(Science Times) It's 6:15 a.m. and you are five minutes into your first run in weeks. The temperature is perfect and the sun is just emerging, turning the sky into a stained glass masterpiece. Then you feel a twinge in your knee stepping onto a curb. Was there a click? It aches a little, but not badly. Maybe you're just rusty -- or maybe it's the beginning of a meniscus tear. Should you push through, or is your workout done? In an ideal world, you would be able to immediately consult a doctor every time you felt pain during exercise. But in real life, you have to make judgment calls. Most of the time, you can walk it off or push through it. But other times, you risk real injury. Everyone's perception of pain is different, and doctors are loathe to make sweeping statements that might cause someone to exacerbate an injury. But the choices you make in the moment -- or the next day -- can be the difference between a temporary nuisance and a persistent problem, said Beth Darnall, an expert in pain psychology at Stanford University and a former ultramarathoner. "It actually might not have been a big deal, but suddenly we've created a big deal because we pushed through an additional 5 miles," she said. So we turned to a few pain and movement experts for tips on what to watch out for the next time your shoulder starts to complain at the gym. How to Spot Worrisome Pain Whether you are playing a pickup game or running a marathon, there are a few key signs to consider when you feel pain during exercise. -- Was there a sound? If a sudden pain is accompanied by a crack or pop, you probably need to stop what you're doing, said Mallory Fox, a movement specialist in Scottsdale, Arizona. Not every popping sound is a snapped ligament or tendon, but don't take the chance. -- Is it unstable or wobbly? Joints warrant special attention because they have more soft tissue that can cause persistent problems. If a joint won't support your weight or feels like it's buckling, it's a strong sign that something is wrong. -- Is it swollen? If you experience a lot of swelling within about 10 minutes, that's usually a sign of a more serious injury, said Conor O'Donovan, a physical therapist at the Shirley Ryan AbilityLab, a rehabilitation research hospital in Chicago. -- Does the pain change how you move? Has your gait shifted or are you favoring one side? Even subtle changes can be a signal that it's time to stop, Darnall said. If you're not sure, have a friend watch you move and look for subtle changes. -- Is your range of motion affected? A healthy knee, for instance, should bend about 160 degrees, Dr. O'Donovan said, but a meniscus injury might cause it to catch a little or perhaps stop around 90 degrees. -- How much pain do you feel, and what type? Broadly speaking, more serious injuries tend to cause sharp or throbbing pain, and a dull ache is more likely to be a temporary muscle issue, Dr. Fox said. But most of time, intensity is more telling than pain type, Dr. O'Donovan said. Track How Your Pain Changes None of these rules are hard and fast, the experts said. Sometimes, the best approach is to wait and see how your pain changes. It sounds obvious, but pain should lessen over time. Fox suggested checking in a day after feeling workout pain and then every day for a week, writing down the pain's severity, from 1 to 10. Look for the same list of issues -- swelling, loss of movement -- but notice whether they change or move. If the pain moves or doesn't improve with proper treatment, it may be time to talk to a physical therapist or even a physician. "Having a sore foot right after you exercise is one thing," Fox said. "But having that persistent pain for five days where nothing has helped" is a red flag. That said, muscle soreness, especially after a new form of exercise, can flare up as much as a day later and last for a week. Such delayed onset pain and may get worse for a couple of days, but it shouldn't feel sharp and shouldn't swell or limit your range of movement. But even the experts sometimes misread the signs. O'Donovan once ignored a nagging back pain after a soccer game even as it worsened and his legs began to swell. Eventually, he learned he had dangerous blood clots and spent five days in the intensive care unit. "I did not read the pain cues correctly and wrote it off," he said. The lesson? "If it's getting progressively worse, something's going on." Chronic Pain Can Change the Rules If you have persistent pain, some of the advice can be different, and you should follow your own comfort levels. It's important to set goals and guardrails before exercising, Darnall said. Whether you are running a 5K or just playing with your kids, plan out how often to take rests -- and stick to them, even if you feel OK. If you have preexisting pain, most experts do not recommend pushing through it without the guidance of a medical professional. Instead, pace yourself -- going easy at first and then progressively harder -- and regularly check for changes in the quality or location of the pain. One way to know if you've pushed it too far is if you experience a "pain hangover," Darnall added, which can cause trouble sleeping that night and tiredness or limited your range of motion the next day. But that's not to say you should avoid working out. One thing that experts know for sure, Darnall said, is that some kind of regular movement is healthy for all of us and exercise can be an excellent treatment for pain. "It helps reduce stress, it helps you sleep better, it improves mood," she said. "You don't want to do too much, but you also don't want to stop." O'Donovan, who also works with chronic pain patients, agreed. "Your body's built to move," he said. "It's healthy to move. And a little bit of increase in pain is OK." This article originally appeared in The New York Times.