This study deals with the influence of time of day on neuromuscular efficiency in competitive cyclists during continuous exercise versus continuous rest. Knee extension torque was measured in ultradistance cyclists over a 24h period (13:00 to 13:00 the next day) in the laboratory. The subjects were requested to maintain a constant speed (set at 70% of their maximal aerobic speed obtained during a preliminary test) on their own bicycles, which were equipped with cyclosimulators. Every 4h, torque developed and myoelectric activity were estimated during maximal isometric voluntary contractions of knee extensors using an isokinetic dynamometer. Mesenteric temperature was monitored by telemetry. The same measures were also recorded while the subjects were resting awake until 13:00 the next day. During activity, torque changed within the 24h period (p < .005), with an acrophase at 19:10 and an amplitude of 7.8% around the mean of 70.7%. At rest, a circadian rhythm was observed in knee extensor torque (p < .05), with an acrophase at 19:30 and an amplitude of 6% around the mean of 92.3%. Despite the standardized conditions, the results showed that isometric maximal strength varied with time of day during both a submaximal exercise and at rest without prior exercise. The sine waves representing these two rhythms were correlated significantly. Although at rest the diurnal rhythm followed muscular activity (i.e., neurophysiological factors), during exercise, this rhythm was thought to stem more from fluctuations in the contractile state of muscle.
Circadian rhythms have formed the subject of many researches in man during bed rest or usual routine, but have been little studied during continuous and sustained physical exercise. This study deals with the influence of time of day on biological markers in competitive cyclists during continuous physical exercise versus continuous rest. Ultra-distance cyclists were studied over a 24 h period (13:00 to 13:00 h the next day) in the laboratory. The subjects were requested to maintain a constant speed (set at 65% - 70% of their maximal aerobic speed obtained during a preliminary test) on their own bicycles which were equipped with home trainers. Workload, core temperature and heart rate were monitored continuously. The same measures were also recorded while the athletes were resting awake until 13:00 h the next day. Results show that in both situations, core temperature and heart rate exhibited significant circadian variations (p < 0.001). Furthermore, during exercise, an accentuation of amplitude and mean of every rhythm (p < 0.05) with a phase lag (p < 0.05) were observed. Despite a strenuous and continuous physical exercise requiring special physiological adaptations, the rhythmic variations observed at rest persisted, which highlighted the influence of biological clocks.
Reçu le 12 octobre 2000 RÉSUMÉ L'objectif de ce travail a consisté, à partir d'une partie des résultats de deux études différentes, à esti mer les effets de la privation de sommeil sur les fluc tuations circadiennes du fonctionnement du système musculo-squelettique. Dans chacune des études (EA S et EPS), les moments musculaires isométriques maximaux et les activités électromyographiques des muscles impliqués ont été enregistrés à sept reprises au cours du nycthémère à l'aide d'un ergomètre isocinétique : contractions volontaires brèves de flexion du coude pour Eas dont les sujets ont dormi à des horaires stan dardisés, et d'extension de la jambe pour EPS dont les sujets ont été complètement privés de sommeil. Les résultats montrent que les moments musculaires iso métriques maximaux obtenus durant EA S et EPS fluc tuent significativement au cours des 24 h (p < 0,005) avec des maximums respectifs situés en début de soi rée (18:30 et 19:30 h), soulignant l'importance des horloges biologiques. Les activités électromyogra phiques des muscles agonistes ne présentent pas de fluctuations journalières pour EAS, au contraire de celles de EPS qui montrent des valeurs plus faibles le matin que le soir (77,2 % et 95,2 %, respectivement ; p < 0,01). Une diminution du niveau d'éveil du sys tème nerveux central liée à la privation complète de sommeil pourrait expliquer ce phénomène. SUMMARY Circadian changes in muscular efficiency : sleep consent versus sleep deprivation The influence of time of day on muscular perfor mance was studied. From part of the results of two different studies (EAS et EPS), the effects of sleep depri vation were appreciated. Seven times over the 24-h period, developed torque and myoelectric activity were estimated during maximal isometric voluntary contractions using an isokinetic dynamometer : elbow flexion for EAS in standardised sleep, and knee exten sion for EPS in complete sleep deprivation. The results showed nycthemeral changes in torque in both condi tions (p < 0.005), with maximal values recorded at the beginning of night. Although during sleep deprivation (EPS) the rhythm followed neurophysiological factors, during EAS this rhythm was accounted for by the variations in the contractile state of muscle.
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