The most likely explanation is that somehow your arm swing action while sprinting is raising your venous blood pressure enough to cause bruising. Another example of this phenomenon is when someone coughs hard it can cause bruising and/or petechiae (very small bruises) on their face and neck due to increased venous blood pressure.

I would get a few things checked to make sure it's not a health issue. Bruising can be caused either by platelet or clotting factor deficiency. So first, get a platelet count. Also, check a stool hemoccult to make sure there's no blood in your stool.

Clotting factors are made in the liver. Get your liver enzymes and bilirubin checked.

Certain clotting factors (II, VII, IX and X) depend on vitamin K1 to work. Start supplementing with vitamin K1 and see if that helps the bruising problem.

Finally, there is a common clotting factor deficiency called "von Willebrand's disease" (VWD), which is sometimes difficult to diagnose ( http://www.nhlbi.nih.gov/health/dci/Diseases/vWD/vWD_Diagnosis.html ). You may need to consult an internist or hematologist to rule in or rule out the diagnosis.

Edit, 7/25/2010: I forgot to mention, sometimes anti-inflammatory meds (NSAIDS), such as aspirin, ibuprofen (Motrin) and naproxen (Aleve) interfere with platelet function enough to cause easy bruising. So avoid these meds.

Are the bruises painful to the touch?

I get these too. For me, they are bruises caused by the Frisbee, and frequently I would not recall having being struck particularly hard at those spots. Sometimes it takes a few hours for me to notice the bruises after the fact.

I'm speculating here, but it may be weakened blood vessels (struck by the Frisbee, perhaps), that finally rupture due to the centrifugal force exerted on your forearms when you later do HIIT sprints.

i asked about bruising as well a while back. some people said sometimes vita e and fish oil can thin the blood to the point where you get easy bruising. another said that after a big weight loss capallaries are closer to the skin's surface and so can get bruised more easily.

Seems like the most common cause of easy bruising and weak capillaries is not enough iron in the blood. Another potential cause is insufficient vitamin C. But there are probably many other less well known causes. One person I saw mentioned he bruised easy after a large amount of weight loss but didn't know why.

As for weight lifting, the weights themselves can produce bruising or close to surface capillary breakage on any place in the body that comes in contact with some surface. For instance, I often get some kind of weird red lines of capillary damage when my shoulders press against shoulder pads while doing leg lifts. My legs are apparently much stronger then the skin on top of my shoulders! If you are sitting on a bench while lifting weights with arms, this can cause bruising on any part of the lower body that touches the bench. However, beyond that, seems that heavy lifting in general can cause bruising in some people even in areas that did not come in contact with another surface.

In addition, it is said that getting old leads to easier bruising. Anyway, I would get it checked out just in case, but I have a suspicion that it's not going to be a big deal and might even be related to the weight loss. Your skin has been through a lot getting all stretched out to cover that fat and then having to snap back into smaller form when the fat was gone. You probably grew a lot of extra capillaries to feed all that fat and now that fat is gone! Some things have definitely changed in your body and you may not be totally acclimated yet. Congrats on your weight loss! YOu are probably 10 times healthier now than you once were.
-Eva