If you're a woman who runs, there's a factor that might be at play that has nothing to do with your shoes, your mileage, or whether you skipped your strength training (though let's be honest, we've all been there).
It's your menstrual cycle.
More specifically: ovulation.
Research shows that injury risk, especially for ligaments and soft tissue, spikes during the ovulatory phase of your cycle. This isn't some fringe theory. Studies have found significantly higher rates of ACL injuries during ovulation compared to other phases of the menstrual cycle.
So what's actually happening in your body?
During ovulation, estrogen peaks. And while estrogen is primarily known as a reproductive hormone, it also has a significant effect on your connective tissue, collagen turnover, and ligament laxity.
Here's the mechanism: estrogen binds to receptors in your ligaments and tendons, influencing how collagen (the protein that gives these structures their strength and stability) is produced and organized. When estrogen is high, it can increase the activity of enzymes that break down collagen and alter the cross-linking of collagen fibers. The result? Your ligaments become more elastic and less stiff.
In simpler terms: higher estrogen can make your ligaments looser and your joints slightly less stable.
Why is this a problem for runners?
Think of your ligaments as the stabilizing cables of a suspension bridge. They're meant to hold your joints in proper alignment while still allowing movement. When those cables become too loose, the bridge can sway more than it should.
For runners, this means:
Your ligaments aren't providing their usual level of joint support. When you land from a stride, especially during speed work or downhill running, your knee or ankle might shift slightly out of its ideal position. Normally, tight ligaments would prevent this micro-movement. But with increased laxity, there's more "give" in the system.
This extra movement, even if it's just a millimeter or two, puts abnormal stress on the ligament itself and surrounding structures. It's like repeatedly bending a paperclip at a slightly wrong angle. Once or twice? Fine. But do it hundreds of times during a run, and you're setting yourself up for a strain or even a tear.
The problem is compounded during high-intensity or high-impact activities. When you're sprinting, cutting, or doing plyometrics, you're generating much more force through your joints. Your body relies on split-second neuromuscular responses to keep everything stable. But if your ligaments are looser than usual, your muscles and nervous system have to work harder to compensate. And sometimes, they can't compensate fast enough.
That's when injuries happen.
For some women, this translates to a subtle but real increase in injury vulnerability during this roughly 3 to 5 day window each month. You might notice things like joints that feel unusually loose, coordination that's just slightly off, or a higher chance of straining a tendon during explosive movements.
Does this mean you should skip running during ovulation?
Absolutely not.
But it does mean you can be smarter about how you run during this window.
Think of ovulation as a yellow light in your training cycle: not a stop sign, but a signal to be more intentional. High-risk activities during this phase include sprint workouts, aggressive downhill running, sharp direction changes, plyometrics, and maximal strength lifts. Steady aerobic runs? Those are generally low risk.
Here's the good news: this isn't about your cycle being a liability. It's about having more data to work with.
If you're already doing the foundational work (strength training, proper fueling, prioritizing recovery), you're building resilience that protects you in every phase of your cycle. A well-fueled, well-strengthened runner can absolutely handle ovulation. You're just adjusting the intensity dial slightly, not shutting down.
How to use this information:
If you track your cycle (and if you don't, this might be a good reason to start), you can use your ovulation days strategically. Dial back the intensity on speed work by 5 to 10%. Focus on form over pace. Prioritize sleep and nutrition even more than usual.
Your body is giving you information. The question is whether you're listening.
Next week, I'll break down how each phase of the menstrual cycle affects endurance, recovery, and performance, and give you a practical framework for adjusting your training throughout the month.
Until then: trust your body, train smart, and know that understanding your cycle isn't a limitation. It's a competitive advantage.
Dr. Sulin
Disclaimer: The information provided in this newsletter is for educational purposes only and is not intended as medical advice. Always consult with your healthcare provider before making changes to your training, nutrition, or health routine.