Biomedical subjects

A Veicsteinas

Publications and source records attributed to A Veicsteinas.

At least 37 records · Page 2Linked to original sources

PubMed1996

Effects of high altitude acclimatization on heart rate variability in resting humans.

The sympatho-vagal nerve interaction at the heart was studied by means of power spectrum analysis of heart rate variability in seven Caucasians (aged 27-35 years) in resting supine and sitting positions before and during 35 days of a sojourn at 5050 m above sea level (asl) and in six Sherpas (aged 22-30 years) at high altitude only. A high frequency peak (HF)-central frequency between 0.20 and 0.33 Hz, a low frequency peak (LF)-central frequency between 0.08 and 0.14 Hz, and a very low frequency component (< 0.05 Hz), no peak observed, were found in the power spectrum in both positions and independent of altitude. The peak powers, as a percentage of the total power, were affected by both body position and altitude. At sea level the change from a supine to a sitting position yielded a decrease in percentage HF from 25 (SEM 1.9)% to 6.2 (SEM 1.5)% (P < 0.05) and a significant increase in the ratio between LF and HF powers (LF:HF) from 1.7 (SEM 0.4) to 6.9 (SEM 1.6). At altitude compared to sea level in the supine position, percentage HF decreased from 25% to 10.9 (SEM 1.0)% (P < 0.05) and the LF:HF ratio increased from 1.7 to 4.8 (SEM 0.7) (P < 0.05). No changes occurred at altitude in the sitting position either in the peak powers or in the LF:HF ratio, but the central frequency of HF peak increased significantly from 0.25 (SEM 0.02) Hz to 0.32 (SEM 0.01) Hz. In the Sherpas comparable results to the Caucasians were found in both body positions. The high LF:HF ratios observed at altitude in both body positions and groups would suggest that hypoxia caused a shift of sympatho-vagal nerve interaction at rest toward a dominance of the sympathetic system, which was found at sea level only in the sitting position. An acclimatization period of 10 days higher than 2850 m asl and 1 month at 5050 m asl did not modify the interactions of the autonomic systems.

Acclimatization↗

PubMed1996

Surface mechanomyogram reflects muscle fibres twitches summation.

The aim of this work is to define the pattern of summation of the muscle fibre twitches in the surface mechanomyogram (MMG) generation process. For this purpose, two groups of muscle fibres of the extensor digitorum communis (EDC) were stimulated using needle electrodes. To these two artificial (because made by different muscle fibre types) motor units (MU1 and MU2), we administered: (a) separate stimulations: 3 and 9 Hz (MU1), 8 and 20 Hz (MU2) for 5 s; (b) simultaneous stimulation: 3 Hz (MU1) + 8 Hz (MU2); 9 Hz (MU1) + 20 Hz (MU2) for 5 s. The mechanomyograms, recorded during separate stimulation of MU1 and MU2, were linearly summated for the generation of a mechanomyographic signal to be compared with the one detected during (b) stimulation procedure. The bispectrum and the bicoherence of the generated MMG (MMGg) and of the MMG recorded during simultaneous (MMGs) stimulation were calculated for the detection of the quadratic non-linearity in the system responses. It was found that the MMGg and MMGs presented difference in the bispectrum and bicoherence index only when the 9-20 Hz stimulation pair was considered In conclusion, our data indicate that the MMG derives from the summation of the active muscle fibres twitches and that the latter is linear only for very low firing rates. This is to be carefully considered when studies on MMG modelling will be undertaken.

Electric Stimulation↗

PubMed1995

Biologic effects of prolonged exposure to ELF electromagnetic fields in rats: II. 50 Hz magnetic fields.

To provide possible laboratory support to health risk evaluation associated with long-term, low-intensity magnetic field exposure, 256 male albino rats and an equal number of control animals (initial age 12 weeks) were exposed 22 h/day to a 50 Hz magnetic flux density of 5 microT for 32 weeks (a total of about 5000 h). Hematology was studied from blood samples before exposure to the field and at 12 week intervals. Morphology and histology of liver, heart, mesenteric lymph nodes, and testes as well as brain neurotransmitters were assessed at the end of the exposure period. In two identical sets of experiments, no significant differences in the investigated variables were found between exposed and sham-exposed animals. It is concluded that continuous exposure to a 50 Hz magnetic field of 5 microT from week 12 to week 44, which makes up approximately 70% of the life span of the rat before sacrifice, does not cause changes in growth rate, in the morphology and histology of liver, heart, mesenteric lymph nodes, testes, and bone marrow, in hematology and hematochemistry, or in the neurotransmitters dopamine and serotonin.

Animals↗

PubMed1995

Loss of muscle oxidative capacity after an extreme endurance run: the Paris-Dakar foot-race.

We measured changes in maximal oxygen uptake capacity (VO2max), ventilation, heart rate, plasma lactate and speed at the end of an incremental exercise test as a consequence of a relay foot race from Paris to Dakar in 6 subjects. Additionally, anthropometric measurements were taken and muscle biopsies from M. vastus lateralis were obtained before and after the race. The latter were analyzed with morphometric methods for fiber size, capillarity and muscle ultrastructural composition. Weight specific VO2max was significantly reduced from 62.4 to 60.5 ml/min.kg after the race while absolute VO2max and the other endurance related functional variables remained unchanged. Body fat, thigh cross-sectional area and thigh volume showed tendential reduction immediately after the race but regained pre-race values within a few days. Fiber size and capillarity were not affected by the race. Volume density of total mitochondria was significantly reduced from 6.98 to 4.89% of fiber volume. Both subsarcolemmal and interfibrillar mitochondria were significantly reduced by 59 and 21%, respectively. The volume density of satellite cell was increased about three-fold whereas the content of lipofuscin remained constant. It is concluded that extreme endurance events such as a multi-stage relay race may induce a considerable loss of oxidative capacity of skeletal muscle tissue.

Adipose Tissue↗

PubMed1994

Effect of acclimatization to high altitude (5,050 m) on motor unit activation pattern and muscle performance.

This study evaluated the influence of acute and chronic hypoxia (up to 40 days at 5,050 m) on the muscle strength, fast-to-slow fiber motor unit (MU) ratio, and MU activation strategy. These factors were studied by measuring the maximal voluntary contraction (MVC) of the elbow flexors and analyzing surface electromyogram (EMG) and soundmyogram (SMG) of the isometrically contracting biceps brachii. From the time and frequency domain analysis of EMG and SMG, the root mean square and mean frequency of the spectra were calculated. SMG and EMG were recorded during 4-s exercises in the 20-100% MVC range before departure; after 2, 15, and 40 days at high altitude; and after the return to sea level. MVC or root mean square and mean frequency at all levels of effort were not significantly different among all experimental conditions. In the present study where optimal living conditions (i.e., food, physical activity, and environmental temperature) were ensured, acute and chronic hypoxia (at least up to 40 days) did not affect the maximal force output or fast-to-slow fiber MU ratio and MU activation pattern during isometric exercise.

Acclimatization↗

PubMed1993

Biologic effects of prolonged exposure to ELF electromagnetic fields in rats. I. 50 Hz electric fields.

A three-year investigation was conducted on the biological effects of high-intensity electric field exposures of rats for up to 18% of their life span. Two hundred and forty adult male rats, divided into groups of 20 animals each, were exposed at ground potential for 8 h/day at 25-kV/m and 100-kV/m 50-Hz electric fields or were sham exposed for 280, 440, and 1240 h. The corresponding ages at sacrifice were 140, 164, and 315 days. An additional group of 40 rats was investigated under similar experimental conditions after 440 h of exposure at floating potential. Independent of exposure duration, mode of grounding, and field strength, no statistical differences in body weight, morphology, and histology of the liver, heart, mesenteric lymph nodes, and blood variables (hematology and serum chemistry) were found in comparison with sham-exposed animals. Plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone (TS) at sacrifice varied widely among experimental animals in the same group but did not differ in exposed compared with sham-exposed rats. A nonsignificant tendency toward a decrease in the testes/body weight ratio was found after 1240 h of exposure. Microscopic examination of a large number of specimens showed no quantitative or qualitative statistical differences in testes alterations either among exposed animals or between exposed and their corresponding sham-exposed groups. We conclude that 50-Hz electric field exposure, even of long duration at very high field strengths, does not induce harmful effects on tissues with high cellular turnover rates and does not impair the reproductive function of rats. Moreover, after exposure, all variables investigated were well within the normal physiological range.

Animals↗

PubMed1993

Body position affects the power spectrum of heart rate variability during dynamic exercise.

The power spectrum analysis of R-R interval variability (RRV) has been estimated by means of an autoregressive method in six men in supine (S) and sitting (C) postures at rest and during steady-state cycle exercise at about 14%, 28%, 45%, 67% of the maximal oxygen consumption (% VO2max). The total power of RRV decreased exponentially as a function of exercise intensity in a similar way in both postures. Three components were recognized in the power spectra: firstly, a high frequency peak (HF), an expression of respiratory arrhythmia, the central frequency (fcentral) of which increased in both S and C from a resting value of about 0.26 Hz to 0.42 Hz at 67% VO2max; secondly, a low frequency peak (LF) related to arterial pressure control, the fcentral of which remained constant at 0.1 Hz in C, whereas in S above 28% VO2max decreased to 0.07 Hz; and thirdly, a very low frequency component (VLF; less than 0.05 Hz, no fcentral). The power of the three components (as a percentage of the total power) depended on the body posture and the metabolic demand. HF% at rest was 30.3 (SEM 6.6) % in S and 5.0 (SEM 0.8) % in C. During exercise HF% decreased by about 30% in S and increased to 19.7 (SEM 5.5) % at 28% VO2max in C. LF% was lower in S than in C at rest [31.6 (SEM 5.7) % vs 44.9 (SEM 6.4) %; P < 0.05], remaining constant up to 28% VO2max.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Energy cost and efficiency of riding aerodynamic bicycles.

Traction resistance (Rt) was determined by towing two cyclists in fully dropped posture on bicycles with an aerodynamic frame with lenticular wheels (AL), an aerodynamic frame with traditional wheels (AT), or a traditional frame with lenticular wheels (TL) in calm air on a flat wooden track at constant speed (8.6-14.6 m.s-1). Under all experimental conditions, Rt increased linearly with the square of air velocity (v2a); r2 equal to greater than 0.89. The constant k = delta Rt/delta v2a was about 15% lower for AL and AT (0.157 and 0.155 N.s2 x m-2) than for TL bicycles (0.184 N.s2 x m-2). These data show firstly, that in terms of mechanical energy savings, the role of lenticular wheels is negligible and, secondly, that for TL bicycles, the value of k was essentially equal to that found by others for bicycles with a traditional frame and traditional wheels (TT). The energy cost of cycling per unit distance (Cc, J.m-1) was also measured for AT and TT bicycles from the ratio of the O2 consumption above resting to speed, in the speed range from 4.7 to 11.1 m.s-1. The Cc also increased linearly with v2a, as described by: Cc = 30.8 + 0.558 v2a and Cc = 29.6 + 0.606 v2a for AT and TT bicycles. Thus from our study it would seem that AT bicycles are only about 5% more economical than TT at 12.5 m.s-1 the economy tending to increase slightly with the speed.(ABSTRACT TRUNCATED AT 250 WORDS)

Aerobiosis↗

PubMed1993

Kinetics of heart rate and catecholamines during exercise in humans. The effect of heart denervation.

To elucidate the role of factors other than the nervous system in heart rate (fc) control during exercise, the kinetics of fc and plasma catecholamine concentrations were studied in ten heart transplant recipients during and after 10-min cycle ergometer exercise at 50 W. The fc did not increase at the beginning of the exercise for about 60 s. Then in the eight subjects who completed the exercise it increased following an exponential kinetic with a mean time constant of 210 (SEM 22) s. The two other subjects were exhausted after 5 and 8 min of exercise during which fc increased linearly. At the cessation of the exercise, fc remained unchanged for about 50 s and then decreased exponentially with a time constant which was unchanged from that at the beginning of exercise. In the group of eight subjects plasma noradrenaline concentration ([NA]) increased after 30 s to a mean value above resting of 547 (SEM 124) pg.ml-1, showing a tendency to a plateau, while adrenaline concentration ([A]) did not increase significantly. In the two subjects who became exhausted an almost linear increase in [NA] occurred up to about 1,300 pg.ml-1 coupled with a significant increase in [A]. During recovery an immediate decrease in [NA] was observed towards resting values. The values of the fc increase above resting levels determined at the time of blood collection were linearly related with [NA] increments both at the beginning and end of exercise with a similar slope, i.e. about 2.5 beats.min-1 per 100 pg.ml-1 of [NA] change.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Muscle sound and electromyogram spectrum analysis during exhausting contractions in man.

The changes in the soundmyogram (SMG) and electromyogram (EMG) frequency content during exhausting contractions at 20%, 40%, 60% and 80% of the maximal voluntary contraction (MVC) were investigated by the spectral analysis of the SMG and EMG detected from the biceps brachii muscles of 13 healthy men. The root mean squares (rms) of the two signals were also calculated. Throughout contraction the EMG rms always increased while this was true only at 20% MVC for the SMG. A marked decrease was detected at 60% and 80% MVC. With fatigue the EMG spectra presented a compression towards the lower frequencies at all exercise intensities. The SMG showed a more complex behaviour with a transient increase in its frequency content, followed by a continuous compression of the spectra, at 60% and 80% MVC, and a nearly stable frequency content at lower contraction intensities. This study suggested that different aspects of the changes in the motor unit's activation strategy at different levels of exhausting contractions can be monitored by SMG and EMG signals.

Adult↗

PubMed1992

Heart rate overshoot at the beginning of muscle exercise.

A characteristic notch in the heart rate (fc) on-response at the beginning of square-wave exercise is described in 7 very fit marathon runners and 12 sedentary young men, during cycle tests at 30% and 60% of maximal oxygen consumption (VO2max). The fc notch revealed a fc overshoot with respect to the fc values predicted from exponential beat-by-beat fitted models. While at 30% of VO2max all subjects showed a fc overshoot, at 60% of VO2max it occurred in the marathon runners but not in the sedentary subjects. The mean time of occurrence of the fc overshoot from the onset of the exercise was 16.7 (SD 4.7) s and 12.2 (SD 3.2) s at 30% of VO2max in the runners and the sedentary subjects respectively, and 23.8 (SD 8.8) s at 60% of VO2max in the runners. The amplitude of the overshoot, with respect to rest, was 41 (SD 12) beats.min-1 and 31 (SD 4) beats.min-1 at 30% of VO2max in the runners and the sedentary subjects respectively, and 46 (SD 19) beats.min-1 at 60% of VO2max in the runners. The existence and the amplitude of the fc overshoot may have been related to central command and muscle heart reflex mechanisms and thus may have been indicators of changes in the balance between sympathetic and parasympathetic activity occurring in fit and unfit subjects.

Adult↗

PubMed1992

Soundmyogram analysis during sustained maximal voluntary contraction in sprinters and long distance runners.

The aim of this study was to describe the influence of the different fiber type composition of the vastus lateralis muscle on the soundmyogram (SMG) time and frequency domain characteristics. The SMG was recorded from the vastus lateralis belly during exhausting maximal voluntary contraction (MVC) of the leg extensors in 7 sprinters (SPR) and 7 long distance runners (LDR). Seven sedentary males (SED) were investigated in the same experimental conditions. In the SPR the effort time was shorter and the MVC was greater while the SMG root mean square and the SMG frequency content, at the onset of contraction, were higher than in the SED and LDR, respectively. Throughout exertion the SMG RMS showed clear reduction for SPR and SED only and the SMG power spectra presented a compression towards the lower frequencies. The reported phenomena were less pronounced in the LDR than in SPR and SED. These results can be explained when the differences in the % of fast twitch fibers area, belonging to stronger and more fatiguing F motor units, in the three groups of subjects are considered and suggest that SMG characteristics reflect the muscle fiber typing.

Adolescent↗

PubMed1990

The influence of exercise intensity on the power spectrum of heart rate variability.

The power spectral analysis of R-R interval variability (RRV) has been estimated by means of an autoregressive method in seven sedentary males at rest, during steady-state cycle exercise at 21 percent maximal oxygen uptake (%VO2max), SEM 2%, 49% VO2max, SEM 2% and 70% VO2max, SEM 2% and during recovery. The RRV, i.e. the absolute power of the spectrum, decreased 10, 100 and 500 times in the three exercise intensities, returning to resting value during recovery. In the RRV power spectrum three components have been identified: (1) high frequency peak (HF), central frequency about 0.24 Hz at rest and recovery, and 0.28 Hz, SEM 0.02, 0.37 Hz, SEM 0.03 and 0.48 Hz, SEM 0.06 during the three exercise intensities, respectively; (2) low frequency peak (LF), central frequency about 0.1 Hz independent of the metabolic state; (3) very low frequency component (VLF), less than 0.05 Hz, no peak observed. The HF peak power, as a percentage of the total power (HF%), averaged 16%, SEM 5% at rest and did not change during exercise, whereas during recovery it decreased to 5%-10%. The LF% and VLF% were about 50% and 35% at rest and during low exercise intensity, respectively. At higher intensities, LF% decreased to 16% and VLF% increased to 70%. During recovery a return to resting values occurred. The HF component may reflect the increased respiratory rate and the LF peak changes the resetting of the baroreceptor reflex with exercise. The hypothesis is made that VLF fluctuations in heart rate might be partially mediated by the sympathetic system.

Adult↗

PubMed1990

Energy cost of walking with hip joint impairment.

The energy cost of walking was measured in 12 patients (age 39-73 years) with hip joint impairment and 10 healthy controls during unassisted walking (2-6 km.h-1) on a level treadmill surface and on a 5% incline. The energy cost of locomotion in most patients increased up to 50% and 70% during level-surface and uphill walking, respectively. This difference between patients and controls was probably due to the increased external mechanical work. The energy cost of walking, although related to pain experienced during walking but not to hip joint range of motion or to joint status evaluated radiographically, provides an additional variable when defining the conditions of disability and functional impairment in individuals with this pathological condition. [Gussoni M, Margonato V, Ventura R, et al: Energy cost of walking with hip joint impairment.

Adult↗

PubMed1990

Spectral analysis of muscular sound during isometric contraction of biceps brachii.

The frequency content of muscular sound (MS), detected by placing a contact sensor transducer over the belly of the biceps brachii during 10 isometric contractions of 4 s each [10-100% of maximal voluntary contraction (MVC)] in seven sedentary men, was analyzed by the maximum entropy spectral estimation and the fast Fourier transform methods. With increasing %MVC, the power spectrum of the MS enlarges and tends to be multimodal beyond 30% MVC. Independent of the method, the mean frequency is approximately 11 Hz at the lower tasks, and then it increases up to 15 Hz at 80% MVC and to 22 Hz at 100% MVC. When the effort is increased the relative power in the 15- to 45-Hz bandwidth (range of firing rate of the motor units with fast-twitch fibers) from 20% reaches 55% of the power in the 6- to 45-Hz bandwidth (firing rate range of motor units with slow- and fast-twitch fibers). Our results obtained by the two different modeling approaches confirm the reliability of the sound signal. Moreover, it appears that from the MS the motor unit activation pattern can be retrieved.

Adult↗

PubMed1989

Muscular sound and force relationship during isometric contraction in man.

The contracting muscle generates a low frequency sound detectable at the belly surface, ranging from 11 to 40 Hz. To study the relationship between the muscular sound and the intensity of the contraction a sound myogram (SMG) was recorded by a contact sensor from the biceps brachii of seven young healthy males performing 4-s isometric contractions from 10% to 100% of the maximal voluntary contraction (MVC), in 10% steps. Simultaneously, the electromyogram (EMG) was recorded as an index of muscle activity. SMG and EMG were integrated by conventional methods (iSMG and iEMG). The relationship between iSMG and iEMG vs MVC% is described by parabolic functions up to 80% and 100% MVC respectively. Beyond 80% MVC the iSMG decreases, being about half of its maximal value at 100% MVC. Our results indicate that the motor unit recruitment and firing rate affect the iSMG and iEMG in the same way up to 80% MVC. From 80% to 100% MVC the high motor units' discharge rate and the muscular stiffness together limit the pressure waves generated by the dimensional changes of the active fibres. The muscular sound seems to reflect the intramuscular visco-elastic characteristics and the motor unit activation pattern of a contracting muscle.

Adult↗

PubMed1989

Plasma norepinephrine and heart rate dynamics during recovery from submaximal exercise in man.

The time course of heart rate (HR) and venous blood norepinephrine concentration [NE], as an expression of the sympathetic nervous activity (SNA), was studied in six sedentary young men during recovery from three periods of cycle ergometer exercise at 21% +/- 2.8%, 43% +/- 2.1% and 65% +/- 2.3% of VO2max respectively (mean +/- SE). The HR decreased mono-exponentially with tau values of 13.6 +/- 1.6 s, 32.7 +/- 5.6 s and 55.8 +/- 8.1 s respectively in the three periods of exercise. At the low exercise level no change in [NE] was found. At medium and high exercise intensity: (a) [NE] increased significantly at the 5th min of exercise (delta [NE] = 207.7 +/- 22.5 pg.ml-1 and 521.3 +/- 58.3 pg.ml-1 respectively); (b) after a time lag of 1 min [NE] decreased exponentially (tau = 87 s and 101 s respectively); (c) in the 1st min HR decreased about 35 beats.min-1; (d) from the 2nd to 5th min of recovery HR and [NE] were linearly related (100 pg.ml-1 delta [NE] congruent to 5 beats.min-1). In the 1st min of recovery, independent of the exercise intensity, the adjustment of HR appears to have been due mainly to the prompt restoration of vagal tone. The further decrease in HR toward the resting value could then be attributed to the return of SNA to the pre-exercise level.

Adult↗

PubMed1989

Changes of muscular sound during sustained isometric contraction up to exhaustion.

The sound (SMG) generated by the biceps muscle during isometric exercise at 20, 40, 60, and 80% of maximum voluntary contraction (MVC) up to exhaustion has been recorded by a contact transducer and integrated (iSMG), together with the surface electromyogram (EMG) in eight young untrained men. At the onset of exercise, iSMG and integrated surface EMG (iEMG) amplitude increased linearly with exercise. iSMG remained constant for 253 +/- 73 (SD), 45 +/- 16, 21 +/- 5, and 0 s at the four levels of contraction. Then iSMG increased linearly at 20% MVC, fluctuated at 40% MVC, and decreased exponentially at 60 and 80% MVC. iSMG exhaustion-to-onset ratio was 5.0 at 20%, 1.0 at 40%, and 0.2 at 60 and 80% MVC. On the contrary, independently of exercise intensity, iEMG increased with time, being 1.4 higher at exhaustion than at the onset. The nonunivocal iSMG changes with time and effort of exercise suggest that the sound may be a useful tool to acquire different information to EMG and output force during muscle contraction up to fatigue.

Adult↗