So it is or was a posterior compartment syndrome at the origin of my soleus and gastrocnemius muscles. This article helped me, because it explains the swelling, and it also explains the numbness I experienced in my toes during the workout on the very day that I noticed the swelling. This article really zeroed-in on the specific type of compartment syndrome. It was last Wednesday, at a club run, when I ran 8 miles fairly hard on what was so far the hottest day this year. The numbness happened on the hills during the last 2 miles. It was painfully numb, especially in the outside of my left foot. And on that day I was wearing Mizuno Precision shoes.
I took Thursday off and iced the swelling several times. The ice seemed to bring it down a little. Thinking I needed to protect my lower leg muscles and bones, I ran 4 miles on Friday and 8 miles on Saturday wearing the same shoes. On both days I felt a heaviness in my shin, as if I was carrying extra fluid around in there. I continued to ice. Then on Sunday I ran 8 miles barefoot. Barefoot, my shin felt normal the entire run, and in the evening, as I sat watching the passing of Christopher on The Sopranos and icing, I noticed the swelling was almost gone.
This incident made me remember that when I was in high school, circa 1976, I had compartment syndrome in both legs. It was so painful I was prescribed some steroid-based med and it ruined my cross-country season during my junior year.
A few other images that helped my figure this out:
(OK, the one above didn't really help me figure it out, I just liked the photo....)