F. Bonde-Petersen

Skeletal muscle tension, flow, pressure, and EMG during sustained isometric contractions in humans

SummaryIn five healthy males sustained isometric torques during elbow flexion, knee extension, and plantar flexion correlated positively with intramuscular tissue pressure (MTP) in the range 0–80% of the maximal voluntary contraction (MVC). During passive compression of the muscle at rest 133-Xenon muscle clearance stopped when MTP reached diastolic arterial pressure (DAP) indicating that the muscle vascular bed was occluded. However, during sustained contractions this relation between DAP, flow and MTP was not seen. In two cases 133-Xenon clearance from M. soleus did not stop in spite of an 80% maximal contraction and MTP stayed below DAP. In other cases MTP would reach as high as 240 mm Hg before clearance was zero. In the deeper parts of the muscles MTP during contraction was increased in relation to the more superficial parts. The mean values for the %MVC that would stop MBF varied between 50 and 64% MVC for the investigated muscles. Mean rectified EMG (MEMG) showed a high correlation to MTP during sustained exhaustive contractions: When MEMG was kept constant MTP also remained constant while the exerted force decreased; when force was kept constant both MEMG and MTP increased in parallel. This demonstrated that muscle tissue compliance is decreasing during fatigue. Muscle ischemia occuring during sustained isometric contractions is partly due to the developed MTP, where especially the MTP around the veins in the deeper parts of the muscle can be considered of importance. However, ischemia is also affected by muscle fiber texture and anatomical distorsion of tissues.

The effect of weight-lifting exercise related to muscle fiber composition and muscle cross-sectional area in humans.

SummaryIsometric and dynamic strength and endurance of knee extensors were tested in 18 young males. The relative composition of slow (ST) and fast twitch (FT) fibers in the vastus lateralis muscle was registered from needle biopsies. Thigh muscle volume was evaluated from ultrasonic measurements. Six subjects served as controls, six trained with 50%, and six with 80% dynamic strength three times per week for 7 weeks with 20 and 12 repetitions per session, respectively. The training load was adjusted to the increases in strength observed during training. Dynamic strength increased by 42.3% in the 80% group (p< 0.01). In the control group and 50% group no significant increases were observed. Dynamic endurance: Controls showed no change. There was an over-all increase in the 50% group, while the 80% group only increased dynamic endurance for heavier loads. Isometric strength and endurance and fiber composition did not change in any group. In the 50% group the area of FT-realtive to ST-fibers increased 12.4% (p>0.05). Dynamic strength relative to muscle cross section increased by 30% in the 80% group (p<0.01) positively correlated to relative content of FT fibers.The present results confirm the specificity of training and indicate that a high content of FT fibers is a prerequisite for a successful strength training.

Local muscle blood flow and sustained contractions of human arm and back muscles

The endurance during sustained contraction of elbow, flexors, elbow extensors, and back extensors was tested in 3 human subjects. The force level used was varied between ca. 15 and ca. 75% of maximal isometric strength (IS). The clearance of133Xe from contracting muscles was registered during and after the endurance test. In this way it was possible to determine whether muscle blood flow (MBF) was increased or had stopped during the contraction. Experiments with artificial ischaemia of the upper arm together with MBF measurements showed that MBF was of no importance for continuing sustained contractions above a certain force level, which was 50, 25, and 40% of IS for elbow flexors, elbow extensors and back extensors, respectively. However, the level where longer lasting (>15 min) sustained contraction is possible is directly related to MBF. These levels were 22, 15, and 20% IS for elbow flexors, elbow extensors, and back extensors, respectively.

Cardiovascular responses to heat stress and blood volume displacements during exercise in man

SummarySubjects exercised in the upright position at approximately 50% of maximal oxygen consumption in four situations: in 25‡ C air, in 45‡ C air [mean skin temperature (

Central venous pressure and plasma arginine vasopressin during water immersion in man

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A simple force platform.

sk) 35‡ C], in 35‡ C water immersed to the level of the xiphoid process, and finally wearing a suit perfused with 35‡ C water. The water immersion prevented gravitational shifts of blood volume to the legs. In this situation the forearm blood flow (FBF) rose continually with increasing core temperature (Tes) in contrast to the attenuation in rise above 38‡ C Tes in 45‡ C air. The differences were significant above 38.6‡ C Tes in experiments in eight subjects. The effects of immersion on cardiac output (CO), stroke volume (SV), and heart rate (HR) were studied in five of the subjects in relation to Tes, since the rate of rise of Tes was different in the four situations. CO and SV tended to be higher during both rest and exercise in the water than in the other three conditions, while HR rose in the same manner with increasing core temperature, except that it was lower in 25‡ C air, where Tes was lower. Thus, the prevention of hydrostatic shifts of peripheral venous volume permitted the maintenance of a higher SV and peripheral blood flow, and enhanced the ability of the circulation to deal with the combined exercise and heat stress.

Cardiovascular effects of simulated zero-gravity in humans.

SummaryDuring prolonged heavy exercise a gradual upward drift in heart rate (HR) is seen after the first 10 min of exercise. This “secondary rise” might be caused by a reduction in stroke volume due to reduced filling of the heart, which is dependent upon both hemodynamic pressure and blood volume. Swimming and bicycling differ with respect to hydrostatic pressure and to water loss, due to sweating. Five subjects were studied during 90 min of bicycle exercise, and swimming the leg kick of free style. The horizontal position during swimming resulted in a larger cardiac output and stroke volume. After the initial rise in heart rate the “secondary rise” followed parallel courses in the two situations. The rises were positively related to the measured increments in plasma catecholamine concentrations, which continued to increase as exercise progresssed. The secondary rise in HR could not be explained by changes in plasma volume or in water balance, nor by changes in plasma [K]. The plasma volume decreased 5–6% (225–250 ml) within the first 5 to 10 min of exercise both in bicycling and swimming, but thereafter remained virtually unchanged. The sweat loss during bicycling was four times greater than during swimming; but during swimming the hydrostatic conditions induced a diuresis, so that the total water loss was only 25% less than during bicycling.

Central venous pressure and plasma arginine vasopressin in man during water immersion combined with changes in blood volume

SummaryThe influence of increased central venous pressure (CVP) on the plasma concentration of arginine vasopressin (pAVP) was examined in 7 healthy males subjected to water immersion (WI) up to the neck following overnight food- and fluid restriction. During WI the subject sat upright in a pool (water temperature=35.0‡ C) for 6 h. In control experiments the subject assumed the same position outside the pool wearing a water perfused garment (water temperature=34.6‡ C). CVP increased markedly during WI and after 20 min of immersion it attained a level which was significantly higher than the control value (10.9±1.5 (mean ± SE) vs. 2.2±1.3 mm Hg, p<0.01). This increase was sustained throughout the 6 h WI period. Simultaneously, after 20 min pAVP during WI was significantly lower than control values (1.8±0.3 vs. 2.2±0.3 pg·ml−1, p<0.05) and sustained throughout WI. Systolic arterial pressure increased significantly by 7–10 mm Hg (p<0.05) after 2 h of WI, while diastolic arterial pressure was unchanged. Heart rate was decreased by 10 bpm throughout immersion. There was no change in plasma osmolality when comparing control with immersion. A pronounced osmotic diuresis, natriuresis and kaliuresis occurred during WI, counteracting an acute significant increase in plasma volume of 6.5±1.9% (P<0.01 within 20 min of immersion). We conclude that an increase in CVP due to WI is accompanied by suppressed pAVP.