So yesterday, I started HIIT for the first time. I just started running again this week and wanted to test it out. I didn't stretch long, maybe 1-2 minutes. Walked for about 2 minutes, then began my first set of 4 sprints. 25 seconds into my 4th sprint, my calves locked up, and I was unable to run anymore. I also felt pain when I was walking. I walked for about 7 minutes and then as I sat down in my car, sharp pains shot through my calves and I felt like the world was going to end. Can anyone give me tips on how to avoid those cramps?