Modélisation rhéologique de la cinétique cardio-respiratoire: exercice sous-maximal et environnement froid

Abstract

Il est admis que l'environnement thermique modifie, entre autres, la réponse cardio-respiratoire à l'exercice physique. La modélisation rhéologique permet d'introduire la notion de cinétique dans l'évolution de variables physiologiques, telles FC, vO2·kg−1, vCO2·kg−1, vO2/FC, R et Tre, lorsque 8 subjets sont soumis à un exercice physique constant (v=0,4 v max), de longue durée (2 h), suivi d'une période de récupération (une heure) dans 2 conditions thermiques différentes (20°C et 0°C). Les résultats montrent qu'une modélisation basée sur l'introduction d'une seule exponentielle n'est pas suffisante pour couvrir les phénomènes observés. L'usage de modèles plus complexes convient, en effet, à expliquer dans le cas présent les phénomènes mesurés, et ce, d'une part d'un point de vue mathématique, mais surtout du point de vue physiologique. En cours d'exercice, le milieu ambiant froid diminue, en règle générale, la vitesse d'adaptation ainsi que l'amplitude de variation imprimées à l'organisme par le stress physique. Seule l'adaptation rapide de FC semble accélérée en hypothermie ambiante. Notons en outre que pour des températures basses et pour des exercices d'intensité relativement faible, l'économie du système respiratoire n'est pas favorablement influencée suite à une hyperventilation. En période de récupération, les phénomènes adaptatifs rapides sont favorisés en ambiance thermique froide, tandis que les phénomènes de récupération lente sont accélérés dans une ambiance de confort. Thermal modification of the environment induces variations of the cardio-respiratory responses to exercise (Patton and Vogel, 1984; Vogelaere et al., 1986; Quirion et al., 1986). The use of rheological models introduces the concept of kinetics with a view of describing the evolution of physiological variables, such as heart rate (FC), oxygen consumption (vO2·kg−1), carbon dioxide release vCO2·kg−1), oxygen pulse (vO2/FC), respiratory quotient (R) and rectal temperature (Tre), when eight male subjects are submitted to a submaximal exercise (v=0.4 v max). The physical exercise was performed in a temperature chamber at two different ambient thermal conditions (20°C and 0°C). The experiment performed in semi-nude conditions extended over two hours and was followed by a passive rest of one hour. The results show that one exponential function (model (0,1)) insufficiently quantifies the time course of the observed variations (Table II). From both the mathematical and physiological point of view more complex models such as (1,2), consisting of two exponential functions and (1,2)*, composed of one exponential and a linear trend are required for the description of the measured phenomena in the present case (Tables III and IV). The choice of the model (1,2)* for the interpretation of the evolution of the various observed physiological variables during the exercise period is based on the fact that prolonged exercise does not allow for a “steady state” (Vogelaere et al., 1981; Shephard, 1987). The concept of steady state negates in fact the existence of “fatigue” (Green, 1987). On the contrary, post exercise recovery implies that the body's functions must reach rest steady state values after a certain recuperation time. The latter phenomenon can be best described by the model (1,2) consisting of a superposition of two exponential functions. The first exponential with a short time constant T1 can be assimilated with the “alactic” recovery time (Elsner and Carlson, 1962; Hollman, 1963), whereas the second one, characterized by a longer retardation time T2 can be related to the slower “lactic” recovery (Vogelaere and S'Jongers, 1984). In general the velocity v and the amplitude of adjustment Δy induced by the physical stress condition are reduced in cold acclimation. Only the fast adaptation of heart rate seems to be accelerated in a hypothermic environment. As a result of the cold induced bradycardia the energetic dissipation function Q−1 of the cardiac system is systematically reduced in a cold environment during both the exercise and recovery periods. Furthermore, an exercise of relatively low intensity performed in the cold induces an increase of the activity of the respiratory system owing to hyperventilation. On the other hand, fast adjustment phenomena are favoured in a cold environment during recovery, whereas slow adaptation responses are accelerated in normal thermal conditions. Transferring this statement into practice it is preconceived that passive post exercise recovery will be enhanced by exposing subjects to a cold environment for two minutes at most and afterwards continuing the recovery at normal room temperature. Finally, if it is intended to maintain a physiological variable at a constant level during exercise, the work load has to be progressively reduced. The relaxation time τ provides the theoretical time for which the initial work intensity has to be reduced by 37% for the observed variable to keep a constant value. This space of time can be used as an indicator for the interindividual appearance of fatigue without the necessity of exposing subjects to real physical exhaustion.