External Power Output Research Articles

Propulsion technique in hand rim wheelchair ambulation.

Six male subjects took part in a pilot study on a stationary wheelchair ergometer. They propelled the ergometer at a speed of 0.55, 0.83, 1.11 and 1.39 m/s. The speed increased every 3 min. Inertia and friction force were adjusted proportional to body weight. Every third minute 750 samples of the torque and velocity signals were digitized at a sampling rate of 100 Hz. From the signals mean external power output (Pmean), peak power (Ppeak), mean torque (Mmean) and peak torque (Mpeak), work/cycle, 'time-to-peak torque' (TTP), cycle duration (CT), push time (PT) and recovery time (RT) were determined in relation to mean velocity (speed). For the mean velocity range studied, analysis of variance (P less than 0.05) revealed significant increments in Ppeak, Mpeak, Pmean, Mmean and work/cycle with increasing mean velocity, whereas CT and PT showed a significant decrease. TTP showed a decrease with speed which, however, was not statistically significant. The RT showed no significant variation as well. Our previous research into propulsion techniques mainly focused on movement frequency and timing and was conducted during wheelchair ambulation on a motor driven treadmill. Despite considerable interindividual variation in terms of movement pattern, current and previous studies showed similar trends in the timing pattern (cycle, push, recovery duration) with respect to speed. Theoretical considerations regarding variations in peak torque and work/cycle with respect to velocity are supported by the current results. Both torque and work/cycle are important technique parameters and of relevance in speed regulation. The data also suggest that wheelchair ambulation can be validly simulated and studied with the special purpose wheelchair ergometer.(ABSTRACT TRUNCATED AT 250 WORDS)

Matching between feline left ventricle and arterial load: optimal external power or efficiency.

We tested the hypothesis that the feline left ventricle normally works at optimal external power as opposed to optimal efficiency by (re)analyzing data from five isolated, blood-perfused cat hearts and 39 open-thorax cats. In the isolated hearts, we measured pump function, external steady power, myocardial oxygen consumption, and efficiency. Optimal external power and optimal efficiency were found at different left ventricular outputs (6.94 +/- 0.33 and 8.35 +/- 0.37 ml/s, respectively; P less than 0.001). In the in situ cat hearts the working point was found at an output of 4.72 +/- 0.32 ml/s, whereas optimal external power was found at 4.84 +/- 0.26 ml/s. These values were not significantly different. Assuming that the point of optimal efficiency was located at the same fraction of the maximal unloaded left ventricular output (Fmax) as in the isolated hearts, i.e., 0.7, we found the point of optimal efficiency for the in situ heart at a flow of 5.83 +/- 0.32 ml/s, which was significantly different (P less than 0.001) from the flow in the working point. Our data therefore indicate that the left ventricle in the open-thorax cat is matched to the arterial load such that its external power output rather than efficiency is optimized.

Speed dependence of the efficiency of heat engines

We have studied the efficiency and power output of a simple model of an irreversible heat engine as a function of cyclic operating frequency. The model adopted is defined by a reversibly cycled working substance coupled to heat source and sink by thermally conducting walls. The maximum operating frequency corresponding to zero external power output is associated with the traversal of a limit cycle in the working substance. Closed form expressions for the maximum operating frequencies are derived for the special cases of isothermal and polytropic limit cycles.

The effect of external loading upon power output in stair climbing.

Previous studies have examined man's ability to produce external power output during maximal repetitive work cycles of short duration. It appears, however, that there were methodological limitations which would inherently mask man's true capacities. Consequently, we examined the effect of variable external loads upon external power output as measured by running upstairs. Fourteen male subjects (16-31 years of age) who regularly participated in competitive sports performed maximal stair step tests under five experimental loading conditions (no external load, 10.1, 19.2, 24.2, and 29.2 kg). Significant increases (P < 0.05) in external power output were found. External power output increased from a mean of 15.9 W x kg-1 +/- 1.0 (unloaded condition) to 18.5 W x kg-1 +/- 1.5 (external load of 29.2 kg). This is the first modern investigation demonstrating that external loads effect external power output as measured by this technique.

Left and right ventricular pump function and consequences of having two pumps in one heart. A study on the isolated cat heart.

SUMMARY We compared pump function of left and right ventricles in isolated ejecting cat hearts in which the natural series arrangement between the two sides of the heart was broken. The relationship between mean ventricular pressure and output obtained by varying the arterial load, the so-called pump function graph, and the mean external power output found at the various load levels, was taken as the basis for the comparison. The way in which pressures and flow generated by the right ventricle are changed by alterations in resistance and compliance of the loading arterial system, is qualitatively similar to what has been found in a previous study for the left side of the heart, in spite of the structural differences. The right ventricular pump function curve differs from the left ventricular curve in the absolute scale for the mean pressure axis, but if the scale is adjusted appropriately, the two curves look alike. Both ventricles appear matched to the input impedances of their respective arterial systems because both of them generate the maximum external power under the normal loading conditions. Right ventricular pump function is dependent on the left ventricular contraction mode. The presence of an isovolumic beat on the left side of the heart enhances right ventricular pump function. This is in contrast to the very small effect of the right ventricular contraction pattern on the pump function of the left side of the heart. Circ Res 46: 564-574, 1980

Effect of running velocity on external mechanical power output.

The mechanical power output and work done in running were measured for eight athletic runners by means of a strain gauge platform and 16mm cine camera. The angle between the line connecting the centre of gravity of the body with the foot on the ground and the horizontal line was about 110 degrees at the moment of foot contact with the ground, independent of running velocity. The external work by velocity change (Wf) increased curvilinearly with the running velocity up to about 630 Joule kg-1 at maximum velocity. The mechanical power output per body weight in forward direction (Pf) increased as the 2nd power of the running velocity and the following equation was obtained Pf=0·436V2·01 f where Pf was expressed in Wkg-1 and Vf in ms-1. At the velocity higher than 6ms-1, the mechanical power by forward velocity changes (Pf) increased gradually greater than that against gravity (Pv) but at the lower velocity than 5ms-1 Pf was always less than Pv.

Reduction of Wind Resistance and Power Output of Racing Cyclists and Runners Travelling in Groups

Abstract Wind resistance is responsible for most of the metabolic cost of cycling (80–90%), whereas for runners the effect is much less (4–8%). In groups of cyclists and runners, those behind consume less energy, being partially shielded from the wind. Experimental data are presented showing reduction of group wind resistance and power output for cyclists and runners. For cyclists, external power output was reduced over 30% at racing speeds. For runners, energy consumption decreased only 2–4% at middle and long distance speeds. A method of predicting the speed of groups of cyclists and runners is shown. By travelling in a group, cyclists can increase their speed about 0 9–1.8 m s−1, while runners can improve only about 01 m s−1. The use of pace lines in cycle racing is by far the most important race tactic. In running, systematic use of pacing is not yet fully utilized although the benefits are potentially significant.

MAN AS A SOURCE OF MECHANICAL POWER

It is deduced from the published literature that the usable external power output of the body is limited in the following manner for the reasons stated : (1) In single movements (of duration less than 1 sec) to less than 6 h.p. ; by the intrinsic power production of muscle, and by the difficulty of coupling a large mass of muscle to a suitably matched load. (2)In brief bouts of exercise (0-1—5 min) to 2—0-5 h.p. ; by the availability in the muscles of stores of chemical substances that can yield energy by hydrolysis (3)In steady-state work (5 min to 150 min or moro) to 0-5-0-4 h.p. ; by the ability of the body to absorb and transport oxygen. (4)In long-term work, lasting all day, to perhaps 0-2 h.p. ; by wear and tear of muscles, the need to eat, etc. All these figures refer to champion athletes ; ordinary healthy individuals can produce less than 70-80 per cent as much power