Abstract
The aim of this study was to determine the effect of time of day on performance, pacing, and hormonal and metabolic responses during a 1000-m cycling time-trial. Nine male, recreational cyclists visited the laboratory four times. During the 1st visit the participants performed an incremental test and during the 2nd visit they performed a 1000-m cycling familiarization trial. On the 3rd and 4th visits, the participants performed a 1000-m TT at either 8 am or 6 pm, in randomized, repeated-measures, crossover design. The time to complete the time trial was lower in the evening than in the morning (88.2±8.7 versus 94.7±10.9 s, respectively, p<0.05), but there was no significant different in pacing. However, oxygen uptake and aerobic mechanical power output at 600 and 1000 m tended to be higher in the evening (p<0.07 and 0.09, respectively). There was also a main effect of time of day for insulin, cortisol, and total and free testosterone concentration, which were all higher in the morning (+60%, +26%, +31% and +22%, respectively, p<0.05). The growth hormone, was twofold higher in the evening (p<0.05). The plasma glucose was ∼11% lower in the morning (p<0.05). Glucagon, norepinephrine, epinephrine and lactate were similar for the morning and evening trials (p>0.05), but the norepinephrine response to the exercise was increased in the morning (+46%, p<0.05), and it was accompanied by a 5-fold increase in the response of glucose. Muscle recruitment, as measured by electromyography, was similar between morning and evening trials (p>0.05). Our findings suggest that performance was improved in the evening, and it was accompanied by an improved hormonal and metabolic milieu.
Highlights
Performance during a short-distance time trial (TT), such as a 1000-m cycling TT, may be optimized when an ‘‘all-out’’ pacing is used [1,2]
The mean values for PO, Paer, Pan, Integrated EMG (iEMG), V O2 and HR were not different between evening and morning, a moderate effect size was found for all parameters with higher values observed in the evening (Table: 1)
There was no main effect for time of day or interaction for all of th:ese variables, but there was a tendency for Paer and V O2 values at the 600- and 800-m sections to be higher in the evening than in the morning (p = 0.07 and 0.06, respectiv:ely)
Summary
Performance during a short-distance time trial (TT), such as a 1000-m cycling TT, may be optimized when an ‘‘all-out’’ pacing is used [1,2]. The underlying mechanism explaining the greater energy expenditure and improved performance in the evening is not fully understood, but a slightly increased core temperature in the evening could increase nerve conduction velocity and vasodilatation, which in turn would increase muscular supply and substrate elimination, thereby improving glycogenolysis and glycolysis [6] While these results would support that performance during a short-distance cycling TT might be increased in the evening, due to an increase in the aerobic and/ or anaerobic energy expenditure, to the best of our knowledge no study has measured the effect of time of day on performance and associated pacing alterations during a TT
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