Today my running went round and round, around a track that is. I drove to a high school and ran intervals on the track. I ran 5 or 6 quarter-mile intervals followed by a quarter-mile jog/walk recovery. My intervals were around 9:30 minutes/mile and my recovery jogs were around 11:15 minutes/mile.
I had planned on doing one more interval after the one I was doing, when my right ham cramped. I immediately stopped and walked a short distance to a bleacher and stretched the ham. That helped but the ham was still tight, and it felt hard to my touch. I rubbed it for a minute and stretched it some more, but it was still hard. I walked the rest of the way around the track, stretched the ham some more, and then drove home. The ham wasn't sore and had no pain -- just a muscle cramp. I've been home for an hour, and the ham is still slightly hard but not nearly as much as it was when it first cramped.
I wanted to wear my wife's heart-rate monitor during the intervals, but I forgot to take it. I'll use it next week. I don't want my HR to go past 119 (80% of my maximum which is approximately 149). I'd like my recovery HR to be about 89 (60%).
Later: My ham feels fine now. After the hardness went away, my buttock was sore. I did a few buttock stretches and it immediately felt fine.