ORIGINAL ARTICLE J. J. Forsyth Æ T. Reilly Circadian rhythms in blood lactate concentration during incremental ergometer rowing Accepted: 16 January 2004 / Published online: 26 February 2004 Ó Springer-Verlag 2004 Abstract This study examined whether circadian rhythms affect lactate threshold (Th lac ) during rowing exercise. Eleven male, endurance-trained athletes [mean (SD) age 29.5 (6.1) years] rowed at 0200, 0600, 1000, 1400, 1800 and 2200 hours under the same experimental conditions. Capillary blood (25 ll) was obtained from the tip of the toe during the last 30 s of a continuous, multi-stage, 3-min, incremental protocol on the Con- cept II ergometer. To determine Th lac , a curve-fitting procedure (the D max method), a visual method (Th lac-vis ) and the fixed blood lactate concentration of 4.0 mmol l )1 (Th lac-4 mM ) were used. Circadian rhythms were appar- ent for oxygen consumption and heart rate at Th lac using the D max method (P=0.02 and P=0.04 respectively), with the acrophases at 2139 hours and 2032 hours respectively coinciding in phase with that of core body temperature. The conclusion is that tests should be completed at the same time of day at which the athlete usually trains, to ensure precision of Th lac determina- tion, especially when the D max method is used to deter- mine Th lac . Keywords Chronobiology Æ Anaerobic threshold Introduction Circadian rhythms constitute changes in physiological activity displaying a period of 24 h (Minors and Wa- terhouse 1981). The most distinctive rhythm observed is that of core body temperature, with a peak occurring at 1800 hours and a trough at 0600 hours (A ˚ kerstedt 1979). The circadian rhythm in temperature persists during submaximal exercise (Reilly and Brooks 1982), as does that of heart rate (HR; Reilly et al. 1984), oxygen consumption ( _ V _ O 2 ; Reilly and Brooks 1982; Giacomoni et al. 1999) and pulmonary ventilation ( _ V E ; Reilly and Brooks 1982). The lactate threshold (Th lac ), defined as the point at which blood lactate increases exponentially with exercise intensity (Yeh et al. 1983), has not been investi- gated thoroughly with respect to circadian rhythms. Marti et al. (1998) tested 20 well-trained runners and found no significant differences in submaximal running speed at Th lac when subjects were tested between 0830 hours and 0900 hours compared to responses between 1030 hours and 1130 hours. It is unlikely that a diurnal variation will be revealed since only two time points were compared, and a difference of only 2 h sep- arated these time points. In addition, Th lac was recorded as the running velocity inducing a blood lactate con- centration of 4 mmol l )1 (Th lac-4 mM ). The usefulness and reproducibility of using this method for representing a maximum lactate steady state, and distinguishing dif- ferences between individuals have been questioned (Stegmann et al. 1981). It may be more appropriate to assess whether the shape of the lactate response curve changes rhythmically with time of day, rather than measuring a fixed blood lactate concentration, which could be affected by changes in diet or training status (Hughes et al. 1982; Yoshida 1984). Sekir et al. (2002) studied the effect of time of day on the relationship be- tween Th lac and the ventilatory threshold (Th vent ). They reported no significant time of day effects for Th lac using the log–log method, or for Th vent using the V-slope method. There was, however, some variation due to time of day in the strength of the relationship between Th lac and Th vent (determined using correlation analysis) when exercise time and _ V E were used as variables to describe the thresholds. Only three measurements were taken at 0700 hours, 1400 hours and 1900 hours. If a circadian rhythm exists for blood lactate response to exercise, then it might be explained by increases in J. J. Forsyth (&) Sport and Exercise Sciences, North East Wales Institute of Higher Education, Plas Coch, Mold Road, Wrexham, Wales, LL20 7UR, UK E-mail: j.forsyth@newi.ac.uk Fax: +44-1978-290008 T. Reilly Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, UK Eur J Appl Physiol (2004) 92: 69–74 DOI 10.1007/s00421-004-1059-8