Any athlete’s worst fear—professional or amateur—is injuring themselves to a point that prevents them from continuing to work out. But, the reality is, exercise injuries are far more common than we think.

According to the Centers for Disease Control and Prevention (CDC), hundreds of thousands of exercise injuries occur each year. Among these, knee injuries are the most common.

However, experts agree that the best thing to do if you fall victim to a knee injury is to seek help (or use an effective topical cream like this one) and to know the signs that signal something serious.

Knee Injuries

You’re on a run and trip over a crack on the sidewalk, or you’re playing tennis and your knee turns at an unnatural angle while the rest of your leg stays in place. If you play any sport, particularly contact or collision sports, you’re no stranger to knee injuries.

You’re also in good company because according to a study published by the Society for Academic Emergency Medicine, the knee is the most commonly injured joint among athletes.

Knee injuries are caused by trauma or damage to the underlying structure of the knee, which could include ligaments, tendons, cartilage, or bursa (a fluid-filled sac that protects the joint from friction). Together, these tissues surround the bone and allow the knee to move with ease. Because the knee is such a complex series of mechanisms, a lot can happen to it.

The most common knee injuries include the following:

• Anterior cruciate ligament (ACL) damage is usually the result of sudden changes in direction, jumps, abrupt stops, and high-speed impact.
• A torn meniscus usually happens after a sudden twist in the leg.
• Knee bursitis, a condition in which the bursa becomes inflamed, is often a result of constant kneeling.
• Patellar tendinitis happens when the tendons that connect the front of the thigh to the shin become inflamed. The impact of running and jumping tends to cause this injury.
• Fractures, i.e., broken bones, are often the result of a fall or a car accident.

After you’ve sustained a knee injury, it’s natural to feel concerned. How do you know if you should walk off your injury or rush to the doctor’s office for an examination? Will a few days of rest be enough for your knee to heal naturally, or do you need to start evaluating options for knee surgery?

Here are seven reasons that should prompt an immediate visit to your doctor’s office.

You’re experiencing swelling.

One of the most obvious signs that a knee injury has occurred is excessive swelling that you can see or feel. This “pain freeze” cream has helped many people feel relief from their nagging pain. Based on the reviews, it’s the most effective on the market.

Swelling is most indicative of a serious cartilage or ligament injury to the knee, according to Taylor Brown, M.D., an orthopedic surgeon at Houston Methodist in Texas.

The amount of swelling that is considered “serious” is subjective. A small amount of swelling may not be cause for concern. If you notice a subtle difference between the sizes of your knees, you probably don’t need to sound the alarm just yet. It’s only extreme swelling you need to worry about.

When we talk about extreme swelling, what we mean is when one knee is significantly larger than the other. According to Dr. David Geier, a doctor specializing in orthopedic surgery, significant swelling “could be a sign of a torn ACL or a patella dislocation with a little piece of bone knocked off, and it should be checked out.”

If your knee doesn’t immediately appear swollen and you’re unsure, “feel the injured and normal knee at the same time, with a hand on each knee to compare the two,” he says. “A swollen knee will feel like a large, warm, fluid-filled balloon.”

If you experience any of the above, he recommends using a topical cream or using the RICE principle. Rest with crutches and activity reduction, ice for 20 to 30 minutes several times a day.

Compression with a soft knee sleeve brace (like this top seller) will help, and elevating your knee above the level of the heart. Over-the-counter pain medication can also provide relief until the swelling goes down.

Aaptiv has workouts specifically to help work around your knee injury. Learn more about Aaptiv here.

If your swelling is minor, you might be tempted to reach for the ice and Advil, but research suggests that some amount of swelling could be good for you. A Cleveland Clinic study confirmed that swelling is caused by a rush of cells called macrophages to the affected area.

Macrophages help repair your injury by ingesting damaged cells. This process results in swelling as fluid rushes into the holes in the tissue left by the macrophages, and this is key to muscle regeneration. Pain and swelling are an adaptive body response. While we often view them as an inconvenience, they are natural signals that we need to slow down.

When an injury causes swelling, the pain and stiffness prevent the athlete from using the injured part of their body. This is where the first part of PRICE – protection – comes into play. By protecting your knee from further injury, you have a better chance of minimizing the damage.

In addition to the severity of the swelling, the speed with which the swelling occurs could indicate a serious injury. If the knee immediately blows up and becomes the size of a grapefruit, then you’ve probably sustained a severe injury.

If you’re unsure or you have swelling that doesn’t subside after 72 hours, please consult with your doctor.

You notice an obvious deformity.

If you notice your knee jutting outward in a way it never has before, take note. A dislocated or fractured patella (kneecap) can cause injuries like this, explains Brian Schwabe, C.S.C.S., board-certified sports physical therapist based in Los Angeles.

While some deformities occur over time, when the deformity is a result of an injury, it could be the result of a fracture or chronic wear on the knee joint. If you are already experiencing any kind of misalignment in your lower extremities, then you could be more prone to this type of injury.

If you notice a bone deformity after an injury, he recommends seeking the assistance of an orthopedic doctor immediately. The doctor will likely take x-rays as well as perform a visual diagnosis to determine if you need surgery.

You felt or heard a “pop”.

Sometimes, a “popping” sound after a movement can indicate something is out of place.

“Oftentimes this type of sound upon injury is indicative of a ligamentous injury,” says Schwabe. Not all ligamentous injuries are full tears, though, and not all require surgery.

However, if you also experience excessive swelling and instability, get it checked out by an M.D. “If you only have a mild sprain then you will be able to rehab it with physical therapy,” he says.

The pop is what you hear as a result of a ligament tear, a meniscus tear, strained tendons, or a dislocated kneecap. Usually what happens is you’ll simultaneously hear and feel the pop.

If you’re not familiar with the meniscus, it’s the cartilage in the knee that functions as a shock absorber. Each knee has two menisci, and they’re shaped like horseshoes. The medial meniscus is on the inside of the knee, and the lateral meniscus is on the outside.

The popping sensation can be difficult to explain, but you’ll certainly know it when it happens to you. Though the sound originates in the knee, you’ll hear it as though it was directly in your ear. It’s as if the sensation travels up your body, and when it gets to your brain, it becomes audible.

Depending on the severity of the tear, your age, and your overall health, you may able to heal on your own. You could start by using a brace and keeping your knee elevated and protected until you can get in to see a sports medicine doctor. However, a pop will almost always bring you down to the ground, and it’s not recommended that you put any weight on it until you can stabilize it.

If the pain is severe, you’ll want to visit a doctor as soon as possible. Surgery is often the recommended course of action, but physical therapy and other non-invasive remedies may be prescribed as alternatives.

You experience greater than normal movement.

If you feel like your range of motion in the knee is higher than usual, something might be out of place or injured. This is known as joint instability.

You may feel like your knee will buckle or fold with walking, twisting, or weight-bearing.

“The sensation of the knee giving away may occur with simple daily activities or upon return to sport following what was thought to be a minor injury,” explains Dr. Brown.

“Additional episodes of joint instability may worsen the condition of a knee after an injury.” Tears of the ACL or MCL and patellar (kneecap) dislocations could also create joint instability, he says.

Hundreds of thousands of exercise injuries occur each year. Among these, knee injuries are the most common.

The best plan of action is to make an appointment with an orthopedic surgeon.

Wear a compression-sleeve-type brace or a knee brace with hinges in the interim, says Dr. Brown.

This may provide some comfort or level of support. Use crutches if you feel like you could fall and suffer additional injury because of knee joint instability.

Another type of hypermobility can occur when the kneecap moves out of place. This condition is called a patella dislocation. It’s usually painful when it happens, and it might be sore the next day. If it pops back in by itself, the injury might not be serious. The true test is whether you feel confident that you can resume regular activity later that day or the next day.

According to Dr. Geier, if the knee resituates itself naturally or if you can put it back in place with little effort, then you may be able to “see how it goes” and avoid a visit to the doctor.

However, if it’s difficult to get the knee back in place or you need assistance to do so, then you’re bound feel pain and most likely swelling as well. If this is the case, Dr. Geier recommends consulting with a doctor. If anything, he says, you’ll get peace of mind.

When the kneecap dislocation is accompanied by swelling, this could indicate that a piece of cartilage or bone got knocked out of place. If the knee feels “loose” when you resume activity, a visit to the doctor may be in order. Your doctor may take x-rays, recommend an MRI, or suggest a visit to a physical therapist.

Knee instability is often caused by weak surrounding muscles. Working on balance and isolation exercises can significantly strengthen the knee and prevent injury. Aaptiv has hundreds of workouts on the app that can build a strong base for your core and lower body. Sign up for Aaptiv today and get a free trial.

You can’t put weight on it.

If it’s immensely painful to stand or put any weight on your injured knee, there’s definitely something serious going on.

“Inability to bear weight after a knee injury could be caused by a fracture, bone contusion, cartilage injury or ligament tear,” explains Dr. Brown. “Initial treatment includes using crutches or a steerable scooter to take pressure off the injured limb and minimize additional damage to the knee.”

If this sounds like you, listen to what your body is trying to tell you. Dr. Brown warns that the old adage of “no pain, no gain” doesn’t apply in this situation.

He advises seeking orthopedic evaluation as soon as possible—even that same day if you can.

After a minor injury like a twist or fall, you might not be able to put weight on your leg immediately. That’s not necessarily a cause for alarm, but if you don’t feel like you’ve recovered within a few minutes, this could indicate something more serious.

It’s similar to when you injure a wrist or ankle. If you fall on your wrist or twist your ankle, you may experience searing pain for a moment or two. When the injury isn’t severe, you’re able to shake it off and resume your life after a few minutes at the most. However, if you’ve experienced a sprain, the pain doesn’t subside. Instead, it gets worse.

Think of a knee injury in the same way. You may experience agony for a moment, but pay attention to how long it takes to go away. If it doesn’t go away within minutes and you’re not able to put any weight on the knee without feeling excruciating pain, then you might need a doctor to help you consider treatment options.

Also, watch out for numbness after the injury. This could be a result of a pinched or damaged nerve and blood vessels. William Blahd Jr., M.D., reports that this symptom could indicate an ACL or MCL tear, a kneecap dislocation, a fractured kneecap, or loose bone.

You can’t straighten your knee or leg.

If you have trouble straightening your leg or it hurts to do so, you probably have a serious knee injury.

To test this, start in a seated position and try to lift your lower leg using your own leg muscles. “You may still be able to bear weight and walk slowly and carefully without assistance, but will probably require assistance to lift your lower leg and fully extend your injured knee,” says Dr. Brown.

“Patella fractures, quadriceps tendon tears, and patellar tendon tears all tend to be associated with an inability to straighten the leg.”

Use a knee immobilizer to hold the knee in a straight position and help with pain relief. This also makes it easier to move about until your appointment with an orthopedic surgeon, he adds.

Another test is to lie down and try to straighten your leg. The goal is to get your knee to lie flat. This might be difficult or impossible to do due to the pain, but what you should watch out for is a total inability to straighten your knee.

Some of the problems that could cause a locked knee include meniscus tears or a torn ACL. The key is to determine whether you can’t straighten your knee due to sheer pain or physical blockage. If you feel blocked, then we recommend getting in touch with a doctor sooner rather than later.

On the flip side, you might have trouble bending your knee. If your knee feels stuck in any way, and the feeling doesn’t go away within a few hours, you could be experiencing something more severe.

Your knee keeps buckling.

When you got hurt, did it feel like your knee gave way underneath you? We’re not talking about a temporary sensation of weakness or some wobbliness, but more like your shin and thigh bone weren’t connecting or staying in place. A buckling knee could signal an ACL tear or a cruciate ligament injury.

It could start with a small pop or crack, or you could feel it after a strenuous exercise session or a soccer game.

Either way, your knee feels unstable or weak. Your knee may give out momentarily, which is scary because you might not trust yourself to walk safely when this happens. According to Dr. Howard Luks, an orthopedic surgeon, this could indicate a tear in the patella or the quadriceps tendon.

These tendons are on the front of your knees and offer a lot of support to keep you upright. If they’re not in top form, you’re bound to feel weak in the knees (cue the terrible SWV song from the ‘90s), and you might not be able to walk at all. If this is the case, you could need surgery.

On the other hand, the weakness could be temporary, caused by wear and tear, improper footwear, or a strain. If you’re a weekend warrior or you’re over the age of 30 (sorry!), you might experience this symptom on a regular basis.

However, if the weakness is recurring and happens every time you exercise, it’s possible you have a time bomb that’s waiting to happen. When in doubt, seek advice from a physician.

This could also signal a torn ACL or a large meniscus tear. These two common injuries are both serious and could cause this symptom to occur, says Aideen Turner, physical therapist and CEO of Virtual Physical Therapists.

“If your knee buckles under you when you are walking, then it usually indicates instability,” she says.

“A lot of people have ACL tears, but if the knee still has functional stability, then surgery is not necessarily needed. But the buckling indicates damage to the cartilage, which means surgery should be performed.”

No matter what you’re going through, remember that a setback is a setup for a comeback. Whether your knee heals on its own, you need some rehab, or you have to resort to surgery, the key is to keep your entire body strong to ensure that your knees have the support and structure they need to keep you moving.

Don’t let your knee stop you from working out. Join Aaptiv today and get classes for meditation, stretching and more.

To help prevent knee injuries in the first place, we recommend taking the following precautions:

1. Warm up before exercising. You can try one of our stretching routines on the Aaptiv app.
2. Use proper gear designed for your chosen activity. Many sports have shoes designed specifically for playing them, and we recommend using those specialized shoes.
3. Keep the muscles surrounding your knees strong.
4. Use the correct exercise techniques. Subscribers to the Aaptiv app get the benefit of having virtual trainers guide them through workouts, ensuring that they’re doing the exercises safely.

There’s no magic formula for preventing knee injuries, but taking some precautions before you step out onto the field or the court and knowing the signs of serious injury can help protect your joints.