The intended audience for this post is clinicians, particularly those interested in biomechanics.
The hamstrings can induce knee extension? The soleus can induce knee extension? The gastrocnemius can induce ankle dorsiflexion? What am I talking about?
The potential for muscles/tendons to influence motion and force at joints should be well understood by now, but we really (as with most things in healthcare) don’t know as much as people think. This is a very brief post, but should get your brain working a bit…
When a patient presents with a biomechanical overload injury of a muscle or tendon, I think most clinicians would agree that if you work directly on that body part, you may get some palliative relief, but you usually have not gotten to the root of the problem. More often, the root of the problem lies with some weakness, or tightness somewhere else which is causing the painful part to be overloaded.
For example, if a patient presented with a biomechanical overload injury (overuse) of the quads, or patellar tendon, one of the first things I look at is the ankle dorsiflexion ROM. Reduced ankle dorsiflexion ROM has been shown to be a major player in patellar tendinopathy (here and here). This is intuitive and if you don’t understand why it’s intuitive, read here.
On the other hand, what if I told you I also look at the strength and endurance of the soleus on patient with quad overuse injuries? What if we talked about the idea that the soleus is a major player in creating knee extension, thus it helps out with quads in creating knee extension during midstance? After you called me an idiot for not knowing my anatomy (the soleus doesn’t even cross the knee joint, how could it cause knee extension?), we could discuss…
In an open chain, the soleus creates ankle plantar flexion. However, in a closed chain situation, at midstance, the soleus’ force acts to pull the tibia posteriorly, thus exerting an extension moment on the knee. This has been shown in a few different modelling studies here, here and here, and it’s force in knee extension is not small either; this modelling study found that the soleus was a major player (larger than rectus femoris) in knee extension during gait. So, it would be intuitive to now say that weakness in the soleus can contribute to overload on the quads.
Here is a quick video of an OpenSim model of how the soleus works
Thus, we see the problem with thinking in classic functional anatomy terms where one looks at a picture of the hamstrings and reads a book or two and then thinks that the hamstrings cause hip extension and knee flexion, when in fact, this isn’t the entire picture. For example, if the hip extension moment arm for the hamstrings is significantly greater then moment arm for knee flexion, the greater hip extension (in closed chain) force will result in knee extension by pulling the femur posteriorly. Again, this has been shown in various studies (here, here and here) and is particularly evident in the stiff knee “crouch gait” seen in cerebral palsy.
Continuing with this theme, it has also been shown that the gastrocnemius can exert a net dorsiflexion moment at the ankle in a closed chain by creating a larger amount of knee flexion, in effect, pulling the body downward, creating ankle dorsiflexion (here).
By no means am I trying to say that I think the hamstrings are a primary knee extensor, or that the gastrocnemius is an ankle dorsiflexor. These are merely potential internal joint moments brought up by modelling studies.
We have all seen pictures whereby EMG based muscle firing patterns during gait are supposed to contribute to how we move.
Undoubtedly, this is great for muscle activity, however, the real potential for a muscle to produce torque at a joint is just as much dependent on it’s length/tension curve, force velocity curve, moment arm, muscle path vector and momentum of the body part it is moving.
As usual, I would love to hear your feedback on this, partly practical, partly theoretical discussion.