Time-of-day effects on biochemical responses to soccer-specific endurance in elite Tunisian football players

This study aimed to investigate footballers' diurnal variation of performance during the Yo-Yo intermittent recovery test and the associated biochemical responses. Fifteen male footballers (17.3 +/- 0.3 years, 69.1 +/- 4.2kg, 179.7 +/- 3.6cm) performed two randomised Yo-Yo tests at 07:00h and 17:00h. Blood samples were collected before and 3min after each test for the assessment of metabolic responses. Resting oral temperature and rating of perceived exertion (RPE) after and peak heart rate during the Yo-Yo test were recorded at both times-of-day. Core temperature and performances during the Yo-Yo test increased from the morning to the evening (P<0.0005 and P=0.01, respectively) without significant time-of-day effects on peak heart rate and RPE. Moreover, pre- and post-Yo-Yo test biochemical parameters (high-density lipoprotein, triglycerides, glucose, creatine-kinase) were higher at 17:00h than 07:00h (160.45 +/- 18.68 vs. 173.73 +/- 14.48 before and 191.18 +/- 21.13 vs. 219.27 +/- 27.74IU center dot L-1 after the Yo-Yo test at 07:00h and 17:00h, P=0.032 and P<0.0005, respectively for creatine-kinase). Only post-exercise lactate levels were higher in the evening (9.82 +/- 0.65 vs. 10.86 +/- 0.33mmol center dot L-1, P<0.0005) with all biochemical variables being increased after the exercise (P<0.0005). These findings suggest a possible link between the diurnal fluctuation of metabolic responses and the related pattern of specific-endurance performances in footballers. Therefore, the higher biochemical responses observed in the evening could explain, partially, the greater performance and metabolic solicitation at this time-of-day.