Why do muscles stiffen after sitting for a long time?

WHy is that? No website seems to have the answer, or at least a detailed answer. It happens to me and pretty sure it happens to almost everybody.

IANAD, but I remember reading in college that it had to do with lactic acid build-up and a lack of oxygen. I’m not exactly sure why this would happen as a result of inactivity, so we’ll let someone else explain the details.

I don’t know but I know this is not correct. If you had lactic acid build-up after exercising, it would have been metabolized after sitting a long time.

Refreshing this thread. There’s still no sure answer for this, yet so many people complain about waking up stiff or being stiff from a long movie, car-ride, desk-job or etc.

Guess: when you’re “at rest,” some of your muscles are actually tensed, even if just a little. So, tensing a muscle for minutes/hours on end would lead to tightness and discomfort.

I dont know if they stiffen if they are actually in a fully relaxed position.
It’s usually when they are in an odd position, unless we are talking about after an injury or something?

I get pretty still just driving for 2 hours, it works loose pretty quickly but never fails to surprise me.

Normally, muscle fibers are in a cycle of tensing and resting. Different ones at different times. Hence muscle tone is related to the current proportion tensed up. When this cycle goes awry you get tics, spasms, etc.

When you move and relax., the proportion in the tensed part of their cycle changes.

Certain activities can mess this up. E.g., a soldier standing at attention for a long time on a warm day. Too many are spending too high a proportion of time tensed. But sitting isn’t too much like that, esp. in the lower body.

I wonder if the underlying problem is related to the “other circulatory system”. Waste products leaking into the surrounding lymphatic space builds up since that needs motion to cycle the fluids around in the extremities.