Effect of time-of-day on neuromuscular properties of knee extensors after a short exhaustive cycling exercise

We investigated the effect on neuromuscular properties of the knee extensors of an exhaustive cycling exercise performed at 6AM and 6PM. Eleven well trained cyclists performed a maximal aerobic exercise consisting in a sustained effort at 95% of their maximal power until exhaustion ( T-lim). Isometric maximal voluntary contraction ( MVC) and electromyographic activities ( root mean square: RMS) of the vastus lateralis were recorded before ( pre-test) and immediately after ( post-test) the cycling exercise. The MVC/RMS ratio was calculated and used as an index of neuromuscular efficiency ( NME). Rectal temperature was recorded continuously throughout the test session. Results show that Tlim cycling exercise was not time-of-day dependent ( p = 0.42). A significant interaction effect of time of day and the cycling exercise was demonstrated for MVC ( p < 0.02) and NME ( p < 0.05). In pre-test, MVC and NME were higher in the late afternoon than in the morning (+ 6.8%); however both parameters were more affected by the Tlim exercise at 6PM than at 6AM. These results suggest that following cycling Tlim exercise, neuromuscular properties of the knee extensors are more affected in the evening than in the morning. However, circadian rhythm of some biomechanical parameters implicated in the pedalling movement may modulate the effect of these diurnal variations.