I've never heard of it happening. I would imagine that, due to the attention brought into sensations and the moment, I would feel it long before I were to experience tissue damage from a meditative posture. I am more concerned with my regular posture, while I am much more present in my intellect (in my head), typing this message.

I've heard that the absolute maximum time for a single sit should be four hours, due to this danger. Sorry but I don't have a source for that information.

It's like going to the gym. A person can train progressively or just go crazy the first weeks/months and get injured. I've read about monks getting hemorrhoids from too much sitting.

So with meditation, like exercise, a lot of the time in the initial phase is spent just encountering a grumpy, unwilling mind which constantly just wants to go to the couch and watch TV or get into some other trouble-solving pleasure-producing activity (e.g., fantasizing). That's natural, I think.

But once a person becomes very keen and willing, then the effort changes. One can become very natural, sensible and somewhat paced about the effort.

So one thing that fascinates me about the jhanas, particularly equanimity, is the body's ability to stay light and fluid for hours, to get up from a sit fluidly. (This is rare and I would not just jump out of any sit to try to show, "Yo, I'm sooo comfy and meditative." It happens naturally or it doesn't. It's ok.) But I think there's some research that can happen here. What is happening in the body when the mind is fully placid and investigating?

Anyway, good luck with your training.

As an emergency department physician I agree: the risk would seem real, yet I haven't heard of it happening, though I don't work at a emergency department near a major meditation center. Given the total retreat time racked up by people I know, I would think someone would have mentioned it at some point if it had occurred to them.

I think part of the trick on meditation retreats is that, at least on all the vipassana retreat I have been on, there is an approximately equal amount of walking and sitting, such that one gets in about 8 hours of sitting and also 8 hours of walking, alternating 45 minutes sitting, 45 minutes walking, or 1 hour sitting, 1 hour walking, which is really quite a lot of walking, as, were you to walk at a slow pace, say 1 mile/hour, you would walk 8 miles, which is definitely something, and were you to walk at a good clip like I do, say 3 miles/hour, you end up walking 24 miles/day, which would be vastly more DVT prophylaxis than you find in, say, an ortho rehab floor, and doesn't give the very long time in bed or in an airplane or car seat that is well-known to cause DVTs and PEs (lung blood clots, for those not medical, very bad things, in other words).

The most I ever sat in a row was 4 hours one time, and that was during the A&P, and I noticed no ill effects and strangely little pain at the time (one of those stage-dependent things).

Lotus position isn't difficult for me, but I made a habit of regular sitting in a chair like a normal englishman for meditation. Millions of Indians sit in lotus posture, and it's nothing to do with meditation. It's the climate, it's so hot there people didn't bother using chairs. Sitting on the floor in northern europe during winter for the sake of convention would be absurd, and it was always silly to me watching people spend years torturing their legs before they have had any real fire for meditation, even.

I have a friend who knows about fitness of legs and all this and he squints whenever he sees someone sat cross-legged. On the other hand since it seems to have been the preferred sit since nearly eternity it's hard to argue. I think something about it may let energy flow more unabated, as once in a while when doing something completely different and have said cross-legged, I suddenly zoned out and started inclining to meditation.

I did a search on medscape and came up with this small but interesting study. It doesn't give a definitive answer to your question (or the experimenters' questions), but it does suggest:

1. Meditation may improve left ventricle ejection compared to control (relaxation).
2. " " arterial elasticity " "
3. ...reduced peripheral resistance...

I honestly don't understand the statistical maneuvers they describe, and can't say for sure what the outcomes mean for DVT, but it suggests a hyptothesis for meditation reducing cardiovascular risks in general. Thought it might interest you.