A new study paints a subtler picture.
Written by: Matt Fitzgerald
One of the first lessons students are taught in sports medicine classes is that the cause of pain in one part of the body may originate in some other part of the body. In 2000, Michael Fredericson at Stanford University suggested that iliotibial pain, a common knee injury in runners, might be caused by weak hip abductors. The study in which Fredericson presented this idea was highly influential, inspiring many other researchers to further investigate the relationship between hip muscle strength and knee injuries in runners over the past decade.
In a 2007 study, researchers found that collegiate female runners with patellofemoral pain, the most common knee injury in runners, were weaker in the hip abductor and hip external rotator muscles on the side of the injured leg than on the other side. These muscles were also weaker than those in uninjured runners. But a more recent study by Belgian researchers reported that differences in hip muscle strength did not predict the likelihood of knee injuries in a pool of initially uninjured runners. This finding suggests that weak hip abductors might be an effect of knee injuries, not a cause.
Now along comes yet another study in these series, this one conducted by scientists at the Mayo Clinic in Rochester, Minn. Like the Belgian study, this new one featured a prospective design, meaning it looked at injury rates over a period of time in an group of initially uninjured runners. In this case the subjects were high school runners. The researchers measured strength in hip abduction (lifting the leg out to the side), hip adduction (pulling the leg inward toward the body), hip external rotation, and hip internal rotation in a total of 98 runners. They then tracked the incidence of patellofemoral injuries throughout a competitive season. Finally, they looked for differences in the strength measurements of the runners who got injured and those who did not.
Five of the 98 runners developed patellofemoral pain. There were some interesting differences between these runners and those who escaped injury. First, the runners who got injured actually had greater hip abductor strength initially. Also, their hip abductors were significantly stronger than their adductors, whereas in the runners who did not get hurt this difference was smaller. When the researchers looked at the relationship between strength in external and external rotation, they found that runners who did not get hurt exhibited greater strength in external rotation relative to internal rotation than the runners who developed knee pain. These findings suggest that the balance of strength in opposing muscle groups in the hips may be a more important determinant of knee injury risk in runners than the absolute strength in any single muscle group.
The Mayo Clinic team also found that the strength of the hip external rotators decreased after the onset of knee pain. This finding confirms the suggestion of the earlier Belgian study that weak hip abductors may be primarily an effect of knee injuries instead of a cause. But the new study also found that hip external rotation decreased after injury. These strength losses may be a matter of central inhibition. In other words, the brain does not allow the muscles to contract with as much force as a way of protecting the injured knee. If this is so, strengthening exercises may not help the injured runner overcome knee pain.
All of this leaves the picture rather cloudy. Should runners strengthen their hip muscles to prevent knee injuries? Should they strengthen their hip muscles once they’ve emerged? If so, how? The total body of research in this area offers no definitive prescriptions. But if we combine the evidence that is available with some common sense, we can make some good choices.
Muscle weakness in itself is rarely the cause of injury. The problem is muscle imbalance. If a certain muscle group becomes much stronger or weaker than the muscles that work in opposition to it, movements become unaligned and damage occurs. Since hip muscle imbalances appear to exist in runners at risk for knee injuries, you should work on strengthening your hip muscles. But you must do so in a balanced manner, with exercises involving both abduction and adduction, and both internal and external rotation.[sig:MattFitzgerald]