Muscle fatigue is defined as a loss of tension development during constant stimulation. Although the relationship is not well documented, muscle fatigue has been inferred from electromyogram (EMG) signals. The purpose of this study was to determine the relationship between the amplitude and duration of single motor unit action potentials (MUAPs) and the loss of tension development (fatigue) in the medial gastrocnemius muscles of cats. Single motor units were fatigued by continuous stimulation at 10 or 80 Hz or with trains of 40-Hz stimuli. When motor units were stimulated at 10 Hz and with trains at 40 Hz (low frequency), tension declined and remained depressed during recovery. The changes in the MUAP correlated poorly with changes in tension. During and after stimulation at 80 Hz (high frequency), changes in the amplitude and duration of MUAPs correlated highly with changes in tension development. Since the EMG signal is dependent on a summation and cancellation of individual MUAPs, the EMG provides a reasonable estimate of high-frequency fatigue but an unreliable measure of low-frequency fatigue.
The temperature profile and the local rate of heat transfer from the wall were measured a t 0.453, 1.13, 4.12, and 9.97 tube diameters downstream from a step increase in wall temperature for air in fully developed turbulent flow a t Reynolds numbers of 15,000 and 65,000 in a 1.52-in. tube. The velocity profile and the pressure were also measured a t these lengths.Radial and longitudinal temperature gradients, radial heat fluxes, and eddy diffusivities for heat and momentum transfer were computed from the measurements, The longitudinal temperature gradients a t all radii were found to differ significantly from the mixed mean temperoture gradient. Although the radial heat flux was a maximum at the wall, the radial heatflux density, in terms of which the eddy diffusivity for heat transfer is usually defined, was found to go through a maximum near the wall and then to decrease almost linearly across the thermal boundary layer, The eddy diffusivity for heat transfer was found to be independent of length in the thermal entrance region and hence a function only of the fluid motion, as previously hypothesized.This paper presents the results of an experimental investigation of heat transfer in the region following a step change in wall temperature in fully developed turbulent flow in a tube. Following such a change in wall temperature, the temperature field in the fluid and the rate of heat transfer from the wall change rapidly, while the velocity field and rate of momentum transfer chan e only slightly, owing to changes in J e physical properties of the fluid with temperature.The length of tube required for the temperature of the fluid at the center of the tube to change some small arbitrary amount is called the thermal entrance region. The thermal entrance region has particular interest because of the high heat transfer coefficients attainable and has received recent attention in connection with heat transfer in nuclear reactors and other compact equipment. The step change in wall temperature in fully developed flow has particular theoretical interest since Tribus and Klein (20) have shown that a knowledge of the resulting temperature field permits computation of the rate of heat transfer for any wall-temperature distribution.Previous experimental investigations of heat transfer in the thermal entrance region have apparently been limited to measurements of the surface temperature, the mixed mean temperature, and the heat flux at the wall. Analytical representations have all assumed that eddy dihsivities measured in fully develo ed boundary layers are in the thermal entrance region.In this investigation the time-mean temperature and velocity fields were measured as well as the heat flux at the wall in order to provide insight into Page 268directly appicable P the mechanism of heat transfer in the entrance region and to test the postulates of previous investigators. The measurements were made in air at Reynolds numbers of approximately 15,000 and 65,000. Temperature differences of less than 30°F. were utilized to minimize...
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