Blog : News

20

December

Forward Lean, Knee Pain & Lever Arms (and a bit of cynicism at the end)


If you’ve ever read some of my other blog posts, you would know that I loathe the prescription of “make sure you lean forward when you run” promoted by all the “one size fits all” proprietary running techniques out there (Pose, Evolution, Chi etc)


I firmly believe that running technique is uniquely individual, as demonstrated by the following post…

While I can’t stand the often promoted “forward lean” instructions for running, there is a time and place for it. For example, when a runner is suffering from knee pain – more specifically, patellofemoral pain syndrome (PFPS), a forward maybe justified…but not necessarily. This post gets a bit detailed at times. If you get confused, consult this video – it may help explain.

There are many factors to consider when a runner is suffering from knee pain (more specifically, “PatelloFemoral Pain Pyndrome – PFPS). One of the biggest factors is the stability of the hip in the transverse plane (i.e internal vs. external rotation). However, I plan on making a post on that at a different time

Anyway, I digress…

The point of this article was to talk about PFPS and the “forward lean”. One of the many factors to be considered is trunk position. Recent research has shown that an increased trunk flexion angle angle (bending forward) can be associated with a decrease in knee extensor moment which results in less compressive force on the patellofemoral joint (a joint in the knee). Keep in mind, the forward lean reduces the extensor moment, but doesn’t change the knee flexion angle. (For those about to get mad at me for using “bending” forward and “leaning” forward interchangeably, I find that most people who are told to “lean” forward end up “bending” forward anyway. More on that later…)

So, why does bending/leaning forward reduce the strain on the patellofemoral joint?

Well, very briefly…there are two competing ground reaction forces that are contributing to the external flexion moment at the knee (a force causing it to bend): 1) the vertical ground reaction force and 2) the anterior to posterior ground reaction forces. Essentially, when you land, there is a vertical force being applied to the knee because your body is going downward when you land, and there is also a posterior force being applied because you are also going forward when you land. The quadriceps exert an extension moment in order to combat the two external flexion moments that are being applied. Hopefully, they sort of cancel out and the knee doesn’t buckle.

Unfortunately, the extensor moment at the knee that is exerted by the quads also puts a large load on the patellofemoral joint.

In order to reduce the extension moment on the knee from the quadriceps you need to move the load somewhere else. In this case, by bending the trunk forward, you move the center of mass more forward (closer to the knee), thus reducing the moment arm for the body’s center of mass force on the knee. In this case, you move the load from the knee on to the hip and low back. See figure 1 below

 

Forward lean and lever arms


Before we get any further, if you don’t know what a lever arm is, please look at figure 2 here…

 

Lever Arm Explained

So, here’s the caveat. If the objective is to reduce the moment arm for the body weight to the knee, is leaning forward the best strategy? Well, maybe for some, but certainly not for everyone.


As you can see in figure 1, the lever arm for the knee is reduced by leaning forward, however, the lever arm for the hip and the low back muscles is increased, thus increasing the load for the glutes and the low back muscles. Unfortunately, this is not mentioned in a couple studies that found reduced load on the knee by leaning forward.

It’s a trade-off.

I have included this video to help explain thing – perhaps if is more clear on a video…


Next time someone tells you that you should “lean forward”, please explain all this to them, or refer them to this blog. If you want to reduce the lever arm from the cernter of mass of the body to the knee, another strategy would be to not overstride. Landing with the foot closer to the center of mass, I think, may be a better strategy.

In the end, there are various ways of dealing with injuries and performance. If you think that getting a gait analysis from just anyone is a good idea, you may be in for a surprise. Can they explain joint moments, lever arms, and force vectors? Do they understand detailed anatomy and the functional aspects of it? Do they read research, or do they read blogs? Do they have a “one size fits all” approach?

Sorry if I seem harsh, but I’m getting tired of clients telling me they’ve gotten a gait analysis done, only to find out it was from a source who knows nothing about anatomy, or biomechanics, but they are a “good runner”. I hope you don’t take your broken car to someone who is a “good driver”, but knows nothing about how a car operates.

 

 

- Kevin Maggs, ,

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