| 1. |
- Eiken, Ola, et al.
(författare)
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Cardiac responses to lower body negative pressure and dynamic leg exercise
- 1985
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Ingår i: European Journal of Applied Physiology and Occupational Physiology. - 0301-5548 .- 1432-1025. ; 54:5, s. 451-455
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Tidskriftsartikel (refereegranskat)abstract
- Cardiac responses to dynamic leg exercise at 0, 50, and 100 W in the supine position were investigated with and without the lower portion of the body exposed to a pressure of -6.6 kPa (Lower Body Negative Pressure, LBNP). Resting values for heart rate (HR) and stroke volume (SV) were considerably higher and lower, respectively, during LBNP than in the control condition. At the transition from rest to the mildest exercise during LBNP SV showed a prompt increase by about 40%, but no significant change in the control condition. HR, which increased by 17 beats X min-1 in the control condition, showed during LBNP no change initially and subsequently a small but significant drop below its resting value. Steady-state values for HR at the various levels of exercise were not significantly affected by LBNP, whereas corresponding values for SV were considerably lowered, so that exercise values for cardiac output were about 3 l X min-1 less during LBNP than in the control condition. The reductions in SV and cardiac output indicate residual pooling of blood in intra- and extramuscular capacitance vessels of the legs. With a change from rest to exercise at 100 W during LBNP mean systolic ejection rate (MSER) increased by 67%, the relations between SV and MSER suggesting that ventricular performance was maintained by a combination of the Frank-Starling mechanism and enhanced contractile strength.
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| 2. |
- Eiken, Ola
(författare)
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Effects of increased muscle perfusion pressure on responses to dynamic leg exercise in man
- 1988
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Ingår i: European Journal of Applied Physiology and Occupational Physiology. - 0301-5548 .- 1432-1025. ; 57:6, s. 772-776
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Tidskriftsartikel (refereegranskat)abstract
- Ventilatory, cardiovascular and metabolic functions and work performance were studied in men performing incremental-load dynamic leg exercise until exhaustion. Part I: Responses to supine exercise were investigated in 8 subjects during exposure of the lower body to subatmospheric pressure at -6.67 kPa (-50 mm Hg) (Lower Body Negative Pressure, LBNP). Due to curtailment of stroke volume, cardiac output was reduced by LBNP over a wide range of work intensities, including heavy loads: ventilation, oxygen uptake and blood lactate concentrations increased with work load, but at lower rates than in the control condition. Part II: In 9 subjects, work performance was compared in three conditions: supine exercise with and without LBNP, and upright exercise. Performance in supine exercise was enhanced by LBNP, and was further improved in upright exercise. In supine exercise, the LBNP-induced reduction in blood lactate and enhancement of work performance are attributed to a more efficient muscle blood flow resulting from increased local perfusion pressure. This strongly suggests that the primary limitation of work performance was set by the peripheral circulation in working muscles rather than by cardiac performance. A similar mechanism may, in part, explain why work performance in dynamic leg exercise was greater in the upright than in the supine posture. It is also concluded that supine leg exercise during LBNP is a useful model of upright exercise, with regard to the central circulation and the circulation in working muscles.
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| 3. |
- Mekjavic, I. B., et al.
(författare)
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The pattern of breathing during hypoxic exercise
- 1987
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Ingår i: European Journal of Applied Physiology and Occupational Physiology. - 0301-5548 .- 1432-1025. ; 56:6, s. 619-622
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Tidskriftsartikel (refereegranskat)abstract
- Breathing pattern was studied in six subjects in normoxia (FIO2 = 0.21) and hypoxia (FIO2 = 0.12) at rest and during incremental work-rate exercise. Ventilation (V) as well as mean inspiratory flow (VT/TI) increased with exercise intensity and were augmented in the hypoxic environment, whereas the ratio between inspiratory (TI) and total (Ttot) breath durations increased with exercise intensity but was unaffected by hypoxia. The relationship of tidal volume (VT) and inspiratory time duration (TI) showed linear, coinciding ranges for the normoxic and hypoxic conditions up to VT/TI values of about 2.5 1.s-1. At higher VT/TI values TI continued to decrease, whereas VT tended to level off, an effect which was more evident in the hypoxic condition. The results suggest that the hypoxic augmentation of exercise hyperpnea is primarily brought about by an enhancement of central inspiratory drive, the timing component being largely unaffected by the hypoxic environment, and that at low to moderate levels of exercise hyperpnea inspiratory off-switch mechanisms are essentially unaffected by moderate hypoxia.
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