I injured my forearm muscles (flexors), and found this article about how boom diameter can affect forearm pain, and what you can do about it.
Its from the Journal of Sports Science and Medicine, where they setup a land based windsurfer and hooked the guys up with electrodes to measure the effects of different boom diameters...
Click the PDF link at the top right of this page:www.jssm.org/vol6/n1/17/v6n1-17abst.php
Or, direct:http://www.jssm.org/vol6/n1/17/v6n1-17pdf.pdf
In summary:Select to expand quote
The important activity of the flexor muscles in the fingers in windsurfing compared to other muscle group activity is thus clearly established; it now seems it could be interesting to study this activity to reveal the positions in which it is the strongest.
Our study confirms the results of previous studies by showing the flexor muscle activity in the fingers is stronger when the hand is in pronation. This would explain the pain felt by most windsurfers in their forearms. These pains are linked to an accumulation of lactic acid associated with weak localized oxygenation.
To hold off the presence of these pains and fatigue, the surfer frequently changes hand positions (palm up or down). This intermittent change improves the quality of vascular circulation.
To rest, the windsurfer can also hold the wishbone in the crook of their arm or with the armpit of the following arm, but there is a risk of nerve and vascular compression and this position cannot be held very long. The use of a wishbone which is small in diameter (28 mm) seems to decrease the muscular activity of the flexors in the fingers, and thus using this type of wishbone can avoid muscular pain in the forearm for as long as possible.
Your thoughts / experience?
I found skinny booms reduced my forearm and wrist pain to nil, nothing. Not one iota of pain, compared to fatter booms where I always got some sort of discomfort after an hour or so on the water.
Of course another way is to use mega stable sails and not too big a board so you are not wrestling the gear...... and then when in the harness you are hardly touching the boom and not continually trimming the sail. So it comes down to lots of money to buy heaps of gear so you're always on the ideal setup 
I was doing slalom racing around 20 years ago when the sails were not as stable as they are now, and to be more competitive, I also began wearing a weight jacket. Over a period of six months or so I developed "compartmental syndrome" in my forearms and got to the point where sailing became impossible.
Compartmental Syndrome is similar to carpal tunnel, whereby the muscles in the wrist / forearm become inflamed and then constrict blood flow to the hands leaving no strength at all to grip the boom.
I had surgery on both arms and was unable to sail for six months or more as a result.
In modern race sails the draft is much more stable and hence stress on the arms seems much less. Nevertheless I have switched to the narrowest diameter boom I could find (Streamlined), which also has the advantage of a very wide front end, and very straight boom arms. This enables you to sail in the less-powerful, but more comfortable position of front hand palm-down. That is how I sail 90% of the time and I have no such problems now.
Believe it or not my arms cramp up in about half an hour with a skinny boom, whereas with a normal boom I can sail for hundreds of k's...
So much so that I'm getting my latest carbon boom regripped so that the grip is fatter!
People are different.
I hear you brother, went out one christmas when i was just starting. It was blowing full on 30 knots and here was me trying to hold on with a 5.8 ( im 67kgs) without a harness. After i was rescued, and all my gear packed away by others i managed to get home. I had serious case of tennis elbow, golfers elbow or whatever you want to call it, both on top of the formarm and underneath. It was painful to say the least, i went to see a phyiso for treatment.
My advice would be to see someone and in the meantime buy some support bandages, dont ignore it thinking it will repair itself
When we came back from 3 months in Europe last year I went for a distance effort in the GTC. Soft hands + long distance = bleeding hands. So I bought some gloves for the next time I went out, after 15mins or so my forearms pumped up and were very sore, couldn't sail for very long until my blisters healed and I could discard the gloves.
I have a 40mm wide boom & get muscle stiffness after sailing but i think that is more from the few times I have to use the strength ( or lack of ) in the arms when i'm unhooked ,gybing,waterstarting ,jumping, etc & streches after help that. I'm hooked in most of the time & use my body to control the sail..why do you have to grip the boom tightly with your hands..?
I have to agree that I noticed my forearm pain (arm pump) completely disappeared when I got my new skinnier (29mm) boom, but alas it broke only after 3 months. Big wide head also made palm down grip much more comfortable as Chris mentioned.
Nebs - there's always one freak of nature.
the only pain i get in my arms is after sailing for a few hours and my muscles are just sore from sailing in general .. what is it about the diamiter that causes pain?
I have read the article you mention. I think it makes sense and it was one of a number of things that prompted me to recently upgrade to a narrower diameter boom.
There are a multitude of potential forearm problems which can be associated with windsurfing. I've been having increasing problems with what I believe is chronic exertional compartment syndrome which is what Chris Adamson describes. This may well not be what is troubling you but I run into major problems with both forearms, and is at it's worst if I don't ease myself into a session/warm up gradually. Free sailing it's not too bad but I'm really struggling slalom racing and by the end of a heat often have difficulties gripping the boom - it's very disheartening especially when I don't feel 'tired'. The other time I struggle is wave riding on a long wave. As it happens I have an appointment with a sports medicine doctor this week to discuss the problem. I have done lots or reading and will have to have a chat to Chris about his surgery. It is very common in motorbike racing:http://www.motocrossactionmag.com/me2/dirmod.asp?sid=&nm=&type=news&mod=News&mid=9A02E3B96F2A415ABC72CB5F516B4C10&tier=3&nid=9B767DC2DD8E4A11B6C914E5A0E9FAF8
Diameter must matter, because sailing with gloves, even very thin ones, in cold weather, makes a huge difference.
I find anything away from straight-line sailing useful to rest the arms. Going freestyle (front-to-back or pushing the rig) rests the entire set of muscles. Frequent gybes and tacks also shift the workload to other muscles for a few seconds.
EDIT: woops, also changing under-over hand grip also helps a tiny bit. Limited though.
Just to add to the mix:
I think tight winter wetsuits on your forearms don't help either as well as they restrict venous return from you forearm muscles and they become engorged and your arms pump up leaving no grip strength - particularly if its on off gusty strong wind.
Whenever I wear short arms I never get it and I have no idea what diameter my boom is
A simple test - put your forearm up in front of you, with your hand bent over as if on a boom - change the diameter of your grip (extend and contract fingers) and watch (or feel with your other hand) the forearm muscle changing shape. The larger the diameter, the more the muscle stretches out. If you are in an office, your co-workers will just think you've lost it for a while, don't worry about them 
So I think this means a narrower boom will cause less lactic acid build up that gives you the pumped feeling.
I developed a cure for sore arms once.
However i left the serum on the No. 31 bus and never got it back.
So I think this means a narrower boom will cause less lactic acid build up that gives you the pumped feeling. 
I suggest that the problem with wearing gloves isn't the extra thickness, it's the lack of grip as you have two interfaces (hand -> glove -> boom) instead of one (hand -> boom).
This means less grip, so you need to clench your fist harder to compensate, so your forearms cramp up.
Agree with the comments regarding wetties reducing circulation though.
I particulary like:
"Our study confirms the results of previous studies"
Also
"To rest, the windsurfer can also hold the wishbone in the crook of their arm or with the armpit of the following arm, but there is a risk of...." getting slammed face first into their sail at the slightest gust of wind.
I use gloves (NP) but not standard yachty gloves found I actually grip the boom lighter as the palms have gripy dots all over them so there is very little slippage.
I got a case of tennis elbow a few years ago and it takes a long time to heal and windsurfing used to aggrevate it. I had to wear a brace for about a year, but it does eventually heal completely. I was advised to do forearm excersizes which helped my sailing endurance a lot. Using only a weight bar with no weights on it, hold the bar as if you were about to do a curl but starting with arms horizontal (bit like you were holding the boom both palms up) Curl the bar but only from the wrist, you will soon feel the burn. Do the same excersize palm facing down curling your wrist backwards. I am not a qualified fitness instructor, it just worked for me
