According to this research paper, leaning forwards during a run may be a way to reduce frontal knee pain, since it effectively causes the body’s weight to be borne by the hip area rather than the knee area.

Various potential problems with this advice have been discussed here and here.

In the abstract of the abovementioned article, the authors write that “sagittal-plane trunk flexion has a significant influence on hip and knee energetics during running. Increasing forward trunk lean during running may be utilized as a strategy to reduce knee loading…”

The problem I see isn’t with the research article, but with well-meaning people using the phrase “leaning forward” as a shorthand for sagittal-plane trunk flexion. Sagittal-plane trunk flexion isn’t the only way to lean forward: we can also achieve this by lumbar spine flexion. A true sagittal-plane trunk flexion creates forward lean as a function of flexion at the hip, rather than at the spine (think “sitting up straight”).

In summary, there are (at least) two possible ways to move the center of gravity forward: at the trunk, and at the hips.

Typically, a runner with lumbar spine flexion is compensating for a weak/badly synchronized gluteus maximus/psoas major system—more on this later—with excessive abdominal flexion, thus putting a lot of strain on the back extensors during late stance and pushoff phase of running gait.

Furthermore, a person can be in a state of chronic sagittal-plane trunk flexion because of a loss of hip extension, meaning that their hip flexors are so tight that their glutes are weakened. A runner who leans forward because of this problem will typically have strained lower back muscles.

The solution isn’t to unilaterally achieve sagittal-plane trunk flexion. The solution is to create it in function of resolving biomechanic problems at the hip.

This problem hides a question: How do we give advice that is tailored in such a way that it promotes people to make the right choice biomechanically speaking (flexing the hip), rather than the wrong one (flexing the back)?

In the comments section of a great article on the topic at runningreform.com, Mike Andersen suggests that using the term “lean” might be a bad way to go, and a better term would be to use the word “angle.”

By changing the angle at which our whole body leans forward, we maintain the same saggital plane trunk flexion while achieving a forward tilt at the hips and ankle simultaneously, because the whole body is in line.

If I could only give a single piece of advice—and I never would, which is why I’d rather give this long explanation—it would be to “sit up straight when you run, and once you can sit up straight, sit up straight and forward.”

Leaning forward means that the hip moment arm increases: as the weight travels forward in relation to the hips, you need stronger and stronger hips to maintain speed without falling. A similar thing happens with squatting. The deeper the squat, the further forward the weight needs to be:

Therefore, in my opinion, it’s easier (and safer, in terms of the unintended consequences of our advice) to look at this problem in the inverse: we understand a lack of forward lean not as a cause of knee pain but as a result of hip dysfunction. If we have frontal knee pain, it is likely because we’re not leaning forward enough, and we’re not doing that likely because we have weak hips. If, with weak hips, we decide the solution is to lean forwards (rather than strengthening the hips, and having the increase in angle be a function of that), then we’ll force a situation where we have to compensate in order to maintain that angle, and the easiest way of doing that is by flattening the lower back—in other words, by hunching forward.

The most basic solution to this problem is simple: strengthen the hips and develop hip mobility.

(This, however, is not necessarily the whole solution for people who have a more complex gait pathology).

Why is this the basic solution? Let’s look at how the hips are structured.

The two most powerful hip muscles are the psoas major and the gluteus maximus. They work in opposition to each other: the psoas major pulls the femur up and slightly to the inside, and the gluteus maximus pulls the femur down and slightly to the outside. The important part is that the psoas major, which anchors to the lumbar vertebrae, also manages to pull the spine down and towards the femur. (This is why you see forward trunk lean in people with a tight psoas major).

We can see the interaction in full force in this slow-motion video of Dennis Kimetto’s record-breaking marathon run (although we shouldn’t forget that there are many other muscles at play). Ultimately, it is the correct interaction of all the muscles, and not just the two I singled out here, that will lead to a good running gait.

In the video, it is plainly clear how when the gluteus maximus is engaged during the pushoff phase, the erector spinae (along with associated back muscles such as the quadratus lumborum) maintains the curvature of Kimetto’s back. On the opposite side, which is in full flexion, that same curvature is maintained by the psoas major on the front side.

Kimmeto’s forward lean is established by a powerful gluteus maximus and erector spinae on one side pulling the leg back, and by a powerful psoas major on the other side, pulling spine down and forwards.

This keeps the pelvis tilted forward at all times during the running stride. 

If Kimmeto’s psoas major wasn’t as effective at maintaining the curvature on the swing side, we would see one of two things happen: he either maintains speed by flexing the lumbar spine, putting immense strain on erector spinae and associates when that leg comes down (and gets hurt at mile 10), or, more likely, his competitor Emmanuel Mutai leaves him in the dust. There is no way that Kimmeto could both (a) not get hurt, and (b) maintain that forward lean without the gluteus maximus and the psoas major working together effectively.

UPDATE: I’m working on a post about good, transferable exercises/drills that develop the interaction of the hip flexors and extensors.