Do Weak Hips Cause Knee Pain?
You’ve probably heard that hip weakness will increase risk of patellofemoral pain- or pain in front of the knee. It’s still a very common idea out there in the running community.
Through research we’ve learned a lot about hip strength and how it does (or doesn’t) relate to pain. In this post, I’ll discuss why hip weakness seems to be a result of pain, rather than the cause.
Just like we learned in science class: Correlation does not equal causation.
Patellofemoral pain and hip strength
When talking about the relationship between hip weakness and patellofemoral pain pain, it’s important to note the difference between prospective and cross sectional studies:
Prospective studies look at a group of people over time. In this case, researchers measure hip strength in the group, and follow them over a set period of time to see injury rates between strong vs. weak individuals.
Cross sectional studies compare 2 groups of people: A group in pain vs. a group without pain. Then researchers measure hip strength in each group and look for differences between the groups.
In the simplest terms, prospective data follows a group over time to look for risk factors, and cross sectional studies look at different groups at a snapshot in time to see what they key differences are between the groups.
People with knee pain have weaker hips
To figure out if there’s a correlation between pain and hip weakness, the experiment design is pretty simple.
Here is a good example. A research group found a group of people who had patellofemoral pain and compared them to a control group without pain. They looked at many variables, one of which was strength. Once the data is collected, they performed an analysis to see if there were differences in strength between the painful vs. pain free group.
What they found is that the group who had patellofemoral pain was also weaker in their hip muscles.
For a long time, findings like this lead us to believe that weakness was the CAUSE of pain. But here’s the issue, we had no way to tell if these people were weak before they had pain. That is why prospective data is also important. It’s a prime example of correlation does not equal causation.
Individuals with weak hips don’t have higher rates of injury
After reading the paragraph above, you might think that weakness causes pain. But that’s because we only had information about AFTER the onset of patellofemoral pain. We don’t know anything about their strength before they had pain.
To really figure out if weakness is a cause of pain, we need to find a group of people who are weaker in their hips, and see if they end up with more pain than a healthy control group. Prospective studies on hip strength and patellofemoral pain do this exact comparison.
Here’s one example. Youri Thijs and colleagues found a cohort of runners to follow over a 10 week period. They took baseline measurements of hip strength before the study began. Then they followed the group for 10 weeks throughout a running program.
As you might expect, some of the runners developed patellofemoral pain. The researchers sought to discover if these individuals had weakness at the beginning of the study which could have caused their pain.
What they found was that the injured runners had no differences in strength compared to the runners without pain.
In other words, the runners who were weak at the beginning of the study did not have a higher rate of patellofemoral pain than the stronger individuals. There are more studies that have similar findings. You can find an overview of all the data here.
So why does pain cause weakness?
We used to think that hip weakness caused aberrant movement at the knee, which caused more strain on the anterior knee and would eventually lead to pain. But as we discussed above, hip weakness doesn’t seem to cause injuries. So what’s really happening?
Some research on iliotibial band (ITB) pain can help theorize what might be happening.
Theory one: pain can cause inhibition of muscles:
The idea that pain can cause inhibition of the hip muscles is found in the ITB pain research.
Fairclough et al suggest that the tissues of the distal ITB are highly innervated, and compression of these tissues can have an inhibitory effect on the proximal hip muscles.
The exact mechanism isn’t fully understood, but there’s something about distal ITB pain that causes the hip muscles to produce less force. It’s possible that it’s a protective mechanism. Perhaps if the muscles produce less force, then less stress is placed on the painful area.
The anterior knee is a sensitive area as well. The leading theory is that pain in the anterior knee can have a similar inhibitory effect on the hip muscles. But as of January 2024, we don’t know for sure why this happens.
Theory two: pain causes individuals to offload the painful area
Have you ever had pain that causes a limp? Limping is an example of altered mechanics in response to pain. Limping helps to take stress off the painful area, which can allow it to recover.
Kinematic data shows that runners with patellofemoral pain display less motion through their running stride compared to healthy controls. It’s theorized that this decrease in motion is a protective mechanism to put less stress on the painful tissues.
These altered movement patterns result in less stress on the painful area. So it’s possible that pain causes altered movement, or avoidance of painful activities all together. This decrease in stress/use of the painful side can lead to weakness over time.
It’s a cycle: pain leads to offloading the painful area. Offloading the painful area makes it weaker. Weakness makes it harder to use the painful limb normally, which leads to more offloading, more weakness etc…
But the truth is we don’t totally know why pain results in weakness. More research will help to figure it out.
So should I stop strengthening my hips?
Nope! You can keep strengthening, but don’t expect it to prevent injuries. We do see that strength training can improve running economy. I’ve talked about that here.
Hip strengthening is still an effective treatment for knee pain. When combined with knee strengthening, hip strengthening is more effective at treating knee pain than knee strengthening alone.
Summary:
Hip weakness doesn’t seem to be predictive of future running injuries
After the onset of pain, people with knee pain have weak hips compared to healthy controls
Knee pain is likely a cause of hip weakness, rather than the other way round.
We’re not 100% sure why pain causes weakness. It could be an inhibitory mechanism, or a result of offloading the painful area which causes it to become weak
Hip strengthening is still beneficial, but not for preventing injuries
Hip strengthening is a good treatment for knee pain
If you’re struggling with patellofemoral pain or knee pain, book a free phone call to see if we’d be a good fit for your rehab journey!
We offer in-home outpatient rehab in the Greater Chicago area. No referral necessary, just call and we can get started!
