Sexual arousal usually isn't voluntary. The conscious mind is complicit in it, but a lot of sexual arousal goes on in the sympathetic nervous system. In addition, impulses from the brain during the REM phase of sleep cause erections, whether you're dreaming about sex or about a test you forgot to study for. Heavy lifting or straining to have a bowel movement can also produce an erection.
I can honestly say that I have never gotten an erection from heavy lifting nor from dropping the kids off at the pool. Though arousal may not be "voluntary", the act of dropping a deuce pretty much counteracts any arousal that may occur.
Among men, there is no consistent relationship between the size of the flaccid penis and its full erect length.
In one study of 80 men, researchers found that increases from flaccid to erect lengths ranged widely, from less than a quarter inch to 3.5 inches longer.
Whatever the clinical significance of these data may be, the locker-room significance is considerable. You can't assume that a dude with a big limp penis gets much bigger with an erection. And the guy whose penis looks tiny could surprise you with a big erection.
Ha! Just like I told everyone in the locker room back in Junior High!
In an MRI picture, the penis looks distinctly boomerang-like, as noted by a French researcher who studied men and women having sex inside an MRI scanner.
OK, who volunteers to have sex inside an MRI scanner with some "researcher" watching you? (Not me, I'm claustrophobic)
Here's how to avoid penile fracture: don't use your penis too roughly.
Boy, did I learn that lesson early. I ain't gonna bump no more with no big fat woman...
Almost all Jewish and Muslim males in the world have circumcised penises, and together they account for about 70 percent of all circumcised males globally.
You know, with that common bond...you'd think that Jews and Muslims could get along.
(courtesy of Ace Of Spades left column)
In a closely-related matter, it's a good idea to avoid focusing solely on your Johnson: