Made up of two major muscles - the soleus and the gastrocnemius - the calf muscles "allow us to get up on our toes," said Carl Stanitski, chief of the department of orthopedic surgery at the Children's Hospital in Detroit and professor of orthopedics at Wayne State University.

With inactivity, calf muscles are underused and become flabby, the reason that they can feel so tender when exercise resumes. The soreness results from stretching the calf muscles and, in the worst cases, from severely tearing them.

The worst calf muscle tears often happen to weekend athletes and aging athletes, usually men 35 and older. "You are going for a shot, you put the muscles in a tremendous stretch, and it will feel like someone has hit you in the back of the calf with the ball," Stanitski said. "That's an acute calf muscle tear."