In this article, I’ll explain why knee swelling occurs.
(In Part 2, I'll explain what to do for knee swelling-- rather than ignoring it.)
[ read time ~ 5 minutes ]
The onset of knee swelling is often sneaky. It comes on gradually without an obvious trauma or injury.
This was the case for my client Bill*. He was 72 years old and had no plans of giving up his active life. Bill had sustained a knee injury playing basketball as a teenager, and he switched from running to cycling in his 30s to avoid knee pain. Over the past several years, he noticed some periodic knee pain that was worse on his left. And following a 5-day cycling tour, his pain was even more obvious and the swelling was undeniable.
Bill began working with me, and during his first session, his left knee measured 41.5 centimeters around (just above his knee cap) compared to 40.5 centimeters on his right knee. (While up to 0.3 centimeters is an expected measurement discrepancy, a difference of at least 0.5 centimeters is significant for swelling.)
Bill’s left knee was a full centimeter bigger. Along with his inability to bend his left knee as far as could bend his right knee, this measurement made it clear that his left knee was swollen.
Why does knee swelling occur?
In most cases when a bone hasn’t been broken and nothing has been torn (like a ligament), swelling occurs when there’s weakness inside the knee-- specifically weakness on the moving surfaces.
Movement happens inside your knee between the end of the femur (i.e., thigh bone) and the top of the tibia (i.e., shin bone), as well as between the kneecap (i.e., the patella) and the groove on the end of the thigh bone. The surfaces of these bones are all covered with a layer of cartilage.
Strong, healthy cartilage is a slick, hard type of connective tissue that protects the underlying bone--similar to the way a bumper protects the frame of the car. Cartilage also decreases the friction during movement.
Cartilage maintains its strength through motion and the right amount of pressure being placed on it. This is similar to how a bone heals when it has a fracture. Too much force-- such as jumping on a broken leg-- causes the bone to become weaker and even more injured. But bone healing is slower when there are no forces applied to the bone; so frequently someone with a stable lower leg fracture wears a walking boot to protect the injury yet allow some force to facilitate healing. Once the bone has healed, it stays strong with an adequate amount of pressure.
Applying the right amount of pressure keeps bones healthy and in an analogous way, it strengthens cartilage.
Walking is an example of applying pressure to knee cartilage.
With each step, the impact of a person’s body weight is absorbed up through that leg. In most people without knee pain, this is one type of pressure that stimulates the knee cartilage to stay strong.
Without this force-- in the case of astronauts (who move around in space without impact and grow weaker) and in the case of people who become more sedentary (who sit more and walk less)-- both bone and cartilage strength diminishes.
Too little activity causes cartilage to weaken.
But cartilage can also weaken with too much force. Let’s say you remain active, and you add jumping rope to your exercise routine. (Jumping places a force through your legs that’s over three times your bodyweight.) If you haven’t been jumping routinely, this might mean that several minutes of jumping rope will start to apply too much pressure to the cartilage in your knees.
Too much activity also causes cartilage to weaken.
Why is weak cartilage a problem?
Weak cartilage may not be an immediate problem-- just like a weak ankle may not feel like an urgent issue. However, if your ankle is weak, when you step on the edge of a rock or uneven pavement, it may bend too far in one direction. Your ankle will likely hurt immediately, and it soon swells up. Your ankle is injured. Had your ankle been stronger, you might have been able to keep it from bending so far in one direction.
The parts of your ankle that are injured when you injure it have nerve supply, so a pain signal is quickly sent to your brain. But cartilage doesn’t have nerve supply. If you apply forces to weak cartilage, it has no way to quickly tell your brain that it’s getting hurt.
When cartilage isn’t strong enough to withstand an exercise (such as running or jumping) or an activity (like taking the stairs, or even something like sitting with bent knees for a long time) there is often a delay of at least several hours before swelling occurs. It’s not obvious that an injury is happening to the cartilage while it’s occurring.
A knee with weak cartilage may not feel “full” or look “puffy” until the next day. By this time, it might be hard to identify what caused the swelling.
While the swelling may resolve on its own, chances are high that repeating the same exercises or activities will cause the swelling to return, and possibly even worsen.
Explaining Knee Swelling
Knee swelling-- even if there wasn’t a trauma or an obvious injury-- indicates that the cartilage inside has become weaker. This weakness may be due to too little activity or too much activity. Sometimes it’s a combination of both: an extended period of time with less exercising followed by resuming previous exercises too quickly.
Swelling is a sign that what you’re doing is too hard on your weakened cartilage.
My client Bill was bothered by his knee pain, but he was not initially worried about the swelling. It was an annoying sensation, but he found ways to ignore his knee swelling. However, his knee pain didn’t improve until his swelling began to resolve.
Read Part 2 to learn what Bill did to eliminate his knee swelling.
Learn more about delayed knee symptoms: The 90 Day Knee Arthritis Remedy.
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