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The aim of the present study was to examine the effects of time of day on stroke parameters and motor organization in front-crawl swimmers. In a randomized order, fourteen regional swimmers (age: 18.7 ± 1.6 years) performed maximal front crawls over 12.5m during two experimental sessions; the morning sessions were conducted between 07:00 and 09:00 h and the evening experiments were conducted between 17:00 and 19:00 h. Stroke parameters (swim velocity, stroke rate and stroke length), motor organization (arm stroke phases and arm coordination) were calculated from aerial and underwater side-view cameras. Arm coordination was quantified in terms of an index of coordination (Idc). Results showed that oral temperature was significantly higher in the evening 36.8 ± 0.2°C than in the morning 36.1 ± 0.2°C (p < 0.001), with a morning-evening differences of -0.7 ± 0.1°C. Performance was also higher in the evening (7.4 ± 0.6 s) than in the morning (8.0 ± 0.8 s) (p < 0.001), with a morning-evening differences of 0.55 ± 0.30 s. Likewise, values of swim velocity and stroke rate were higher in the evening than in the morning (p < 0.001) with morning-evening differences of -0.10 ± 0.04 m.s -1 and -3.99 ± 2.91 cycles.min -1 , respectively. Percentage IdC increased significantly (p < 0.01) between the morning (-5.1 ± 6.5%) and evening (-1.6 ± 7.0%). It is concluded that maximal swimming trials are performed better in the evening than the morning, and that this might be explained by better stroke parameters and motor organization at this time.

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