I was talking with a female running partner once, and she asked if a woman's menstrual cycle had any effect on racing. Apparently, not much. When an Austrian research team tested 19 eumenorrheic women during two distinctly different menstrual phases, they found no significant differences in the women's exercise performances on such measures as heart rate, oxygen capacity, power, and lactate threshold. Their conclusion, according to Medicine And Science In Sports And Exercise: "We did not find performance changes associated with menstrual cycle. Our data do not support findings that the menstrual cycle influences lactate "thresholds" and ventilatory "thresholds."