Circadian rhythms in human muscular efficiency: continuous physical exercise versus continuous rest. A crossover...

Area of Science:

• Sports Science
• Human Physiology
• Chronobiology

Background:

• Understanding diurnal variations in neuromuscular performance is crucial for optimizing athletic training and competition schedules.
• Previous research suggests time-of-day effects on physiological functions, but specific impacts on cyclists' neuromuscular efficiency during sustained activity are less understood.

Purpose of the Study:

• To investigate the influence of time of day on neuromuscular efficiency in competitive cyclists.
• To compare circadian rhythms in knee extension torque during continuous submaximal exercise versus continuous rest.

Main Methods:

• Measured knee extension torque and myoelectric activity in ultradistance cyclists over a 24-hour period using an isokinetic dynamometer.
• Subjects performed maximal isometric voluntary contractions every 4 hours during continuous cycling at 70% of maximal aerobic speed and during rest.
• Monitored mesenteric temperature via telemetry.

Main Results:

• Significant circadian rhythm in knee extension torque observed during both exercise (acrophase 19:10, amplitude 7.8%) and rest (acrophase 19:30, amplitude 6%).
• Isometric maximal strength varied significantly with time of day under both conditions.
• The diurnal rhythms during exercise and rest were significantly correlated.

Conclusions:

• Isometric maximal strength exhibits a time-of-day variation in competitive cyclists, even under standardized laboratory conditions.
• While rest rhythms may be influenced by neurophysiological factors, exercise-induced rhythms are likely more related to fluctuations in muscle contractile state.
• These findings highlight the importance of considering diurnal variations for performance optimization in cycling.