Kine, C. E., Devlin, T. M., Zielinski, M. R., Moore, T. A., Durstine, J. L., Davis, J. M., & Youngstedt, S. D. (2009). Time of habitual training does not alter circadian rhythm of swim performance. ACSM 56th Annual Meeting, Seattle, Washington, Presentation Number 2747.

"Some studies have found better athletic performance at the usual time of training relative to other times of day. These results could reflect either a change in the circadian system or a non-circadian adaptation that results in better performance at the accustomed time of training. However, other studies have provided equivocal or negative results."

This study examined if the circadian rhythm of swim performance differed between individuals who habitually trained at a certain time(s) of day compared to those who did not. Swimmers (N = 25) were assessed for 50-55 consecutive hours in the laboratory while adhering to a three-hour ultra-short sleep/wake cycle, a common chronobiological technique. Ss performed six maximal 200-m swims, with each trial separated by nine hours. Performances were z-transformed and expressed relative to time of day. The time of habitual training for each swimmer was obtained via questionnaire prior to the study. Estimates of swim performance rhythm amplitude (i.e., range of performance), acrophase (i.e., time of peak performance), and mesor (i.e., fitted mean value) were obtained via cosinor analysis and compared between groups.

Twelve Ss reported no usual time of training. The remainder (N = 13) had habitual swim times of training daily between 0600-0900 hours, 1600-1800 hours, or at both times. Those who reported a habitual time of training had lower mesors (i.e., faster swim time), but did not have different performance rhythm acrophases or amplitudes compared to those without a specific time of training. No group differences were found in swim performances when analyzed via an analysis of covariance.

Implication. This study found minimal evidence of different swim performance rhythm characteristics between those who habitually trained at a particular time of day versus those who did not. No evidence was found for superior performance that was specific to the usual time of training. This study does not discount the possibility that such an advantage might be observed under usual diurnal conditions, due to variations in diet, sleep, rest, and performance preparation.

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