Sebastien Racinais Ph.D. ASPETAR, Qatar Orthopedic and Sports Medicine Hospital, Exercise and Sports Science Department, Doha, Qatar
Muscle contraction influences Temperature ...
Heat production.Basically, mechanical efficiency of the muscle contraction is close to 0.2 whereas most of the energy produced (i.e., 70 to 80 %) is thermal energy. Even at rest, the muscle metabolic activity produce thermal energy (thereafter called heat in this review). The fact that passive exercise doesn’t elevate muscle temperature suggests that movement friction doesn’t add to the heat gain in the muscle (Gonzales-Alonso et al 2000).
When a contraction is initiated, heat production starts as soon as the muscle is activated. This initial heat production occurs even if the oxidative reactions are blocked, suggesting the implication of the anaerobic metabolism in it.
During the first minutes of exercise, heat production by contracting human skeletal muscle increases (Gonzales-Alonso et al 2000, Krustrup et al 2001). This observation could be a consequence of the decreasing rate in the utilisation of PCr and the increasing rate of oxidative phosphorylation when exercise continues (Bangsbo et al 2001), since heat production is lower when ATP is provided by PCr hydrolysis and glycogenolysis compared to oxidative phosphorylation (Walsh and Woledge 1970, Curtin and Woledge 1978, Gonzales-Alonso et al 2000).
Heat production persists thereafter during several minutes. This delayed heat production mainly disappears in hypoxic muscle, suggesting the implication of the aerobic metabolism in it.
Heat release. Muscle temperature is the net result of a regional heat balance, caused by locally generated metabolic heat, heat carried to and away by the blood, heat lost to superficial tissues by conduction, and local heat storage (Saltin et al 1968).
Most of the heat produced during the first seconds of exercise seems to be accumulated in the contracting muscle whereas, thereafter, most of the heat produced is transported to the body core by the blood or lymph drainage (Gonzales-Alonso et al 2000). This increasing heat release is probably linked to the higher muscle temperature increasing the muscle-to-blood temperature gradient (Krustrup et al 2001).
The release of the heat produced at the level of the exercising muscle induces an increase in body core temperature. Muscle temperature rises rapidly from resting levels close to 35°C and within 3-5 min is above the rectal temperature, leading thereafter an increase in rectal temperature (Saltin et al 1968), rectal temperature representing a good index of the core temperature (Saltin and Hermansen 1966).
This heat production by working muscle as a consequence of the muscle contractions in the early exercise will represent also a factor affecting the subsequent exercise. This first paragraph will focus on muscle temperature and muscle function. The following second, third and fourth paragraphs will focus on the consequence of the body core temperature increase.
...and Temperature influences Muscle contraction
Muscle power. Leg immersion in a hot bath leads an increase in muscle temperature allowing an improvement of the vertical jump height (Bergh and Ekblom 1979, Davies and Young 1985). Equivalent data were observed during short and intensive cycle exercise (< 30 sec), with an increase in power output after a warm bath (Bergh and Ekblom 1979, Sargeant 1987) and a decrement of it after a cold bath (Sargeant 1987, Crowley et al 1991).
From the studies relating modifications in short-duration exercise performance associated to muscle temperature recording, it is possible to estimate a range of variation in muscle force and/or power per degree of variation in muscle temperature (Table 1). Dynamic force production seems to be more dependant of muscle temperature than isometric force production (Binkhorst et al 1977, Bergh and Ekblom 1979).
Furthermore, a decrease in muscle temperature leads more alteration in high velocity pedalling (Sargeant 1987) or throw (Oksa et al 1995) than in the same move performed at lower frequency.
Even if an increase in muscle temperature above basal values have only a few (Asmussen et al 1976, Bergh and Ekblom 1979) or no effect (Clarke et al 1958, Binkhorst et al 1977, Davies and Young 1983) on maximal isometric force, this parameter is clearly impaired by cold bath (Clarke et al 1958, Davies and Young 1983).
By considering that the data are dependant of both the exercise considered (i.e., isometric or dynamic contraction) and the methodology used to modify muscle temperature (i.e., water bath, environmental chamber or physical exercise), variation in muscle performance seems to range from 3 to 5% per 1°C of variation in muscle temperature (Table 1).
Table 1: Variation (%) in muscle force/power for 1°C of variation in muscle temperature.
||Variation (%) in
for 1°C of variation
in muscle temperature
to modify muscle
|Oksa et al (1995)
||60 min in a room at 10°C
||Cycle ergo meter
||Water bath 12-18°C (45 min)
Water bath 44°C (45 min)
|Bergh and Ekblom (1979)
||Leg force 0°.s-1
||Cooling by water bath and warming by exercise
||Leg force 90°.s-1
||Leg force 180°.s-1
|Oksa et al (1996)
||Room at 10°C (60 min)
A voluntary contraction is the result of both a voluntary muscle drive (i.e., involving both supraspinal and spinal structures) and the mechanical response to this drive (i.e., exitation-contraction coupling).
In order to isolate the effect of muscle temperature on muscle contractility, some studies investigated the mechanical response to the muscle to an electrical stimulation of the motor nerve.
An increase in muscle temperature reduces the time to reach the peak of tension and the half-relaxation time of an electrically evoked twitch (Davies and Young 1983, 1985, Segal et al 1986). At the opposite, a decrease in muscle temperature slows down these two parameters (Davies and Young 1983).
These data are in accordance with the variations in performance observed for voluntary actions.
Environmental temperature. In line with the observation done after local cooling and/or warming, environmental temperatures are also able to modify muscle function.
A cold environment markedly decrease muscle power (Hackney et al 1991, Oksa et al 1995, 1996, 2000) whereas the effects of a hot environment are equivocal. From one hand, Dotan and Bar-Or (1980) failed to observe variations in the power output produced by children during a Wingate cycling test (30 sec) in hot (38-39°C, 25-30% rh), warm and humid (30°C, 85-90% rh) or control (22-23°C, 55-60% rh) environments.
These data were later confirmed with adults by Backx et al (2000) who failed also to observe significant difference in power production within control (22°C, 30% rh), warm and humid (30°C, 85% rh) and hot (40°C, 40% rh) environments. At the opposite, with a protocol close to Backx et al (2000), Falk et al (1998) observed higher peak power in hot environment (35°C, 30% rh) than in control conditions (22°C, 40% rh).
These data were confirmed by Ball et al (1999), who recorded higher peak power output during a 30 sec cycling test in warm condition (30°C, 55% rh) in comparison to control condition (19°C, 40% rh) and by observation of Linnane et al (2004) that mean power output during an equivalent cycling test (30 sec) performed in control environment (21°C) is significantly increased if the test was preceded by a hot exposure (44°C).
All these data come from power output recorded during cycling test, but the explanations for these different sensibilities to a warm environment is still unclear.
An interesting explanation could be the recently developed hypothesis of a threshold in the beneficial effect of increasing body temperature (Racinais et al 2004b, 2005a, 2006). Basically, the endogenous diurnal increase in body temperature (i.e., circadian rhythm) could represent a passive warm-up affecting human body as a warm environment.
In line with this hypothesis, it is generally observed an increase in muscular power during the day in parallel to the circadian increase in body temperature (Reilly and Down 1986, 1992, Bernard et al 1998, Melhim 1993, Racinais et al 2004b and 2005a).A recent study conducted in a warm and humid environment (28°C, 63% rh) failed to show any circadian variation in muscle power, suggesting that the warm and humid environment may have blunted the circadian rhythm in muscular power (Racinais et al 2004a). This study raised intriguing questions about the cause of this stability.
Two suggestions seemed reasonable: (i) a long-term adaptation to the environmental conditions or (ii) an instantaneous effect of the testing conditions.
Some recent studies comparing diurnal variation (i.e., morning versus afternoon) in muscle power in the same subject in function of the environmental temperature observed an interaction effect within time-of-day and environmental condition (Racinais et al 2004b, 2005a) showing that it was the testing condition which allowed to blunt the diurnal variation in muscle power.
Interestingly, a warm environment allows to improve muscle power (Racinais et al 2004b) and contractility (2005a) in the morning only, when body temperature were at their lowest.
This suggests that these two passive warm-up effects have no additional effect on the improvement in muscle power. That suggests the existence of a threshold for passive warm-up effects, above which additional sources of warm-up have no additional effect, since neither extremely hot exposure (76°C, 27% rh for 30 min) nor the diurnal increase in body temperature leaded a modification in muscle power in moderately warm and humid environment (Racinais et al 2006).
From Muscle contraction to repeated or prolonged exercise
Effect of exercise duration. This introduction points the possibility of an improvement in muscle power production after heating the muscle by a hot bath (e.g., Bergh and Ekblom 1979, Davies and Young 1985, Sargeant 1987) or in warm environment (e.g., Falk et al 1998, Ball et al 1999, Linnane et al 2004). However, an elevated body temperature seems to be a limiting factor for performance of prolonged, intermittent, high-intensity running in warm environment (Morris et al 2000).
Furthermore, it exist also a lot of evidence pointing hyperthermia has a key limiting factor for long duration exercise in warm environment (see the other chapters of this website). If an increase in muscle temperature may improve power production during short duration exercise but an increase in body core temperature limit prolonged exercise, the question is to known how and when the shift from improvement to impairment in physical ability occurs.
The model of repeated exercise could provide some answer about this shift. Indeed, the repetition of maximal exercise allows to investigate the evolution of the performance with considering heat produced during the first exercise(s) leading an increase in body core temperature in the following exercise(s), especially in hot or warm and humid environment. With a recovery period of 4 min within two cycling sprint of 30 sec, Ball et al (1999) observed an increase in peak power output in warm environment (30°C, 55% rh) in comparison of neutral environment (19°C, 40% rh) for the both sprint.
However, the relatively long recovery period allowed (i.e., 4 min) and the low number of repetition (i.e., only 2 sprints) may prevent to observe any hyperthermia induced by the first sprint. Interestingly, equivalent data were also observed with shorter recovery period (i.e., 30 sec) within five sprints of 15 sec (Falk et al 1998) or three sprints of 30 sec (Backx et al 2000). These studies observed either an improvement in power output in warm environment (Falk et al 1998) or not (Backx et al 2000) but both of them failed to observed significant effect of the environmental condition on the performance decrement with the exercise repetition.
Taking together, these data suggest that a warm environment have no deleterious effect on the repetition of short-duration exercise. However, Linnane et al (2004) recorded a significantly higher mean power output during a 30 sec cycling sprint after a warm exposure (+6% for 1°C of increase in rectal temperature) but failed to observe significant difference in a second sprint performed 4 min after. Two hypotheses could be advanced to explain this observation. Firstly, it is possible that an increase in muscle temperature following the first sprint "cancelled" out the passive warm-up effect of a warm-environment on subsequent sprints (Racinais et al 2005b).
Secondly, even in neutral environment, the higher power production during the first sprints could lead to higher metabolic disturbances, greater PCr depletion and less subsequent PCr repletion that in turn might decrease the following sprints (Bogdanis et al 1996, Dawson et al 1997). In the consideration of damageable effects leading by the higher power production of the first sprint, we have also to consider the specificity of a warm environment.
In this special condition, and from our point of view, the mean damageable effect is may be the rise in body core temperature since an elevation in muscle temperature seems to be benefic for muscle power production (see Introduction) whereas an elevation in central temperature seems to limit the capacity to exercise (see the other chapters of this website) and to provide an adapted neural drive to the muscle (see the other chapters of this website). A recent study confirmed that the power decrement during a repeated-sprints exercise is higher during hyperthermia (induced by an exercise in hot environment, 40°C) than in control condition (Drust et al 2005). Furthermore, the authors concluded: ‘The reduction in power output appeared despite elevations in muscle temperature that are expected to enhance high intensity performance.
Metabolic factors such as elevated muscle lactate and extra-cellular potassium do not seem to explain the reduced performance. This may suggest that the impairments in exercise capacity are related to temperature dependent mechanisms associated with the attainment of a high core temperature.’ (Drust et al 2005). The shift from improvement to impairment in human ability to exercise in warm environment seems to be related to the apparition of new consequences of the increase in body core temperature when exercise prolonged.
As presented in the table 2, this shift can be easily put in relation to the exercise duration (Table 2, part A, B and C). The studies reported in the part A observed an improvement in human performance in warm environment or following different passive warm-up of the body. All the performances improved have the common characteristics to be of short duration from less than 1 sec (i.e., vertical jump) to 30 sec (i.e., cycling exercise).
The studies reported in the part B failed to observed a marked effect of a warm environment on human performance. All these performances have the common characteristics to be a repetition of short exercise or to have an intermediary duration (e.g., 5 min).
The studies reported in the part C are based on exercise lasting more than 10 min and observed a decrement in human performance in warm environment. The ability of human to perform a long duration exercise is generally considered to be limited by a warm environment and its effect on body temperature, dehydration and/or central nervous system (see the other chapters of this website).
In summary, human body temperature influence the physiologic functioning of different organs in a different way and the specific temperature of the brain (motor command) and the muscle (contractile function) have to be considered in addition to central body core temperature.
Table 2: Different effects of an increase in body temperatures on human physical abilities in function of exercise characteristics and duration. ↑ improvement, = no significant effect, ↓ decrement.
||Effect of body temperatures increase
|Part A: positive effect on human physical abilities
|Segal et al (1986)
||↑ muscle contractility
||< 1 sec
||Muscle in vitro, water bath
|Davies and Young (1985)
||↑ muscle force
||< 1 sec
|↑ muscle contractility
|Binkhorst et al (1977)
||↑ muscle force
|Bergh and Ekblom (1979)
||↑ power output
||Water bath and exercise
|↑ muscle force
|Falk et al (1998)
||↑ peak power output
||5 * 15 sec
||↑ power output
|Linnane et al (2004)
||↑ mean power output
||Water bath and warm environment
|Ball et al (1999)
||↑ peak power output
||2 * 30 sec
|Part B: no effect on human physical abilities
|Dotan and Bar-Or (1980)
||= power output
|Backx et al (2000)
||= power output
||2 * (3 * 30 sec)
|Linnane et al (2004)
||= mean power output
||2nd sprint of 30 sec
||Water bath before warm environment
|Falk et al (1998)
||= peak power output
||2nd series of
|5 * 15 sec
|Hue et al (2003)
||= power production
|Part C: negative effect on human physical abilities
|Voltaire et al (2002)
||↓ incremental exercise duration
||~10 to 15 min
|Gonzalez-Alonso et al (1999)
||↓ time to exhaustion
||~28 to 63 min
||Water bath before warm environment
|Nybo and Nielsen (2001)
||↓ time to exhaustion
||~50 min or more
|See the other chapter of this website for more examples concerning long duration exercise
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