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Objectives: To determine the effect of unknown exercise duration and an unexpected increase in exercise duration on fat oxidation during cycling exercise. Method: Ten sports education students completed three bouts of bicycle ergometer cycling at 70％ of their peak oxygen uptake. In the first trial, they were told to pedal a bicycle ergometer for 20 minutes and were stopped at 20 minutes （20 MIN）. In the second trial, they were told to cycle for 10 minutes, but at 10 minutes were told to cycle for another 10 minutes（10 MIN）. In the third trial, they were not told how long they would be cycling but were stopped after 20 minutes （unknown, UN）. During each of the cycling bouts, psychological （rate of perceived exertion and affect scale） and physiological （heart rate, oxygen consumption, CO2 production, ventilation, and respiratory exchange ratio） parameters were recorded. The trial order was random for each subject. Results: There were no significant differences in fat oxidation, psychological and physiological parameters among the three trials during the first-half 10 minutes. However, in the second-half 10 minutes, fat oxidation was significantly higher in 10 MIN and UN than 20 MIN. Heart rate, ventilation, and respiratory exchange ratio were significantly lower in 10 MIN and UN than 20 MIN. In terms of oxygen consumption and CO2 production, 10 MIN was significantly lower than UN and 20 MIN. The rate of perceived exertion in 10 MIN and affect scale in UN showed significant difference when compared with UN and 20 MIN, respectively. Conclusions: Unexpected increase in exercise duration （10 MIN） and unknown exercise duration（ UN） influenced fat oxidation, suggesting that subjects will be more economical in their use of physiological resources in order to maintain a reserve in anticipation of a longer exercise bout and greater physiological demand. |
