Amenorrhea Secondary

The goal of this clinical trial is to determine how changes in exercise and psychosocial stress may influence the risk of menstrual cycle irregularities in female runners and cyclists. The main questions it aims to answer are: * Does the implementation of exercise or psychosocial stress effect circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length? * Is there an additive effect of combined exercise and psychosocial on circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length? Participants will be asked to do the following over the \~3 month enrollment period: * attend a laboratory visit at the beginning and end of the study to have their resting metabolic rate, aerobic fitness, and body composition tested * monitor their menstrual cycle length, daily perceived stress levels, physical activity, and diet each month * provide several urine and saliva samples each month * either maintain their usual physical activity and lifestyle habits (control group), increase the duration of their weekly running or cycling mileage by 30% (exercise stress group), complete cognitive function tasks designed to be stressful (psychosocial stress group), or increase the duration of their weekly running or cycling mileage by 30% and complete cognitive function tasks designed to be stressful (exercise + psychosocial stress group) during the final month of enrollment. Researchers will compare control, exercise stress, psychosocial stress, and exercise + psychosocial stress groups to see if there is an effect on circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length.