Why Do ACL Injuries Spike in the Days Before a Period? The Ligament Laxity Research

If you tear an ACL without contact, the odds that it happened during a specific window of your menstrual cycle are not random. A body of work stretching back over two decades has found that non-contact ACL ruptures cluster in the pre-ovulatory and ovulatory phases, when estrogen peaks. Wojtys and colleagues, working out of the University of Michigan, were among the first to document this pattern, showing that injuries occurred disproportionately during the follicular and ovulatory phases rather than being evenly spread across the cycle (see [1]). Later reviews pooling multiple studies reached a similar conclusion: the risk is not flat month to month.

The proposed mechanism is knee joint laxity. Estrogen receptors sit on the ACL itself, and controlled measurements have shown that anterior knee laxity fluctuates across the cycle, rising around ovulation when estrogen is highest (see [2]). A more lax ligament under a high-velocity cutting or landing load has less passive restraint, and the muscles have to do more of the work of stabilizing the joint at exactly the moment the ligament is least able to help. This is a signal worth understanding, not a reason to fear a normal hormonal rhythm.

Here is the part that matters for prevention, and it is where the honest framing has to come in. The laxity story explains a piece of the risk, but neuromuscular control explains more of it and is the piece you can actually influence. Landing mechanics, hip and knee alignment on deceleration, and eccentric hamstring strength are all trainable, and structured neuromuscular training programs have been shown to reduce non-contact ACL injury rates substantially across athletic populations (see [3]). No pill changes your landing pattern. Consistent, well-coached movement work does. If you track your cycle alongside training load, you can note it in /body and watch how the two lines sit against each other over months rather than guessing from a single bad session.

None of this means restricting training during any phase — the evidence does not support sitting out days of the month. It means the mechanism is real, the modifiable lever is neuromuscular quality, and the hormonal window is context, not destiny.

This is general information, not medical advice — talk to a qualified clinician about your own situation.