It's when a muscle doesn't fit it into its 'space' or 'fascia'. Sometimes happens from inflammation after a fracture, where it's called 'acute' and requires serious immediate surgery. In my case, it's 'chronic', and involves the muscle that runs down the front of both shins. basically, when I exercise (cycling but also running and rowing - even going up stairs- really aggravate it), I get a rapid onset of pain in my shin, which is why it's often mistaken for shin splints. Other symptoms are numbness that starts in my toes and works its way up my lower leg. Good news, is pain usually goes away once I stop pedalling or if I can spin gently (although sometimes it can last for days). I have it because I'm flat-footed so my lower leg structure is all out of alignment, but people sometimes get it if they build up a muscle too quickly. In my case, I can't put too much power on the pedals, and can't ride out of the saddle at all; it also affects my driving where using the clutch pedal can set it off. Personally, I think everyone who has been struggling with 'shin splints' for any period of time should talk to their GP about the possibility of compartment syndrome - it's turning out to be quite common in runners. I was misdiagnosed for fifteen years and am releived that finally something can be done about this. Only problem is the only really conclusive test for it is really a surgical procedure in itself and no fun at all. Still, its no price to pay to be pain free!
