The repeated-bout effect: influence on biceps brachii oxygenation and myoelectrical activity

This study investigated biceps brachii oxygenation and myoelectrical activity during and following maximal eccentric exercise to better understand the repeated-bout effect. Ten men performed two bouts of eccentric exercise (ECC1, ECC2), consisting of 10 sets of 6 maximal lengthening contractions of the elbow flexors separated by 4 wk. Tissue oxygenation index minimum amplitude (TOI(min)), mean and maximum total hemoglobin volume by near-infrared spectroscopy, torque, and surface electromyography root mean square (EMG(RMS)) during exercise were compared between ECC1 and ECC2. Changes in maximal voluntary isometric contraction (MVC) torque, range of motion, plasma creatine kinase activity, muscle soreness, TOI(min), and EMG(RMS) during sustained (10-s) and 30-repeated isometric contraction tasks at 30% (same absolute force) and 100% MVC (same relative force) for 4 days postexercise were compared between ECC1 and ECC2. No significant differences between ECC1 and ECC2 were evident for changes in torque, TOI(min), mean total hemoglobin volume, maximum total hemoglobin volume, and EMG(RMS) during exercise. Smaller (P < 0.05) changes and faster recovery of muscle damage markers were evident following ECC2 than ECC1. During 30% MVC tasks, TOI(min) did not change, but EMG(RMS) increased 1-4 days following ECC1 and ECC2. During 100% MVC tasks, EMG(RMS) did not change, but torque and TOI(min) decreased 1-4 days following ECC1 and ECC2. TOI(min) during 100% MVC tasks and EMG(RMS) during 30% MVC tasks recovered faster (P < 0.05) following ECC2 than ECC1. We conclude that the repeated-bout effect cannot be explained by altered muscle activation or metabolic/hemodynamic changes, and the faster recovery in muscle oxygenation and activation was mainly due to faster recovery of force.     

The isometric force that induces maximal surface muscle deoxygenation

To determine the external force that induces maximal deoxygenation of brachioradialis muscle 32 trained male subjects maintained isometric contractions using the elbow flexor muscles up to the limit time (isotonic part of the isometric contraction, IIC) and beyond that time for 120 s (anisotonic part of the isometric contraction). During IIC each subject maintained relative forces of either 25% and 70% maximal voluntary contraction (MVC), 50% and 100% MVC, or 40% and 60% MVC. Muscle oxygenation was assessed using a near infrared spectroscope, and expressed as a percentage of the reference value (ΔO_2rest) which was the difference between the minimal oxygenation obtained after 6 min of ischaemia at rest and the maximal reoxygenation following the release of the tourniquet. During IIC at 25% MVC, muscle oxygenation decreased to 17 (SEM 3)% ΔO_2rest, then it levelled off [25 (SEM 1)% ΔO_2rest]. After the point at which target force could not be maintained, reoxygenation was very weak. During IIC at 40%, 50%, 60%, and 70% MVC, the lowest muscle oxygenation values were obtained after 15–20 s of contraction and corresponded to −18 (SEM 6), −59 (SEM 12) −31 (SEM 6), and −29 (SEM 6)% ΔO_2rest, respectively. For the contraction at 100% MVC, the lowest oxygenation [−19 (SEM 9)% ΔO_2rest] was obtained while force was decreasing (69% MVC). During the anisotonic part of the isometric contractions, the greatest reoxygenation rate was obtained after 50% MVC IIC (P < 0.001). Our results showed that during isometric elbow flexions between 25% and 100% MVC, there was no linear relationship between external force and muscle oxygenation, and that the maximal deoxygenation of the brachioradialis muscle was obtained at 50% MVC. Accepted: 16 February 1998     

Sweating responses and the muscle metaboreflex under mildly hyperthermic conditions in sprinters and distance runners

To investigate the effects of different training methods on nonthermal sweating during activation of the muscle metaboreflex, we compared sweating responses during postexercise muscle occlusion in endurance runners, sprinters, and untrained men under mild hyperthermia (ambient temperature, 35°C; relative humidity, 50%). Ten endurance runners, nine sprinters, and ten untrained men (maximal oxygen uptakes: 57.5 ± 1.5, 49.3 ± 1.5, and 36.6 ± 1.6 ml·kg(-1)·min(-1), respectively; P < 0.05) performed an isometric handgrip exercise at 40% maximal voluntary contraction for 2 min, and then a pressure of 280 mmHg was applied to the forearm to occlude blood circulation for 2 min. The Δ change in mean arterial blood pressure between the resting level and the occlusion was significantly higher in sprinters than in untrained men (32.2 ± 4.4 vs. 17.3 ± 2.6 mmHg, respectively; P < 0.05); however, no difference was observed between distance runners and untrained men. The Δ mean sweating rate (averaged value of the forehead, chest, forearm, and thigh) during the occlusion was significantly higher in distance runners than in sprinters and untrained men (0.38 ± 0.07, 0.19 ± 0.03, and 0.11 ± 0.04 mg·cm(-2)·min(-1), respectively; P < 0.05) and did not differ between sprinters and untrained men. Our results suggest that the specificity of training modalities influences the sweating response during activation of the muscle metaboreflex. In addition, these results imply that a greater activation of the muscle metaboreflex does not cause a greater sweating response in sprinters.     

Inherent allocation patterns and potential growth rates of herbaceous climbing plants

We tested the hypothesis that herbaceous climbing plants, unlike non-climbing herbs, maximize height growth and leaf area, with minimal expenditure in support structures. The enhanced investment in leaf area was expected to result in high relative growth rates in terms of biomass increment. Four leguminous herbaceous climbers from nutrient-poor sites and four non-leguminous herbaceous climbers from nutrient-rich sites, were compared with non-climbing, self-supporting leguminous and non-leguminous herbaceous species from similar habitats. Plants were grown in hydroponic cultures in controlled environment chambers. All climbers had inherently taller shoots than self-supporting plants when compared at an equal amount of total plant dry weight, due to longer stems per unit of support biomass. In contrast to the hypothesis, the relative growth rates of all climbers were relatively low compared to the range found for self-supporting species. The biomass allocation patterns of the non-leguminous climbers were similar to those of the self-supporting species. Leguminous climbers allocated more biomass to support tissue and less biomass to leaves than non-climbers. As a result, height growth was even more emphasized in leguminous climbers than in non-leguminous climbers. Climbing legumes had high rates of net carbon gain, which partly compensated the lower relative leaf weight. We conclude that leguminous herbaceous climbers maximize height growth by a large investment in support biomass, enabling them to keep a large proportion of their leaves in the better illuminated environment at the top of the vegetation canopy.     

Muscle function in men and women during maximal eccentric exercise

This study assessed muscle fatigue patterns of the elbow flexors in untrained men and women to determine if sex differences exist during acute maximal eccentric exercise. High-intensity eccentric exercise is often used by athletes to elicit gains in muscle strength and size gains. Development of fatigue during this type of exercise can increase risk of injury; therefore, it is important to understand fatigue patterns during eccentric exercise to minimize injury risk exposure while still promoting training effects. While many isometric exercise studies have demonstrated that women show less fatigue, the patterns of fatigue during purely eccentric exercise have not been assessed in men and women. Based on the lack of sex differences in overall strength loss immediately post-eccentric exercise, it was hypothesized that women and men would have similar relative fatigue pattern responses (i.e., change from baseline) during a single bout of maximal eccentric exercise. Forty-six subjects (24 women and 22 men) completed 5 sets of 10 maximal eccentric contractions on an isokinetic dynamometer. Maximal voluntary isometric contraction strength was assessed at baseline and immediately following each exercise set. Maximal eccentric torque and contractile properties (i.e., contraction time, work, half relaxation time, and maximal rate of torque development) were calculated for each contraction. Men and women demonstrated similar relative isometric (32% for men and 39% for women) and eccentric (32% for men and 39% for women) fatigue as well as similar deficits in work done and rates of torque development and relaxation during exercise (p > 0.05). Untrained men and women displayed similar relative responses in all measures of muscle function during a single bout of maximal eccentric exercise of the elbow flexors. Thus, there is no reason to suspect that women may be more vulnerable to fatigue-related injury.     

Measuring lifting forces in rock climbing: effect of hold size and fingertip structure

This study investigates the hypothesis that shallow edge lifting force in high-level rock climbers is more strongly related to fingertip soft tissue anatomy than to absolute strength or strength to body mass ratio. Fifteen experienced climbers performed repeated maximal single hand lifting exercises on rectangular sandstone edges of depth 2.8, 4.3, 5.8, 7.3, and 12.5 mm while standing on a force measurement platform. Fingertip soft tissue dimensions were assessed by ultrasound imaging. Shallow edge (2.8 and 4.3 mm) lifting force, in newtons or body mass normalized, was uncorrelated with deep edge (12.5 mm) lifting force (r < .1). There was a positive correlation (r = .65, p < .05) between lifting force in newtons at 2.8 mm edge depth and tip of bone to tip of finger pulp measurement (r < .37 at other edge depths). The results confirm the common perception that maximum lifting force on a deep edge ("strength") does not predict maximum force production on very shallow edges. It is suggested that increased fingertip pulp dimension or plasticity may enable increased deformation of the fingertip, increasing the skin to rock contact area on very shallow edges, and thus increase the limit of force production. The study also confirmed previous assumptions of left/right force symmetry in climbers.     

Kinetic model for isometric contraction in smooth muscle on the basis of myosin phosphorylation hypothesis.

A kinetic model was proposed to simulate an isometric contraction curve in smooth muscle on the basis of the myosin phosphorylation hypothesis. The Ca2+-calmodulin-dependent activation of myosin light-chain kinase and the phosphorylation-dephosphorylation reaction of myosin were mathematically treated. Solving the kinetic equations at a steady state, we could calculate the relationship between the Ca2+ concentration and the myosin phosphorylation. Assuming that two-head-phosphorylated myosin has an actin-activated Mg2+-ATPase activity and that this state corresponds to an active state, we computed the time courses of the myosin phosphorylation and the active state for various Ca2+ transients. The time course of the active state was converted into that of isometric tension by use of Sandow's model composed of a contractile element and a series elastic component. The model could simulate not only the isometric contraction curves for any given Ca2+ transient but also the following experimental results: the calmodulin-dependent shift of the Ca2+ sensitivity of isometric tension observed in skinned muscle fibers, the disagreement between the Ca2+ sensitivity of myosin phosphorylation and that of isometric tension at a steady state, and the disagreement between the time course of myosin phosphorylation and that of isometric tension development.     

Effects of mechanical vibration applied in the opposite direction of muscle shortening on maximal isometric strength

Most studies about human responses to mechanical vibrations involve whole-body vibration and vibration applied perpendicularly to the tendon or muscle. The aim of the present study was to verify the effects of mechanical vibration applied in the opposite direction of muscle shortening on maximal isometric strength of the flexor muscles of the elbow due to neural factors. Conventional isometric training with maximal isometric contractions (MVCs) and isometric training with vibrations were compared. Nineteen untrained males, ages 24 +/- 3.28 years, were divided into 2 training groups. Group 1 performed conventional isometric training and group 2 isometric training with mechanical vibrations (frequency of 8 Hz and amplitude of 6 mm). Both groups executed 12 MVCs with a duration of 6 seconds and 2-minute intervals between the repetitions. The subjects trained 3 times per week for 4 weeks. The strength of the group subjected to vibrations increased significantly by 26 +/- 11% (p < 0.05), whereas the strength of the group with conventional isometric training increased only 10 +/- 5% (p < 0.05). These data suggest that training with vibrations applied in the opposite direction of muscle shortening enhances the mechanism of involuntary control of muscle activity and may improve strength in untrained males. Since these findings were in untrained males, further studies with athletes are necessary in order to generalize the results to athletes' training, although it seems that it would be possible.     

The effects of two different frequencies of whole-body vibration on knee extensors strength in healthy young volunteers: a randomized trial

The aim of this study was to investigate the effects of two different frequencies of whole-body vibration (WBV) training on knee extensors muscle strength in healthy young volunteers. Twenty-two eligible healthy untrained young women aged 22-31 years were allocated randomly to the 30-Hz (n=11) and 50-Hz (n=11) groups. They participated in a supervised WBV training program that consisted of 24 sessions on a synchronous vertical vibration platform (peak-to-peak displacement: 2-4 mm; type of exercises: semi-squat, one-legged squat, and lunge positions on right leg; set numbers: 2-24) three times per week for 8 weeks. Isometric and dynamic strength of the knee extensors were measured prior to and at the end of the 8-week training. In the 30-Hz group, there was a significant increase in the maximal voluntary isometric contraction (p=0.039) and the concentric peak torque (p=0.018) of knee extensors and these changes were significant (p<0.05) compared with the 50-Hz group. In addition, the eccentric peak torque of knee extensors was increased significantly in both groups (p<0.05); however, there was no significant difference between the two groups (p=0.873). We concluded that 8 weeks WBV training in 30 Hz was more effective than 50 Hz to increase the isometric contraction and dynamic strength of knee extensors as measured using peak concentric torque and equally effective with 50 Hz in improving eccentric torque of knee extensors in healthy young untrained women.     

Mechanomyographic and electromyographic responses to unilateral isometric training

The purpose of this investigation was to examine the effects of unilateral, isometric training of the forearm flexors on strength and the mechanomyographic (MMG) and electromyographic (EMG) responses of the biceps brachii in the trained and untrained limb at three joint angles. Seventeen adult females (mean age +/- SD = 21 +/- 2 years) were randomly assigned to a control (CTL; N=7) or a training (TRN; N=10) group. The TRN group performed isometric training of the non-dominant forearm flexors on a Cybex II Dynamometer at a joint angle such that the Cybex lever arm was positioned 60 degrees above the horizontal plane. The training consisted of 3 to 5 sets of 8, 6-second repetitions at 80% of maximal voluntary contraction 3 times per week for 8 weeks. The results indicated a significant increase in flexed arm circumference as well as isometric strength in the trained limb at all three joint angles. There were, however, no changes in MMG or EMG amplitude in the trained or untrained limb and no cross-training effect for strength or flexed arm circumference. These findings suggested that the increased strength may have been due to factors associated with hypertrophy, independent of neural adaptations in the biceps brachii. Furthermore, hypertrophy may have had counteractive effects on the MMG signal that could be responsible for the lack of a training-induced change in the MMG amplitude.     

Maximum bilateral contractions are modified by neurally mediated interlimb effects

The force exerted by a single limb during a maximal bilateral contraction has been found to be less than the force associated with a maximal unilateral contraction. The purpose of this study was to determine whether this bilateral deficit is due to neural mechanisms. For one experiment, three groups of subjects (untrained, cyclists, and weight lifters) performed maximal one- or two-limb isometric tasks for which the two-limb combinations were either both legs or the left arm and the right leg. The untrained subjects exhibited a bilateral deficit, the cyclists did not, and the weight lifters produced a bilateral facilitation. Although the changes in electromyogram did not completely parallel the changes in force, variability in the filtered electromyogram associated with the maximal contraction was too great for reliable interpretation. The arm-leg task demonstrated that the bilateral deficit affects only homologous contralateral muscles. For the second experiment, two groups of subjects (bilateral deficit and bilateral facilitation) performed maximal left leg contractions while the right leg either rested or was activated with electrical stimulation. All subjects produced an increase in the maximal voluntary left leg force during right leg electromyostimulation. The magnitude of the increase was greatest for the bilateral facilitation subjects. These results suggest that interlimb interactions during maximal bilateral contractions are mediated by neural mechanisms.     

Physiological modifications of local haemodynamic conditions during bilateral isometric contractions

Nine subjects (five women and four men) simultaneously performed two isometric contractions sustained until exhaustion at different relative forces: 40% of maximum voluntary contraction (MVC) for the right elbow flexors; 50% MVC for the left elbow flexors. Contraction of the left elbow flexors commenced at 50% of the limit time (maximum maintenance time) of isometric contraction of the right elbow flexors. Increase in heart rate during concomitant contraction of the left elbow flexors led to an increase in blood flow to the right elbow flexors. Under these conditions, the limit time of isometric contraction of the right elbow flexors was prolonged with respect to the limit time obtained for an isolated isometric contraction at the same relative tension. The difference was more significant in the female (+40%, P less than 0.05) than in the male subjects (+20%, P greater than 0.05).     

Noninvasive optical characterization of muscle blood flow,oxygenation, and metabolism in women with fibromyalgia

Introduction

Women with fibromyalgia (FM) have symptoms of increased muscular fatigue and reduced exercise tolerance, which may be associated with alterations in muscle microcirculation and oxygen metabolism. This study used near-infrared diffuse optical spectroscopies to noninvasively evaluate muscle blood flow, blood oxygenation and oxygen metabolism during leg fatiguing exercise and during arm arterial cuff occlusion in post-menopausal women with and without FM.

Methods

Fourteen women with FM and twenty-three well-matched healthy controls participated in this study. For the fatiguing exercise protocol, the subject was instructed to perform 6 sets of 12 isometric contractions of knee extensor muscles with intensity steadily increasing from 20 to 70% maximal voluntary isometric contraction (MVIC). For the cuff occlusion protocol, forearm arterial blood flow was occluded via a tourniquet on the upper arm for 3 minutes. Leg or arm muscle hemodynamics, including relative blood flow (rBF), oxy- and deoxy-hemoglobin concentration ([HbO₂] and [Hb]), total hemoglobin concentration (THC) and blood oxygen saturation (StO₂), were continuously monitored throughout protocols using a custom-built hybrid diffuse optical instrument that combined a commercial near-infrared oximeter for tissue oxygenation measurements and a custom-designed diffuse correlation spectroscopy (DCS) flowmeter for tissue blood flow measurements. Relative oxygen extraction fraction (rOEF) and oxygen consumption rate (rVO₂) were calculated from the measured blood flow and oxygenation data. Post-manipulation (fatiguing exercise or cuff occlusion) recovery in muscle hemodynamics was characterized by the recovery half-time, a time interval from the end of manipulation to the time that tissue hemodynamics reached a half-maximal value.

Results

Subjects with FM had similar hemodynamic and metabolic response/recovery patterns as healthy controls during exercise and during arterial occlusion. However, tissue rOEF during exercise in subjects with FM was significantly lower than in healthy controls, and the half-times of oxygenation recovery (Δ[HbO₂] and Δ[Hb]) were significantly longer following fatiguing exercise and cuff occlusion.

Conclusions

Our results suggest an alteration of muscle oxygen utilization in the FM population. This study demonstrates the potential of using combined diffuse optical spectroscopies (i.e., NIRS/DCS) to comprehensively evaluate tissue oxygen and flow kinetics in skeletal muscle.     

Effects of training on functional variables of muscles in reared Atlantic salmon Salmo salar smolts: connection to downstream migration pattern

The relative amount of muscle contraction regulating dihydropyridine and ryanodine receptors in the swimming muscles of trained reared Atlantic salmon Salmo salar smolts was compared with those of untrained and wild smolts. After an optimized 2 week training period, i.e. swimming with a velocity of 1·5 body lengths per second for 6 h per day, the level of both receptors was significantly higher in the muscles of trained S. salar than in the untrained ones before they were released into the natural environment. This difference persisted after downstream migration in the river. The highest level of receptors was observed in wild S. salar. Swimming performance was also higher in trained fish compared to untrained ones. Furthermore, swimming performance was positively associated with the level of receptors in both red and white muscle types. Downstream migration after release into the wild was significantly slower in trained smolts than in untrained fish. This indicates that trained smolts were most probably swimming harder against the current in the river than untrained smolts. The possible advantages for a slower migration in the river are discussed. This study shows that the prerequisites for effective contraction of the swimming muscles are better met in trained S. salar compared to untrained fish, and the muscles of trained smolts more closely resemble those of wild smolts. The results also imply that the capacity of untrained, reared smolts to swim against the current is not equal to that of their trained or wild counterparts which affects the downstream migration pattern of S. salar smolts.     

Leaves of Lianas and Self-Supporting Plants Differ in Mass per Unit Area and in Nitrogen Content

Abstract: The aim of this study was to test the hypothesis that the reduction in supporting tissues in climbers compared to self-supporting plants is also true for their leaves, and that climbers generally require higher leaf nitrogen than self-supporting plants to accomplish fast growth. This hypothesis was tested using paired samples of both growth forms with assessment of leaf area index above the sampled plants (LAI_a) in a tropical rain forest in Gabon. The sampling protocol ensured that within a highly fluctuating low canopy environment, the growth conditions were identical for each pair sampled. The results confirmed the hypothesis. Lianas had significantly lower leaf mass per unit leaf area (LMA) than their supporters. Liana leaves also contained significantly more nitrogen than host tree leaves. The differences in nitrogen concentration between liana and tree leaves reversed for the most shaded sites, when nitrogen was expressed on a leaf area base (N_area). Significant regression between leaf nitrogen and LAI_a was found for the climbers on the shaded sites but not for their supporters. This indicated better acclimation of climbers to prevailing light conditions. Better nitrogen allocation at low LMA, together with lower carbon costs for building supporting tissues, makes lianas highly competitive, especially where high nitrogen availability is assured.     

Changes in sEMG parameters among trunk and thigh muscles during a fatiguing bilateral isometric multi-joint task in trained and untrained subjects

This study aimed to explore changes in the electrical activity distribution among synergist muscles involved in the maintenance of this bilateral multi-joint task. It also tested relations between changes in surface electromyographic (sEMG) parameters with endurance time. Eighteen subjects, trained and untrained in hiking, performed a submaximal (50% of maximal contraction) isometric hiking test until exhaustion. The electrical activity of main superficial muscles implicated in this posture was recorded bilaterally. Trained subjects sustained the hiking position for 315 ± 82 s, versus 225 ± 68 s for untrained subjects. Patterns of electrical activity and mean power frequency (MPF) were different between populations. MPF shift in abdominal muscles was higher than in other synergists for both groups. Although typical changes in sEMG parameters were observed, few relations with endurance time were found, and for untrained subjects only. Changes in the relative contribution among synergists were observed, mainly for trained subjects. It is hypothesized that the task (a complex multi-joint posture involving numerous joints and muscles) may allow some variability in the contribution of synergist muscles during fatigue especially for the trained group. This probably explains the absence of relationship between endurance time and sEMG changes for trained subjects.     

The relation between spectral changes of the myoelectric signal and the intramuscular pressure of human skeletal muscle

The purpose of the present study was to investigate if the intramuscular pressure generated during an isometric muscle contraction is important for the appearance of EMG spectral changes accompanying localized muscular fatigue. The EMG and intramuscular pressure of the left biceps brachii in eight volunteers were recorded during standardized isometric contractions by means of intramuscular wire electrodes and infusion catheters, respectively. Spectral changes were elicited by a submaximal contraction and the intramuscular pressure at which the induced spectral changes were able to recover was determined. It was found that significant recovery was possible only if the intramuscular pressure dropped below a level of about 2.7 kPa (20 mm Hg). It is concluded that the intramuscular pressure during a sustained isometric contraction is relevant for the generation of fatigue induced spectral changes, and that measurement of the intramuscular pressure makes possible predetermination of whether or not an isometric muscle contraction is liable to result in localized muscular fatigue.     

The Pattern of Activation in the Sartorius Muscle of the Frog

The development of isometric twitch tension has been compared with the redevelopment of isometric tension in the fully active frog sartorius muscle following release. At 0°C the rate of rise of isometric twitch tension is the same as that for the muscle in the fully active state at the same tension but not until about 40 msec. after the stimulus and then only for a few milliseconds. The rates of rise of tension in the twitch and in the redevelopment of tension in the fully active muscle appear to be nearly the same at low tensions. Substitution of nitrate for chloride in the Ringer's solution bathing the muscle retards the development of tension during the early part of the contraction phase of the twitch and the effect reaches a maximum within 3 minutes after changing the solutions. These observations have been discussed in connection with some possible patterns of activation and the hypothesis has been advanced that the rate of activation of a sarcomere is determined mainly by the rate at which the transverse component of the link between excitation and contraction is propagated inwards from the periphery to the center of the fiber. This hypothesis has been discussed in relation to others concerning the nature of excitation-contraction coupling.     

Contribution of prostaglandins to the dilation that follows isometric forearm contraction in human subjects: effects of aspirin and hyperoxia.

In 11 healthy volunteers, we evaluated, in a double-blind crossover study, whether the vasodilation that follows isometric contraction is mediated by prostaglandins (PGs) and/or is O2 dependent. Subjects performed isometric handgrip for 2 min at 60% maximal voluntary contraction (MVC), after pretreatment with placebo or aspirin (600 mg orally), when breathing air or 40% O2. Forearm blood flow was measured in the dominant forearm by venous occlusion plethysmography. Arterial blood pressure was also recorded, allowing calculation of forearm vascular conductance (FVC; forearm blood flow/arterial blood pressure). During air breathing, aspirin significantly reduced the increase in FVC that followed contraction at 60% MVC: from a baseline of 0.09 +/- 0.011 [mean +/- SE, conductance units (CU)], the peak value was reduced from 0.24 +/- 0.03 to 0.14 +/- 0.01 CU. Breathing 40% O2 similarly reduced the increase in FVC relative to that evoked when breathing air; the peak value was 0.24 +/- 0.03 vs. 0.15 +/- 0.02 CU. However, after aspirin, breathing 40% O2 had no further effect on the contraction-evoked increase in FVC (the peak value was 0.15 +/- 0.02 vs. 0.16 +/- 0.02 CU). Thus the present study indicates that prostaglandins make a substantial contribution to the peak of the vasodilation that follows isometric contraction of forearm muscles at 60% MVC. Given that hyperoxia similarly reduced the vasodilation and attenuated the effect of aspirin, we propose that the stimulus for prostaglandin synthesis and release is hypoxia of the endothelium.     

Enhanced oxygen extraction and reduced flow heterogeneity in exercising muscle in endurance-trained men

The aim of this study was to investigate the effects of endurance training on skeletal muscle hemodynamics and oxygen consumption. Seven healthy endurance-trained and seven untrained subjects were studied. Oxygen uptake, blood flow, and blood volume were measured in the quadriceps femoris muscle group by use of positron emission tomography and [15O]O2, [15O]H2O, and [15O]CO during rest and one-legged submaximal intermittent isometric exercise. The oxygen extraction fraction was higher (0.49 +/- 0.14 vs. 0.29 +/- 0.12; P = 0.017) and blood transit time longer (0.6 +/- 0.1 vs. 0.4 +/- 0.1 min; P = 0.04) in the exercising muscle of the trained compared with the untrained subjects. The flow heterogeneity by means of relative dispersion was lower for the exercising muscle in the trained (50 +/- 9%) compared with the untrained subjects (65 +/- 13%, P = 0.025). In conclusion, oxygen extraction is higher, blood transit time longer, and perfusion more homogeneous in endurance-trained subjects compared with untrained subjects at the same workload. These changes may be associated with improved exercise efficiency in the endurance-trained subjects.