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1 Faculty of Health Sciences, School of Human Kinetics, 2 Faculty of Medicine, and 3 Faculty of Sciences, University of Ottawa, Ottawa K1N 6N5; and 4 Human Protection and Performance Group, Defence Research and Development Canada-Toronto, Toronto, Ontario, Canada M3M 3B9
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Seven
subjects (1 woman) performed an incremental isotonic test on a
Kin-Com isokinetic apparatus to determine their maximal oxygen
consumption during bilateral knee extensions
(
O2 sp). A multisensor thermal
probe was inserted into the left vastus medialis (middiaphysis) under
ultrasound guidance. The deepest sensor (tip) was located ~10 mm from
the femur and deep femoral artery (Tmu 10), with
additional sensors located 15 (Tmu 25) and 30 mm
(Tmu 40) from the tip. Esophageal temperature (Tes) was measured as an index of core temperature.
Subjects rested in an upright seated position for 60 min in an ambient
condition of 22°C. They then performed 15 min of isolated bilateral
knee extensions (60% of O2 sp) on a
Kin-Com, followed by 60 min of recovery. Resting Tes was
36.80°C, whereas Tmu 10, Tmu 25, and
Tmu 40 were 36.14, 35.86, and 35.01°C, respectively.
Exercise resulted in a Tes increase of 0.55°C above preexercise resting, whereas muscle temperature of the exercising leg
increased by 2.00, 2.37, and 3.20°C for Tmu 10,
Tmu 25, and Tmu 40, respectively.
Postexercise Tes showed a rapid decrease followed by a
prolonged sustained elevation ~0.3°C above resting. Muscle
temperature decreased gradually over the course of recovery, with
values remaining significantly elevated by 0.92, 1.05, and 1.77°C for
Tmu 10, Tmu 25, and Tmu 40, respectively, at end of recovery (P < 0.05). These
results suggest that the transfer of residual heat from previously
active musculature may contribute to the sustained elevation in
postexercise Tes.
heat load; thermoregulation; hyperthermia; heat content; heat balance
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A NUMBER OF STUDIES HAVE EXAMINED muscle temperature (Tmu) profiles for resting conditions under different thermal conditions (4, 7, 8, 17, 20-22, 24, 27, 30, 31). The study by Ducharme and Tikuisis (8), however, was the only study to present a mean Tmu profile for a group of subjects, as opposed to single-depth measurements. Despite the large number of studies, there is still no consistent description of resting Tmu profile. There also have been a number of studies that have reported Tmu response during exercise (1, 2, 5, 6, 19, 23-27), of which the study by Saltin et al. (25) seems to be the only one to examine changes in Tmu profile (i.e., Tmu measured at multiple depths). Although, these experiments were not designed to show the time course change in tissue temperature gradients, their measurement of individual intramuscular temperature response did show large variations in the temperature at the superficial, mid-, and deep muscle sites. Furthermore, they showed significant variation in the rates of temperature change during muscle activity.
There are no studies that have examined changes in Tmu during the postexercise period. Several have reported postexercise Tmu response (1, 23, 25); however, none has specifically addressed these responses. In short, there remains a lack of information regarding the kinetics of heat exchange between muscle and the core of the body and within a given mass of muscle tissue. This information is critical to our understanding of the underlying mechanism responsible for the sustained, postexercise elevation in core temperature. In previous work, core temperature has been shown to remain elevated by ~0.4°C for a prolonged period after cessation of exercise (16, 28) performed in different thermal environments. However, the actual mechanism responsible for this increase in core temperature remains unresolved.
Tissue temperature at any given time is ultimately determined by the relative rates of heat production and heat loss. For example, regional Tmu at any point in time is the result of regional differences in metabolic rate (9), conductive heat loss to adjacent tissue (9, 10), and deep and peripheral convective blood flow (9, 29). As such, it would be expected that both regional temperature profile and the rate of temperature change would differ during resting, exercise, and postexercise recovery. The following study was designed to measure intramuscular temperature profile during rest, exercise, and postexercise recovery. In contrast to the findings of previous studies, we hypothesized that the tissue temperature profile will be consistent among subjects as the probe position is standardized within the muscle of all subjects. Furthermore, in conjunction with a postexercise decrease in heat loss, subsequent to a decrease in skin blood perfusion, and an attenuation of sudomotor activity during the postexercise recovery (14), we hypothesized that convective heat transfer between muscle and core will significantly influence postexercise core temperature response.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects.
Subsequent to approval of the project by the University Human Research
Ethics Committee, seven healthy subjects (6 men, 1 woman) consented to
participate in the study. Mean values (±SD) of the subjects' age,
height, body mass, maximal oxygen consumption during bilateral
concentric knee extensions (O2 sp), and
body fat content were 24 ± 5 yr, 1.8 ± 0.5 m,
85.6 ± 6.1 kg, 2.1 ± 0.9 l/min, and 10.9 ± 2.3%, respectively.
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O2) was determined
by open-circuit analysis by using an automated gas-collection system
(Quinton Instrument, Seattle, WA; model Q-Plex 1 cardiopulmonary
exercise system). Skin blood flow was measured by laser-Doppler
velocimetry (PeriFlux System 5000, main control unit; PF5010 LDPM,
operating unit; Perimed, Stockholm, Sweden) from the left midanterior
forearm. The laser-Doppler flow probes (PR 401 angled probe, Perimed)
were taped to cleaned skin, in an area that superficially did not
appear to be highly vascular and from where consistent readings were
noted (18). Sweat rate was estimated from a
5.0-cm2 ventilated capsule placed on the upper back.
Anhydrous compressed air was passed through the capsule over the skin
surface at a rate of 1 l/min. Water content of the effluent air was
measured at known barometric pressure by using the readings from an
Omega HX93 humidity and temperature sensor (Omega Engineering,
Stamford, CT). Sweat rate was calculated from the product of the
difference in water content between effluent and influent air and the
flow rate. This value was normalized for the skin surface area under the capsule and expressed in milligrams per minute per centimeter squared.
Subjects performed an incremental isotonic test (constant angular
velocity, increases in force output) on the Kin-Com isokinetic apparatus to determine their O2 sp. The
exercise consisted of bilateral, concentric knee extension over a range
of 70° from perpendicular, with the subject sitting (hip angle
between 90 and 110°) and the long axis of the thigh in the horizontal
plane. The force output was increased by 15 N every 2 min until
fatigue, whereas the angular velocity was maintained at 58.3°/s
throughout the test. The results of the test were used to establish the
work rate for the experimental trial. The experimental trial was
conducted in the morning after a 24-h period without heavy or prolonged physical activity. On arrival at the laboratory at 0800, subjects were
appropriately instrumented. Subjects then rested in a semirecumbent position for 60 min at an ambient temperature of 22°C, of which the
final 20 min were recorded as representative of the baseline resting
values. At 2 min before exercise, the subjects were secured to the
Kin-Com isokinetic apparatus at the level of the torso and ankles.
Subjects then performed 15 min of exercise, as described above,
consisting of bilateral, concentric knee extension over a range of
70° from perpendicular against a dynamic resistance sufficient to
elicit a heat load of 4.78 kJ/kg. Exercise was followed by 60 min of
seated rest.
The total energy expended (Mtotal) as a result of exercise,
during the period from onset of exercise until the time at which O2 returned to preexercise values, was
calculated from the sum of the energy expended by using the following
equation (expressed in kJ)
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(1) |
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Ex/rest is the rate of energy
expenditure during exercise and recovery, RER is the respiratory
exchange ratio, ec is the caloric equivalent (in kJ/l
O2) for carbohydrates, and ef is the caloric
equivalent (in kJ/l O2) for fat.
The minute values were summed for the entire period as described above.
The mechanical work (W) done during each contraction of the exercise
phase was measured and recorded by using the Kin-Com isokinetic
machine. This was calculated from the force exerted and the angular
displacement during the knee extension
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(2) |
is the angular displacement.
The total work done (Wtotal) was the sum of the work
accomplished during each of the contractions during the 15 min of exercise.
Mechanical efficiency (ME) was defined as the Wtotal
completed during the 15-min exercise period divided by the
Mtotal minus the energy expended under resting conditions
(Mrest) (Mtotal 
Mrest).
Thus
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(3) |
O2 during the 5 min preceding the
exercise bout. These values were calculated and expressed in kilojoules
by using the aforementioned equation.
The total heat load generated by the exercise (HLex) for
each subject was calculated by subtracting the Mrest and
the energy equivalent of the total mechanical work done
(Wtotal) from the Mtotal. Values are expressed
in kilojoules
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(4) |
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(5) |
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(6) |
sk, and | |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Baseline Tes and sk were 36.80 ± 0.30 and 31.66 ± 0.89°C, respectively. Resting
Tmu was significantly lower than Tes (i.e., 36.14 ± 0.29, 35.86 ± 0.31, and 35.01 ± 0.33°C for
Tmu 10, Tmu 25, and Tmu 40,
respectively) (Fig. 1). It should be
noted that the increase in muscle tissue temperature before the onset
of exercise was likely due to the preparation of the subject for the
exercise portion of the experimental trial.
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The Tmu profiles, expressed as a function of the position
of the placement of the temperature relative to the radius of the thigh
(r/rsk), show a parabolic profile for
mean resting tissue temperature profile (Fig.
2). This parabolic form of
Tmu profile was observed consistently in the data of all
seven subjects (Fig. 3). During resting,
the deep (36.14°C) and mid-Tmu (35.86°C) were significantly different from superficial (35.01°C) Tmu.
As depicted in Fig. 4, the greatest
tissue temperature difference (1.13°C) was between the deep and
superficial sections of the muscle, with a 0.84°C temperature
gradient between the mid- and superficial muscle. Mean tissue
temperature difference between deep and midmuscle was only 0.28°C.
Furthermore, the muscle-to-core temperature gradient was equal to
0.66, 0.94, and 1.79°C in relation to Tmu 10, Tmu 25, and Tmu 40, respectively
(P < 0.05).
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Exercise tissue temperature response.
After the onset of exercise, Tes increased gradually,
reaching a maximum rate of increase of 0.05 ± 0.02°C/min
between 6 and 9 min of exercise, and, subsequently, the rate decreased
over the balance of the exercise period (Table
2). In contrast, Tmu at all
measured points increased rapidly during the initial period of exercise
followed by a gradual reduction in the rate over the balance of the
exercise period. Superficial muscle (Tmu 40) showed the
greatest rate of temperature increase (0.61 ± 0.19°C/min). This
value was significantly higher than the rate measured in deep muscle
(0.22 ± 0.09°C/min). After the initial 3 min of exercise, the
rate of Tmu change decreased gradually and was similar at all three intramuscular sites until the end of exercise. Exercise resulted in a 0.55°C (end-exercise Tes of 37.35°C)
increase in core temperature above baseline resting values. In
contrast, Tmu increased by 2.09, 2.37, and 3.20°C above
baseline resting values for Tmu 10, Tmu 25,
and Tmu 40 measurement points, respectively, with
end-exercise values similar at all three measured sites (i.e., 38.23, 38.23, and 38.21°C for Tmu 10, Tmu 25, and
Tmu 40, respectively). sk increased continuously during exercise to an end-exercise value that was significantly elevated above baseline rest (32.79°C,
P < 0.05). The increase in sk was
paralleled by an increase in nonevaporative heat loss (i.e., Fig.
5). Forearm skin blood flow increased
continuously during the course of the exercise.
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Postexercise tissue temperature response.
Tes decreased rapidly (0.04°C/min) during the initial
minutes after the cessation of exercise (Table 2), after which there was a rapid decrease in the rate of temperature decrease to negligible values. At ~5 min of recovery, Tes reached an elevated
value 0.35°C above baseline resting values (P < 0.05). For the duration of recovery, the rate of decrease of
Tes remained at ~0.001°C/min. Tes decreased
to 37.11°C by the end of the 60-min recovery period (~0.3°C above
baseline). Tmu showed a similar high rate of temperature decrease during the initial 5 min of exercise recovery, although the
rates were ~2 to 2.7 times greater than the rate measured for
Tes. Unlike the response in Tes,
Tmu for all measured sites decreased continuously during
the initial 30 min of recovery. However, the rates of Tmu
decay were reduced for the duration of recovery. In the final 15 min of
recovery, superficial muscle demonstrated an elevated rate of
temperature decrease above deep muscle (P < 0.05).
Muscle tissue temperature at the end of the postexercise recovery
period remained significantly elevated above baseline resting values
by 0.92, 1.05, and 1.77°C for
Tmu 10, Tmu 25, and
Tmu 40, respectively (P <0.05) (Fig. 1). sk and whole body nonevaporative heat loss
decreased to baseline resting values within ~20-25 min of
recovery. Similarly, forearm skin blood flow decreased to baseline
resting values within 10 min of the termination of the exercise. In
contrast, both thigh nonevaporative heat loss and thigh skin
temperature remained significantly elevated from preexercise values for
the duration of the recovery period (P < 0.05).
Heat load and heat loss response. The workload resistance was adjusted for each subject, according to the individual heat load and mechanical efficiency of the leg-extension exercise. The mechanical efficiency varied from 5.98 to 15.96%, whereas the average for the group was 9.93 ± 1.32%. The average actual workload was one at which the heat production was 4.78 ± 0.38 kJ/kg. The Mtotal and Wtotal were 585.87 ± 53.72 and 41.48 ± 4.68 kJ, respectively. Thus the average heat load generated as a result of the exercise was 391.73 ± 38.93 kJ. The average total dry heat loss during the exercise period was 15.93 ± 5.98 kJ, whereas, during the 60 min of postexercise recovery, this value was 37.90 ± 18.80 kJ.
The kinetics of heat load generation at rest, over the 15 min of exercise, and over the first 10 min of recovery are presented in Fig. 5. The corresponding evolution of dry heat loss is also shown. Thus during the 5 min preceding exercise, the dry heat loss defined relative to heat load, that is, minus the resting levels, was essentially zero. During exercise, the dry heat loss increased at a rate of 0.14 kJ/min to a maximal level of 2.19 kJ/min after 15 min of exercise. At cessation of exercise, the sensitive heat loss dropped exponentially to a level ~1.0 kJ/min above initial resting values and remained elevated for the next 10 min. The heat production, on the other hand, increased to 17.08 kJ/min after 2 min of exercise and continued to rise at a rate of ~0.78 kJ/min for the next 13 min to a maximum heat production of 27.71 kJ/min. Immediately postexercise, the heat load returned exponentially to resting levels within 5 min.| |
DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In this study, an attempt was made to specifically evaluate the kinetics of heat exchange in muscle tissue during and after exercise by using a multisensor thermal probe positioned at a predetermined internal marker. In contrast to previous studies, we observed similar individual and group Tmu profiles during resting, exercise, and subsequent resting recovery. Furthermore, we observed a sustained elevation of core temperature for the duration of the recovery period that is consistent with previous findings (16, 28). Specifically, Tes showed a rapid decrease in the first minutes of exercise recovery followed by a prolonged sustained elevation of ~0.3°C. Of particular importance was the observation that deep Tmu decreased during the early stages of exercise recovery to values equal to that of Tes. Subsequently, deep Tmu remained relatively unchanged from Tes for the duration of recovery. This supports the hypothesis that the postexercise recovery of core temperature may be, to a large degree, influenced by the residual heat load of muscle.
Tissue temperature response: resting. Different shapes of limb temperature profile have been reported for resting conditions during different thermal stresses between individuals, whereas we noted a consistent parabolic profile of Tmu in all subjects (4, 7, 8, 17, 20-22, 24, 27, 31). This could arise from inconsistency in the specific placement of the internal probe in the muscle. There is a wide variation in recorded muscle tissue temperature due to the proximity of the probe to the surface and to such structures as large arteries and bone (10). This could have been a source of significant variation in the recorded internal temperature. The differences in the specific heat of these tissues, as well as the differing blood flow and hence the convective effect within these structures, influence the rates of temperature change in adjacent regions of the muscle. Therefore, consistent placement of the probe is critical. Thus an attempt was made to minimize the variation in temperature recording resulting from individual anatomic differences.
Tmu response: exercise. From the onset of exercise until the late phases of exercise, there is a gradual change in the Tmu profile from one parabolic in the form seen at rest to a zero gradient or homogenous temperature profile across the muscle. As shown in Fig. 4, the large temperature gradient that existed between the deep and midportions of the muscle and the superficial muscle was rapidly eliminated during the first 5 min of exercise. By the end of exercise, the gradient between the different muscle depths was nonexistent, as temperatures across the radial axis of the muscle became homogenous. This effect was not observed earlier by Saltin et al. (24). In that case, it was noted that both mid- and superficial Tmu remained generally lower than deep Tmu. Similarly, superficial Tmu remained lower than mid-Tmu, whereas the temperature gradient between mid- and deep muscle seemed to remain relatively constant throughout exercise. The gradient between the superficial muscle increased relative to that between mid- and deep muscle. The different response to that observed in our study may be attributed to a number of factors, which may include the following: 1) differences in the measurement site (i.e., vastus lateralis); 2) differences in the muscle mass implicated in the exercise activity; 3) differences in ambient conditions; and 4) difference in work intensity. For example, it would be expected that the temperature profile of the vastus lateralis would be different from that of the medialis, as it is less affected by the circulation in the femoral artery and vein. The relative influence of convective heat exchange would be considerably different between these muscles.
The high rate of Tmu increase in the early stages of exercise is consistent with previous studies (1, 3, 24, 25). Aulick et al. (3) showed, at the beginning of exercise, that heat gained by the leg (local metabolic heat production plus vascular heat delivery from the viscera) exceeded heat loss, and femoral vein blood temperature rose rapidly. In this study, the superficial regions of the muscle demonstrated the largest rate of Tmu increase (0.53°C/min) that was 1.4 and 1.6 times the rate of temperature increase for the deep and midmuscle, respectively. On the other hand, based on visual observation of the work by Saltin and coworkers (24), it would seem that the rate of temperature increase was greater in deep muscle compared with superficial muscle. During the course of exercise, the muscle-to-core temperature gradient increased progressively (Fig. 4), from1.15°C at rest to +0.90°C
by the end of exercise. Also, despite the rapid increase in muscle heat
content (as represented by increased Tmu) to values exceeding that for core, the rate of temperature increase of core remained consistently lower than that of muscle. Therefore, this would
suggest that the rate of heat accumulation within the core region is
attenuated to a large degree by an increase in the rate of whole body
heat loss (i.e., evaporative and nonevaporative heat loss). For
example, Aulick et al. (3) previously noted that, as limb
sweat rate, cutaneous blood flow, and muscle-to-skin temperature
differences increased during exercise, the active leg became a more
effective vehicle for heat dissipation, and that femoral venous
temperature eventually reached a plateau during steady state.
Furthermore, Gisolfi and Robinson (11) showed that much of
the heat produced by active leg muscles is rapidly transported to
surface veins and that this muscle heat is potentially lost across the
leg surface. In this study, muscle-to-skin temperature gradient
remained elevated during the course of the exercise by ~5.2°C, and
skin blood flow and sweat rate increased gradually during the course of
the exercise. Furthermore, it has previously been shown that, during
leg work, the inactive upper limbs also act as an avenue for vascular
heat loss from the central circulation (15), which would
further attenuate the increase in core temperature.
Tmu response: postexercise. Few studies have graphically presented muscle tissue temperature response during the postexercise period, and, even so, no specific discussion was presented with regard to these data (1, 23, 25). It is clear in this study that, during the transition from exercise to postexercise resting recovery, the Tmu profile across the radial axis of the muscle remains constant (i.e., linear profile, see Figs. 2 and 3). During the course of the 60-min recovery, all three sites showed a similar rate of temperature change, although superficial muscle showed a significantly greater rate of temperature decrease toward the later stages of recovery (P < 0.05).
Deep Tmu decreased during the early stages of exercise recovery to values equal to that of Tes, after which deep Tmu remained relatively unchanged from Tes for the duration of recovery, with the deep muscle-to-core temperature gradient no greater than ~0.02°C. The lack of a difference in temperature gradient between muscle and core suggests equilibration of heat distribution within the body. Thus changes in surface heat loss (i.e., evaporative and nonevaporative heat loss) will change the rate of whole body cooling. Therefore, the rate of core temperature decay is limited by the rate at which heat is lost at the skin-air interface. All Tmu values remained significantly elevated above baseline resting values at the end of recovery. That was paralleled by a significant increase in Tes of ~0.3°C (P < 0.05). Aikas et al. (1) have shown a similar postexercise increase in Tmu of the previously active muscle, although Tes showed a rapid decrease to values below baseline rest within a short period (~15 min) after cessation of exercise. This is more difficult to explain if one believes that the convective arterial flow plays a major role in muscle cooling postexercise. Our results are more consistent with those of Saltin et al. (25), who, on the other hand, did observe a sustained increase in postexercise Tes, whereas Tmu remained significantly elevated above baseline resting values. Thoden et al. (28) previously showed a prolonged postexercise elevation (0.4-0.5°C) in Tes after dynamic exercise. It was subsequently shown that an increase in the postexercise hypotensive response, induced by exercise of increasing intensity, was paralleled by an increase (~0.4°C) in the magnitude of the postexercise elevation in Tes (13). It was suggested that the postexercise Tes response may be defined to a large degree by the gradient between the periphery and core and that the convective transfer of residual heat from previously active musculature may contribute to the sustained elevation in postexercise Tes. Our observation of a prolonged elevation in Tmu at values elevated above esophageal provides further evidence to support this conclusion. Thus, in the absence of a postexercise increase in heat loss response (14), Tes would remain elevated as long as 1) the heat content of muscle remains higher or equal to that of core; 2) the postexercise hypotensive effect persists; or 3) a combination of the two.Summary. In the present study, exercise was performed such that the dynamic resistance during the bilateral knee-extension exercise was sufficient to elicit a heat load of 4.78 kJ/kg. Thus it can be assumed that the rate of heat production and accumulation in muscle was comparable between subjects. Thus the variation in Tmu profile observed between the transition from rest to exercise and exercise to resting recovery was not only the result of the change in metabolic heat production but also the result of changes in the convective heat transfer between blood and muscle and conductive heat transfer within the muscle and skin surface. Furthermore, as with previous studies that have shown that tissue heat content and compartmental heat exchange are significantly influenced by convective heat exchange during rest (9) and exercise (15), our findings suggest that postexercise core temperature response (and the rate of temperature decay) is significantly influenced by convective heat transfer between muscle and core.
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ACKNOWLEDGEMENTS |
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We acknowledge the technical support of Carolyn Proulx and Normand Boulé.
This research was supported by the Natural Sciences and Engineering Research Council of Canada (to G. P. Kenny).
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FOOTNOTES |
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Address for reprint requests and other correspondence: G. P. Kenny, School of Human Kinetics, Univ. of Ottawa, Rm. 372, Montpetit Hall, P.O. Box 450 Station A, 125 Univ., Ottawa, Ontario, Canada K1N 6N5 (E-mail: gkenny{at}uottawa.ca).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published February 21, 2003;10.1152/japplphysiol.01107.2002
Received 3 December 2002; accepted in final form 7 February 2003.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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), 15 mm
(Tmu 25; 
), and 30 mm
(Tmu 40; 
) from femur and deep femoral
artery] and esophageal (
) temperature response during
rest, exercise (Ex), and postexercise recovery. Vertical dotted lines
represent the start (time = 0 min) and end (time = 15 min) of
exercise. * Significant difference from baseline resting values,
P < 0.05. 1 Value for esophageal
temperature not significantly elevated from baseline.
), Ex (
), and postexercise (post-Ex; 
)
recovery at selected periods as a function of the placement of the
temperature sensors relative to the radius of the thigh. r,
Radius (cm); rsk, radius of the thigh (cm);
r/rsk, relative radius.
, deep muscle to
midmuscle; 
significantly different from the deep to midmuscle temperature
gradient; § significantly different from baseline resting;
* superficial-to-core temperature gradient significantly different
from deep-to-core temperature gradient.
) and dry heat loss (
)
responses during baseline resting, exercise, and postexercise recovery.
Vertical dotted lines represent the start (time = 0 min) and end
(time = 15 min) of exercise.