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Hip weakness is sometimes blamed for the cause of knee pain in the physical rehab, sports, and fitness fields. However, emerging studies and reviews show that there is lack of strong evidence that correlates muscle weakness with pain, which could put that belief to rest. A recent study published in the British Journal of Sports Medicine reviewed 24 papers that examine the relationship between hip strength and knee pain — more specifically, patellofemoral pain (PFP). Michael Skovdal Rathleff, Ph.D., from the Department of Health Science and Technology at Aalborg University in Denmark, and his colleagues found “moderate-to-strong evidence from prospective studies indicates no association between isometric hip strength and risk of developing PFP.”
Researchers divided the 24 papers into separate meta-analyses, cross-sectional, gender specific, and different ways of testing. “There were only three prospective studies, and they all used isometric strength testing,” said Rathleff in an online interview with Guardian Liberty Voice. “Isometric strength testing is done in a fixed position where your joint or muscle don’t change angle or length during testing.” While the researchers found “moderate evidence” from cross-sectional studies that adults with PFP have lower isometric hip strength than those with no pain, there is limited evidence suggesting that adolescents with PFP “do not have the same strength deficits as adults with PFP.” A limited number of prospective studies also show that there may be no association between isometric hip strength and risk of developing PFP. “Therefore, hip weakness may not be the cause of knee pain — in fact, it is more likely to be a result.
“I think adequate strength is important and we should not neglect strength. But the questions is how many with poor hip strength would develop knee pain while lying in their couch watching Netflix?” Rathleff speculated. “There is always the aspect of knee loading when talking about development of knee pain. I think it will be more precise to say that good strength might make you able to cope with more knee loading before developing knee pain. For example, a runner with good overall hip and knee strength might be able to run 30 miles per week without problems because the muscles work well and help absorb the load. However, a runner with poor hip strength might only be able to run 10 miles per week before developing knee pain. So hip strength modifies how much load you can tolerate. However, these two examples might not be as relevant as they used to be after our review suggested that poor hip strength is not a risk factor after all.”
Knee pain, or almost any kind of pain, is quite complex. It is not a one-way street as it was once thought among medical professionals. “The brain can boss nerves around, tell them how sensitive to be,” wrote Paul Ingraham, who is a former registered massage therapist and currently a science writer. During stress, particularly negative stress, the brain might request “more information” from the peripheral nerves, telling them to make more signals in response to smaller stimuli. Sometimes it might do the opposite. Pain signals do not just go up to the brain; they also go down. “This two-way functionality in the pain system is the main difference between modern pain science and old-school pain science.” Thus, knee pain does not necessarily have to be a biomechanical problem, such as hip or leg weakness.
“Pain is a difficult field. Poor strength will not cause pain per se, but strengthening the muscles might help reduce the pain,” said Rathleff. “The way we think it might work is that poor strength or your knee pain (you could develop poor strength after you develop patellofemoral pain as we write in our review) might cause you to move slightly different. This movement pattern (caused by pain and e.g. low hip strength) might irritate certain joint structures. The joint structures could then send nociceptive information to the brain, which we then process and interpret as pain. If we then strengthen the muscles and start to move better, this might reduce the irritation of the joint structures. They will stop sending nociceptive information to the brain and our knee pain will go away. However, I must stress that this is a really simple explanation and pain is not simple. This relationship between joint irritation and pain might be modified by your mood if you catastrophize about your pain.”
The belief that muscle weakness can lead to pain permeated throughout the medical, sports, and fitness profession for many years. Most of this belief is held in the false assumption of cause and effect.”There is a pervasive issue among clinicians and even researchers in confusing correlation with causation, or by determining what they believe is factual information based on inductive reasoning where a number of observations wrongly confirms a given belief,” stated Dr. Jonathan Fass, DPT, who is a private physical therapist in Riyadh, Saudi Arabia. “In reality, we can never say with 100 percent certainty that any number of observations ‘proves’ an idea, only that it has yet to be disproved. While that may seem counter-intuitive, it is logical.”
“If you’ve ever heard the phrase ‘Past performance is no guarantee of future results’ in investment advice, this is the reason why,” Fass continued. “Basing a future assumption on past data is poor reasoning. No matter how the market had been performing before the recent financial crisis, there was no way that you could have inferred that it will continue to perform well the day before the mortgage bubble burst. Blaming the glute medius or any other muscle for PFP just because you find that there is weakness or dysfunction of that muscle appears to cause pain is like assuming that current results were predictive of past performance, which is the opposite — yet faulty reasoning — of the previous example. For the same reason that we cannot tie two observations together and decide that one necessarily caused the other or that they are even related, we cannot look back and decide that a finding is causative of an initial problem. It would be as if we decided that a lack of chemotherapy (the treatment) was the cause of a patient’s cancer (the disease) simply because the addition of chemotherapy resolved the cancer.”
Rathleff hopes that their recent review will help clinicians understand that hip weakness is not necessarily the cause of knee pain, yet hip strength is important to the overall health of muscles and joints. “It is still important that they assess and treat it when they see their patients, but they should not think they have ‘cured’ the cause of their pain just by increasing the patients hip strength. This is clinically important because finding the cause of someone’s pain will help you treat the cause and help the patients avoid that the pain returns. If we all go around thinking that poor strength is the root of all evil, we will rarely start looking for additional causes of their knee pain.”
By Nick Ng