Maybe you've heard this one before: "No pain, no gain."

When it comes to athletic training, sometimes that pain is not just well-earned soreness from a solid workout. Sometimes it's a genuine injury.

How can an athlete — or someone who's just trying to stay in shape — tell the difference?

Mike Thomason, 60, acknowledges he had a hard time discerning whether he was truly injured or just sore from working out earlier this year. Thomason, who lives in Ardmore, Ala., competes as a Masters track athlete in the shot put, discus, and hammer and weight throws.

He had been looking forward to 2010, because he had moved into the men's 60 to 64 age group. He hoped to take advantage by being at the young end of the age bracket.

"I started throwing (training) seriously in March of this year, and I noticed after my workout sessions that my (right) bicep was sore," he said.

He was also puzzled that he couldn't throw the discus and the shot, which are lighter in the 60 to 64 age group, farther than the heavier implements used in the 55 to 59 age group.

"In my practice sessions, I had to throw the discus eight to 10 times and the shot at least six to eight times before my arm muscle soreness would go away," he said. "I thought I had pulled or strained my arm; so for two weeks, no throwing at all."

When he returned to training, so did the pain. Because the pain was a "dull, persistent ache," he figured that it was arthritis in the shoulder. After having subpar performances in some local Masters track meets, he noticed that he had trouble tucking in his shirt with his right arm.

That was the last straw. He finally visited a doctor, who recommended an MRI.

"Two weeks later, I had shoulder surgery for a partial tear in my rotator cuff and ruptured biceps connector," Thomason said. "I guess what drove me to think I was injured versus just in pain was the persistence of the situation."

Tips to help decide

Thomason's story can be instructive for clues on how to determine whether soreness is simple pain or an injury that needs attention. It's also a frustrating example of how the clues often don't lead to a definitive conclusion.

Asymmetry: One clue was the "asymmetrical" nature of the pain: It only occurred in one arm and not the other. That often indicates that the soreness is a real injury, experts say.

"If both legs are sore in the same spot, that is nearly always normal muscle soreness from a hard workout," said Craig Godwin, a Masters track coach from Eugene, Ore. "If there is pain in one leg and the other feels fine in the same spot, that is certainly something to be concerned about."

(But he cautions that this advice often doesn't apply to throwers. "Since field events are inherently asymmetrical, asymmetrical soreness can also be normal," Godwin said.)

Joints vs. muscles: Others say that pain in a joint — for example, the knees, ankles or shoulders — is different from muscle soreness and should be checked out if it persists. "Pain in a tendon, joint, bone is not normal or desired and should be treated as an injury," Godwin said.

Even though Thomason's injury was ultimately diagnosed as a rotator-cuff problem, the pain mainly presented itself in the biceps area, which made the issue appear to be related to the strain of stressful workouts. So when in doubt, let a medical professional decide if you are injured.

Swelling: Swelling is a key indicator of an injury that needs medical attention, says Allan Tissenbaum, an orthopedic surgeon with Washington Orthopedics and Sports Medicine in Washington, Pa. "Any time there's swelling, that's an injury," said Tissenbaum, who is a Masters athlete and a former Masters world champion in the 100-meter dash. "Stop the game, stop training, stop what's going on. And anytime you feel a pop or hear a pop, that's something to be paid attention to."