Thigh muscle activation distribution and pulmonary V̇o2 kinetics during moderate, heavy, and very heavy intensity cycling exercise in humans

Exercise modality, but not exercise training, alters the acute effect of exercise on endothelial function in healthy men

We used a within-subject crossover design to examine the impact of exercise modality, i.e., resistance (RT) and endurance (END), on the acute impact of exercise on endothelial function. Then, we examined whether a 4-wk period of chronic exercise training altered the acute exercise-induced change in endothelial function in healthy individuals. Thirty-four healthy, young men (21 ± 2 yr) reported to our laboratory and completed assessment of endothelial function [using the brachial artery flow-mediated dilation test (FMD)] before and immediately after a single bout of RT (leg-extension) or END (cycling). Subsequently, participants completed a 4-wk period of training (12 sessions), followed by evaluation of the FMD before and after a single bout of exercise. Following a 3-wk washout, participants repeated these experiments with the different exercise modality (in a balanced crossover design). An exercise × modality interaction effect was found (P < 0.001). Post hoc pairwise analyses revealed a decrease in FMD after END (P < 0.001) but not after RT (P = 0.06). Four weeks of exercise training improved resting FMD after END and RT (P = 0.04) but did not alter the acute effect of exercise on FMD (exercise × modality × training effect: P = 0.63), an effect independent of the modality of exercise (exercise × training interaction: P = 0.46 and P = 0.11 in RT and END, respectively). These distinct changes in FMD following acute exercise may relate to the different prolonged physiological responses induced by endurance versus resistance exercise. Specifically, endurance exercise, but not resistance exercise, causes a decrease in brachial artery endothelial function, which was unaffected by 4 wk of chronic exercise training.NEW & NOTEWORTHY We found that resistance and endurance exercise modalities lead to different endothelial function responses after a single bout of exercise. Endothelial function increased after an acute bout of resistance exercise, whereas it decreased after an acute bout of endurance exercise. Four weeks of chronic exercise training did not affect the acute endothelial function response.

Correlation analysis between lower limb muscle architectures and cycling power via ultrasonography

The primary purpose was to examine the relationship between the muscle architectural characteristics of short and long-distance cyclist—including muscle thickness, fascicle angle, and fascicle length—of the anterior thigh and posterior leg and its impact in 20-s cycling power. The secondary purpose was to clarify the muscle variables that predict the cycling power by using ultrasonography to measure the muscle architectural characteristics. Twenty-four varsity cyclists participated in this study, of whom 12 were short-distance cyclists and 12 were long-distance cyclists. B-mode ultrasonography was used to measure muscle architecture parameters. A cycle ergometer was used to measure the cycling power. The rectus femoris, vastus medialis, and medial head of gastrocnemius were significantly thicker in short-distance cyclists than in long-distance cyclists at every site (p < 0.05). Our analysis revealed that the rectus femoris fascicle length at the 30% level of the thigh was a significant independent predictor of the 20-s cycling power in short-distance cyclists, while the rectus femoris fascicle angle at the 50% level was that of the 20-s cycling power in long-distance cyclists. These findings highlight the significance of rectus femoris muscle architecture to cycling power.

Oxygen Uptake Kinetics in Youth: Characteristics, Interpretation, and Application

Pulmonary oxygen uptake ( V˙O2 ) kinetics, which describes the aerobic response to near instantaneous changes in metabolic demand, provides a valuable insight into the control and coordination of oxidative phosphorylation during exercise. Despite their applicability to the highly sporadic habitual physical activity and exercise patterns of children, relatively little is known regarding the influence of internal and external stimuli on the dynamic V˙O2 response. Although insufficient evidence is available during moderate-intensity exercise, an age-related slowing of the phase 2 time constant (τ) and augmentation of the V˙O2 slow component appears to manifest during heavy-intensity exercise, which may be related to changes in the muscle phosphate controllers of oxidative phosphorylation, muscle oxygen delivery and utilization, and/or muscle fiber type recruitment patterns. Similar to findings in adults, aerobic training is associated with a faster phase 2 τ and smaller V˙O2 slow component in youth, independent of age or maturity, indicative of an enhanced oxidative metabolism. However, a lack of longitudinal or intervention-based training studies limits our ability to attribute these changes to training per se. Further, methodologically rigorous studies are required to fully resolve the interaction(s) between age, sex, biological maturity, and external stimuli, such as exercise training and exercise intensity and the dynamic V˙O2 response at the onset and offset of exercise.

Oxygen Uptake and Lactate Kinetics in Patients with Chronic Obstructive Pulmonary Disease during Heavy Intensity Exercise: Role of Pedaling Cadence

Oxygen uptake slow component ([Formula: see text]_sc) is associated with lactate accumulation, likely a contribution of poorly oxidative muscle fibers. We aimed to test the hypothesis that higher muscle tension during slow pedaling rates would yield more prominent [Formula: see text]_sc in healthy subjects, but not in COPD patients. Eight severe COPD patients and 8 age-matched healthy individuals performed 4 rest-heavy exercise transitions at 40 and 80 RPM. Work rates at the two cadences were balanced. Venous blood was sampled for measurement of lactate concentration at rest and every 2 minutes until the end of exercise. [Formula: see text] kinetics were analyzed utilizing nonlinear regression. [Formula: see text] phase II amplitudes at the two cadences were similar in both groups. In healthy individuals, [Formula: see text]_sc was steeper at 40 than 80 RPM (46.6 ± 12.0 vs. 29.5 ± 11.7 mL/min², p = 0.002) but not in COPD patients (16.2 ± 14.7 vs. 13.3 ± 7.6 mL/min²). End-exercise lactate concentration did not differ between cadences in either group. In healthy individuals, greater slow-cadence [Formula: see text]_sc seems likely related to oxidative muscle fiber recruitment at higher muscular tension. COPD patients, known to have fast-twitch fiber predominance, might be unable to recruit oxidative fibers at high muscle tension, blunting [Formula: see text]_sc response.

Elevated baseline work rate slows pulmonary oxygen uptake kinetics and decreases critical power during upright cycle exercise

Critical power is a fundamental parameter defining high-intensity exercise tolerance, and is related to the phase II time constant of pulmonary oxygen uptake kinetics (τV˙O2). Whether this relationship is causative is presently unclear. This study determined the impact of raised baseline work rate, which increases τV˙O2, on critical power during upright cycle exercise. Critical power was determined via four constant-power exercise tests to exhaustion in two conditions: (1) with exercise initiated from an unloaded cycling baseline (U→S), and (2) with exercise initiated from a baseline work rate of 90% of the gas exchange threshold (M→S). During these exercise transitions, τV˙O2 and the time constant of muscle deoxyhemoglobin kinetics (τ[HHb + Mb] ) (the latter via near-infrared spectroscopy) were determined. In M→S, critical power was lower (M→S = 203 ± 44 W vs. U→S = 213 ± 45 W, P = 0.011) and τV˙O2 was greater (M→S = 51 ± 14 sec vs. U→S = 34 ± 16 sec, P = 0.002) when compared with U→S. Additionally, τ[HHb + Mb] was greater in M→S compared with U→S (M→S = 28 ± 7 sec vs. U→S = 14 ± 7 sec, P = 0.007). The increase in τV˙O2 and concomitant reduction in critical power in M→S compared with U→S suggests a causal relationship between these two parameters. However, that τ[HHb + Mb] was greater in M→S exculpates reduced oxygen availability as being a confounding factor. These data therefore provide the first experimental evidence that τV˙O2 is an independent determinant of critical power. Keywords critical power, exercise tolerance, oxygen uptake kinetics, power-duration relationship, muscle deoxyhemoglobin kinetics, work-to-work exercise.

The effects of priming exercise on the V̇O2 slow component and the time-course of muscle fatigue during very-heavy-intensity exercise in humans

We hypothesized that prior exercise would attenuate the muscle fatigue accompanied by oxygen uptake slow-component (V̇O_2SC) behavior during a subsequent very-heavy (VH)-intensity cycling exercise. Thirteen healthy male subjects performed tests to determine the critical power (CP) and the fixed amount of work above CP ([Formula: see text]) and performed 6 square-wave bouts until 3 or 8 min, each at a work rate set to deplete 70% [Formula: see text] in 8 min, with a maximal isokinetic effort before and after the conditions without (VH_CON) and with prior exercise (VH_EXP), to measure the cycling peak torque decrement. The V̇O_2SC magnitude at 3 min (VH_CON = 0.280 ± 0.234, VH_EXP = 0.116 ± 0.109 L·min^-1; p = 0.04) and the V̇O_2SC trajectory were significantly lower for VH_EXP (VH_CON = 0.108 ± 0.042, VH_EXP = 0.063 ± 0.031 L·min^-2; p < 0.01), leading to a V̇O_2SC magnitude at the eighth minute that was significantly lower than VH_CON (VH_CON = 0.626 ± 0.296 L·min^-1, VH_EXP = 0.337 ± 0.179; p < 0.01). Conversely, peak torque progressively decreased from pre-exercise to 3 min (Δtorque = 21.5 ± 7.7 vs. 19.6 ± 9.2 Nm) and to 8 min (Δtorque = 29.4 ± 15.8 vs. 27.5 ± 12.0 Nm) at VH_CON and VH_EXP, respectively, without significant differences between conditions. Regardless of the condition, there was a significant relationship between Δtorque and the V̇O_2SC (R²: VH_CON = 0.23, VH_EXP = 0.25; p = 0.01). Considering that "priming" effects on the V̇O_2SC were not accompanied by the muscle force behavior, these findings do not support the hypothesis of a "causal" relationship between the time-course of muscle fatigue and V̇O_2SC.

Effect of acute nitrate ingestion on V̇O2 response at different exercise intensity domains

While nitrate supplementation influences oxygen uptake (V̇O2) response to exercise, this effect may be intensity dependent. The purpose of this study was to investigate the effect of acute nitrate supplementation on V̇O2 response during different exercise intensity domains in humans. Eleven men ingested 10 mg·kg−1 body mass (8.76 ± 1.35 mmol) of sodium nitrate or sodium chloride (placebo) 2.5 h before cycling at moderate (90% of gas exchange threshold; GET), heavy (GET + 40% of the difference between GET and peak oxygen uptake (V̇O2peak), Δ 40) or severe (GET + 80% of the difference between GET and V̇O2peak, Δ 80) exercise intensities. Volunteers performed exercise for 10 min (moderate), 15 min (heavy) or until exhaustion (severe). Acute nitrate supplementation had no effect on any V̇O2 response parameters during moderate and severe exercise intensities. However, the V̇O2 slow amplitude (nitrate: 0.93 ± 0.36 L·min−1 vs. placebo: 1.13 ± 0.59 L·min−1, p = 0.04) and V̇O2 slow gain (nitrate: 5.81 ± 2.37 mL·min–1·W−1 vs. placebo: 7.09 ± 3.67 mL·min–1·W−1, p = 0.04) were significantly lower in nitrate than in placebo during the heavy exercise intensity. There was no effect of nitrate on plasma lactate during any exercise intensity (p > 0.05). Time to exhaustion during the severe exercise intensity was also not affected by nitrate (p > 0.05). In conclusion, acute nitrate supplementation reduced the slow component of V̇O2 only when performing heavy-intensity exercise, which might indicate an intensity-dependent effect of nitrate on V̇O2 response.

Sex differences in the oxygen delivery, extraction, and uptake during moderate-walking exercise transition

Previous studies in children and older adults demonstrated faster oxygen uptake (V̇O2) kinetics in males compared with females, but young healthy adults have not been studied. We hypothesized that young men would have faster aerobic system dynamics in response to the onset of exercise than women. Interactions between oxygen supply and utilization were characterized by the dynamics of V̇O2, deoxyhemoglobin (HHb), tissue saturation index (TSI), cardiac output (Q̇), and calculated arteriovenous O2 difference (a–vO2diff) in women and men. Eighteen healthy active young women and men (9 of each sex) with similar aerobic fitness levels volunteered for this study. Participants performed an incremental cardiopulmonary treadmill exercise test and 3 moderate-intensity treadmill exercise tests (at 80% V̇O2 of gas exchange threshold). Data related to the moderate exercise were submitted to exponential data modelling to obtain parameters related to the aerobic system dynamics. The time constants of V̇O2, a–vO2diff, HHb, and TSI (30 ± 6, 29 ± 1, 16 ± 1, and 15 ± 2 s, respectively) in women were statistically (p < 0.05) faster than the time constants in men (42 ± 10, 49 ± 21, 19 ± 3, and 20 ± 4 s, respectively). Although Q̇ dynamics were not statistically different (p = 0.06) between groups, there was a trend to slower Q̇ dynamics in men corresponding with the slower V̇O2 kinetics. These results indicated that the peripheral and pulmonary oxygen extraction dynamics were remarkably faster in women. Thus, contrary to the hypothesis, V̇O2 dynamics measured at the mouth at the onset of submaximal treadmill walking were faster in women compared with men.

Locomotor Muscle Fatigue Does Not Alter Oxygen Uptake Kinetics during High-Intensity Exercise

The V˙O2 slow component (V˙O2sc) that develops during high-intensity aerobic exercise is thought to be strongly associated with locomotor muscle fatigue. We sought to experimentally test this hypothesis by pre-fatiguing the locomotor muscles used during subsequent high-intensity cycling exercise. Over two separate visits, eight healthy male participants were asked to either perform a non-metabolically stressful 100 intermittent drop-jumps protocol (pre-fatigue condition) or rest for 33 min (control condition) according to a random and counterbalanced order. Locomotor muscle fatigue was quantified with 6-s maximal sprints at a fixed pedaling cadence of 90 rev·min⁻¹. Oxygen kinetics and other responses (heart rate, capillary blood lactate concentration and rating of perceived exertion, RPE) were measured during two subsequent bouts of 6 min cycling exercise at 50% of the delta between the lactate threshold and V˙O2max determined during a preliminary incremental exercise test. All tests were performed on the same cycle ergometer. Despite significant locomotor muscle fatigue (P = 0.03), the V˙O2sc was not significantly different between the pre-fatigue (464 ± 301 mL·min⁻¹) and the control (556 ± 223 mL·min⁻¹) condition (P = 0.50). Blood lactate response was not significantly different between conditions (P = 0.48) but RPE was significantly higher following the pre-fatiguing exercise protocol compared with the control condition (P < 0.01) suggesting higher muscle recruitment. These results demonstrate experimentally that locomotor muscle fatigue does not significantly alter the V˙O2 kinetic response to high intensity aerobic exercise, and challenge the hypothesis that the V˙O2sc is strongly associated with locomotor muscle fatigue.

The slow component of pulmonary O2 uptake accompanies peripheral muscle fatigue during high-intensity exercise

During constant-power output (PO) exercise above lactate threshold (LT), pulmonary O2 uptake (V̇o2p) features a developing slow component (V̇o2pSC). This progressive increase in O2 cost of exercise is suggested to be related to the effects of muscle fatigue development. We hypothesized that peripheral muscle fatigue as assessed by contractile impairment would be associated with the V̇o2pSC. Eleven healthy men were recruited to perform four constant-PO tests at an intensity corresponding to ∼Δ60 (very heavy, VH) where Δ is 60% of the difference between LT and peak V̇o2p. The VH exercise was completed for each of 3, 8, 13, and 18 min (i.e., VH3, VH8, VH13, VH18) with each preceded by 3 min of cycling at 20 W. Peripheral muscle fatigue was assessed via pre- vs. postexercise measurements of quadriceps torque in response to brief trains of electrical stimulation delivered at low (10 Hz) and high (50 Hz) frequencies. During exercise, breath-by-breath V̇o2p was measured by mass spectrometry and volume turbine. The magnitude of V̇o2pSC increased ( P < 0.05) from 224 ± 81 ml/min at VH3 to 520 ± 119, 625 ± 134, and 678 ± 156 ml/min at VH8, VH13, and VH18, respectively. The ratio of the low-to-high frequency (10/50 Hz) response was reduced ( P < 0.05) at VH3 (−12 ± 9%) and further reduced ( P < 0.05) at VH8 (−25 ± 11%), VH13 (−42 ± 19%), and VH18 (−46 ± 16%), mirroring the temporal pattern of V̇o2pSC development. The reduction in 10/50 Hz ratio was correlated ( P < 0.001, r2 = 0.69) with V̇o2pSC amplitude. The temporal and quantitative association of decrements in muscle torque production and V̇o2pSC suggest a common physiological mechanism between skeletal muscle fatigue and loss of muscle efficiency.

Electromyographic, mechanomyographic, and metabolic responses during cycle ergometry at a constant rating of perceived exertion

Ten subjects performed four 8-min rides (65%-80% peak oxygen consumption) to determine the physical working capacity at the OMNI rating of perceived exertion (RPE) threshold (PWCOMNI). Polynomial regression analyses were used to examine the patterns of responses for surface electromyographic (EMG) amplitude (EMG AMP), EMG mean power frequency (EMG MPF), mechanomyographic (MMG) AMP, and MMG MPF of the vastus lateralis as well as oxygen consumption rate, respiratory exchange ratio (RER), and power output (PO) were examined during a 1-h ride on a cycle ergometer at a constant RPE that corresponded to the PWCOMNI. EMG AMP and MMG MPF tracked the decreases in oxygen consumption rate, RER, and PO, while EMG MPF and MMG AMP tracked RPE. The decreases in EMG AMP and MMG MPF were likely attributable to decreases in motor unit (MU) recruitment and firing rate, while the lack of change in MMG AMP may have resulted from a balance between MU de-recruitment as PO decreased, and an increase in the ability of activated fibers to oscillate. The current findings suggested that during submaximal cycle ergometry at a constant RPE, MU de-recruitment and mechanical changes within the muscle may influence the perception of effort via feedback from group III and IV afferents.

Muscle deoxygenation in the quadriceps during ramp incremental cycling: Deep vs. superficial heterogeneity

Muscle deoxygenation (i.e., deoxy[Hb + Mb]) during exercise assesses the matching of oxygen delivery (Q̇O2) to oxygen utilization (V̇O2). Until now limitations in near-infrared spectroscopy (NIRS) technology did not permit discrimination of deoxy[Hb + Mb] between superficial and deep muscles. In humans, the deep quadriceps is more highly vascularized and oxidative than the superficial quadriceps. Using high-power time-resolved NIRS, we tested the hypothesis that deoxygenation of the deep quadriceps would be less than in superficial muscle during incremental cycling exercise in eight males. Pulmonary V̇O2 was measured and muscle deoxy[Hb + Mb] was determined in the superficial vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF-s) and the deep rectus femoris (RF-d). deoxy[Hb + Mb] in RF-d was significantly less than VL at 70% (67.2 ± 7.0 vs. 75.5 ± 10.7 μM) and 80% (71.4 ± 11.0 vs. 79.0 ± 15.4 μM) of peak work rate (WR(peak)), but greater than VL and VM at WR(peak) (87.7 ± 32.5 vs. 76.6 ± 17.5 and 75.1 ± 19.9 μM). RF-s was intermediate at WR(peak) (82.6 ± 18.7 μM). Total hemoglobin and myoglobin concentration and tissue oxygen saturation were significantly greater in RF-d than RF-s throughout exercise. The slope of deoxy[Hb + Mb] increase (proportional to Q̇O2/V̇O2) in VL and VM slowed markedly above 70% WR(peak), whereas it became greater in RF-d. This divergent deoxygenation pattern may be due to a greater population of slow-twitch muscle fibers in the RF-d muscle and the differential recruitment profiles and vascular and metabolic control properties of specific fiber populations within superficial and deeper muscle regions.

Muscle activation during exercise in severe acute hypoxia: role of absolute and relative intensity

The aim of this study was to determine the influence of severe acute hypoxia on muscle activation during whole body dynamic exercise. Eleven young men performed four incremental cycle ergometer tests to exhaustion breathing normoxic (FIO2=0.21, two tests) or hypoxic gas (FIO2=0.108, two tests). Surface electromyography (EMG) activities of rectus femoris (RF), vastus medialis (VL), vastus lateralis (VL), and biceps femoris (BF) were recorded. The two normoxic and the two hypoxic tests were averaged to reduce EMG variability. Peak VO2 was 34% lower in hypoxia than in normoxia (p<0.05). The EMG root mean square (RMS) increased with exercise intensity in all muscles (p<0.05), with greater effect in hypoxia than in normoxia in the RF and VM (p<0.05), and a similar trend in VL (p=0.10). At the same relative intensity, the RMS was greater in normoxia than in hypoxia in RF, VL, and BF (p<0.05), with a similar trend in VM (p=0.08). Median frequency increased with exercise intensity (p<0.05), and was higher in hypoxia than in normoxia in VL (p<0.05). Muscle contraction burst duration increased with exercise intensity in VM and VL (p<0.05), without clear effects of FIO2. No significant FIO2 effects on frequency domain indices were observed when compared at the same relative intensity. In conclusion, muscle activation during whole body exercise increases almost linearly with exercise intensity, following a muscle-specific pattern, which is adjusted depending on the FIO2 and the relative intensity of exercise. Both VL and VM are increasingly involved in power output generation with the increase of intensity and the reduction in FIO2.

High-field (11.75T) multimodal MR imaging of exercising hindlimb mouse muscles using a non-invasive combined stimulation and force measurement device

We have designed and constructed an experimental set-up allowing electrical stimulation of hindlimb mouse muscles and the corresponding force measurements at high-field (11.75T). We performed high-resolution multimodal MRI (including T2 -weighted imaging, angiography and diffusion) and analysed the corresponding MRI changes in response to a stimulation protocol. Mice were tested twice over a 1-week period to investigate the reliability of mechanical measurements and T2 changes associated with the stimulation protocol. Additionally, angiographic images were obtained before and immediately after the stimulation protocol. Finally, multislice diffusion imaging was performed before, during and immediately after the stimulation session. Apparent diffusion coefficient (ADC) maps were calculated on the basis of diffusion weighted images (DWI). Both force production and T2 values were highly reproducible as illustrated by the low coefficient of variation (<8%) and high intraclass correlation coefficient (≥0.75) values. Maximum intensity projection angiographic images clearly showed a strong vascular effect resulting from the stimulation protocol. Although a motion sensitive imaging sequence was used (echo planar imaging) and in spite of the strong muscle contractions, motion artifacts were minimal for DWI recorded under exercising conditions, thereby underlining the robustness of the measurements. Mean ADC values increased under exercising conditions and were higher during the recovery period as compared with the corresponding control values. The proposed experimental approach demonstrates accurate high-field multimodal MRI muscle investigations at a preclinical level which is of interest for monitoring the severity and/or the progression of neuromuscular diseases but also for assessing the efficacy of potential therapeutic interventions.

Influence of thigh activation on the VO₂ slow component in boys and men

PURPOSE

During constant work rate exercise above the lactate threshold (LT), the initial rapid phase of pulmonary oxygen uptake (VO₂) kinetics is supplemented by an additional VO₂ slow component (VO₂Sc) which reduces the efficiency of muscular work. The VO₂Sc amplitude has been shown to increase with maturation but the mechanisms are poorly understood. We utilized the transverse relaxation time (T₂) of muscle protons from magnetic resonance imaging (MRI) to test the hypothesis that a lower VO₂ slow component (VO₂Sc) amplitude in children would be associated with a reduced muscle recruitment compared to adults.

METHODS

Eight boys (mean age 11.4 ± 0.4) and eight men (mean age 25.3 ± 3.3 years) completed repeated step transitions of unloaded-to-very heavy-intensity (U → VH) exercise on a cycle ergometer. MRI scans of the thigh region were acquired at rest and after VH exercise up to the VO₂Sc time delay (ScTD) and after 6 min. T₂ for each of eight muscles was adjusted in relation to cross-sectional area and then summed to provide the area-weighted ΣT₂ as an index of thigh recruitment.

RESULTS

There were no child/adult differences in the relative VO₂Sc amplitude [Boys 14 ± 7 vs. Men 18 ± 3 %, P = 0.15, effect size (ES) = 0.8] during which the change (∆) in area-weighted ΣT₂ between the ScTD and 6 min was not different between groups (Boys 1.6 ± 1.2 vs. Men 2.3 ± 1.1 ms, P = 0.27, ES = 0.6). A positive and strong correlation was found between the relative VO₂Sc amplitude and the magnitude of the area-weighted ∆ΣT₂ in men (r = 0.92, P = 0.001) but not in boys (r = 0.09, P = 0.84).

CONCLUSIONS

This study provides evidence to show that progressive muscle recruitment (as inferred from T₂ changes) contributes to the development of the VO₂Sc during intense submaximal exercise independent of age.

Metabolic and cardiovascular responses during voluntary pedaling exercise with electrical muscle stimulation

PURPOSE

We aimed to test the effect of additional electrical muscle stimulation (EMS) during moderate-intensity voluntary pedaling exercise on metabolic and cardiovascular responses.

METHODS

Eleven healthy male subjects performed moderate-intensity pedaling exercise at a constant workload (80% of ventilatory threshold) for 20 min while EMS was applied to thigh muscles from 5 to 10 min and from 15 to 20 min during the exercise.

RESULTS

A significantly higher oxygen uptake (VO₂), heart rate, and respiratory gas exchange ratio were observed during the exercise periods with EMS despite the constant workload. These changes were accompanied by an elevated blood lactate concentration, suggesting the existence of additional fast-twitch motor unit (MU) recruitment during the exercise with EMS.

CONCLUSION

Our data suggest that the use of intermittent EMS during a constant load exercise mimics the high-intensity interval training, possibly due to additional fast-twitch MU recruitment and co-contractions of the quadriceps and hamstrings muscles, leading to higher anaerobic metabolism and a lower mechanical efficiency.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

The effect of priming exercise on O2 uptake kinetics, muscle O2 delivery and utilization, muscle activity, and exercise tolerance in boys

This study used priming exercise in young boys to investigate (i) how muscle oxygen delivery and oxygen utilization, and muscle activity modulate oxygen uptake kinetics during exercise; and (ii) whether the accelerated oxygen uptake kinetics following priming exercise can improve exercise tolerance. Seven boys that were aged 11.3 ± 1.6 years completed either a single bout (bout 1) or repeated bouts with 6 min of recovery (bout 2) of very heavy-intensity cycling exercise. During the tests oxygen uptake, muscle oxygenation, muscle electrical activity and exercise tolerance were measured. Priming exercise most likely shortened the oxygen uptake mean response time (change, ±90% confidence limits; -8.0 s, ±3.0), possibly increased the phase II oxygen uptake amplitude (0.11 L·min(-1), ±0.09) and very likely reduced the oxygen uptake slow component amplitude (-0.08 L·min(-1), ±0.07). Priming resulted in a likely reduction in integrated electromyography (-24% baseline, ±21% and -25% baseline, ±19) and a very likely reduction in Δ deoxyhaemoglobin/Δoxygen uptake (-0.16, ±0.11 and -0.09, ±0.05) over the phase II and slow component portions of the oxygen uptake response, respectively. A correlation was present between the change in tissue oxygenation index during bout 2 and the change in the phase II (r = -0.72, likely negative) and slow component (r = 0.72, likely positive) oxygen uptake amplitudes following priming exercise, but not for muscle activity. Exercise tolerance was likely reduced (change -177 s, ±180) following priming exercise. The altered phase II and slow component oxygen uptake amplitudes in boys following priming exercise are linked to an improved localised matching of muscle oxygen delivery to oxygen uptake and not muscle electrical activity. Despite more rapid oxygen uptake kinetics following priming exercise, exercise tolerance was not enhanced.

Muscle metabolism and activation heterogeneity by combined 31P chemical shift and T2 imaging, and pulmonary O2 uptake during incremental knee-extensor exercise

The integration of skeletal muscle substrate depletion, metabolite accumulation, and fatigue during large muscle-mass exercise is not well understood. Measurement of intramuscular energy store degradation and metabolite accumulation is confounded by muscle heterogeneity. Therefore, to characterize regional metabolic distribution in the locomotor muscles, we combined 31P magnetic resonance spectroscopy, chemical shift imaging, and T2-weighted imaging with pulmonary oxygen uptake during bilateral knee-extension exercise to intolerance. Six men completed incremental tests for the following: (1) unlocalized 31P magnetic resonance spectroscopy; and (2) spatial determination of 31P metabolism and activation. The relationship of pulmonary oxygen uptake to whole quadriceps phosphocreatine concentration ([PCr]) was inversely linear, and three of four knee-extensor muscles showed activation as assessed by change in T2. The largest changes in [PCr], [inorganic phosphate] ([Pi]) and pH occurred in rectus femoris, but no voxel (72 cm3) showed complete PCr depletion at exercise cessation. The most metabolically active voxel reached 11 ± 9 mM [PCr] (resting, 29 ± 1 mM), 23 ± 11 mM [Pi] (resting, 7 ± 1 mM), and a pH of 6.64 ± 0.29 (resting, 7.08 ± 0.03). However, the distribution of 31P metabolites and pH varied widely between voxels, and the intervoxel coefficient of variation increased between rest (∼10%) and exercise intolerance (∼30-60%). Therefore, the limit of tolerance was attained with wide heterogeneity in substrate depletion and fatigue-related metabolite accumulation, with extreme metabolic perturbation isolated to only a small volume of active muscle (<5%). Regional intramuscular disturbances are thus likely an important requisite for exercise intolerance. How these signals integrate to limit muscle power production, while regional "recruitable muscle" energy stores are presumably still available, remains uncertain.

Metabolic and neuromuscular responses at critical power from the 3-min all-out test

The purpose of this study was to determine the specific metabolic and neuromuscular responses at critical power (CP) from the 3-min all-out test. Nine men (mean ± SD: aged 23.7 ± 3.3 years) performed an incremental test for the determination of peak oxygen consumption (V̇O2peak) and gas exchange threshold. CP was estimated for each subject from the 3-min all-out test. Oxygen consumption (V̇O2), the ventilation versus carbon dioxide production ratio (V̇E/V̇CO2 ratio), electromyographic (EMG) amplitude, and EMG mean power frequency (MPF) were examined during exhaustive rides at CP for each subject. There was no significant difference between the V̇O2 at exhaustion (40.6 ± 7.5 mL·kg−1·min−1) and V̇O2peak (42.9 ± 7.3 mL·kg−1·min−1). Furthermore, there were significant increases in EMG amplitude and the V̇E/V̇CO2 ratio during the exhaustive rides at CP. There was, however, no significant change in EMG MPF over time. Therefore, the current findings indicated that the 3-min all-out test overestimated CP and the demarcation between the heavy- and severe-intensity domains. Specifically, the V̇O2, ventilatory, and EMG amplitude responses were consistent with those observed during continuous exercise in the severe exercise intensity domain. It is likely that the ventilatory and EMG amplitude responses were associated with a common mechanism of fatigue that is different from what affects EMG MPF.

Skeletal muscle volume following dehydration induced by exercise in heat

Background

Intracellular skeletal muscle water is redistributed into the extracellular compartment during periods of dehydration, suggesting an associated decline in muscle volume. The purpose of this study was to evaluate skeletal muscle volume in active (knee extensors (KE)) and less active (biceps/triceps brachii, deltoid) musculature following dehydration induced by exercise in heat.

Methods

Twelve participants (seven men, five women) cycled in the heat under two conditions: (1) dehydration (DHYD) resulting in 3% and 5% losses of estimated total body water (_ETBW), which was assessed by changes in body mass, and (2) fluid replacement (FR) where 3% and 5% losses of _ETBW were counteracted by intermittent (20 to 30 min) fluid ingestion via a carbohydrate-electrolyte beverage. During both conditions, serum osmolality and skeletal muscle volume (assessed by magnetic resonance imaging) were measured at baseline and at the 3% and 5% _ETBW loss measurement points.

Results

In DHYD, serum osmolality increased at 3% (p = 0.005) and 5% (p < 0.001) _ETBW losses, while FR decreased serum osmolality at the 5% loss of _ETBW time point (p = 0.009). In DHYD, KE muscle volume declined from 1,464 ± 446 ml to 1,406 ± 425 ml (3.9%, p < 0.001) at 3% _ETBW loss and to 1,378 ± 421 ml (5.9%, p < 0.001) at 5% _ETBW loss. The largest decline in KE volume in DYHD occurred in the mid-belly (31 ml, p = 0.001) and proximal (24 ml, p = 0.001) regions of the grouped vasti muscles. There were no changes in volume for the biceps/triceps (p = 0.35) or deltoid (p = 0.92) during DHYD. FR prevented the loss of KE muscle volume at 3% (1,430 ± 435 ml, p = 0.074) and 5% (1,431 ± 439 ml, p = 0.156) _ETBW loss time points compared to baseline (1,445 ± 436 ml).

Conclusions

Following exercise in the heat, the actively contracting muscles lost volume, while replacing lost fluids intermittently during exercise in heat prevented this decline. These results support the use of muscle volume as a marker of water loss.

Slow component of VO2 kinetics: mechanistic bases and practical applications

The V·O₂ slow component, a slowly developing increase in V·O₂ during constant-work-rate exercise performed above the lactate threshold, represents a progressive loss of skeletal muscle contractile efficiency and is associated with the fatigue process. This brief review outlines the current state of knowledge concerning the mechanistic bases of the V·O₂ slow component and describes practical interventions that can attenuate the slow component and thus enhance exercise tolerance. There is strong evidence that, during constant-work-rate exercise, the development of the V·O₂ slow component is associated with the progressive recruitment of additional (type II) muscle fibers that are presumed to have lower efficiency. Recent studies, however, indicate that muscle efficiency is also lowered (resulting in a "mirror-image" V·O₂ slow component) during fatiguing, high-intensity exercise in which additional fiber recruitment is unlikely or impossible. Therefore, it seems that muscle fatigue underpins the V·O₂ slow component, although the greater fatigue sensitivity of recruited type II fibers might still play a crucial role in the loss of muscle efficiency in both situations. Several interventions can reduce the magnitude of the V·O₂ slow component, and these are typically associated with an enhanced exercise tolerance. These include endurance training, inspiratory muscle training, priming exercise, dietary nitrate supplementation, and the inspiration of hyperoxic gas. All of these interventions reduce muscle fatigue development either by improving muscle oxidative capacity and thus metabolic stability or by enhancing bulk muscle O2 delivery or local Q·O₂-to-V·O₂ matching. Future honing of these interventions to maximize their impact on the V·O₂ slow component might improve sports performance in athletes and exercise tolerance in the elderly or in patient populations.

Determinants of performance in 1,500-m runners

Our aim was to investigate the relationship between physiological variables (not previously studied) and performance in elite 1,500-m runners. We assessed eight male athletes with an average personal best time of 233.3 ± 6.9 s (110% of the world record) for the 1,500-m race. Ventilatory measurements, maximal oxygen consumption VO2max maximal vastus lateralis muscle deoxygenation (∆[deoxy(Hb+Mb)])max via near-infrared spectroscopy (NIRS), and maximal velocity (V (max)) were obtained during an incremental treadmill test. During subsequent constant-speed exercise at Vmax, we determined the time to exhaustion (Tlim), end-exercise blood lactate concentration ([La]b(max)), VO2 and ∆[deoxy(Hb+Mb)] kinetics parameters. The mean VO2max, [La]b(max) and Vmax were 70.2 ± 3.9 mL kg(-1) min(-1), 12.7 ± 2.4 mmol L(-1), 21.5 ± 0.5 km h(-1), respectively. VO2 at Vmax showed a significant negative correlation with Tlim, whereas [La]b(max) was positively correlated with Tlim. Race speed was found to significantly correlate with ∆[deoxy(Hb+Mb)](max) (79% of maximal value obtained during a transient limb ischemia), ∆[deoxy(Hb+Mb)] slow component (22.9 ± 9.3% of total amplitude) and [La]b(max) at Vmax. [La]b(max) at Vmax was also significantly correlated with ∆[deoxy(Hb+Mb)] slow component, suggesting a greater release of oxygen from the hemoglobin due to the Bohr effect. We conclude that both the maximal capacity of muscle to extract O2 from the blood and the end-exercise blood lactate accumulation are important predictors of best performance in 1,500-m runners.

Influence of all-out and fast start on 5-min cycling time trial performance

PURPOSE

To examine the influence of two different fast-start pacing strategies on performance and oxygen consumption (VO2) during cycle ergometer time trials lasting ∼5 min.

METHODS

Eight trained male cyclists performed four cycle ergometer time trials whereby the total work completed (113 ± 11.5 kJ; mean ± SD) was identical to the better of two 5-min self-paced familiarization trials. During the performance trials, initial power output was manipulated to induce either an all-out or a fast start. Power output during the first 60 s of the fast-start trial was maintained at 471.0 ± 48.0 W, whereas the all-out start approximated a maximal starting effort for the first 15 s (mean power: 753.6 ± 76.5 W) followed by 45 s at a constant power output (376.8 ± 38.5 W). Irrespective of starting strategy, power output was controlled so that participants would complete the first quarter of the trial (28.3 ± 2.9 kJ) in 60 s. Participants performed two trials using each condition, with their fastest time trial compared.

RESULTS

Performance time was significantly faster when cyclists adopted the all-out start (4 min 48 s ± 8 s) compared with the fast start (4 min 51 s ± 8 s; P < 0.05). The first-quarter VO2 during the all-out start trial (3.4 ± 0.4 L·min(-1)) was significantly higher than during the fast-start trial (3.1 ± 0.4 L·min(-1); P < 0.05). After removal of an outlier, the percentage increase in first-quarter VO2 was significantly correlated (r = -0.86, P < 0.05) with the relative difference in finishing time.

CONCLUSIONS

An all-out start produces superior middle distance cycling performance when compared with a fast start. The improvement in performance may be due to a faster VO2 response rather than time saved due to a rapid acceleration.

Computational Model of Cellular Metabolic Dynamics in Skeletal Muscle Fibers during Moderate Intensity Exercise

Human skeletal muscles have different fiber types with distinct metabolic functions and physiological properties. The quantitative metabolic responses of muscle fibers to exercise provide essential information for understanding and modifying the regulatory mechanisms of skeletal muscle. Since in vivo data from skeletal muscle during exercise is limited, a computational, physiologically based model has been developed to quantify the dynamic metabolic responses of many key chemical species. This model distinguishes type I and II muscle fibers, which share the same blood supply. An underlying hypothesis is that the recruitment and metabolic activation of the two main types of muscle fibers differ depending on the pre-exercise state and exercise protocols. Here, activation measured by metabolic response (or enzymatic activation) in single fibers is considered linked but distinct from fiber recruitment characterized by the number (or mass) of each fiber type involved during a specific exercise. The model incorporates species transport processes between blood and muscle fibers and most of the important reactions/pathways in cytosol and mitochondria within each fiber type. Model simulations describe the dynamics of intracellular species concentrations and fluxes in muscle fibers during moderate intensity exercise according to various experimental protocols and conditions. This model is validated by comparing model simulations with experimental data in single muscle fibers and in whole muscle. Model simulations demonstrate that muscle-fiber recruitment and metabolic activation patterns in response to exercise produce significantly distinctive effects depending on the exercise conditions.

Methodological validation of the dynamic heterogeneity of muscle deoxygenation within the quadriceps during cycle exercise

The conventional continuous wave near-infrared spectroscopy (CW-NIRS) has enabled identification of regional differences in muscle deoxygenation following onset of exercise. However, assumptions of constant optical factors (e.g., path length) used to convert the relative changes in CW-NIRS signal intensity to values of relative concentration, bring the validity of such measurements into question. Furthermore, to justify comparisons among sites and subjects, it is essential to correct the amplitude of deoxygenated hemoglobin plus myoglobin [deoxy(Hb+Mb)] for the adipose tissue thickness (ATT). We used two time-resolved NIRS systems to measure the distribution of the optical factors directly, thereby enabling the determination of the absolute concentrations of deoxy(Hb+Mb) simultaneously at the distal and proximal sites within the vastus lateralis (VL) and the rectus femoris muscles. Eight subjects performed cycle exercise transitions from unloaded to heavy work rates (>gas exchange threshold). Following exercise onset, the ATT-corrected amplitudes (Ap), time delay (TDp), and time constant (τp) of the primary component kinetics in muscle deoxy(Hb + Mb) were spatially heterogeneous (intersite coefficient of variation range for the subjects: 10–50 for Ap, 16–58 for TDp, 14–108% for τp). The absolute and relative amplitudes of the deoxy(Hb+Mb) responses were highly dependent on ATT, both within subjects and between measurement sites. The present results suggest that regional heterogeneity in the magnitude and temporal profile of muscle deoxygenation is a consequence of differential matching of O2 delivery and O2 utilization, not an artifact caused by changes in optical properties of the tissue during exercise or variability in the overlying adipose tissue.

The relationship between muscle deoxygenation and activation in different muscles of the quadriceps during cycle ramp exercise

The relationship between muscle deoxygenation and activation was examined in three different muscles of the quadriceps during cycling ramp exercise. Seven young male adults (24 ± 3 yr; mean ± SD) pedaled at 60 rpm to exhaustion, with a work rate (WR) increase of 20 W/min. Pulmonary oxygen uptake was measured breath-by-breath, while muscle deoxygenation (HHb) and activity were measured by time-resolved near-infrared spectroscopy (NIRS) and surface electromyography (EMG), respectively, at the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). Muscle deoxygenation was corrected for adipose tissue thickness and normalized to the amplitude of the HHb response, while EMG signals were integrated (iEMG) and normalized to the maximum iEMG determined from maximal voluntary contractions. Muscle deoxygenation and activation were then plotted as a percentage of maximal work rate (%WR(max)). The HHb response for all three muscle groups was fitted by a sigmoid function, which was determined as the best fitting model. The c/d parameter for the sigmoid fit (representing the %WR(max) at 50% of the total amplitude of the HHb response) was similar between VL (47 ± 12% WR(max)) and VM (43 ± 11% WR(max)), yet greater (P < 0.05) for RF (65 ± 13% WR(max)), demonstrating a "right shift" of the HHb response compared with VL and VM. The iEMG also showed that muscle activation of the RF muscle was lower (P < 0.05) compared with VL and VM throughout the majority of the ramp exercise, which may explain the different HHb response in RF. Therefore, these data suggest that the sigmoid function can be used to model the HHb response in different muscles of the quadriceps; however, simultaneous measures of muscle activation are also needed for the HHb response to be properly interpreted during cycle ramp exercise.

Pulmonary O2 uptake kinetics as a determinant of high-intensity exercise tolerance in humans

Tolerance to high-intensity constant-power (P) exercise is well described by a hyperbola with two parameters: a curvature constant (W') and power asymptote termed "critical power" (CP). Since the ability to sustain exercise is closely related to the ability to meet the ATP demand in a steady state, we reasoned that pulmonary O(2) uptake (Vo(2)) kinetics would relate to the P-tolerable duration (t(lim)) parameters. We hypothesized that 1) the fundamental time constant (τVo(2)) would relate inversely to CP; and 2) the slow-component magnitude (ΔVo(2sc)) would relate directly to W'. Fourteen healthy men performed cycle ergometry protocols to the limit of tolerance: 1) an incremental ramp test; 2) a series of constant-P tests to determine Vo(2max), CP, and W'; and 3) repeated constant-P tests (WR(6)) normalized to a 6 min t(lim) for τVo(2) and ΔVo(2sc) estimation. The WR(6) t(lim) averaged 365 ± 16 s, and Vo(2max) (4.18 ± 0.49 l/min) was achieved in every case. CP (range: 171-294 W) was inversely correlated with τVo(2) (18-38 s; R(2) = 0.90), and W' (12.8-29.9 kJ) was directly correlated with ΔVo(2sc) (0.42-0.96 l/min; R(2) = 0.76). These findings support the notions that 1) rapid Vo(2) adaptation at exercise onset allows a steady state to be achieved at higher work rates compared with when Vo(2) kinetics are slower; and 2) exercise exceeding this limit initiates a "fatigue cascade" linking W' to a progressive increase in the O(2) cost of power production (Vo(2sc)), which, if continued, results in attainment of Vo(2max) and exercise intolerance. Collectively, these data implicate Vo(2) kinetics as a key determinant of high-intensity exercise tolerance in humans.

Muscle fiber recruitment and the slow component of O2 uptake: constant work rate vs. all-out sprint exercise

The slow component of pulmonary O(2) uptake (Vo(2)) during constant work rate (CWR) high-intensity exercise has been attributed to the progressive recruitment of (type II) muscle fibers. We tested the following hypotheses: 1) the Vo(2) slow component gain would be greater in a 3-min all-out cycle test than in a work-matched CWR test, and 2) the all-out test would be associated with a progressive decline, and the CWR test with a progressive increase, in muscle activation, as estimated from the electromyogram (EMG) of the vastus lateralis muscle. Eight men (aged 21-39 yr) completed a ramp incremental test, a 3-min all-out test, and a work- and time-matched CWR test to exhaustion. The maximum Vo(2) attained in an initial ramp incremental test (3.97 ± 0.83 l/min) was reached in both experimental tests (3.99 ± 0.84 and 4.03 ± 0.76 l/min for all-out and CWR, respectively). The Vo(2) slow component was greater (P < 0.05) in the all-out test (1.21 ± 0.31 l/min, 4.2 ± 2.2 ml·min(-1)·W(-1)) than in the CWR test (0.59 ± 0.22 l/min, 1.70 ± 0.5 ml·min(-1)·W(-1)). The integrated EMG declined by 26% (P < 0.001) during the all-out test and increased by 60% (P < 0.05) during the CWR test from the first 30 s to the last 30 s of exercise. The considerable reduction in muscle efficiency in the all-out test in the face of a progressively falling integrated EMG indicates that progressive fiber recruitment is not requisite for development of the Vo(2) slow component during voluntary exercise in humans.

Skeletal muscle fatigue precedes the slow component of oxygen uptake kinetics during exercise in humans

The mechanisms determining exercise intolerance are poorly understood. A reduction in work efficiency in the form of an additional energy cost and oxygen requirement occurs during high-intensity exercise and contributes to exercise limitation. Muscle fatigue and subsequent recruitment of poorly efficient muscle fibres has been proposed to mediate this decline. These data demonstrate in humans, that muscle fatigue, generated in the initial minutes of exercise, is correlated with the increasing energy demands of high-intensity exercise. Surprisingly, however, while muscle fatigue reached a plateau, oxygen uptake continued to increase throughout 8 min of exercise. This suggests that additional recruitment of inefficient muscle fibres may not be the sole mechanism contributing to the decline in work efficiency during high-intensity exercise.

Critical speed in the rat: implications for hindlimb muscle blood flow distribution and fibre recruitment

Critical speed (CS) constitutes an important metabolic and performance demarcator. However, active skeletal muscle blood flow distribution specifically surrounding CS remains unknown. We tested the hypotheses that CS could be accurately determined in the running rat and that measurement of hindlimb inter- and intramuscular blood flow below and above CS would support that the greatest muscle fibre recruitment above, relative to below, CS occurs in the predominantly glycolytic muscles. Seven male Sprague-Dawley rats performed five constant-speed tests to exhaustion at speeds between 95 and 115% of the speed that elicited to determine CS. Subsequent constant-speed tests were performed at speeds incrementally surrounding CS to determine time to exhaustion, V(O2), and hindlimb muscle blood flow distribution. Speed and time to exhaustion conformed to a hyperbolic relationship (r(2) = 0.92 ± 0.03) which corresponded to a linear 1/time function (r(2) = 0.93 ± 0.02) with a CS of 48.6 ± 1.0 m min(-1). Time to exhaustion below CS was ∼ 5× greater (P < 0.01) than that above. Below CS V(O2) stabilized at a submaximal value (58.5 ± 2.5 ml kg(-1) min(-1)) whereas above CS (81.7 ± 2.5 ml kg(-1) min(-1)) increased to (84.0 ± 1.8 ml kg(-1) min(-1), P > 0.05 vs. above CS). The 11 individual muscles or muscle parts that evidenced the greatest blood flow increases above, relative to below, CS were composed of ≥ 69% Type IIb/d/x muscle fibres. Moreover, there was a significant correlation (P < 0.05, r = 0.42) between the increased blood flow above expressed relative to below CS and the percentage Type IIb/d/x fibres found in the individual muscles or muscle parts. These data validate the powerful CS construct in the rat and identify that running above CS, relative to below CS, incurs disproportionate blood flow increases (indicative of recruitment) in predominantly highly glycolytic muscle fibres.

Age- and sex-related differences in muscle phosphocreatine and oxygenation kinetics during high-intensity exercise in adolescents and adults

The aim of this investigation was to examine the adaptation of the muscle phosphates (e.g. phosphocreatine (PCr) and ADP) implicated in regulating oxidative phosphorylation, and oxygenation at the onset of high intensity exercise in children and adults. The hypotheses were threefold: primary PCr kinetics would be faster in children than adults; the amplitude of the PCr slow component would be attenuated in children; and the amplitude of the deoxyhaemoglobin/myoglobin (HHb) slow component would be reduced in children. Eleven children (5 girls, 6 boys, 13 +/- 1 years) and 11 adults (5 women, 6 men, 24 +/- 4 years) completed two to four constant work rate exercise tests within a 1.5 T MR scanner. Quadriceps muscle energetics during high intensity exercise were monitored using (31)P-MRS. Muscle oxygenation was monitored using near-infrared spectroscopy. The time constant for the PCr response was not significantly different in boys (31 +/- 10 s), girls (31 +/- 10 s), men (44 +/- 20 s) or women (29 +/- 14 s, main effects: age, p = 0.37, sex, p = 0.25). The amplitude of the PCr slow component relative to end-exercise PCr was not significantly different between children (23 +/- 23%) and adults (17 +/- 13%, p = 0.47). End-exercise [PCr] was significantly lower, and [ADP] higher, in females (18 +/- 4 mM and 53 +/- 16 microM) than males (23 +/- 4 mM, p = 0.02 and 37 +/- 11 microM, p = 0.02), but did not differ with age ([PCr]: p = 0.96, [ADP]: p = 0.72). The mean response time for muscle tissue deoxygenation was significantly faster in children (22 +/- 4 s) than adults (27 +/- 7 s, p = 0.01). The results of this study show that the control of oxidative metabolism at the onset of high intensity exercise is adult-like in 13-year-old children, but that matching of oxygen delivery to extraction is more precise in adults.

Evaluation of individual skeletal muscle activity by glucose uptake during pedaling exercise at different workloads using positron emission tomography

Skeletal muscle glucose uptake closely reflects muscle activity at exercise intensity levels <55% of maximal oxygen consumption (V̇o2max). Our purpose was to evaluate individual skeletal muscle activity from glucose uptake in humans during pedaling exercise at different workloads by using [18F]fluorodeoxyglucose (FDG) and positron emission tomography (PET). Twenty healthy male subjects were divided into two groups (7 exercise subjects and 13 control subjects). Exercise subjects were studied during 35 min of pedaling exercise at 40 and 55% V̇o2max exercise intensities. FDG was injected 10 min after the start of exercise or after 20 min of rest. PET scanning of the whole body was conducted after completion of the exercise or rest period. In exercise subjects, mean FDG uptake [standardized uptake ratio (SUR)] of the iliacus muscle and muscles of the anterior part of the thigh was significantly greater than uptake in muscles of control subjects. At 55% V̇o2max exercise, SURs of the iliacus muscle and thigh muscles, except for the rectus femoris, increased significantly compared with SURs at 40% V̇o2max exercise. Our results are the first to clarify that the iliacus muscle, as well as the muscles of the anterior thigh, is the prime muscle used during pedaling exercise. In addition, the iliacus muscle and all muscles in the thigh, except for the rectus femoris, contribute when the workload of the pedaling exercise increases from 40 to 55% V̇o2max.

Influence of priming exercise on pulmonary O2 uptake kinetics during transitions to high-intensity exercise from an elevated baseline

It has been suggested that the slower O2 uptake (V̇o2) kinetics observed when exercise is initiated from an elevated baseline metabolic rate are linked to an impairment of muscle O2 delivery. We hypothesized that “priming” exercise would significantly reduce the phase II time constant (τ) during subsequent severe-intensity cycle exercise initiated from an elevated baseline metabolic rate. Seven healthy men completed exercise transitions to 70% of the difference between gas exchange threshold (GET) and peak V̇o2 from a moderate-intensity baseline (90% GET) on three occasions in each of the “unprimed” and “primed” conditions. Pulmonary gas exchange, heart rate, and the electromyogram of m. vastus lateralis were measured during all tests. The phase II V̇o2 kinetics were slower when severe exercise was initiated from a baseline of moderate exercise compared with unloaded pedaling (mean ± SD τ, 42 ± 15 vs. 33 ± 8 s; P < 0.05), but were not accelerated by priming exercise (42 ± 17 s; P > 0.05). The amplitude of the V̇o2 slow component and the change in electromyogram from minutes 2 to 6 were both significantly reduced following priming exercise (V̇o2 slow component: from 0.47 ± 0.09 to 0.27 ± 0.13 l/min; change in integrated electromyogram between 2 and 6 min: from 51 ± 35 to 26±43% of baseline; P < 0.05 for both comparisons). These results indicate that the slower phase II V̇o2 kinetics observed during transitions to severe exercise from an elevated baseline are not altered by priming exercise, but that the reduced V̇o2 slow component may be linked to changes in muscle fiber activation.

Progressive recruitment of muscle fibers is not necessary for the slow component of VO2 kinetics

The "slow component" of O2 uptake (VO2) kinetics during constant-load heavy-intensity exercise is traditionally thought to derive from a progressive recruitment of muscle fibers. In this study, which represents a reanalysis of data taken from a previous study by our group (Grassi B, Hogan MC, Greenhaff PL, Hamann JJ, Kelley KM, Aschenbach WG, Constantin-Teodosiu D, Gladden LB. J Physiol 538: 195-207, 2002) we evaluated the presence of a slow component-like response in the isolated dog gastrocnemius in situ (n=6) during 4 min of contractions at approximately 60-70% of peak VO2. In this preparation all muscle fibers are maximally activated by electrical stimulation from the beginning of the contraction period, and no progressive recruitment of fibers is possible. Muscle VO2 was calculated as blood flow multiplied by arteriovenous O2 content difference. The muscle fatigued (force decreased by approximately 20-25%) during contractions. Kinetics of adjustment were evaluated for 1) VO2, uncorrected for force development; 2) VO2 normalized for peak force; 3) VO2 normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected VO2 was considered, was observed in all muscles when VO2/peak force and VO2/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for VO2/peak force (0.18+/-0.06, means+/-SE) and for VO2/force-time (0.22+/-0.05) compared with uncorrected VO2 (0.04+/-0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of VO2 kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.

Electromyographic analysis of pedaling: a review

Although pedaling is constrained by the circular trajectory of the pedals, it is not a simple movement. This review attempts to provide an overview of the pedaling technique using an electromyographic (EMG) approach. Literature concerning the electromyographic analysis of pedaling is reviewed in an effort to make a synthesis of the available information, and to point out its relevance for researchers, clinicians and/or cycling/triathlon trainers. The first part of the review depicts methodological aspects of the EMG signal recording and processing. We show how the pattern of muscle activation during pedaling can be analyzed in terms of muscle activity level and muscle activation timing. Muscle activity level is generally quantified with root mean square or integrated EMG values. Muscle activation timing is studied by defining EMG signal onset and offset times that identify the duration of EMG bursts and, more recently, by the determination of a lag time maximizing the cross-correlation coefficient. In the second part of the review, we describe whether the patterns of the lower limb muscles activity are influenced by numerous factors affecting pedaling such as power output, pedaling rate, body position, shoe-pedal interface, training status and fatigue. Some research perspectives linked to pedaling performance are discussed throughout the manuscript and in the conclusion.