Alert: this is a post about "women's stuff" ;) Sometimes it's scary to think how our reasoning and mood is affected by hormones, it makes me feel like a robot that works on some mysterious chemical reactions. Above all, I wanted to learn a little more about estrogen and progesterone, the two primary sex steroid hormones in the female body, so I did some research to hopefully figure out what connection (if any) exists between muscle stiffness and hormones. It looks like women goes through a lot of hormones fluctuations (drops and rises) especially 1-2 weeks before their menstrual cycle, those cause several symptoms like anger, moodiness, fatigue, hunger, depression... AND joint/muscle stiffness !
It's hard to determine exactly what causes what, but right before a woman's period there is a high concentration of estrogen and progesterone in the blood, that suddenly drop right when the period starts. This drop releases the stiffness and rebalances all the above discomforts.
Here's a more detailed explanation of what happens:
"The graph shows the normal hormonal fluctuation seen with menstruation. Day 1 of the cycle correlates with the first day of menstrual bleeding. The first phase of the menstrual cycle is called the follicular phase. Estrogen and progesterone levels are at their lowest as the uterine lining sheds through menstruation. The follicular phase lasts 10 – 14 days, or until ovulation occurs. Just prior to ovulation estrogen and FSH (follicule stimulating hormone) rise, and LH (luteinizing hormone) surges to stimulate the release of an egg from the ovary. The luteal phase begins when ovulation occurs. Progesterone is exclusively released from the developing egg. Both estrogen and progesterone work in concert together to prepare the uterine lining for pregnancy. If fertilization does not occur, estrogen and progesterone levels decrease causing menstruation to occur." (drlauraruby.com).
So LOW levels of estrogen and progesterone seems to work magic with flexibility, at least in my experience.
Another interesting point this author makes is "estrogen dominance, resulting in changes in the ratio of estrogen to progesterone are the underlying cause of many of the symptoms seen with PMS". So this could solve my question about what causes the stiffness. Or at least I'm getting closer to an answer. I'm sure this can be slightly different from person to person ;)
To show you where I experience the most stiffness: my middle/upper back. They refuse to bend on those pre-cycle days !