Hormonal shifts during menstruation change the way your knee joints behave, how well your body dampens pain signals, and even how you land when you jump or change direction. The drop in estrogen and progesterone that triggers your period loosens the ligaments stabilizing your knee, weakens your body’s built-in pain-suppression system, and may alter your balance. These effects overlap during the same few days, which is why knee pain can seem to appear out of nowhere right around your period and vanish again afterward.
How Hormones Loosen Your Knee Ligaments
Your anterior cruciate ligament, the main stabilizer inside the knee, has receptors for estrogen, progesterone, and a lesser-known hormone called relaxin. When levels of these hormones shift, so does the physical stiffness of the ligament itself. Research measuring knee laxity across the menstrual cycle found that ACL looseness increased when estrogen and progesterone rose from their baseline to their peak values during the cycle.1PubMed Central. Hormonal changes throughout the menstrual cycle and increased anterior cruciate ligament laxity in females That might sound paradoxical if your knee hurts most when those hormones are at their lowest, during menstruation. But the ligament doesn’t snap back to a tighter state the instant hormones drop; there is a lag. And the overall pattern is one of instability: laxity fluctuates rather than holding steady, and the knee has to adapt to a ligament whose tension keeps changing week to week.
Relaxin adds another layer. Relaxin receptors on the ACL promote the breakdown of collagen while suppressing new collagen production. Serum relaxin concentration peaks around days 21 to 24 of the cycle, a window that has also been linked to greater ACL injury risk.2PubMed Central. Menstrual Cycle Hormone Relaxin and ACL Injuries in Female Athletes: A Systematic Review The collagen-weakening effects of relaxin don’t reset overnight when your period starts. Cumulative ligament fatigue from the late luteal phase may carry over into menstruation, contributing to a knee that feels unstable or achy as you bleed.
Your Body’s Pain Brake Is Weakest During Menstruation
Even if your knee’s physical structure stayed exactly the same throughout the month, you would still perceive more pain during your period. That’s because your nervous system’s ability to suppress pain signals fluctuates with your hormones. A review of clinical and laboratory studies found that estrogen fluctuations increase pain intensity and perception, while testosterone tends to raise the pain threshold.3PubMed Central. The Role of Sex Hormones in Pain-Related Conditions During menstruation, estrogen and progesterone are both at rock bottom, and the effects are measurable.
Your body has a built-in mechanism called conditioned pain modulation, essentially a “pain inhibits pain” system in which your brain dials down one pain signal when another is present. Studies testing this system across the menstrual cycle found that it works best around ovulation, when estrogen is high and progesterone is still low, and performs worst during the early follicular phase, which coincides with your period.4PubMed. The influence of menstrual phases on pain modulation in healthy women Another study looking specifically at inhibitory pain mechanisms confirmed the pattern: pain suppression was significantly more effective during ovulation than during menses or the luteal phase.5PubMed. Excitatory and inhibitory pain mechanisms during the menstrual cycle in healthy women
Pain thresholds and tolerance also varied across the cycle in the majority of studies reviewed on the topic, and ovarian hormones were found to have distinct effects on inflammation, stress responses, and the sensory systems that carry pain signals.6PubMed. Ovarian hormones and pain response: a review of clinical and basic science studies The practical upshot is this: a low-grade knee issue that you don’t notice mid-cycle can become genuinely painful during your period, not because the joint has gotten worse, but because your brain’s volume knob for pain is turned up.
Even pain medication may work differently depending on cycle phase. One study found that women not using oral contraceptives got significantly more pain relief from morphine during the follicular phase than during the luteal phase, whereas women on the pill showed no such difference.7PubMed Central. Evaluation of menstrual cycle effects on morphine and pentazocine analgesia The finding is specific to opioid analgesia rather than over-the-counter drugs, but it illustrates how deeply hormonal status shapes the pain experience.
Balance and Proprioception Drop at Menses
Proprioception is your body’s sense of where a joint is in space without looking at it. When proprioception at the knee is poor, muscles fire with less precision, the joint is exposed to more awkward loading angles, and you’re more likely to feel pain during everyday movements like walking downstairs. Research on knee joint position sense found that accuracy was worst during menstruation, when circulating sex hormones are low. One study reported that the average error in sensing knee position was greatest at menses and smallest during the mid-luteal phase.8PubMed. Menstrual cycle and knee joint position sense in healthy female athletes An earlier study testing multiple knee angles reached a similar conclusion: repositioning errors were higher during the menstrual phase compared with the follicular and luteal phases.9PubMed. The effects of menstrual cycle on the knee joint position sense: preliminary study
Dynamic balance tells a similar story. A study using a modified star excursion balance test found that balance performance was lowest during the menses phase, with reach distances in multiple directions being significantly shorter compared with the mid-follicular and mid-luteal phases.10PubMed. Effect of the menstrual cycle on knee joint position sense and dynamic balance When balance is off, the muscles around the knee have to work harder as compensating stabilizers. That extra demand can translate into soreness or a feeling of joint vulnerability, especially if you’re also dealing with less effective pain suppression.
Changes in How You Move
Biomechanical studies that track how women move during landing tasks reveal that movement patterns shift across the cycle in ways that load the knee differently. At ovulation, when estrogen and progesterone rise and the ACL becomes more lax, women tend to show greater knee valgus moment, meaning the knee dips inward, along with increased internal rotation of the thigh bone.11PubMed Central. Jump-landing biomechanics and knee-laxity change across the menstrual cycle in women with anterior cruciate ligament reconstruction During menses, a different pattern emerges: women land harder, generating greater peak vertical ground-reaction force, and the shin bone is more internally rotated at the moment of contact.11PubMed Central. Jump-landing biomechanics and knee-laxity change across the menstrual cycle in women with anterior cruciate ligament reconstruction Landing harder means more impact force traveling through the knee with each step, jump, or stair descent.
A systematic review synthesizing the biomechanical literature concluded that the greatest risk of ACL injury falls within the pre-ovulatory phase of the cycle, and that women with greater ACL laxity during that phase show more knee valgus and tibial rotation during functional tasks.12PubMed Central. Effects of the menstrual cycle on lower-limb biomechanics, neuromuscular control, and anterior cruciate ligament injury risk: a systematic review Meanwhile, research tracking knee movement during side-cutting tasks found that the mid-luteal phase was associated with the lowest biomechanical risk profile for the ACL, with the least knee valgus.13PubMed Central. The effects of sex hormones during the menstrual cycle on knee kinematics In other words, the biomechanical stress on the knee is not uniform across the month: it shifts, and the menses and pre-ovulatory windows tend to be the worst.
One reassuring finding is that raw muscle strength around the knee doesn’t appear to fluctuate with the cycle. A repeated-measures study of physically active women found no significant differences in the hamstring-to-quadriceps strength ratio or in peak torque of knee flexion and extension across menstrual cycle phases.14PubMed. Hamstring-to-quadriceps ratio across menstrual cycle phases in physically active women: a repeated-measures study Your muscles can still produce the same force; the issue is more about coordination, laxity, and how your nervous system manages the joint than about strength itself.
Inflammation and Prostaglandins
Prostaglandins are the chemical messengers that cause your uterus to contract and shed its lining, and they’re the main reason periods are painful. But prostaglandins don’t stay neatly contained in the uterus. They circulate in the bloodstream and can trigger inflammatory responses elsewhere in the body, including in joints. The same prostaglandins responsible for menstrual cramps can contribute to a general state of low-grade systemic inflammation during your period.
Blood markers of inflammation do shift across the cycle, though the pattern isn’t entirely straightforward. One study found that highly sensitive C-reactive protein, a standard marker of inflammation, was highest during the early follicular phase, which is when menstruation occurs.15The Journal of Clinical Endocrinology & Metabolism. Low-Grade Inflammation and Estimates of Insulin Resistance during the Menstrual Cycle in Lean and Overweight Women Another study found that CRP rose at midcycle and in the luteal phase, correlating with progesterone increases.16PubMed. Menstrual cycle-associated changes in blood levels of interleukin-6, alpha1 acid glycoprotein, and C-reactive protein The differences may come down to how sensitive the CRP assay was, how the cycle phases were defined, and the populations studied. The key point is that inflammatory markers are not static throughout the month. If your knee already has mild inflammation from wear, cartilage issues, or overuse, a hormone-driven bump in systemic inflammation during menstruation can push it past the threshold where you notice pain.
When the Cause Is Deeper Than the Cycle Itself
For some people, menstrual-phase knee pain is a sign of something more than normal hormonal fluctuation. Endometriosis, a condition in which tissue similar to the uterine lining grows outside the uterus, can affect far more than the pelvis. A pilot study found that roughly half of women with endometriosis reported leg pain, and the pain was bilateral in about 60 percent of those cases, suggesting involvement of multiple peripheral nerves.17PubMed Central. A pilot study of the prevalence of leg pain among women with endometriosis
Endometriosis can even appear in skeletal muscle and joints directly. A systematic review of endometriosis involving the musculoskeletal system found that about 12 percent of cases involved lower-limb muscles and roughly 6 percent involved joints. Around three-quarters of patients with musculoskeletal endometriosis had symptoms that worsened in sync with their menstrual cycle.18PubMed Central. Endometriosis of the skeletal muscular system (ESMS): a systematic review If your knee pain is severe, doesn’t respond to anti-inflammatory medication, or comes with other endometriosis symptoms like heavy periods, painful intercourse, or chronic pelvic pain, it’s worth raising the possibility with a doctor. Musculoskeletal endometriosis is rare, but it’s underdiagnosed partly because clinicians don’t always think to look for it outside the pelvis.
What Actually Helps
Non-steroidal anti-inflammatory drugs like ibuprofen are often the first line of defense for both menstrual cramps and period-related joint pain, and the mechanism is the same: they block prostaglandin production. A randomized crossover trial comparing ibuprofen, acetaminophen, and placebo found that ibuprofen cut menstrual fluid prostaglandin levels by more than half compared with placebo, and acetaminophen also significantly reduced them, though less potently.19American Journal of Obstetrics and Gynecology. Clinical efficacy and differential inhibition of menstrual fluid prostaglandin F2α in a randomized, double-blind, crossover treatment with placebo, acetaminophen, and ibuprofen in primary dysmenorrhea Taking ibuprofen before your pain peaks, rather than waiting until it’s already severe, tends to work better because you’re suppressing the prostaglandin surge before it fully ramps up.
Hormonal contraceptives are another approach. By flattening the hormonal peaks and valleys across the month, the pill may reduce the cycle-dependent changes in ligament laxity and pain sensitivity. A systematic review of oral contraceptive effects on the musculoskeletal system noted that the pill did not consistently change ACL stiffness, but it did reduce muscle stiffness during passive stretching.20PubMed Central. Effects of orally administered hormonal contraceptives on the musculoskeletal system of healthy premenopausal women—A systematic review The evidence on whether the pill reduces joint pain specifically is still thin, but the rationale is plausible: if the hormonal swings are causing the problem, smoothing them out should help.
Training can also make a difference. A study of female collegiate athletes found that knee control during a challenging single-leg squat improved with practice regardless of menstrual cycle phase, and the best knee control and quadriceps activation occurred during the mid-luteal phase.21PubMed Central. Influence of the Menstrual Cycle and Training on the Performance of a Perturbed Single-Leg Squatting Task in Female Collegiate Athletes The key takeaway is that the knee can be trained to stay better aligned even when hormonal conditions are unfavorable. Strengthening the muscles around the knee and practicing balance work builds a safety net that partially compensates for the laxity and proprioceptive dips that come with menstruation.
Nutritional Gaps That Can Make It Worse
Mineral deficiencies may amplify the whole picture. A case-control study of university students found that serum calcium and magnesium levels were significantly lower in women with premenstrual syndrome, and that each unit decrease in calcium or magnesium raised the risk of PMS by about 19 percent and 14 percent, respectively.22PubMed Central. The Association between the Risk of Premenstrual Syndrome and Vitamin D, Calcium, and Magnesium Status among University Students: A Case Control Study Calcium and magnesium both play roles in muscle contraction and nerve signaling. If you’re already low in either, the normal hormonal dip at menstruation may have outsized effects on muscle cramps, joint stiffness, and pain perception. Interestingly, vitamin D levels did not differ between PMS and non-PMS groups in that study, despite vitamin D often being lumped into the conversation about menstrual symptoms.
This doesn’t mean taking a magnesium supplement will eliminate your knee pain. But if your diet is low in dairy, leafy greens, or nuts, correcting the deficiency removes one contributing factor from a problem that already has plenty of others stacking up at once.
Sleep, Sensitization, and the Feedback Loop
Poor sleep is common in the days around menstruation, and it doesn’t just make you tired. Sleep disruption can amplify pain by feeding into central sensitization, a state in which the nervous system becomes hyperreactive and processes normal signals as painful. Research in women with fibromyalgia found that PMS severity was positively associated with central sensitization and negatively associated with sleep quality, and that central sensitization partially mediated the relationship between PMS severity and poor sleep. The average sleep quality score in that study was well above the threshold considered “poor.”
You don’t need to have fibromyalgia for this feedback loop to matter. Even in otherwise healthy people, a night or two of bad sleep lowers pain thresholds. Combine that with the already-reduced pain inhibition of the menstrual phase, and you have a situation where knee discomfort that would be trivial on a well-rested day mid-cycle becomes genuinely distracting. Prioritizing sleep hygiene in the days leading up to your period is one of the simplest interventions available, even though it gets far less attention than supplements or medication.