The combined effect of time of day and menstrual cycle on lactate threshold.

Purpose: This study examined the isolated and combined effects of time of day and menstrual cycle phase on the determination of the lactate threshold (Tlac) and blood lactate concentration.
Methods: Eleven endurance-trained female athletes (mean age 32.4 ± 6.9 yr) were tested at 06:00 and 18:00 h and at two phases of the menstrual cycle, the midfollicular phase and the midluteal phase. Capillary blood (25 μL) was obtained from the tip of the toe at rest, and during the last 30 s of a continuous, multistage, 3-min incremental protocol on the Concept II rowing ergometer. To determine Tlac, a curve-fitting procedure (Dmax method), a visual method (Tlac-vis), and the fixed blood lactate concentration of 4.0 mmol•L-1 (Tlac-4 mM) were used. Ventilatory threshold (Tvent) was also determined.
Results: In the midluteal phase of the menstrual cycle, Tlac-4 mM occurred at a significantly higher exercise intensity, heart rate, and oxygen consumption than it did in the midfollicular phase. Blood lactate concentration at Tvent and at Tlac using the Dmax method was significantly lower in the midluteal phase. No significant interaction effects (menstrual cycle × time of day) were observed for any of the methods used to determine Tlac or for values of blood lactate concentration at rest and at maximum.
Conclusion: These findings suggest that, when using fixed values of blood lactate in physiologic assessment, consideration should be given to the menstrual cycle phase in which the test is carried out.