Over the last few months of being sheltered-in-place due to the COVID-19 craziness, many of us picked up new hobbies, or revisited old ones, including many A-Team Athletes. I developed the running challenge (which you can access via the app) so we can all get some fresh air and cardio while we have limited access to gyms and (possibly) more free time due to scheduling changes. If you've used the app, you'll note that you are asked to take your heart rate on a regular basis. We do this to track changes over time; reducing your resting heart rate usually means that you are getting more fit. Prior to starting the running challenge, my cardio consisted mostly of long walks, and my heart rate averaged about 68 beats per minute (BPM), which is indicates good level of fitness for women my age. After a few weeks of running about 2-3x per week, my resting heart rate read as low as 63 BPM, indicating an excellent level of fitness according to ACSM guidelines. Not too shabby. Fast forward a few weeks, and my heart rate has shot up to as high as 73 BPM, dropping my fitness level by two categories. Now that I'm done with my MS program, my extra time allows me to dive deep into rabbit holes to explain why I'm seeing this trend in data (arguably my favorite pastime). Now, some of you may already know the relationship between a woman's cycle and her body. We know that body temperature increases during ovulation. Since most of my strength athletes are women, I have paid special attention to how the female cycle affects cravings, metabolism, and strength levels. My body, personally, is very sensitive to the hormonal changes in my cycle, so I pay very close attention to which phase I am in at any given time, and I make changes to my diet and training in order to better coincide with these changes. When I noticed a spike in my body weight that coincided with ovulation, and an increase in heart rate, I decided to look deeper to see if there is a connection. A study published in 2008 showed that resting heart rate and resting systolic blood pressure both go up during ovulation. A more recent study (2017) using wrist-worn devices showed that resting heart rate continues to go up after after ovulation, through the luteal phase, and was elevated significantly (3.8 BPM) compared to the menstrual phase. So how can we use this information as athletes? Well for one, tracking heart rate as a tool for measuring fitness is only useful if our heart rate is a reliable source of information. We are not able to compare heart rate readings from one phase of our menstrual cycle against the heart rate readings from another phase. Now that we have these tools in our figurative toolbox, we know that we must compare our heart rate readings from month-to-month, not week-to-week, in order to determine our fitness levels and compare trends over time. Before I go down a rabbit hole of determining how heart rate variability during cardiovascular exercise (rather than during rest) is affected by the menstrual cycle, I will sign off. That's a topic for another day.