2max) was measured during the exercise protocol; heart rate (HR) and blood pressure (BP) were measured before exercise, during exercise (at 2-min intervals), and at the first minute of post-exercise recovery and subsequently at 2-min intervals until the recovery of BP. Results indicated that young adults had lower systolic blood pressure (SBP) recovery ratio (P < 0.05), lower SBP recovery time (P < 0.001), higher SBP% decline in 1, and 3 min (P < 0.001), and higher DBP% decline in 1, and 3 min (P < 0.05, <0.001) than the older adults, thus indicating faster BP recovery in young than older adults. A bivariate correlation test, revealed significant associations (P < 0.001, <0.01) between age and BP recovery parameters: percentage SBP decline in 1 and 3 min (27 and 39%), percentage DBP decline in 1 and 3 min (14 and 26%), third minute SBP ratio (22%), and SBP recovery time (72%). After controlling for factors affecting BP recovery such as resting SBP, percentage HR decline, VO_2max and delta SBP, the observed correlations reduced in SBP recovery time (29%; P < 0.002) but disappeared (P > 0.01) in the other BP recovery parameters. These data indicate the need to take into account, factors affecting BP recovery when interpreting the effect of age on BP responses after exercise in future investigations.     

Hyperoxia does not increase peak muscle oxygen uptake in small muscle group exercise

We examined the influence of hyperoxia on peak oxygen uptake (V˙O _2peak) and peripheral gas exchange during exercise with the quadriceps femoris muscle. Young, trained men (n=5) and women (n=3) performed single-leg knee-extension exercise at 70% and 100% of maximum while inspiring normal air (NOX) or 60% O₂ (HiOX). Blood was sampled from the femoral vein of the exercising limb and from the contralateral artery. In comparison with NOX, hyperoxic arterial O₂ tension (P_aO ₂) increased from 13.5 ± 0.3 (x ± SE) to 41.6 ± 0.3 kPa, O₂ saturation (S_aO ₂) from 98 ± 0.1 to 100 ± 0.1%, and O₂ concentration (C_aO ₂) from 177 ± 4 to 186 ± 4 mL L^–1 (all P < 0.01). Peak exercise femoral venous PO ₂ (P_vO ₂) was also higher in HiOX (3.68 ± 0.06 vs. 3.39 ± 0.7 kPa; P < 0.05), indicating a higher O₂ diffusion driving pressure. HiOX femoral venous O₂ saturation averaged 36.8 ± 2.0% as opposed to 33.4 ± 1.5% in NOX (P < 0.05) and O₂ concentration 63 ± 6 vs. 55 ± 4 mL L^–1 (P < 0.05). Peak exercise quadriceps blood flow (Q˙_leg), measured by the thermo-dilution technique, was lower in HiOX than in NOX, 6.4 ± 0.5 vs. 7.3 ± 0.9 L min^–1 (P < 0.05); mean arterial blood pressure at inguinal height was similar in NOX and HiOX at 144 and 142 mmHg, respectively. O₂ delivery to the limb (Q˙_leq times C_aO ₂) was not significantly different in HiOX and NOX. V˙O _2peak of the exercising limb averaged 890 mL min^–1 in NOX and 801 mL min^–1 in HiOX (n.s.) corresponding to 365 and 330 mL min^–1 per kg active muscle, respectively. The V˙O _2peak-to-P_vO ₂ ratio was lower (P < 0.05) in HiOX than in NOX suggesting a lower O₂ conductance. We conclude that the similar V˙O _2peak values despite higher O₂ driving pressure in HiOX indicates a peripheral limitation for V˙O _2peak. This may relate to saturation of the rate of O₂ turnover in the mitochondria during exercise with a small muscle group but can also be caused by tissue diffusion limitation related to lower O₂ conductance.     

Foreword

Aim: It is not clear how lipolysis changes in skeletal muscle and adipose tissue during exercise of different intensities. We aimed at estimating this by microdialysis and muscle biopsy techniques. Methods: Nine healthy, young men were kicking with both legs at 25% of maximal power (W_max) for 45 min and then simultaneously with one leg at 65% and the other leg at 85%W_max for 35 min. Results: Glycerol concentrations in skeletal muscle and adipose tissue interstitial fluid and in arterial plasma increased (P < 0.001) during low intensity exercise and increased (P < 0.05) even more during moderate intensity exercise. The difference between interstitial muscle and arterial plasma water glycerol concentration, which indicates the direction of the glycerol flux, was positive (P < 0.05) at rest (21 ± 9 μm ) and during exercise at 25%W_max (18 ± 6 μm ). The difference decreased (P < 0.05) with increasing exercise intensity and was not significantly different from zero during exercise at 65% (−11 ± 17 μM) and 85% (−12 ± 13 μm ) W_max. In adipose tissue, the difference between interstitial and arterial plasma water glycerol increased (P < 0.001) with increasing intensity. The net triacylglycerol breakdown, measured chemically from the biopsy, did not differ significantly from zero at any exercise intensity although directional changes were similar to microdialysis changes. Conclusions: Skeletal muscle releases glycerol at rest and at low exercise intensity but not at higher intensities. This can be interpreted as skeletal muscle lipolysis peaking at low exercise intensities but could also indicate that glycerol is taken up in skeletal muscle at a rate which is increasing with exercise intensity.     

Cytokine response to acute running in recreationally-active and endurance-trained men

To compare the cytokine response to exhaustive running in recreationally-active (RA) and endurance-trained (ET) men. Eleven RA men (VO_2max 55 ± 7 mL·min^?1·kg^?1) and 10 ET men (VO_2max 68 ± 7 mL·min^?1·kg^?1) followed a controlled diet and refrained from volitional exercise for 8 days. On the fourth day, participants completed 60 min of treadmill running (65 % VO_2max), followed by intermittent running to exhaustion (70 % VO_2max). Fasting blood was obtained at baseline, after 20, 40 and 60 min of exercise, at the end of intermittent exercise, during 2 h of recovery and on four follow-up days (FU1–FU4). Tumour necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-1 receptor antagonist (IL-1ra) and creatine kinase (CK) were measured. Exercise increased the concentrations of all cytokines and CK, but there were no significant differences between groups. IL-1β increased (2.2–2.5-fold, P < 0.001) during exercise, while TNF-α was increased (1.6–2.0-fold, P < 0.001) during exercise and for 2 h post-exercise. IL-6 (71–84-fold, P < 0.001) and IL-1ra (52–64-fold, P < 0.001) were increased throughout exercise and up to FU1, peaking immediately after exercise and at 1.5–2 h post-exercise, respectively. CK concentrations were increased (P < 0.001) throughout exercise and up to FU4, peaking at FU1, but were not associated with changes in any cytokines. Exhaustive running resulted in modest and transient increases in TNF-α and IL-1β, and more marked and prolonged increases in IL-6 and IL-1ra, but improved training status did not affect this response. Increased CK might indicate either exercise-induced muscle cell disruption or increased cell permeability, although neither appears to have contributed to the increased cytokine concentrations.     

The limit to exercise tolerance in humans: mind over muscle?

In exercise physiology, it has been traditionally assumed that high-intensity aerobic exercise stops at the point commonly called exhaustion because fatigued subjects are no longer able to generate the power output required by the task despite their maximal voluntary effort. We tested the validity of this assumption by measuring maximal voluntary cycling power before (mean ± SD, 1,075 ± 214 W) and immediately after (731 ± 206 W) (P < 0.001) exhaustive cycling exercise at 242 ± 24 W (80% of peak aerobic power measured during a preliminary incremental exercise test) in ten fit male human subjects. Perceived exertion during exhaustive cycling exercise was strongly correlated (r = −0.82, P = 0.003) with time to exhaustion (10.5 ± 2.1 min). These results challenge the long-standing assumption that muscle fatigue causes exhaustion during high-intensity aerobic exercise, and suggest that exercise tolerance in highly motivated subjects is ultimately limited by perception of effort.     

Time to exhaustion at maximal lactate steady state is similar for cycling and running in moderately trained subjects

We compared time to exhaustion (t _lim) at maximal lactate steady state (MLSS) between cycling and running, investigated if oxygen consumption, ventilation, blood lactate concentration, and perceived exertion differ between the exercise modes, and established whether MLSS can be determined for cycling and running using the same criteria. MLSS was determined in 15 moderately trained men (30 ± 6 years, 77 ± 6 kg) by several constant-load tests to exhaustion in cycling and running. Heart rate, oxygen consumption, and ventilation were recorded continuously. Blood lactate concentration and perceived exertion were measured every 5 min. t _lim (37.7 ± 8.9 vs. 34.4 ± 5.4 min) and perceived exertion (7.2 ± 1.7 vs. 7.2 ± 1.5) were similar for cycling and running. Heart rate (165 ± 8 vs. 175 ± 10 min^?1; P < 0.01), oxygen consumption (3.1 ± 0.3 vs. 3.4 ± 0.3 l min^?1; P < 0.001) and ventilation (93 ± 12 vs. 103 ± 16 l min^?1; P < 0.01) were lower for cycling compared to running, respectively, whereas blood lactate concentration (5.6 ± 1.7 vs. 4.3 ± 1.3 mmol l^?1; P < 0.05) was higher for cycling. t _lim at MLSS is similar for cycling and running, despite absolute differences in heart rate, ventilation, blood lactate concentration, and oxygen consumption. This may be explained by the relatively equal cardiorespiratory demand at MLSS. Additionally, the similar t _lim for cycling and running allows the same criteria to be used for determining MLSS in both exercise modes.